Introduction

In Lewis Padgett's1 short story, Private Eye, it is discovered that the whole of the physical environment doubles as a pervasive recording system: On every wall, every tree, every patch of ground people walk upon, their every action and conversation leaves "'fingerprints' of light and sound waves."2 Scientists have learned to "descramble" these naturally created records of people's activities and compile them into video archives containing every individual's entire past.3 Government investigations proceed very differently from those in our own world. Police detectives spend most of their time in a screening room, rewinding and fast-forwarding through each suspect's life.4 When they want a closer look, they can slow or pause their film to examine "every expression of his face, every muscular flection, every breath he [draws]."5 When curious about the experiences that have shaped him, they can instantly transport themselves back into his childhood.6 Even those not currently under such a government microscope know that each moment of their lives is preserved for "[a]n invisible audience from the future."7

The inescapable surveillance that Padgett describes in his obscure 1949 story resembles that described in another more famous science fiction tale published the same year: George Orwell's 1984. Like George Orwell's vision of a technologically transformed future,8 the world imagined by Padgett is one where individual privacy is brought by science to the brink of extinction. But unlike the inhabitants of Orwell's imagined world, who have resigned themselves to living without their privacy, the inhabitants of the world described by Padgett have made an impressive and arguably effective effort to save it. They have countered the threat posed by exotic "past-tracing" technology with a not-so-exotic legal device: a rule that past-tracing evidence can be accessed only for purposes of investigating a "serious crime" and introduced at trial "only if it [has] a direct connection with the crime."9 Although natural barriers no longer stand in the way of paternalistic or distrustful officials, the law continues to do so: Government investigators are allowed to explore people's pasts only when doing so is indispensable to the task of protecting people or of apprehending those who have harmed them.10

The courts and legal thinkers of Padgett's time did not carefully examine the effectiveness of such protections, and there was little reason to worry about this question in 1949, when nothing remotely like "past-tracing" technology played a significant part in their day-to-day lives. But the challenge outlined in this story is a more pressing concern now. While the physics of Padgett's imagined world have remained firmly in the realm of fiction (there are no hidden video recordings encoded in wood, stone, or soil), its privacy-eroding technology is not all that far from becoming reality. Walls, lampposts, and trees do not function as natural video cameras, but new technologies allow public authorities to line them with artificial ones. A growing number of communities throughout the world are doing so on a massive scale. The United Kingdom has led the way. Cameras now encircle the center of London in a "ring of steel," photographing the license plate and driver of every vehicle that enters.11 A massive video surveillance system also watches the interior of the city.12 Other British cities-according to one count, at least 440 in all13-have also covered whole neighborhoods and business districts with cameras. American cities are rapidly following suit. Although most residents and visitors of New York remain oblivious to them, thousands of video cameras,14 many "indistinguishable from lampposts," sit above parks and streets throughout the city and on the campuses of schools and universities.15 The City of Baltimore has installed cameras at "all 106 downtown intersections" and in its Inner Harbor area.16 The Washington, D.C. police department operates cameras that watch over downtown streets, subways, parks, and other public spaces and has plans to substantially expand its video surveillance system in the near future.17 Chicago has recently joined the list of major cities installing cameras over public streets,18 and other American cities are doing so as well.19

The cameras now proliferating in urban spaces are in many respects far more powerful than the video cameras of the past. Most can quickly pan, tilt, or rotate 360 degrees at the command of a far-away control room.20 They can isolate an individual in a business district or subway station, zoom in on him, and scrutinize facial expressions, movements, even reading materials in close detail.21 They often work not as isolated units, but as components of an integrated network of cameras that sends information from many different parts of the city back to a common observation center, which can then analyze the data as a whole or follow a person as he moves from one part of the city to another.22 The digitization of video images and tremendous expansions in computer memory have made it possible for operators to store substantial amounts of visual data and retrieve and search this data when they have a desire to view it.23

In fact, the technology being developed today may one day go beyond the science fiction analogy. Detectives in Padgett's world had comprehensive records of images in video archives. Investigators in our own world have methods of connecting those images to identities and other information beyond what the camera shows. Using facial recognition software, investigators might quickly match a face to a specific name and then (with the aid of other databases) to that person's "medical history, tax records, criminal arrest records, voting records, political affiliations, and any other conceivable type of information."24 Under such a surveillance regime, each life might become not merely an electronic "open book," but an "open web site," which investigators can use not only to rewind or fast-forward through large portions of a person's history, but to link to extensive data on (and perhaps visual recordings of) that person's colleagues, the organizations she belongs to, and various discussions or references regarding her that take place in her absence.25

To be sure, the increasing use of cameras and facial recognition software does not by itself condemn us to live in a world where our pasts can be "traced" by curious officials. The vast majority of images captured by these cameras are likely to receive little scrutiny from the necessarily limited staff that operates them, and most images they do register are likely to be forgotten soon afterwards.26 Facial recognition technology likewise must overcome significant hurdles before it can function as an effective mechanism for instantly tracking and identifying people in public streets.27 But technological developments are rapidly transforming camera systems and facial recognition devices into far more powerful instruments than they once were.28 Although we do not yet live in Padgett's world, "[o]ne need not be a science fiction fan," as mapping expert Mark Monmonier points out, "to envision a future in which cameras as dense as streetlights feed images to central computers with face-recognition algorithms and biometrics software that match pedestrians to their stored profiles and track their movement through streets and parks."29

Paradoxically, the part of Padgett's imagined world that fits least comfortably into the emerging landscape of twenty-first century government surveillance is the part that would probably strike Fourth Amendment scholars as the most familiar: the rule that visual records of our lives can be accessed only for purposes of investigating a "serious crime."30 This is not because there is anything obviously unreasonable about requiring government officials wishing to page through an individual's past first to request a warrant based on probable cause that such an intrusive investigation will uncover evidence of criminal activity. On the contrary, there are good reasons to think that our legal regime should interpose a warrant requirement, or some equivalent legal hurdle, between government authorities and video records of its citizens' day-to-day lives.

The Fourth Amendment protects "[t]he right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures."31 Police must obtain a warrant from a neutral magistrate before they can engage in technologically-primitive searches of an individual's journals, letters, and other evidence of past activities.32 It would be odd to place no such restrictions on their ability to view a more comprehensive and vivid record of a person's history in a video database.33

Indeed, the absence of some such constitutional limitation seems to leave authorities free to engage in a variant of the dragnet searches that the Fourth Amendment was clearly intended to prevent. The drafters of the Bill of Rights gave individuals protection against "unreasonable searches and seizures" in order to assure that people walking down a street, for example, could not be stopped randomly and searched by a government official who had no reason to suspect them of wrongdoing. With comprehensive video archives, authorities would again be able to randomly stop and closely scrutinize numerous people on public streets, doing so this time by pausing on a person's image, enhancing or magnifying detail, and electronically matching aspects of each person's appearance against biometric or other databases. Such silent and invisible searches by far-away camera operators do not limit a person's physical movement34 or subject him to the kind of unsettling physical intrusion that occurs when a police officer stops and frisks him in the street. But the sense that one is at all times subject to close monitoring can be just as unsettling as a brief on-site search. Unlike the individual who is freed from intensive scrutiny after a police frisk, an individual walking through streets laden with cameras can never be sure that the monitoring has ended.35

It would not be surprising, therefore, if courts sought to ensure that such powerful electronic personal searches took place only within constitutional boundaries as strict as those which confine their (more spatially and temporally limited) physical counterparts-perhaps only, as Padgett suggested, when authorities have probable cause to believe that searching a particular person or place is necessary to investigate a "serious crime."36

But while such constitutional limits on wide-scale video surveillance may seem intuitively reasonable and necessary, contemporary Fourth Amendment jurisprudence is ill-equipped to provide or even delineate them for at least two reasons. The first is that mass video surveillance occurs in the public realm-in streets, parks, and highways-where courts have been reluctant to find that individuals have reasonable expectations of privacy, at least in that information which they fail to conceal.37 Unlike random stops and searches by government officials, extensive video surveillance does not dig beneath the visible surface that people project to the world.

As a consequence, contemporary Fourth Amendment jurisprudence differentiates pervasive video surveillance from more familiar mass suspicionless searches in one crucial respect: by holding that it is not a "search" at all.38 Fourth Amendment "searches," according to the Supreme Court's current test, do not include all investigations of the sort an English speaker might describe as a "search."39 As the Supreme Court emphasized in its landmark decision in Katz v. United States, which still provides the key legal test for what counts as a "search," "what a person knowingly exposes to the public . . . is not a subject of Fourth Amendment protection."40 Thus, even when police carefully scan a crowd with binoculars, in search of a particular person, they are not engaging in a Fourth Amendment "search."41 Fourth Amendment interests are implicated only when the government uncovers things that people conceal. Because the Fourth Amendment offers protection only against suspicionless searches and seizures-and not against suspicionless examinations (no matter how rigorous)-public camera networks would seem to be outside of the Fourth Amendment's ambit, at least as long as their focus remains on public space and does not wander into private homes, offices, or other enclosed areas.42

In the context of mass video surveillance, however, this is a strange result. Even a video archive that includes only a person's movements through public settings would inevitably reveal much that he would rather not share with an audience, let alone have incorporated into official records. A person usually cannot enter a psychiatrist's office, marriage counseling center, or infertility clinic except from a public street. It is often in public that people ask others out on a date, join a religious community, or seek resources in a library for educating themselves about medical concerns or social dilemmas. Of course, even in these deeply personal aspects of our public lives, there is at least a small chance we will be photographed or filmed by others nearby. But such third parties are unlikely to know when we will be there or who we are, and they will usually go away with only a brief snapshot of our lives. By contrast, a government agency armed with a comprehensive visual record of our public activities would not have to guess when we might reveal personal information in public, as it could probe our lives after the fact, and might quickly build a more complete picture (for example, figuring out what specific medical or social problem led us to a certain source of help) by looking elsewhere in its substantial database of the recorded images and other information that we leave behind as we move through visually-surveilled public space.

Moreover, making so much of our day-to-day lives available for display intuitively constitutes a much more significant intrusion into our privacy than many briefer and more limited examinations that courts have not hesitated to classify as "searches." The Supreme Court has given force to Fourth Amendment protections, for example, where a principal looks through a student's purse.43 Students "may find it necessary to carry with them a variety of legitimate, noncontraband items," said the Court, "and there is no reason to conclude that they have necessarily waived all rights to privacy in such items merely by bringing them onto school grounds."44 Reviewing a roadblock program on public highways, the Court likewise emphasized that "people are not shorn of all Fourth Amendment protection when they step from their homes onto the public sidewalks. Nor are they shorn of those interests when they step from the sidewalks into their automobiles."45

In the above cases, the Court discussed things or acts hidden from the rest of the world in a car or a container. But the same logic applies with equal force to the activities captured by public cameras: it is difficult (if not impossible) for individuals to avoid providing significant evidence of thoughts and personal interests as they walk on a public street-through their facial expressions, interactions with others, and choices of activities. A detective or spy wishing to build a dossier on an individual's life and personality would probably learn more from examining a searchable database of such images than he would by rummaging through a purse, wallet, or suitcase, especially if he could link from the images to other information about the individual's identity and background. Yet contemporary Fourth Amendment jurisprudence seems to provide protection only against the lesser of these intrusions. Indeed, while public surveillance video systems were first used in the 1960s46 and are becoming more prevalent, federal courts have yet to seriously address the question of how to analyze them under the Fourth Amendment.47

When they do, they will find there is a second reason-apart from the "surface-bound" nature of such surveillance and its focus on open places-that contemporary Fourth Amendment doctrine may fail to give force to the probable cause protections that Lewis Padgett imagined would keep powerful video surveillance technology in check.48 The Fourth Amendment does not bar all warrantless searches; it bars only those that are unreasonable.49 Even if public video surveillance is a search under the Fourth Amendment, one might argue that it is nonetheless reasonable even when left unconstrained by warrant and probable cause requirements. Such arguments have been most likely to gain support when the crime the government investigators are working to prevent is an act of terrorism.50 Many ordinary would-be criminals might be deterred from theft or violent crime simply by the prospect that the police will be able to easily identify them after the fact (although the evidence for the deterrent powers of existing surveillance systems is by no means clear).51 By contrast, suicide bombers are much harder to detect and deter. They are often unintimidated by the prospect of being identified in the aftermath of a bombing and are likely to have done all the damage they want to do-in the form of massive loss of human life and massive damage to individuals' sense of security-before police even begin their investigation. To fight terrorism effectively, one might argue, authorities must closely scrutinize numerous people before they have probable cause for focusing on one person or another, just as airports trying to prevent hijackings examine all passengers and all luggage instead of trying to narrow their search for terrorists on the basis of insufficient information.52

In the past, courts have recognized that the need to avoid such devastating loss of life demands flexibility in interpreting search and seizure requirements and sometimes requires allowing authorities to conduct searches without any "individualized suspicion" of wrongdoing.53 As Justice O'Connor recently noted, courts have been more willing to dispense with individualized suspicion requirements when "even one undetected instance of wrongdoing could have injurious consequences for a great number of people."54 For example, "fires and epidemics [that] ravage large urban areas,"55 train wrecks that cause "great human loss,"56 and plane hijackings that claim "hundreds of human lives"57 are less likely to require these individualized requirements. The threat of terrorism seems to provide just as compelling of a reason to lift warrant and probable cause requirements that might slow the use of public video surveillance to gather (and track) leads needed to uncover and prevent planned terror attacks.

But the battle against terrorism does not easily fit into the model created by courts to deal with the safety threats they addressed in the latter decades of the twentieth century. The searches used by the government to address each of the threats described above were brief and confined to a certain facet of life: weapon checks that take place only at airports,58 periodic and limited housing inspections,59 and brief alcohol and drug tests aimed exclusively at train operators or other employees whose jobs had potentially significant implications for public safety.60 Unlike the threats such searches are meant to detect, the threat posed by terrorism is designed to create, and often does create, a fear of sudden and devastating loss that is not confined to a limited portion of our day-to-day existence, but instead permeates the whole of public life.61 This widespread threat was made clear by the variety of settings and possible scenarios that were the subject of terrorist warnings in the months after September 11, 2001. The government warned that explosives might be used in malls, bridges, apartments, and trains;62 that scuba-diving terrorists might sabotage boats or strike coastal areas;63 that attackers might use trucks or other vehicles as weapons on public roadways;64 and that they might poison water supplies,65 target large holiday gatherings66 and national monuments,67 shoot down airplanes with missiles,68 or use explosives, chemical weapons or radiological bombs to kill thousands in subways or town centers.69 The variety and unpredictability of possible attacks, in method and location, has made some authorities despair of countering them except by suspicionless surveillance techniques that stretch across all of public life, and capture all manner of detail.70

But such a proposal is troubling because even if advanced surveillance technologies showed great promise in countering terrorism,71 unconstrained and pervasive camera networks would protect the public sphere only by changing its character. The use of such technologies might lessen anxiety about violence in public spaces, but it would do so by undermining the forms of freedom that people traditionally seek (and find) in these spaces.72 Generally, the Supreme Court has allowed generalized suspicionless surveillance only in environments such as workplaces, schools, and high security zones, where people are already subject to a substantial degree of oversight and constraint.73 Even in these situations, it has imposed constitutional limits on the scope of such searches to ensure against abuses of discretion.74 Individuals who have to be self-conscious in such controlled environments have been able to find a refuge in other public settings-parks, streets, public squares-for freer and more spontaneous behavior, something they could hardly do if such settings were under a scrutiny even more sustained and extensive than that the courts have allowed in highly-regulated environments.75

This lack of fit between existing jurisprudence on the one hand and emerging threats to privacy and security on the other hand requires a novel analysis of search and seizure protections. Even the significant constitutional thinking that courts have engaged in to fit the Fourth Amendment to twentieth-century developments-new electronic surveillance technologies and security threats distinctive to modern life-needs rethinking if constitutional privacy protections are to work well in twenty-first century conditions. In particular, courts have to think carefully about how to give Fourth Amendment protections greater force in the public sphere.

How can courts extend constitutional privacy protection to open and observable activities in spite of long-standing judicial reluctance to do so? One might suggest that the starting point for an answer lies in strongly reaffirming one of the most widely-repeated statements in the Supreme Court's influential decision in Katz v. United States: "[T]he Fourth Amendment protects people not places."76 After all, one of the key arguments against extending constitutional limits to public camera systems is that the sidewalks, parks, and plazas that these cameras watch over are not private places like the home.77 The best way to respond to this claim, one might argue, is to stress that the Fourth Amendment does not protect the privacy of places, but the privacy of the people in these places, and its protections can move with people as they leave their homes and move from place to place, taking private information with them.78 One can restate this point in the language that Justice Harlan proposed in his concurring opinion in Katz, which the Court has since adopted as its test for what constitutes a "search."79 The Fourth Amendment, one might say, protects privacy anywhere that people reasonably expect to have such privacy.80 Since people reasonably expect to be free from ongoing government surveillance even on sidewalks, plazas, and parks, the Fourth Amendment should have force in these public environments as well as in the home or office and should allow for monitoring of public life only to the extent needed to prevent terrorism or serious crime. This kind of justification is offered by certain powerful critiques of unconstrained public video surveillance. For example, a former justice of the Canadian Supreme Court advised that, under his country's analogue of the Fourth Amendment, courts should dispense with "rigid, formalistic borders between private and public spatial domains" and instead attend to what constitutes a "reasonable expectation of privacy in a given context."81 Christopher Slobogin has likewise built a compelling case for constitutional restriction of public video surveillance by "tak[ing] seriously the Court's admonition that the Fourth Amendment's scope is ultimately defined by 'expectations of privacy society is prepared to recognize as reasonable.'"82

This Article, however, will suggest that a different approach provides a more promising foundation for modern Fourth Amendment jurisprudence. It will argue that, contrary to Katz's famous pronouncement, courts can often best protect privacy in public life by focusing on places rather than the people who act in them. Instead of protecting individual expectations of privacy directly, courts might best protect privacy in public life indirectly by identifying and protecting those features of our society, including those features of public space, that allow anonymity and other privacy-related interests to exist in sufficient measure. This approach better captures what is disturbing about widespread public video surveillance. Such surveillance threatens Fourth Amendment values not simply through its effects on the privacy of any individual activity, but by wholly transforming the public environment in a way that is at odds with core requirements of a free society. This approach is also more helpful than the Katz framework in clarifying the core of the challenge that confronts us as we adapt Fourth Amendment protections to the threats posed by terrorism and other violent crime. This challenge is not to freeze and give force to every existing expectation of privacy that individuals might have had before confronted with such threats. Nor is it to make Fourth Amendment protections fluctuate with Americans' changing (and heterogeneous) preferences about privacy. It is rather to assure that, even as courts allow government officials to hunt more vigorously for evidence of criminal activity or signs of terrorist threats, and use new technologies to do so, they do not compromise those core privacy protections that are integral to a free society.

To be sure, Katz's "reasonable expectation of privacy" test has not been without value in protecting privacy in the face of previous technologically-driven transformations of our environment. It created a useful framework for analyzing bugging, wiretapping, thermal imaging, and other techniques that allow investigators outside of a home, office, or phone booth to somehow look or hear inside. However, this framework is not as useful for analyzing emerging video surveillance systems, which invade our privacy by continuously gathering and analyzing the significant evidence of our thoughts, interests, and actions that we leave in the "outside" world itself.

Part I looks more closely at the development of the Katz test and considers why, although the Katz majority set out to provide a framework that would protect privacy "even in an area accessible to the public,"83 its protections actually extended only to spaces that were in some sense enclosed or marked off by clear boundaries from the outside world (for example, homes, phone booths, and perhaps "virtual spaces" for electronic communications).

Part II explains how recent technological developments-particularly in video surveillance, tracking technology, and biometric identification-have allowed officials to circumvent Katz's protection of private environments by collecting significant information about us that we inevitably leave behind as we move through public space. While courts have not squarely confronted this difficulty, some courts have noted it and expressed the sense that it may require somehow extending Fourth Amendment protections even to activity that is already open to public view.

Part III proposes a solution to this problem. Just as the Supreme Court after Katz (and most notably in Kyllo)84 barred governments from simply circumventing (or eroding) the privacy-protecting features of houses and other traditionally private environments, twenty-first century courts should similarly bar government from technologically nullifying the privacy-protecting features of public space. As this part explains, such a focus on protecting the public environment has an important advantage over Katz's "reasonable expectations test." It frees courts from the burden of making controversial judgments about what kinds of individual activities are sufficiently "intimate" to deserve Fourth Amendment protection. Having fortified the features of both private and public environments that make unmonitored activity possible, courts can leave individuals to decide for themselves what legally permissible activities they would like shielded from observation by others. I also explain here why this task-the task of preserving an environment that is suitable for privacy and liberty-should be a matter of constitutional law, and not only a job for legislators. Although this judicial imperative will necessarily place some limits on the uses majorities make of new surveillance technologies, it need not cast courts in the role of Luddites, bent on denying police valuable crime-fighting technologies that many others (including perhaps criminals) are left free to use. A sound Fourth Amendment jurisprudence for public space would let law enforcement agents make extensive use of new technologies so long as protections for privacy interests are built into the technology itself or provided by a warrant or warrant substitute, which ensures that such technologies are used narrowly for proper ends.

Having examined more closely what this alternative to the Katz test entails, Part IV of the Article then asks whether this approach for replacing Katz's reasonable expectations test is really superior to a revised "reasonable expectations" approach-one that explains why people might in fact reasonably expect protection against unconstrained video surveillance even in public. Although the language of reasonable expectations certainly allows room for a vigorous defense of "public privacy," its ambiguity blurs the clear lines people often depend on to figure out where and when they are free from monitoring and leads courts to confuse situations where privacy interests are absent with very different situations where privacy interests must share space with other important public interests, but deserve vigorous protection at the same time.

Finally, Part V examines the question of when it is reasonable for police or other government officials to use public video surveillance even though it is a search. It argues that while the devastation and unpredictability of terrorism may make virtually every search in public seem a reasonable and necessary one, the need to protect the character of the public sphere requires courts to insist, whenever possible, on statutory, programmatic, or technological constraints that will make video searches as safe as possible for the freedom from government scrutiny that people have traditionally found in streets, parks, and other public spaces.

I. The Katz Revolution: Strengthening and Expanding the Protection of Private Spaces

Science fiction writers are not the only ones who have imagined a world where all forms of individual privacy might be erased by futuristic technologies. In 1928, Justice Louis Brandeis noted that "[i]n the application of a constitution . . . our contemplation cannot be only of what has been but of what may be"85 and went on to describe "what may be." He imagined various threats that scientific advances might one day pose to Fourth Amendment protections: the possibility that "without removing papers from secret drawers," officers might somehow "reproduce them in court" and that "advances in the psychic and related sciences may bring means of exploring unexpressed beliefs, thoughts and emotions."86

Brandeis's purpose in painting this picture of future technology was to show that the Fourth Amendment interpretation of many of his Supreme Court colleagues-an interpretation which equated "searches" only with physical intrusions into a person's home or property-would leave the home and all other centers of privacy entirely unprotected against numerous "[s]ubtler and more far-reaching means of invading privacy [that] have become available to the Government."87 The occasion for this warning was the case of Olmstead v. United States, which addressed the question of whether police officials violated the Fourth Amendment's prohibition against "unreasonable searches" when they intercepted and listened to the defendant's phone calls.88 Unlike Brandeis, the Court's majority (in an opinion by Chief Justice Taft) found that such wiretapping did not violate or even implicate the Constitution.89 Indeed, the Court not only rejected the defendant's claim that the government's wiretapping was an "unreasonable search," it found that there was no "search" at all.90 "There was no entry of the houses or offices of the defendants" and the government could therefore not be viewed as overstepping any constitutional boundary lines.91 The defendant's phone lines were "not part of [the defendant's] house or office any more than are the highways along which they are stretched."92 And no one could reasonably expect Fourth Amendment protection against official scrutiny when he "installs in his house a telephone instrument with connecting wires intend[ing] to project his voice to those quite outside."93 Messages projected to the world outside the home might be heard by those outside the home and, as the Court noted, the police had secured information from the suspect's conversations "by the use of the sense of hearing and that only."94 It would be perverse, the Court emphasized, to place any constitutional hurdles before officials wishing to listen to sounds that are available to them. The meaning of searches and seizures, said Chief Justice Taft, may not be so enlarged as to "forbid hearing or sight."95

For Brandeis, such a view was appropriate only for previous times, when officials interested in seizing a person's "papers and other articles incident to his private life" could do so only by "breaking and entry."96 To prevent such invasions, courts simply had to stop the physical acts that made them possible. But with modern technologies such as wiretapping, police officials could get much of a person's private information even while remaining outside the home. Rather than guarding only against physical invasions, argued Brandeis, the Fourth Amendment must protect against "every unjustifiable intrusion by the Government upon the privacy of the individual."97

While Brandeis lost this specific battle, his insistence that courts must fortify the Fourth Amendment against new technologies was ultimately heeded by the Court. In the 1967 case of Berger v. New York, the Court rejected Olmstead's conclusion that Fourth Amendment protections could not extend to wiretapping.98 Later that year, it expressly overruled Olmstead's "physical trespass" interpretation of the Fourth Amendment in Katz v. United States.99 On the surface, the Katz decision may appear to be a fundamental shift in Fourth Amendment jurisprudence. Before Katz, the Fourth Amendment protected against search and seizure simply by safeguarding certain constitutionally protected areas, most notably the home.100 After Katz, the Fourth Amendment's protection became far broader, because it protected an individual's "reasonable expectation of privacy" not only within certain well-marked zones or enclaves, but everywhere that circumstances might give rise to such an expectation.101

While the above picture is not inaccurate, it portrays Katz and its reasonable expectations test as more revolutionary than it actually was. One reason is that in figuring out how to apply Katz to new fact situations, most courts have relied not on the majority opinion by Justice Stewart, but on the concurrence by Justice Harlan. Harlan's opinion did not so much abandon the doctrine of constitutionally protected areas as update it to take account of new technologies for electronic surveillance.102 More specifically, the doctrine of constitutionally protected areas was modernized by Justice Harlan (and subsequent Supreme Court cases) in two fundamental ways.

First, the Court refined its protection of "constitutionally protected areas" to guard against the kinds of technology that Brandeis was most obviously concerned about in his Olmstead dissent: technology that allowed the government to make intangible and surreptitious "entries" into traditional privacy zones (most notably the home itself).103 In United States v. Karo, for example, the Court barred the government from using a beeper to track a defendant inside his house.104 In Kyllo v. United States, it barred police outside from using forward-looking infrared (FLIR) technology to "see"-in heat measurements-details about the interior of a home.105 As Justice Harlan noted in his Katz concurrence, "electronic as well as physical intrusion into a place that is . . . private may constitute a violation of the Fourth Amendment."106

Second, apart from protecting the home and other places where individuals were traditionally able to exclude others, the Court recognized the importance of new "constitutionally protected areas" where technology has made it possible, desirable, and virtually unavoidable for people to convey information about their personal desires and preferences. Thus, in a world where much communication cannot take place except over public phone lines, and people make calls from public places, one could hardly expect an individual to succeed in avoiding the discussion of family affairs or personal anxieties over such phone lines. Justice Harlan emphasized this point also in his Katz concurrence: "an enclosed telephone booth is an area where, like a home, and unlike a field, a person has a constitutionally protected reasonable expectation of privacy." Moreover, as David Sklansky has emphasized, in the modern world, such "constitutionally protected zones" are not only physical but also virtual. Private activity and communication occurs not only in homes, offices, or enclosed phone booths, but also in Internet chat rooms, web sites, and other electronic environments.108 Even if a government wiretap (or bugging device on a phone booth) intercepts conversations that take place completely outside the home (for example, between a husband and wife who are talking, respectively, from a public phone booth and a cell phone), this fact does not eliminate individuals' privacy interests in such conversations.

Thus, just as Justice Kennedy has sought recently in the context of First Amendment "public forum" doctrine to modernize the notion of a "traditional public forum" where free speech protections have especially strong force vis-a-vis other interests of the public,109 Justice Harlan tried in Katz to preserve but modernize the notion of specific enclaves where Fourth Amendment privacy protections have especially strong force.110

Indeed, Justice Harlan not only tried to modernize the doctrine of constitutionally protected zones, he succeeded-to such an extent that his concurring opinion helped undercut the majority's attempt to give privacy protection stronger force in public life.111 In contrast to Harlan's updating of the doctrine of constitutionally protected zones, the Court's majority opinion in Katz explored another more radical challenge to the Olmstead framework. It had considered the possibility that privacy protections might in a sense be made portable and taken with people as they traveled from place to place.112 Under such a conception of the Fourth Amendment, individuals might have constitutional privacy protection even where they were least able to exclude others from being present or to place limits on what is seen and heard: in parks, streets, and public squares. As one state court later put it in elaborating upon this strand of Katz when analyzing video surveillance: "A person has a 'halo' of privacy wherever he goes and can invoke a protectable right to privacy wherever he may legitimately be . . . be it a public park or a private place . . . ."113 As noted earlier, Katz itself made clear that its goal was to extend Fourth Amendment privacy rights beyond those particular zones or sites traditionally regarded as "private."114 Holding that the Fourth Amendment protects people, not places, it stressed that "what [a person] seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected."115

Such language accords with powerful intuitions about privacy. Many Americans would probably object to the idea that they become fair targets for minute-to-minute surveillance or recording as soon as they walk out of a home or office. The problem with this aspiration in Katz is that it predictably leads to a blurring of the boundaries between what is private and what is public and open to view, and seems to leave both law enforcement officials and others with little guidance as to what is covered by Fourth Amendment protections. After all, to clearly communicate what it is we regard as private outside of the Fourth Amendment context, we often rely on barriers that block sight or hearing. As William Heffernan notes, "a closed door" or "a sealed envelope" provides a cue that what lies behind or inside of it is not to be observed or read.116 If we take away such cues, and insist that certain activities or objects are private and should be safeguarded against observation even when they are visible, we need to provide some substitute method of marking off these activities or objects as deserving of protection against observation.

The Court in Katz suggested that a person might provide signals about what "he seeks to preserve as private" by "knowingly expos[ing]" it or not doing so.117 But it did not clearly define how one might do so other than by concealing or not concealing it.118 It was perhaps this gap in the majority's reasoning that led Justice Harlan to translate the majority's "knowing exposure" test back into the framework of constitutionally protected zones. Justice Harlan acknowledged and endorsed the majority's insistence that the touchstone of Fourth Amendment privacy is what an individual "seeks to preserve as private."119 To capture this notion of individual intent, justice Harlan built his two-part test for identifying a Fourth Amendment search around the notion of individual expectations. Government surveillance of an activity amounted to a search (1) if it intruded upon an actual (subjective) expectation of privacy and (2) if that "expectation [was] one that society is prepared to recognize as 'reasonable.'"120 However, Justice Harlan also made clear that an individual's expectation of privacy was most likely to be reasonable if it arose in a private place. He stressed that one cannot tell what protection the Fourth Amendment offers to people without "reference to a 'place.'"121 While an individual would have a reasonable expectation of privacy in a traditionally private place, such as a home,122 or a "temporarily private place," such as a phone booth,123 such an expectation would not be reasonable for "conversations in the open."124

Courts and commentators have followed Harlan's lead. In United States v. Taborda,125 for example, the Second Circuit noted that the requirement that society recognize an expectation as reasonable "appears to focus less on a person's actions and more on the place in which he acts."126 Likewise, in his influential treatise on search and seizure law, Wayne LaFave takes the position that "[u]nder the Katz expectation of privacy test, particular attention must be given to the nature of the place at which the observed objects or activities are located, for this will bear directly upon whether there was a justified expectation of privacy as to those objects or activities."127

The Supreme Court has also given its support to an interpretation of reasonable expectations tied firmly to particular places. In United States v. Oliver, the Court held that Katz's reasonable expectation of privacy test did nothing to weaken the open fields doctrine.128 This doctrine, adopted by the Court in 1924, provides that the protection of the Fourth Amendment "is not extended to the open fields."129 The Oliver Court stressed that, "as a practical matter [open fields] usually are accessible to the public and the police in ways that a home, an office, or commercial structure would not be" and therefore an "expectation of privacy in open fields is not an expectation that 'society recognizes as reasonable.'"130 While the Court was specifically analyzing fields rather than streets or public squares, its logic seems to apply even more forcefully to the latter areas, to which the police and the public have greater access than they do to the privately-owned fields in Oliver.

By salvaging the concept of constitutionally protected areas, Justice Harlan effectively postponed a more radical challenge to the Olmstead framework, one which could protect anonymity and other forms of privacy even in public. Ironically, it was Justice Harlan himself who later revisited this challenge and provided a starting point for addressing it that was significantly more promising than the one that the majority had sought to provide in Katz.131 Indeed, Justice Stewart's majority opinion may bear as much responsibility as Harlan's concurrence for the failure to extend constitutional privacy protections to public spaces. Even without the gloss imposed by Harlan's concurring opinion, Stewart's focus on "knowing exposure" might have led courts to focus on whether a person bothered to conceal his activities behind a wall or a barrier, because that is the most common signal one can give of whether these activities are private. Stewart's opinion, in any event, did not provide additional guidance as to when activities in public space should count as "private," and this silence helped assure that Harlan's narrower rule would prevail. As Edmund Kitch stressed shortly after Katz was decided, the Court could not successfully redefine Fourth Amendment law without "a limiting principle to replace that of Olmstead."132 Its failure to provide any such limiting principle left future courts nothing to rely upon except the familiar distinction between private and public areas.

Individuals, of course, may still claim Fourth Amendment protection against an unreasonable physical search of their person in public spaces. Courts have held, for example, that a pat-down of outer clothing by a police officer in a street or a park is a search.133 Likewise, when an official searches a handbag or a suitcase, that is a search, because such containers, like walls, provide a clear and accepted means of concealing items from others' view.134 What Katz did not provide was a form of constitutional privacy protection that might protect even those public activities that are visible or audible.

In short, while the Katz majority attempted to fashion constitutional safeguards that would protect us in streets and parks as well as in homes and offices, it could not find a way to do so. As David Sklansky observes, "[t]he decision in Katz seemed to promise a Fourth Amendment that was less tied to specific locations, and therefore somehow more modern. The Justices keep renewing that promise, but they have never figured out how to make good on it."135 Fortunately, as explained below in Part III, the Supreme Court does not need to make good on this promise because there is another, more viable approach to protecting privacy and anonymity in the public sphere. Instead of persisting in trying to sever privacy from location, courts might instead begin protecting those forms of privacy that are distinctive to public places (along with the privacy they already protect in homes and other enclosed areas).

II. The Technological Challenge to Katz: Watching, Tracking, Identifying, and Detecting Private Details in Public Spaces

The lack of safeguards against monitoring in public places was generally of little consequence in the decades after Katz. Although the government was free to track and observe individuals in public places, the use of such tracking was limited by important practical constraints.136 Tracking and observing an individual takes significant effort. Moreover, if an investigator is hoping to find specific information about particular people, substantial time may pass before his surveillance picks up something useful. It may be a substantial burden for him to listen to hours of recorded conversations or sift through piles of intercepted data.137

But one of the hallmarks of new surveillance technologies is the degree to which they lower the costs, both in time and expense, of round-the-clock monitoring. Real-time human monitoring is no longer necessary, as videos and tracking devices can be supplemented with devices that automatically record a person's movements for viewing at a later time. While government monitors might have once needed impossibly large and unmanageable libraries of video footage or other records to capture significant portions of a person's life (and tremendous amounts of tedious labor to search such records) they can now store massive amounts of information in computer memory banks and then have computers rapidly search and return the information they are looking for.138

Taking a page from Lewis Padgett's science fiction world, governments have also used such technologies to transform public space into a medium that itself records movements. Even those individuals who limit their contact with modern electronic technologies-by shunning cell phones or Internet credit card transactions, for example-might now find that they leave a record as they move through public space. Their movements might be recorded by ubiquitous camera networks or by "intelligent transportation systems" that trace the movements of automobiles on public highways. Such transformations bring to physical space many of the same worries that have recently been raised about the tracking of our movements in virtual space (through use of cookies or "web click trails").139 As science fiction writer William Gibson recently noted of such technological transformations, "the street itself seem[s] to have evolved" into a "sensory apparatus."140

Faced with these dramatic alterations in the physical scaffolding for our individual liberties and rights of privacy, the response of American courts has often been surprisingly nonchalant. Indeed, courts have sometimes acted as though these novel and far-reaching technological developments are not really novel at all-but rather more effective and cost-efficient variants of long-accepted methods of police work. Such an analogy of the new to the old is in fact a familiar part of the modern Fourth Amendment jurisprudence and has often appeared in the Supreme Court's applications of the Katz test. Recording a conversation on audiotape, it reasoned in United States v. White, is no more constitutionally problematic than remembering it and writing it down.141 Photographing a public scene with a powerful zoom lens, it said in Dow Chemical Co. v. United States, is no more a breach of constitutional privacy rights than looking at the same scene with binoculars, or for that matter with unaided vision.142 Finally, in United States v. Knotts, the Court found that tracking someone with the aid of a hidden beeper is just a more efficient means of tailing him as he drives down a street.143

Such statements may seem strange in the wake of the Court's recognition in Katz that Fourth Amendment jurisprudence must take adequate account of new technological developments. But they make perfect sense if one accepts the account of Katz presented in Part I. Under this account, technological change acquires constitutional significance not when it makes state monitoring of individuals considerably more extensive or intense but only when it somehow pierces the walls of a house, a telephone booth, or some other enclosed physical, virtual, or communicative space. Where expanding methods of surveillance leave such recognized private zones untouched, courts applying the Katz framework tend to declare them outside the scope of the Fourth Amendment. For example, in Dow Chemical Co., the Supreme Court acknowledged that a sophisticated camera had revealed details of a company's physical plant that would otherwise have remained invisible to government agents.144 But the Court insisted that the use of such magnification technology did not cross the constitutionallysignificant boundary line one crosses when using "[a]n electronic device to penetrate walls or windows."145

But a closer look at recent search and seizure decisions reveals the building blocks for an alternative Fourth Amendment jurisprudence. In a number of cases, courts appear to recognize that, even when official surveillance is focused only on public spaces, it can present a significant threat to core liberty and privacy interests. The most obviously relevant caselaw for purposes of this Article is the caselaw on video surveillance itself.146 But emerging video surveillance systems undermine privacy not only by acquiring images, but also by magnifying details, tracking or reconstructing people's movements, and identifying people by using facial recognition software. It is therefore useful to look at how the law deals with all of these technologies of public surveillance and to understand how its treatment of such public surveillance compares to its treatment of new detection technologies-like X-ray devices or thermal imagers-that do "penetrate walls or windows."147

A. Video Surveillance

There is little dispute that, in some forms, video surveillance can severely undermine privacy and freedom. Individuals will have little space for private action if they constantly feel as though they are being watched by an unseen audience. When George Orwell wanted to describe a society bent on crushing individuality, he made video surveillance a central part of it: "telescreens" extended the government's gaze into homes, workplaces, and street corners.148 And at least some courts have echoed Orwell's dystopian vision of the future when addressing Fourth Amendment challenges to video surveillance. In a 1984 decision, for example, the Seventh Circuit warned that if left unrestricted, "television surveillance . . . could be grossly abused-to eliminate personal privacy as understood in modern Western nations."149 The court did not bar video surveillance entirely. On the contrary, it found that, given the gravity of the threat it was facing, the government had acted permissibly when it gathered surreptitious video footage of terrorists' bomb-making activities.150 But Judge Posner's decision subjected video surveillance within private homes or businesses to strict constitutional limits, intended to ensure that such surveillance takes place only when it is necessary.151 Before receiving a warrant to install cameras in homes or other private places, police must satisfy four criteria analogous to those they must meet under the Wiretap Act.152 They must (1) show that normal (less intrusive) methods have failed or are not worth trying, (2) describe particularly the nonverbal conduct to be surveilled, (3) limit the period of interception to no longer than is necessary to achieve stated objectives, and (4) minimize the interception of conduct unrelated to the objectives of the warrant.153 Six other circuits have since imposed the identical or nearly identical constraints on video surveillance and repeated the Seventh Circuit's warning that video surveillance can be incredibly destructive of privacy and must be carefully limited.154

But such vigilance against video monitoring has been reserved almost entirely for cases where police wish to tape or televise activities within a home or private office. Where defendants have complained of being videotaped in public environments, courts have almost always found the Fourth Amendment inapplicable. Thus, although the Tenth Circuit stressed that "[t]he use of a video camera is an extraordinarily intrusive method of searching"155 and demanded extensive justification from government agents who had used such a surveillance method to monitor the interior of a private business,156 it flatly rejected a complaint about video cameras mounted on telephone poles on a street outside of the defendant's residence.157 In contrast to cameras hidden in a home or office, these cameras captured nothing more than "what any passerby would easily have been able to observe."158 In short, the crucial factor for the Tenth Circuit was whether the surveillance took place "inside" or "outside."159 Indeed, this dichotomy was so important that the Court of Appeals refused to deviate from it even though the inside surveillance (analyzed in the Tenth Circuit's Mesa-Rincon decision) took place in a business where there was only "a 'medium' expectation of privacy,"160 while the outside surveillance (analyzed in the Tenth Circuit's Jackson case) was aimed at the area just outside someone's home.161

A similar stance on the constitutionality of public video surveillance has been adopted by virtually every state and federal court to address the issue.162 With this caselaw as a background, it is not surprising that even ACLU spokesmen who vigorously endorse legislative limits on unrestricted video surveillance systems have sometimes conceded that "it is not unconstitutional to place cameras in public places."163

But a closer look at the caselaw reveals greater nuance in judicial analysis of public video surveillance. First, a number of courts which have objected to video surveillance in enclosed and arguably "private" environments have set forth analyses which appear to raise constitutional doubts about public video surveillance as well.164 The key theme in these cases is that close and sustained scrutiny can constitute a Fourth Amendment search even when casual or incidental observation from passers-by would not be. In State v. Thomas, for example, an Indiana state court found that the government engaged in a search when it surreptitiously videotaped a store clerk's activities behind a cash register even though these activities often occurred in plain view of store customers.165 "Incidental or occasional looks by members of the public," explained the court, should not automatically leave a person vulnerable to "prolonged observation by the government from a non-public vantage point" (in this case, from a video camera recording through a hole in the ceiling).166

Other courts have reached a similar conclusion. In United States v. Taketa, for example, the Ninth Circuit agreed with the government that the defendant had "no general privacy interest" in his colleague's office, but found that "he may have an expectation of privacy against being videotaped in it."167 The video surveillance at issue, said the court, was unlike a physical search of the individual's possessions because it was "directed straight at him, rather than being a search of property he did not own or control," because he was present for the video search, and because the "silent, unblinking lens of the camera was intrusive in a way that no temporary search of the office could have been."168 In State v. Bonnell, the Hawaii Supreme Court found on similar grounds that "[w]hatever the general privacy interest the defendants may or may not have had in the [employee] break room," they did have a constitutional right against being subjected to television surveillance there.169 None of these courts was willing to state that individuals' right against being videotaped extended to parks and streets as well as office space.170 Indeed, the Ninth Circuit recently found that while, under Taketa's holding, "[a] person has a stronger claim to a reasonable expectation of privacy from video surveillance than against a manual search," such protection does not extend to "activities already visible to the public."171 But while cases such as Taketa have resisted extending Fourth Amendment protections to public places, their logic seems to lead in that direction: if the unrelenting gaze of a video camera can corrode an individual's privacy even in places where he has no general privacy interest, it is not clear why such a gaze is any less harmful when it tracks him from block to block than it is when it comes from a fixed camera at a store or workplace. In either environment, someone who knows he is being closely and steadily watched (or knows that he may be subject to such scrutiny) is likely to feel the significant discomfort and loss of freedom that comes from being under an official magnifying glass.

Other cases provide yet another reason to doubt that public video surveillance will remain forever shielded from Fourth Amendment scrutiny.172 While federal and state court cases have been almost unanimous in permitting the government to aim cameras at specific individuals or areas within public places, none of these courts has carefully considered the constitutional implications of mass video surveillance in a town or city. The few courts that have touched on such mass surveillance have hinted that it might well be subject to Fourth Amendment limits. The Vermont Supreme Court, for example, did this even as it rejected a defendant's claim that the government had violated the Fourth Amendment when it taped him attending to his marijuana garden in "unposted, open land."173 Having denied the defendant Fourth Amendment protection, it hastily added that matters might be different "where video surveillance is aimed indiscriminately at public places and captures lawful activities of many citizens in the hope that it will deter crime or capture what crime might occur."174 In other words, public video surveillance will not automatically escape constitutional scrutiny simply because it occurs in public. Whether it triggers the Fourth Amendment will also depend on the scale of the surveillance and the degree to which it is constrained by the need for suspicion of criminal wrongdoing.

The Supreme Court of Alaska recently followed the Vermont Supreme Court in stressing the latter of these two factors. It held that the Fourth Amendment did not shield an employee's actions in plain view of customers from video monitoring,175 but went on to note that such monitoring might well have triggered constitutional protections if it "had not been initiated for a legitimate purpose-the detection of theft-and had not been based on reasonable grounds to believe that [the employee] was stealing."176 In other words, while warrantless videotaping in public is acceptable, suspicionless videotaping apparently is not.

Such a stance is somewhat odd because under existing Fourth Amendment jurisprudence, police are free to observe a person or activity without "reasonable suspicion" of criminal activity, as long as such observation does not amount to a search or seizure.177 It is, after all, only searches and seizures which must be reasonable under the Fourth Amendment.178 By requiring reasonableness even for a nonsearch, courts like those in Cowles and Costin seem to implicitly acknowledge that courts should be on guard against public video surveillance even if they do not impose constitutional constraints on the simplest forms of it.

Such cases also reveal judicial reluctance to equate new and sophisticated mass surveillance systems with the familiar, decades-old practice of pointing a camera at someone. And this reluctance is well-founded. Admittedly, the presence of cameras in public is not new: government-operated networks of public cameras first appeared decades ago. Early video systems were installed in a number of towns in New York and New Jersey, and a network of cameras funded by the New York Times and several local businesses was installed in Times Square in 1973.179 Miami also experimented with video surveillance in the early 1980s.180 But these systems are very different from what is emerging now. The images they captured were often too grainy or blurry to be of any help to police or to be a significant threat to individual privacy. In contrast, the new camera systems are entirely different in scale and scope. The technological advances I have already discussed-miniaturization, digitization, and scientific leaps in computer storage and processing technology-allow authorities to capture and retain substantially more detailed information about activities in public space. As the General Accounting Office has recently noted, some systems allow camera operators to "move" from city block to city block with a joystick and zoom in on activities they wish to scrutinize.181 Recently, the Defense Advanced Research Projects Agency (DARPA) has encouraged development of video technology that can automatically record the movements of thousands of vehicles and search for and identify these "vehicles by size, color, shape, and license tag, or drivers and passengers by face."182 According to DARPA, such technology is meant to recreate and understand wartime encounters (as signified by its name: "3-D Combat Zones"), but privacy advocates worry that it can easily be used for domestic surveillance.183 Whatever one thinks of these impressive technological advances in video surveillance, they are not accurately described as a mere automated equivalent of human vision that captures nothing more than "what any passerby would easily have been able to observe."184 Rather, they change public space into something it would not otherwise be, something which in a sense preserves and processes records of people's movements and activities in a way that primitive cameras (and even primitive networks of cameras) have not done before.

B. Enhancements to Video Surveillance: Tracking, Magnification, and Biometrics

It is not only the expansion of video surveillance itself that poses a challenge to the viability of the Katz test but also the dramatic changes occurring in technologies that supplement and enhance such surveillance. Networks of video cameras function not only as video cameras, but also, when linked together and given the capacity to identify and lock onto a person, as tracking devices. Supplemented with zoom capacities and infrared detectors, they might reveal features of a person that are normally invisible even to bystanders only a few yards away. And with the aid of biometric identification devices, they might also provide investigators with information of a sort that is not normally sensed at all. They might reveal the name of an unknown individual in a photograph or videotape, and investigators might then link this identifying information to other personal information. While such biometric devices are typically used to authenticate or identify unknown people, they can also be used to reconstruct the movements of a known person by searching a large database of footage from public streets and recognizing all places and events where a specific person has appeared on camera in a given day or week.185

Might such potentially invasive technologies trigger Fourth Amendment protections even if unadorned video cameras do not? There are certainly strong intuitive reasons to think that they would. Government observers can learn much more about an individual if, thanks to tracking technology, they have not merely a video snapshot of him, but an ongoing broadcast that shows the places he goes and the associates he meets.186 Biometrie databases allow observers to undermine the anonymity of those they watch. Powerful magnification also allows them to discern small details that a person never intended to make known to the world. If the Fourth Amendment is meant, as the Supreme Court has stated, to prevent government from entering on its own whim into "the privacies of life,"187 then the modern tracking, identification, and magnification technologies would seem to be a matter of constitutional concern.

But, as with video surveillance itself, courts and many commentators have been reluctant to place any Fourth Amendment limits on such technologies except to protect the traditionally private environments of the home or office. And, as with video surveillance, this stance against extending Fourth Amendment protections into the public sphere begins to weaken when judges are confronted with versions of these technologies that do not temporarily undercut privacy or anonymity, but threaten to banish them entirely from public life.

1. Tracking.-In analyzing tracking technology, the Supreme Court might appear (on an initial reading) to have adopted the Katz approach without alteration. First, if a person "knowingly exposes" his movements to others in a public space, he has no grounds for a constitutional complaint when those others (including police) decide to take note of these movements.188 Thus, the Court found no fault with the police when they planted a beeper in a container, arranged for the container to be sold to specific individuals, and then tracked the beeper, and these individuals, as they drove back to their cabin.189 To be sure, police may have had good reason to be suspicious, because the container sought and acquired by the defendants was filled with a "precursor chemical" used in manufacturing illicit drugs. The police had received a tip that the defendants had stolen this chemical before and were recently purchasing additional containers of it.190 But for the Court, such suspicious information was in this case constitutionally irrelevant. "A person traveling in an automobile on public thoroughfares," said the Court, "has no reasonable expectation of privacy in his movements"191 and can raise no Fourth Amendment objection when police electronically follow or retrace those movements even without a good basis fordoing so.192

Apart from the argument that what is in public cannot be private, the Court also had a second, now-familiar Fourth Amendment argument for refusing to hold new technologies for tracking movements through public space unconstitutional.193 To do so, it implied, would confine law enforcement to primitive means for detecting and investigating evidence of crime.194 The police did not need a warrant simply to tail a driver on a public road; therefore, the Court decided that they should not need a warrant to follow the same person on the same roads with the aid of a tracking device capable of monitoring his movements more accurately and efficiently.195 As the Court put it: "[n]othing in the Fourth Amendment prohibited the police from augmenting [their] sensory faculties . . . with such enhancement as science and technology afforded them in this case."196

As the Court hastened to add in Knotts and made even clearer a year later in United States v. Karo,197 these decisions did not mean that police could use the same technological enhancement to electronically follow and monitor people within homes or other private enclaves.198 To search such environments, after all, police would need a warrant, and their ability to "enter" electronically did not relieve them of this burden.199 This, of course, is simply an application of the general principle voiced in justice Harlan's Katz concurrence that "electronic as well as physical intrusion into a place that is . . . private may constitute a violation of the Fourth Amendment," but people cannot expect the same Fourth Amendment protection-whether from familiar or new forms of observation-when "in the open."200

This straightforward application of the Katz test came with a caveat, similar to the one which lower courts have offered for public video surveillance. Just as some of those courts have indicated that their endorsement of public video surveillance should not be read as permitting mass suspicionless surveillance,201 the Court in Knotts stressed that universal, round-the-clock tracking of many citizens might well require a different constitutional analysis.202 Responding to the petitioner's claim that Fourth Amendment protection against beeper tracking was needed to prevent omnipresent monitoring of people's movements, the Court stressed that "if such dragnet-type law enforcement practices . . . should eventually occur, there will be time enough then to determine whether different constitutional principles may be applicable."203

At that time, the Court was able to treat such a warning as nothing more than speculation about an unlikely future.204 It noted that "reality hardly suggests" that this kind of dragnet searching was taking place.205 As Christopher Slobogin points out, it would be harder for the Court to offer the same response now.206 Within a matter of years, police may no longer have to go to the trouble of surreptitiously installing a beeper on each person they wish to follow because people increasingly carry or use tracking devices voluntarily in their everyday lives. For example, more companies are installing location-determining technology in the cell phones they create, and the FCC has recently ordered all companies manufacturing cell phones to do so, to ensure that 911 callers can obtain emergency assistance as quickly as possible.207

More and more people likewise allow themselves to be tracked automatically when they drive their cars on public highways. Electronic tollway systems, such as E-ZPass in the Northeast,208 I-Pass in Illinois,209 and FasTrak in California,210 register the presence of each driver who has installed a tag or transponder in her windshield, so there is no need for the driver to stop and pay.211 Such electronically-facilitated transactions make driving less burdensome, but at the cost of making it less anonymous. This trade-off characterizes numerous other features of evolving "intelligent transportation systems." The same technologies that allow lost drivers to find out where they are, what services are nearby, and how to get where they are going212 also potentially allow unseen government observers to learn or record this information. Devices on roadways that collect invaluable information on traffic, weather, and road conditions can also, in many cases, collect information about the movements and driving habits of particular drivers.213 This has caused worry about whether such technology will make deeper inroads into drivers' privacy,214 especially because the data collected by electronic tollway systems for drivers' convenience has been subpoenaed by private lawyers in divorce cases, state agencies investigating theft and judicial misconduct, and federal agencies, including the FBI.215

Radio transmitting devices may also allow officials and others to trace the paths not only of our phones and automobiles, but of numerous other products we cannot do without. For instance, various companies have been considering the use of radio frequency identification (RFID) tags to track the merchandise people buy.216 Fearful of kidnapping, some people have considered installing trackable computer chips into their own bodies (or those of their children).217 And with networked cameras appearing over numerous cities, authorities can more easily track people as they walk down a street, even if they are not equipped with a device that emits or receives radio signals.

One might argue that the spread of these new tracking technologies gives courts reason to leave them unbounded by any constitutional constraints: if people have voluntarily decided to use cell phones and electronic tollways, and thus, to trade the privacy of their movements for safety and convenience, why should the Fourth Amendment stand in their way? But such an argument does not dispel the concerns that led the Court to qualify its holding in Knotts.218 That people voluntarily submit to some forms of tracking technology-like that pinpointing the location of 911 callers-does not mean that they should be left with no constitutional safeguards against other forms of tracking imposed upon them without their individual consent, such as inescapable tracking by cameras. Nor should such consent be understood to allow government to take any more privacy in return for safety or traffic benefits than is necessary. People might willingly allow themselves to be located when they make a 911 call or require roadside assistance, but also reasonably expect that government officials will need a warrant to track their calls or their cars for any other purpose.

2. Magnification.-The Supreme Court's response to use of magnification with visual surveillance has been very similar. In fact, when the EPA used powerful map-making cameras to take detailed pictures of a chemical plant (magnified by a factor of 240) from an airplane, the Court's response was built around the same two points that formed the core of its Knotts decision on tracking.219 First, said the Court, the government had pointed its camera only toward a public space where police, and others, had a perfect right to cast their eyes. The grounds of the chemical plant were "comparable to an open field" and "as such [were] open to the view and observation of persons" flying overhead.220

Second, technologically expanding or fine-tuning such observation of public space does not become constitutionally impermissible simply because it reveals details invisible to the naked eye. "The mere fact that human vision is enhanced somewhat," said the Court, "does not give rise to constitutional problems."221 While federal courts have not hesitated to hold that use of a telescope to spy upon activities in the home might constitute a search,222 the Court stressed this was not the case here.223

As in Knotts, however, the Court in Dow Chemical qualified this stance in dicta by hinting that some types of magnification devices might raise constitutional problems. The EPA's use of a high-power camera was acceptable, said the Court, in part because the camera had revealed only the equipment and physical layout of the plant it was photographing, and did not capture intimate details such as "a class ring" or "identifiable human faces."224 Unlike the powerful cameras approved of in Dow Chemical, which were pointed only at a plant, the zoom lenses on cameras watching over streets and parks can be used-and likely would be used-to magnify faces, coat pockets, and other aspects of an individual's person without a warrant.225

It is not clear that the zoom capacities of emerging video surveillance would be free of Fourth Amendment limitations, even under the Court's current search and seizure jurisprudence.

3. Biometrics and Face Recognition.-"Facial recognition" technology is designed to help camera operators quickly match an unfamiliar face on a screen with an already identified face in a database, and then perhaps to a name, social security number, and other personal information in other databases.226 While it has recently been touted as a way to identify and apprehend terrorists, it is not clear how effective it is. In the recent past, such identification has been thwarted by facial hair, aging, changes in lighting, and variations in the angle between someone's face and the camera,227 and critics point to recent tests showing poor performance in surveillance at airports and other environments.228 That facial recognition software is still imperfect is clear from DARPA's "Face Recognition Vendor Test," which measures how existing facial recognition systems perform under a number of different conditions.229 The most recent run of this test in 2002 showed that even the most effective current systems had difficulty identifying faces outdoors (the best recognition rate was only 50%) and that their reliability also decreased markedly when the database of faces grew beyond a relatively small number (when the number of the faces in the database was increased from 25 to 3,000, the identification and detection rate dropped from 77% to 56%).230

This imperfection has not stopped law enforcement from continuing to experiment with-and hold out hope for the potential of-facial recognition. Much to the outrage of privacy advocates, the Tampa police used this technology to scan the faces of those attending the 2001 Super Bowl in Tampa Bay, comparing each face with those in a police database.231 Tampa subsequently installed face recognition-equipped cameras in its Ybor City entertainment district, comparing each face it captured to a database of "30,000 photographs of wanted felons and lost children."232 Although the city recently ended the experiment after finding the technology did not help lead police to criminals,233 other municipalities and agencies continue to be interested in facial recognition. Virginia Beach, Virginia has installed and continues to operate a facial recognition system that compares sunbathers and others wandering its beachfront to images of people with outstanding warrants, missing persons, and runaways.234 Other states are using facial recognition systems to guard against fraudulent acquisition of drivers licenses.235 And the flaws of existing facial recognition technologies may well be overcome by future versions of this technology being developed by private vendors and in DARPA's "Human Identification at a Distance" program.236

The Supreme Court has not analyzed facial recognition software as it has tracking or magnification. But commentators have borrowed from the Court's language in other Fourth Amendment cases to build arguments for facial recognition that parallel those that the Court has offered for tracking and magnification.237 First, such commentators say, just as the magnification permitted in Dow Chemical simply enlarged images already visible to ordinary members of the public, so facial recognition uses for raw data only images taken from public settings.238 And one's facial appearance, as the Supreme Court noted in United States v. Dionisio, can hardly be characterized as private since, like one's voice, it is "constantly exposed to the public" and "[n]o person can have a reasonable expectation that . . . his face will be a mystery to the world."239 Thus, the argument goes, the government need not impinge on anyone's privacy to determine what he looks like or to use this publicly-available knowledge of his appearance to locate him on a street or among a group of people.

Proponents often provide a second related argument for leaving facial recognition unrestricted by the Constitution which, not surprisingly, mirrors the arguments invoked by the Court with respect to tracking and magnification. They argue that such technologies are nothing more than new, more effective incarnations of traditional and unquestionably acceptable law enforcement practices. Thus, says one commentator, facial recognition appears constitutionally unproblematic since using the "system is the equivalent of officers observing a crowd and comparing the faces in it to those in a criminal face book; it is just much faster and may be more accurate."240

There are also characteristics of facial recognition that make it privacy-enhancing, and these might appear to allay concerns that this technology runs afoul of the Fourth Amendment's proscription on investigations that are privacy-invasive. Most notably, many commonly used facial recognition methods-like other "biometric" technologies that identify people from biological characteristics-do not compare images; they compare measurements. Facial recognition technology frequently uses a "feature extraction" process241 to take measurements of 80 or more nodal points on the face-such as the "upper ridges of the eye sockets, areas around the cheek bones, sides of the mouth, nose shape, and the position of major features relative to each other"242-and then uses algorithms to translate those measurements into an identifying record or "face print" that consists, not of a picture, but of a numeric string.243 With this record in its database, a facial recognition system then tests for a "match" by taking measurements from a person's face, creating another numeric template, and comparing the new numeric template to the one in its database-with the aid of an algorithm-to see if the degree of similarity between the two "templates" warrants a conclusion that the face it is observing matches the "face print" in its database.244 Because "biometric templates" of faces or fingerprints include only a very limited selection of data about any individual face or fingerprint, no one can reconstruct the appearance of a face or fingerprint from its numeric code, any more than one can reconstruct the contents of a whole novel from an identifying code that consists only in letters selected from specific positions in the text.245 Indeed, such lightning-fast automated comparisons between "face prints," in which no human observer is involved,246 might strike many people as less invasive of privacy than having a police officer stare at faces in a line or a crowd to judge their similarity with a mugshot. It would not be surprising if courts recited such characteristics of facial recognition, and cited United States v. Dionisio,247 to hold that facial recognition in public falls entirely outside of the scope of the Fourth Amendment.

Such an analysis, however, ignores powerful reasons that some forms of facial recognition technology should be subject to Fourth Amendment limits. First, to the extent that facial recognition can easily be used to locate people in videotape footage or to retrace their movements, it would be subject to the Court's caveat in Knotts that tracking might be constitutionally problematic when it is widespread and ongoing.248

Another possible basis for regulating facial recognition technology may be found in the Court's decision in Dow Chemical.249 As noted earlier, the Court, in that decision, refused to classify as a Fourth Amendment "search" the use of a high-power map-making camera, but supported its conclusion by noting that the camera did not capture any "identifiable human faces."250 Of course, video cameras supplemented by a facial recognition system not only might capture identifiable human faces-they are designed to do so-but also identify the faces they record.

But perhaps the most powerful reason for limiting facial recognition technology comes not from Fourth Amendment decisions on tracking and magnification, but from the Court's vigilant defense of anonymity. While Dionisio noted correctly that a person cannot expect his face to "be a mystery to the world,"251 his identity often is. A person's face does not identify him by name to all who see it, or provide observers with other personal information. If it did, authorities equipped with video cameras would have little need for facial recognition technology in the first place. Thus, as the Supreme Court recently noted in the First Amendment context, a person does not lose her right to retain her anonymity as soon as she shows her face.252 Striking down a town ordinance that required all individuals to provide their names to a town before engaging in any door-to-door solicitation or canvassing, the Court squarely rejected the argument that anonymity is lost as soon as one appears in public. "The fact that circulators revealed their physical identities," observed the Court, "[does] not foreclose our consideration of the circulators' interest in maintaining their anonymity [since] . . . [i]n the Village, strangers to the resident certainly maintain their anonymity . . . ."253 Therefore, it is possible that even if Fourth Amendment protection is not triggered when the government records the photograph or "face print" of a person's face, it will be triggered when investigators subsequently use facial recognition or other biometric technology to immediately acquire otherwise difficult-to-acquire identifying information about an unknown individual.

Of course, when we act in public we reveal not only our appearance, but often our identity as well. We respond to our names and show our drivers' licenses or other identifying documents to security guards and store clerks. Airport staff typically verify our identities, usually by glancing at photo identification, before allowing us to check in luggage or proceed to an airport gate. And it is not only facial recognition devices that could recognize us in a crowd, but also an acquaintance or an investigator holding a picture of us. Consequently, when one looks narrowly at a particular activity on a given day, one might argue that our anonymity might have been taken away in that circumstance as much by a chance encounter with a friend or acquaintance as by advanced technology. But what such an observer could not do is remove all opportunities for anonymous or private action in a substantial portion of public space. While an acquaintance might happen to find us in a crowd of thousands at a protest rally, he cannot be relied upon to do so each time we attend such an event, and an airport staff member or security guard is unlikely to remember our identities-or that of the many other people he checks in-after he has verified that we are who we claim to be.

Such concerns about loss of anonymity do not disappear simply because most biometric devices compare numeric codes rather than identifiable images. In the first place, some methods of representing faces for purposes of facial recognition may require face prints to contain information that would allow investigators to reconstruct a person's appearance. For example, instead of measuring well-known facial features, the "eigenface" method encodes a face's global appearance by representing it as a weighted mix of certain face patterns in a database of grayscale images. One analysis notes that in contrast to geometry-based methods, which can match faces "without any need for actual facial images at the comparison point," matching in the eigenface method "is done on a pixel-by-pixel basis."254 Even when the code in a "face print" cannot be used by the police to reconstruct the appearance of someone's face, when used in conjunction with facial recognition technology, it can allow them to pick a face out of a crowd, or out of video footage, and to learn a great deal about the person to whom the face belongs. Even without a photograph of a person, an official might quickly eliminate his anonymity with the aid of an identifying code.

Facial recognition is not the only biometric technology that can make video surveillance a much more potent tool for invading individuals' privacy. To be sure, apart from a person's face, few physical characteristics or personal traits commonly used in biometrics are likely to appear in a video image. Video cameras are ill-suited to capture useful data about a person's fingerprints,255 the three-dimensional geometry of her hand,256 or the distinctive physical attributes of her eyes, such as the vein patterns in her retinas257 or the structure of her irises.258 Without audio capacity, cameras are of course also unable to record or identify the distinctive features of a particular person's voice.259 In general, biometrics systems using these physical features use them not for identification, but for verification of a claimed identity. Before letting someone into a secured building, or into confidential computer files, a security system might ask the would-be entrant for a biometric identifier (like a fingerprint or iris scan), not to determine his name or any other personal information, but only to verify that he is authorized to enter.260

To the extent that such verification systems save any identifying information tying particular people to particular places or activities, they might have the consequence of making existing surveillance systems, such as cameras, much more invasive. If fingerprinting or iris scanning devices frequently record when a particular person has used a specific ATM or entered a specific building, then such data could conceivably aid a video search of that person's movements, in the same way that Intelligent Highway Systems or phone company records might aid such a search by providing information about the location of a person's car or cell phone. Indeed, biometric technology could allow for more inescapable tracking, because while your car or your cell phone can be used by someone else, it is virtually impossible for another person to use your fingerprint or retina.261 And unlike facial recognition technology, which might be thwarted by changes in environment or appearance, biometric techniques such as iris and retina scans, and to a lesser extent fingerprint matching, are almost impossible to deceive or circumvent.262

C. Detection

Another kind of caselaw also casts doubt upon the Katz framework, and it deals with the very threat that Katz was meant to address: the danger that police will use modern technology to somehow circumvent physical barriers that are relied upon to keep information private. As I have noted above, courts that refuse to treat public surveillance as a "search" sometimes explain this result by underscoring how such surveillance differs from the paradigmatic electronic search in which investigators somehow look or listen through a wall or window.263

But in the decades since Katz, difficult questions have arisen, both about whether detection technologies are generally "searches" under the Fourth Amendment and about how much weight courts can continue to place on the distinction between "see through" technologies, which presumably upset reasonable expectations of privacy, and mere "enhancements" of visual observation, which presumably do not.

Some modern detection devices, to be sure, present little problem for the Katz paradigm. Most people agree, for example, that airport officials are conducting Fourth Amendment searches when they use X-ray devices or so-called millimeter scanning devices.264 While clothing and luggage are usually impermeable to visible light (largely to cloak what lies underneath or inside), X-rays and millimeter radiation pass right through these barriers, allowing investigators with the right equipment to view what is on the other side and conduct the equivalent of an "electronic strip search."265

But unlike the detection technologies described above, which can provide a vivid picture of practically everything an individual is hiding in a container or underneath a coat or shirt, many detection devices signal only the presence or absence of a particular substance or object with distinctive physical properties. The "magnetometers" that travelers typically have to pass through at airports, for example, detect only metals. They detect disturbances in the earth's magnetic field and sound an alarm only after detecting the sort of disturbance caused by a metal object of, or above, a certain mass.266

The same basic mechanism is at work in many new, highly-sophisticated instruments for detecting weapons or illegal drugs.267 Many of these devices sense the presence of particular materials by reading magnetic or chemical "signatures." One such device, for example, is a more refined version of a metal detector. It "measures what objects do to the earth's magnetic field," but instead of simply detecting the presence or absence of metal, it compares the measurements of magnetic field disturbance to "known signatures of weapons of similar shape, mass and density to determine the likelihood that the object is a weapon" of a particular kind.268 Another device, called the "Gun Tracker" scans people from a distance.269 When it detects a possible weapon on someone's person, it triggers a video camera, which then follows the suspicious individual and places a red dot at the location on the person's image where the Gun Tracker has located the potential weapon.270

Other devices use "chemical signatures" to detect explosives or narcotics. Perhaps the most familiar "chemical signature" detector is the trained bomb- or drug-sniffing dog. But scientists have recently developed many new mechanical "sniffing" devices. A machine called "the Senior," for example, uses high-speed gas chromatography to quickly search the air around a suspect or his baggage for even the slightest molecular traces of narcotics.271 Other devices "emit puffs of air that pick particles off [airline travelers'] clothing" for instantaneous chemical analysis,272 or detect explosives on the surface of luggage by firing energized neutrons or lasers at it in order to cause a "signature" reaction that will identify even small amounts of explosives or other chemicals of interest.273 Researchers have also been developing "smart dust"-tiny silicon chips, to be dispersed in the air or blended into the paint on the surface of a building or vehicle, that can detect and identify deadly biological or chemical agents nearby.274

At first glance, it may seem as though these devices provide grounds for limiting constitutional privacy protections rather than extending them. Devices focused on drugs or explosives, for example, have made it plausible to think scientists might be able to manufacture devices that can directly "sense" illegal, or at least highly suspicious, activity and in this way spare police the intrusive information gathering that would otherwise be necessary before determining which house or container to search. According to Arnold Loewy, such a technique of searching-epitomized by high-tech chemical detectors and by marijuana-sniffing dogs-approximates the kind of search police would use in a more perfect crime-fighting regime where nonintrusive technology could automatically distinguish criminal from innocent activity. In Loewy's law enforcement Utopia:

[E]ach policeman would be equipped with an evidence-detecting divining rod. He would walk up and down the streets and whenever the divining rod detected evidence of crime, it would locate the evidence. First, it would single out the house, then it would point to the room, then the drawer, and finally the evidence itself. Thus, all evidence of crime would be uncovered in the most efficient possible manner, and no innocent person would be subject to a search. In a real society (such as ours), the fourth amendment serves as an imperfect divining rod.275

The same year that Loewy published this article, the Supreme Court agreed that a canine sniff revealing only the presence of contraband was not the kind of investigation the Fourth Amendment was meant to constrain. In United States v. Place, the Court found-in response to a traveler's complaint about a warrantless use of a drug-sniffing dog to examine his luggage-that there was no invasion of a Fourth Amendment privacy right because "the sniff discloses only the presence or absence of narcotics, a contraband item."276 The Court stressed that the canine sniff was sui generis.277 It was unique in that it targeted only the guilty and left legitimate privacy interests unharmed. The following year, in United States v. Jacobsen,278 the Court found another real-life example of Loewy's "evidence detecting rod" in the chemical tests used by field agents to determine if a white powder was cocaine. Federal Express employees had found the powder inside a damaged package sent by defendants and turned it over to police.279 As it had done with respect to the canine sniff in Place, the Court in Jacobsen stressed that "[a] chemical test that merely discloses whether or not a particular substance is cocaine does not compromise any legitimate interest in privacy."280

Not surprisingly, courts and commentators have since taken note of other focused search techniques that might fit the same model. In his dissent in the Court's recent Kyllo decision, Justice Stevens, the author of the Court's decision in Jacobsen, noted that the rule in Place should apply not only to dog sniffs, but also to devices that "detect [only] the odor of deadly bacteria or chemicals for making a new type of high explosive."281 The Sentor and other devices, one might argue, fit this description. After collecting a sample of the air around a person, or the matter on his clothing, such devices determine only if drugs or explosives are present. And one writer examining facial recognition technology, Alexander Nguyen, has suggested that the rule in Place and Jacobsen might apply to facial recognition technology as well, at least where it destroys the anonymity only of those who police have determined are criminals or have connections to terrorist groups:

Facelt [a well-known device for face recognition] generates a face print and compares it to files in its database of wanted criminals-and, according to company officials, discards the computerized print from its memory if there is no match. In this way, all Facelt really does is answer a simple question: Is the individual being scanned a criminal?. . .

Indeed, FaceIt very closely resembles metal detectors at airports or dog sniffs that the Court has held constitutional in United States v. Place where no search warrant or probable cause was present, or a test by law enforcement officials of white powder to determine whether or not it was cocaine as opposed to sugar or talcum powder which the [C]ourt held constitutional in United States v. Jacobsen.282

Such analogy to Place or Jacobsen is attractive because it suggests a way in which vigorous protection of privacy might be reconciled with increased law enforcement vigilance against hard-to-detect threats. Yet there are a number of reasons that courts should be extremely cautious in extending the Place and Jacobsen model, especially in the case of video surveillance. First, it is rare that devices can single out only illegal activity or materials. As Arnold Loewy stresses, even "so innocuous a device as a magnetometer cannot distinguish permissible metals (coins, keys, etc.) from impermissible ones (guns, knives, etc.)."283 Partly for this reason, courts have invariably held that use of magnetometers at airports or federal buildings is a search.284

Second, a surveillance device will not necessarily cease to intrude upon the privacy of the innocent even if it does detect only contraband. Particularly when the device is extremely sensitive and picks up molecular traces of a chemical, it might find traces of narcotics not only on someone who possesses drugs, but also on someone who had incidental contact with the drug possessor.285 As Peter Bober notes, a police officer using the Senior to sample the air around a person might find trace amounts of narcotics in the environment not because that person possesses drugs, but because the police officer herself has unknowingly carried trace amounts of narcotics on her own person after conducting an earlier drug bust.286 Many devices also occasionally give "false positives." They signal the presence of drugs or explosives even when there are none. For example, after shutting down San Francisco International Airport and searching unsuccessfully for a traveler who tested positive for explosives, but was mistakenly allowed by a guard to proceed, authorities noted that the substance detected was probably fertilizer, which has a chemical signature identical to that of certain explosives.287

There is a third reason why surveillance techniques that pick out only illegal activity might harm the privacy interests of innocent people, even when functioning perfectly. As Michael Adler points out, a search that pierces a house or container wall only to uncover illegal activity can severely undermine the confidence that people have in homes and other private environments more generally.288 Even when the activity they wish to shield is entirely innocent, people may be justifiably unnerved by the state's ability to effortlessly monitor and gather information from environments that are supposed to serve as sanctuaries for freedom.289

Such skepticism about targeted or noninvasive technologies was clearly evident in the Court's recent decision in Kyllo, in which it held that the police engaged in a Fourth Amendment "search" when they spied on the inside of a private residence with a thermal imager.290 To be sure, such a device is in some ways similar to an X-ray or millimeter scanning device. It allows police to "see" in heat measurements things they cannot see with visible light, and therefore allows them to "see" people or things that emit heat from the other side of a wall.291

But forward looking infrared (FLIR) devices like that used in Kyllo do not provide anything approaching a vivid picture. They show only significant differences in temperature, and in most cases, seem to uncover little beyond the possession of high-intensity lamps of the kind needed to grow marijuana indoors. As the dissenting opinion in Kyllo stressed, the surveillance in that case was conducted with "a fairly primitive thermal imager," which merely collected "from the exterior surfaces of [Kyllo's] home" heat measurements showing only "relative differences in emission levels, vaguely indicating that some areas of the roof and outside walls were warmer than others."292

Nonetheless, the Supreme Court squarely rejected the arguments that such technological crudeness made the Fourth Amendment inapplicable. Just as metal detectors might invade the privacy of people carrying coins or other entirely innocent metal objects, FLIR devices might reveal sources of heat that have nothing to do with drug possession. As Justice Scalia noted, it "might disclose, for example, at what hour each night the lady of the house takes her daily sauna and bath."293 More important for the Court was an argument akin to Adler's argument that even unintrusive searches focused narrowly on criminal activities can undermine the sanctity of a private environment and the security that people feel in it. As Scalia stressed, it did not really matter whether the FLIR revealed particularly intimate details: "In the home . . . all details are intimate details."294 To let the state surveil this environment unannounced and without a warrant would weaken the protection traditionally afforded to this most private of all environments.

While Kyllo emphasizes the integrity and historical importance of the home in Fourth Amendment jurisprudence, its position is similar to the positions that other commentators and Justices have taken against uncritically applying Place to permit "dog sniff" type surveillance in public spaces as well. Even for those who do not possess, or plan to possess, any drugs, constant and visible use of police dogs may dampen the freedom they feel in public spaces. Thus, even as he compares dog sniffs to "divining rods" that reveal only criminal activity, Arnold Loewy emphasizes that he does not endorse the "carte blanche use of marijuana-sniffing dogs," in part because of the effect that even errorless dogs would have on innocent people: "[T]he very act of being subjected to a body sniff by a German Shepherd may be offensive at best or harrowing at worst to the innocent sniffee."295 In his dissent in Jacobsen, Justice Brennan likewise warned that "under the Court's analysis in [Place and Jacobsen], law enforcement officers could release a trained cocaine-sensitive dog . . . to roam the streets at random," or put people and houses under the constant watch of machines that detect illicit chemicals, something which would give our society a resemblance to authoritarian societies that refuse to