Outside the terms of the warrant, they used the device to track Jones’s vehicle and his whereabouts for 28 days, gathering 2,000 pages of data. They used that data to further investigate, to charge, and ultimately to convict Jones of possession and conspiracy to distribute cocaine.

When Jones appealed his conviction and the use of the GPS evidence, the government argued that it did not need a warrant to track the movements of his car, as these movements were almost entirely in public. In the landmark case of Katz v. United States (1967), the Supreme Court had said, sensibly for the time, “What a person knowingly exposes to the public … is not a subject of Fourth Amendment protection.” Justice John Marshall Harlan, concurring in that case, mused about Fourth Amendment protection stemming from an “expectation of privacy” that society finds reasonable. In the hard cases ever since, courts and commentators have tried to use the “reasonable expectation of privacy” test to divine the meaning of the Fourth Amendment.

It is a stretch for people to expect privacy in things they expose to the public. So perhaps the Fourth Amendment allows government agents free rein to track people’s movements, even by secretly attaching GPS devices to their cars.

But the Jones Court declined to let advancing technology and the “reasonable expectation” test overrun the Fourth Amendment’s privacy protections. Instead, it relied on property rights to determine when government agents have invaded the right against unreasonable searches and seizures. Jones opens the door to improvements in Fourth Amendment protection — improvement that could come about in a drugsniffing dog case being argued this fall.

“We hold that the Government’s installation of a GPS device on a target vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a ‘search,’ ” Justice Antonin Scalia wrote for a five‐​justice majority of the Jones Court, consistent with a Cato Institute amicus brief in the case. The government “occupied private property for the purpose of obtaining information.”

The Court’s decision was unanimous, but four justices disagreed with Justice Scalia’s rationale. Justice Samuel Alito led this group, arguing strongly against the use of property analysis, or, as he put it, “18th‐​century tort law.” Alito would have used the Katz test, finding that one has a reasonable expectation of privacy in the sum total of one’s public movements. Katz doctrine, he granted, was “not without its own difficulties.”

Justice Sonia Sotomayor concurred separately in a very interesting and sure to be influential opinion. She joined the majority opinion, but took pains also to express agreement with Justice Alito’s concurrence. Importantly, she mused about the weakness of the third‐​party doctrine — the “premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties.” She wrote:

The Court was unanimous in denying government agents authority to track people using high‐​tech devices without a warrant, but it was almost perfectly split in its reasoning. That opens Fourth Amendment law to reform.

PROTECTING PRIVACY — BUT HOW?
A point of agreement between Justices Scalia and Alito was the goal of preserving “that degree of privacy against government that existed when the Fourth Amendment was adopted.” But neither justice was clear about what he meant in saying that. Preserving some past state of affairs with relation to privacy cannot be a clear goal without a command of what the thing is.

In 1967, the year that the Supreme Court decided Katz, scholar Alan Westin characterized privacy in his seminal book Privacy and Freedom as “the claim of individuals, groups, or institutions to determine for themselves when, how, and to what extent information about them is communicated to others.” This is the strongest sense of the word “privacy”: the exercise of power to control personal information. When people can control information about themselves, they will define and protect their privacy as they see fit.

If the Court wants to give individuals the same level of control over personal information as they had in the past, it should examine how people controlled information in the past and see that their ability to do so is maintained in the present. In the late 18th century, people controlled information about themselves by how they arranged the things in the world. Retreating into one’s home and drawing the blinds, for example, caused what happened inside to be private. Lowering one’s voice to a level others could not hear made a conversation private. Draping the body with clothing made the details of its shapes, textures, and colors private.

These actions, in the abstract, prevent others from perceiving things. At base, people protect their privacy by denying others access to the photons, sound waves, particulate remnants, and physical surfaces that reveal themselves and the things around them.

It is not enough, though, for people to withdraw into their homes, lower their voices, or wrap their bodies in clothes. They also rely on law. When people enter their homes, they rely on property rights to prevent others from accessing what goes on within. When people put on clothing, they rely on the law of battery, which bars wrongful physical contact that might strip the body of its wrappings.

Courts have had an easy time with “real world” privacy protection in Fourth Amendment cases. In Terry v. Ohio (1968), for example, a plain‐​clothes police detective observed three men acting strangely and became suspicious that they were casing a store for a stick‐​up. When Officer McFadden grabbed Terry, spun him around, and patted down the outside of his clothing, the Court easily recognized this as a seizure, followed by a search. The seizure and search were reasonable and therefore constitutional.

Consider how physics and law worked in the Terry case. Standing in a place he was legally entitled to be, Officer McFadden had used his eyes to capture the photons bouncing off his suspects and the things around them. Terry and his fellows had not concealed their movements on the street. Visual observation and inference combined to give McFadden an idea that they might be armed.

Because he had reasonable suspicion, Officer McFadden was allowed to touch Terry in a way that would otherwise be a battery. After he seized Terry and turned him, Officer McFadden placed his hands on Terry’s outer garments. He used touch to seek out information that was otherwise concealed from view. The hard resistance and weight of the gun were different from the soft resistance of the human body, of clothing, papers, and such, and the gun was found. Only a year after the Katz decision, the Supreme Court did not resort to the fuzzy “reasonable expectations” analysis. It wrote with confidence and clarity about the seizure of Terry, the search itfacilitated, and their legal import.

THE PHYSICS OF PRIVACY PROTECTION
The Supreme Court has struggled when applying the physics of privacy to information technologies. In Olmstead v. United States, the 1928 wiretapping case that the Court famously got wrong, Olmstead and his colleague in bootlegging had used the telephone to communicate. A microphone in the handset produced a modulated electrical current that varied its frequency and amplitude in response to the sound waves arriving at its diaphragm. The resulting current was transmitted inaudibly and invisibly along the telephone line to the local exchange, then on to the phone at the other end of the circuit.

Importantly, the signal passing along the electric wire was invisible and inaudible to any human. It could not be perceived and was thus private. Chief Justice William Howard Taft described the means by which the government tapped the defendants’ phones: “Small wires were inserted along the ordinary telephone wires from the residences of four of the petitioners and those leading from the chief office” of the conspiracy. These wires carried the signal to a coil and diaphragm controlled by the government, which reproduced the sound of the voices otherwise unheard all along the wire. Government agents transcribed the conversations to use as evidence.

But later in his opinion, Chief Justice Taft denied these facts. Justifying his legal conclusions, he wrote: “There was no searching. There was no seizure. The evidence was secured by the use of the sense of hearing, and that only.” He flatly denied the physical realities of wiretapping while withholding Fourth Amendment protection from telephone conversations.

The Court returned to the issue nearly four decades later in 1967’s Katz v. United States. Katz was an alleged bookie, convicted because FBI agents had bugged the public telephone booth from which he placed his calls. As in Olmstead, the bug converted sound waves to electrical signals. Crucially, the listening device was configured to be invisible to Katz. Unable to see the device, and seeing nobody near the phone booth in which he spoke, Katz believed his conversations were private. And they were — but for the FBI agents using high‐​tech gadgetry to hear what they otherwise could not have heard.

Justice Stewart’s majority opinion reversing Katz’s conviction rested on the physical protection that Katz had given to his oral communications by going into a phone booth. Against the argument that Katz was in public for all to see, he wrote: “[W]hat he sought to exclude when he entered the booth was not the intruding eye — it was the uninvited ear.” The Court gave Katz’s privacy‐​protective actions Fourth Amendment backing.

The “reasonable expectation of privacy” language Justice Harlan used in his solo concurrence has certainly enjoyed repetition, but it was not the holding in the case. He mused about “a twofold requirement, first that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as ‘reasonable.’ ”

This language would change the factual question the majority opinion turned on — Was the information physically and legally available to others? — into a murky two‐​part analysis with a quasi‐​subjective part and a quasi‐​objective part. Judicial administration of the Fourth Amendment has suffered ever since.

THE WEAKNESS OF THE KATZ TEST
Subsequent Courts have not faithfully applied the Katz test, instead presuming what parties’ expectations are and then analyzing their reasonableness. The Katz test has reversed the focus of the Fourth Amendment, turning away from the reasonableness of government action to examine the reasonableness of privacy expectations.

Reasonable expectation doctrine biases Fourth Amendment law against privacy in another way. Courts examine concealment of personal information under the reasonable expectation test, but they do not apply any such analysis when information is left exposed. The plain view doctrine is a simple constitutional rule: if a thing is visible (or otherwise perceivable) by authorities acting within the law, a person cannot make a Fourth Amendment claim against their observing it and acting on the knowledge of it. Plain view is a simple factual question, but plain concealment and privacy get further consideration. It should be that information one conceals from the general public one also conceals from the government.

The actual holding in Katz was based on physical protection given to a telephone conversation. Katz had excluded “the uninvited ear” when he went into a phone booth, including the ears of government agents. When one has arranged one’s affairs using physics and law to conceal information, it is generally unreasonable for government agents to defeat those arrangements. Thus it is reasonable to expect privacy. But reasoning backward from expectations to protection of information, as Justice Harlan did, has utterly confounded courts trying to apply Fourth Amendment doctrine. The challenge will only grow if courts try to square “reasonable expectations” with continuing advances in information technology.

FLORIDA V. JARDINES — THE DOG-SNIFF CASE
It may not be a high‐​tech case, but a drug‐​sniffing dog case, in which the Courtmight revise Fourth Amendment doctrine.

In Florida v. Jardines, a case before the Court this fall, law enforcement received an uncorroborated tip that Joelis Jardines was growing marijuana in her home. Two officers went to the front door of her home with a dog trained to detect narcotics. The dog alerted on the front door, which prompted the officers to seek a warrant, ultimately finding the marijuana plants inside. The question in the case is whether having a dog sniff at the front door of a suspected marijuana grow house is a Fourth Amendment search that requires probable cause.

In Illinois v. Caballes (2005), the Court said that a drug‐​sniffing dog search is no search at all because it only reveals the existence of illegal drugs, something in which nobody can have a “reasonable expectation of privacy.” That is a logical extension of Katz, but it is deeply concerning because it would hold any activity of government agents constitutional if it is sufficiently tailored to discovering only crime.

The Department of Homeland Security (DHS) has various technologies under development that could fit within the Caballes rule even though they are highly invasive. One, called Future Attribute Screening Technology, would examine Americans’ biologic cues — cardiovascular signals, pheromones, electrodermal activity, respiration, and so on — to detect intent to cause harm. This would not invade “reasonable expectation of privacy” under current doctrine because it would indicate only the existence of a criminally guilty mind.

Another DHS program is called the Remote Vapor Inspection System. The RVIS generates laser beams at various frequencies to be aimed at a target vapor. The beams reflected and scattered back to the sensor head reveal spectral signatures that can be matched to the signatures of sought‐​after gasses and particulates. Using RVIS, government agents might remotely examine the molecular content of the air in houses and cars, quietly and routinely explore the gasses exiting houses through chimneys and air ducts, and perhaps even silently inspect any person’s exhaled breath. If RVIS technology is programmed to indicate only on substances produced by wrongdoing, the Caballes rule means that even pervasive, frequent, and secret use would be considered no search.

CONCLUSION
The Court should abandon Caballes and no longer use its parent, the “reasonable expectation of privacy” test, in Fourth Amendment cases. Instead it should use the plain meanings of terms like “search” and “seizure” and the actual holding of Katz v. United States, which turned on physical protection of information — not “expectations” — to administer the Fourth Amendment.

In Jones, the Court opened the door to reforming Fourth Amendment doctrine this way. It should not be courts’ broad pronouncements about expectations of privacy that govern constitutional protection. The way people arrange the things in the world — what they do with their property — controls whether information is private and protected. A person’s car is not the government’s to use for its surveillance purposes. If government agents want to learn a suspect’s whereabouts, they will have to do it another way, or get a warrant.

Joelis Jardines closed and locked the doors to her home, denying all comers the ability to perceive what goes on within, including through the smells produced inside. The Cato Institute argued to the Court as an amicus in Jardines, “It is a search when government agents bring a drug‐​sniffing dog to the front door of a person’s home to examine the home for the presence of drugs. The dog makes perceptible what otherwise was not perceptible.”

It is reasonable to expect that one’s privacy will be maintained when one has placed sufficient physical and legal barriers around personal information. But in an important sense, privacy is beside the point. The Fourth Amendment, after all, is not a privacy management tool. The Fourth Amendment describes the right of individuals, retaining sovereignty not given to the state, to be free of unreasonable searches and seizures no matter what material or social consequences wrongful government action might have.