In his concurrence in United States v. Jones, Justice Alito was not willing to define the 4th Amendment in terms of history, but instead was willing to look into the future:

This case requires us to apply the Fourth Amendment’s prohibition of unreasonable searches and seizures to a 21st-century surveillance technique, the use of a Global Po-sitioning System (GPS) device to monitor a vehicle’s move-ments for an extended period of time. Ironically, the Court has chosen to decide this case based on 18th-century tort law. By attaching a small GPS device ¹ to the underside of the vehicle that respondent drove, the law enforcement officers in this case engaged in conduct that might have provided grounds in 1791 for a suit for trespass to chattels. ² And for this reason, the Court concludes, the installation and use of the GPS device constituted a search. Ante, at 3–4.

This holding, in my judgment, is unwise. It strains the language of the Fourth Amendment; it has little if any support in current Fourth Amendment case law; and it is highly artificial.

…

The Court argues—and I agree—that “we must ‘assur[e] preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted.’ ” Ante, at 5 (quoting Kyllo v. United States, 533 U. S. 27, 34 (2001)). But it is almost impossible to think of late- 18th-century situations that are analogous to what took place in this case. (Is it possible to imagine a case in which a constable secreted himself somewhere in a coach and remained there for a period of time in order to monitor the movements of the coach’s owner? ³ ) The Court’s theory seems to be that the concept of a search, as originally un-derstood, comprehended any technical trespass that led to the gathering of evidence, but we know that this is in-correct.

Yet in Perry, Alito was very concerned about the future effects of same-sex marriage.

The one thing that the parties in this case 20 seem to agree on is that marriage is very important. 21 It’s thought to be a fundamental building block of 22 society and its preservation essential for the 23 preservation of society. Traditional marriage has been 24 around for thousands of years. Same-sex marriage is 25 very new. I think it was first adopted in The 1 Netherlands in 2000. So there isn’t a lot of data about 2 its effect. And it may turn out to be a — a good 3 thing; it may turn out not to be a good thing, as the 4 supporters of Proposition 8 apparently believe. 5 But you want us to step in and render a 6 decision based on an assessment of the effects of this 7 institution which is newer than cell phones or the 8 Internet? I mean we — we are not — we do not have the 9 ability to see the future.

So you got that? GPS on cars? Doesn’t matter what history says, it’s bad. Same-sex marriage? Let’s wait and see.

Update: Solicitor General Verrilli addresses this point later, in a stab to Alito:

And the fourth point I would make, and I do think this is significant, is that the principal

argument in 1967 with respect to Loving and that the Commonwealth of Virginia advanced was: Well, the social science is still uncertain about how biracial children will fare in this world, and so you ought to apply rational basis scrutiny and wait. And I think the Court recognized that there is a cost to waiting and that that has got to be part of the equal protection calculus.

And so — so I do think that’s quite fundamental.