We take it for granted that the past is fixed. History always happened the way we remember it happening. But how do we know for sure that that's the case? If time travel is possible, time travelers could be zipping back and changing things around all the time. How would we even be able to tell if that was the case?

We asked a number of experts on time travel — none of whom have actually traveled in time themselves. (Or if they had, they were keeping it under their hats.)

First of all, it's really difficult to know one way or the other, because "presumably, when a time traveler would change our past, this would also instantaneously change our memories of the past in order to render them consistent with the 'new' past," says Christian Wüthrich, a professor of Philosophy and Science Studies at UCSD. "If the changes to our past occur instantaneously and completely consistently, i.e., involving updates to all memories and record of the past, then we may not just not know it for certain."

But according to the experts, here's how you can tell that someone isn't changing the past all the time.

Time travel is impossible.

We heard this a lot, from many of the experts. As Jon Thaler, a physics professor at the University Illinois at Urbana-Champaign, tells io9: "The problem is that we don't know how to construct a theory that permits time travel. Without a theory, it is difficult to know what phenomena to look for." Thaler wrote the time travel section of the Usenet Physics FAQ, in which he explains that the Theory of Relativity might permit "closed timelike curves" that allow time travel — but the famous "grandfather paradox" (in which you go back and kill your own grandfather as a baby) proves the whole thing is impossible. Thaler tells io9:
"In a nutshell, it appears that "closed timelike curves" — jargon for the physical setup that permits time travel — are incompatible with quantum mechanics." This is basically how physicists interpret the "grandfather paradox," except that this approach is more quantitative, "and therefore the kind of situation that physicists like to analyze."

Even if time travel was possible, you couldn't change the past anyway.

We heard this one a lot, too. Wüthrich says that many philosophers of science assume that the past must be consistent, to avoid those nasty paradoxes. These "consistency constraints" mean that there's only one past, and it's fixed. Therefore, says quantum physics expert Todd Brun with USC, "even if you travel back to the past with the intention of changing history, events will conspire to force you instead to conform to it (and history would already include the presence of time travelers)."

This is why many philosophers of time like stories such as Robert A. Heinlein's "All You Zombies," where it's totally consistent, says Nick Huggett, author of Everywhere and Everywhen: Adventures in Physics and Philosophy. Huggett, a philosophy professor at University of Illinois at Chicago, adds:

It's now not at all clear what it could mean to change the past at all. To say something changes is to say that it was one way at one time, and another way at another time, right? So would you have changed the past? That seems to require that yesterday was first one way, and then another — first you didn't arrive by time travel on that day, and then you did? But in our story you did arrive by time travel, and so the first option never happened, and so the past was not changed by your arrival — again because it didn't make something different happen.

The alternative to this notion is that every time you travel back in time and change stuff, you're creating a new universe, according to Hugh Everett's "many worlds" theory. This new reality would still have a consistent past, which everybody would remember the same way — but it would be the past that resulted from the time traveler's changes.

Says Brun:

There could be multiple universes, and altering the past will cause the universe to branch. Weird and science fictional as that sounds, we have to take it seriously in quantum mechanics, where one of the leading interpretations of the theory is the so-called "Many Worlds" picture, in which every quantum event causes the universe to branch. But even in this case, one cannot go back and change the history of one's own universe.

You might only be able to travel back as far as the point where the first time machine was invented.

Unless someone's invented a time machine and we just don't know about it yet, this would mean that we're safe. Explains Vanderbilt University Physics Professor Thomas J. Weiler:

Time travel to the past, if at all possible, may only go back as far as the first time machine. Since the past of our civilization lacks a time machine, it is immutable, whereas the past of more advanced civilizations may be mutable.

Nobody's showing off

The only clear way to know if time travelers were going back and messing with the past would be if they bragged about it. Which, being people, they probably would. Says Huggett:

Suppose tomorrow you go back in time 2 days (i.e., to yesterday), and sensationally appear on national TV making correct predictions about today. It would be well known today that your future self had affected the past.

Although, even then, we wouldn't know for sure if these boastful time travelers had actually changed anything, since we'd only remember one past.

There would be physical traces

Depending on the method of time travel people were using, you'd expect there to be physical traces, says Huggett. In Carl Sagan's novel Contact, time travel is possible using a path in spacetime that leads to the past.

According to Huggett, this kind of wormhole "requires exotic forms of matter to hold it open, which may be detectable." Also, there's the question of conservation of energy — when you appear in the past, you won't be formed out of matter and energy that were already there, but instead you'll basically be importing energy from the future. This could create traces that might be detectable — if anybody actually was coming backwards in time to our era. So in a nutshell, the results of time travel would be unnoticeable to those of us who are stuck in linear time, but the methods of time travel would probably leave some trace.

Screwing with cause and effect would change the laws of probability

If someone really could go back and change the past, this could mean that effects would precede causes, Brun tells io9. And that, in turn, could mean that everything would go topsy-turvy, leading to logical inconsistencies that we might well notice. Brun explains:

The mixing up of cause and effect can make otherwise improbable events become much more likely. So if we suddenly find the laws of chance violating our common sense estimates of probability, that could mean that time travel is going on nearby. In principle this means that one could detect the existence of time machines — perhaps even before they are ever built! But it's hard to know exactly what to look for.

History proves they're not doing it.

Let's give the last word to Harvard University Physics Professor Gary Feldman: "If time travelers are constantly changing the past, they are not very good at it. Why did they not avoid two disastrous and pointless world wars in the past century?"