# Ask a Physicist: Is Time Travel Possible?

Luke, from California, wrote in last week:

I’m writing a research paper on time travel. Do you think time travel is possible?

Hi Luke,
Are we talking backwards or forwards here?

One of the most interesting aspects of Einstein’s relativity is that, the faster you move, the slower time passes for you. If you’re writing a paper, you’ll want to read our previous article on why time dilation occurs and how to calculate it—but the quick explanation is that it’s a consequence of lightspeed being the universal speed limit. The takeaway, though, is that time dilation makes traveling into the future entirely possible…if you can go fast enough.

 Even though they’re moving at over 120 million miles per hour, the particles in this beam only experience time 1.7% slower than we do. Image Credit: Lawrence Berkeley Nat’l Labs, Public Domain.

For instance, traveling at 95% the speed of light (pretty impossibly fast, but we’re talking hypothetically here) gives a time dilation ratio of about 3.2—meaning that, for every year that passes on your ship, 3.2 years will pass here on Earth.

Once you get up to 99% the speed of light, it’s closer to 7 years, and although you can never get TO the speed of light, you can inch ever closer. The closer you get, the more time dilation you experience—so if you can somehow swing your ship around at those speeds and come home, you could find yourself a thousand years in the future after only a few years of travel time in your ship!

But backwards in time is another story.

As the discussion above illustrates, time is a complicated thing—it’s not a dimension the way people typically picture dimensions. We can travel both directions in any of the 3 spatial dimensions, but time isn’t a space like that, it’s just a mental and mathematical construct that we use to sequence events. The past isn’t “stored” anywhere, except in the present—once a moment passes, that moment ceases to exist.

Let’s say you have a bunch of bouncy balls in a box, and you shake it up. Imagine a snapshot in time of the interior of that box—each ball has a position and a momentum, right? And if you know the laws of physics, you can calculate how things are going to look inside the box one second into the future—which balls are going to bounce off which walls, the angle they’ll come off at, etc. Given the state of things at that snapshot in time, there’s only one place all those balls can end up one second from then.

Likewise, you can calculate backwards, to figure out where the balls must have been one second prior to that—there’s only one way things could have looked one second into the past, to arrive at the present configuration of the system. This is what I mean when I say the past is only “stored” in the present.

So to make time move backwards inside this box, you’d have to catch each ball and throw it back the way it came—all at once. And that’s assuming there’s no air molecules in the box! True backwards time travel would mean precisely reversing the momentum of every particle in the universe at once, except your own. Then, once you’ve wound the clock back far enough, you’d have to do it again, to start time moving forward again. Hopefully you can see why this would be impossible in about three or four different ways.

There’s a reason people have dreamed and written about time travel for hundreds of years—and probably much longer. Who doesn’t have a regret that they wish they could travel back and undo? Who hasn’t wondered what the distant future of Earth, far beyond our lifespans, will hold? But for better or for worse, it seems that we’re stuck in the here and now…at least until we get a handle on near-lightspeed travel.

—Stephen Skolnick