### Ask a Physicist: Wormholes and Time Travel

Keegan from Normal, IL wants to know:
"I have heard a few theories about using wormholes in space to travel from one place to another almost instantly, and I have heard that by doing that you can also travel through time in a similar way. In theory, how does time travel with wormholes work?"

Keegan,
It goes like this: Light travels at a finite speed, just like sound does (as you'll know if you've ever counted the seconds between when you see a lightning strike and the boom of thunder that follows). But because even light has a finite speed, that lightning strike actually happens a few instants before you even see it—as a result, two people different distances from the lightning would disagree on how long ago it happened. This is called the relativity of simultaneity—the idea that, depending on your reference point, the same set of events can seem to happen in a different order.

Now, since nothing moves faster than light, it's impossible to see the same flash of lightning twice. If you COULD travel faster than light, however—and this is where the wormholes come in, allowing you to travel between distant points in less time than it would take moving at the speed of light), you'd be able to witness events that happened in the past—if your wormhole deposited you five light-years from Earth and you turned a telescope toward our planet, you'd see the events of five years ago.

 Wormholes—known technically as Einstein-Rosen Bridges—never before observed but mathematically possible in Einstein's theory of relativity, can act as a sort of shortcut between two points in space, making it seem as though a person has traversed space faster than the speed of light, and potentially allowing a person to see a past image of themselves.
However, this isn't truly time-travel in the sense most people think of it. Instead, it's rather like hearing a rumble of thunder, outracing it in a supersonic jet, then hearing it again. TRUE time travel, as you're imagining it, isn't really possible—time isn't a dimension the way we typically think of them. It's not a space you can move in; it's largely just a tool we use to sequence events. By this logic, traveling backward in time would require stopping and directly reversing the momentum of all the particles in the universe at once, then somehow pulling the same trick again once you've wound the clock far back enough. To say nothing of the task's complexity, this would require more energy than exists in the universe, effectively ruling it out as a possibility anytime soon.

Of course, there's always the other type of time travel, the eminently more possible forward time travel. When you're moving at relativistic velocities—those approaching the speed of light—time seems to slow down for you, relative to a stationary observer. This is called time dilation, and it's a very intriguing concept that results from the math of Einstein's relativity. If you have a set of identical twins and one leaves for a five-year excursion in a rocket traveling 99.5% the speed of light while the other stays here on Earth, time will pass slower for the traveling twin. When they return to Earth, the traveler will find that their twin—and the rest of the world with them—has aged fifty years in the five years that the trip seemed to take—effectively enabling the possibility of traveling to the future! Unfortunately, we're a long way off from being able to utilize this technique, and there's no corresponding way to turn back the clock.

Hope this helps—and keep the questions coming!

1. If you could move one end of an Einstein-Rosen bridge at relativistic speeds, you would end up with a wormhole with one end younger than the other, which would've closer to the time machine people think of. Cf. "Timemaster" by Dr. Robert L Forward.

2. Actually, if you make the huge assumption that you have a stable wormhole, you can turn it into a time machine this way:

1) Move mouth B of the wormhole at high (relativistic) speed relative to mouth A.

2) Return mouth B to the vicinity of mouth A.

This is a consequence of the time dilation effect that you mentioned. This wormhole time machine is described in Kip Thorne's book Black Holes & Time Warps: Einstein's Outrageous Legacy (1994). See also The Physics of Stargates: Time Travel, Parallel Universes, and the Enigma of Wormhole Physics (2010) by Enrico Rodrigo for a very nice explanatory graph of what's going on.

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### The Science of Ice Cream: Part One

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