Rewind
Moving forward in time would be great, but moving backward would be even better. Making piles of money on the stock market is just one of the attractive possibilities.
Astronomers know that it is possible, indeed unavoidable, to at least see things as they were in the past. Whenever we aim a telescope at a distant object, we’re looking back into history. Even at its dazzling speed of travel, the light that falls on earthly mirrors and detectors takes time to get here. We see the Moon as it was 1.3 seconds ago, the Sun as it was eight minutes ago, the Andromeda Galaxy as it was two and a half million years ago. Out at the limit of observable space, we can see the afterglow of the Big Bang: the infant universe as it was almost fourteen billion years ago.
But at those distances it’s hard to see any details interesting at a human scale, and we are naturally more interested in our own history than that of a distant galaxy. And many of us won’t be satisfied by just looking. We would much prefer to go in person.
So could we do it for real?
The scientific answer is a definite Maybe.
Fair warning: after this point, things are going to get weird, even according to standards that find relativistic time dilation perfectly normal. Moving forward in time is fully authorized by a mature theory that is backed up by experiment to great accuracy. Backward, not so much. Einstein recognized nothing in his work that supported the possibility of going back in time.
But Einstein was not the only, nor the last, smart person on Earth.
Among the smartest people currently on Earth is a Caltech professor named Kip Thorne. You may not have heard of him, but you have probably heard of the brilliant wheelchair-bound physicist Stephen Hawking. Kip Thorne makes bets about relativity with Stephen Hawking. Sometimes he wins. Dr. Thorne is the world’s authority on practical time machines. He has written a readable book called Black Holes and Time Warps: Einstein’s Outrageous Legacy. Most of what I know on this topic comes from Dr. Thorne’s lectures and book.
It turns out that there are several theoretical possibilities for making a real time machine. None are supported by experiment, and even the theories are contentious. And let us be clear: the engineering would be incredibly difficult. Even if the theory holds up, we will not be ready to build the first working time machine until a far-off future when our technology is almost unimaginably advanced. We’ll be commuting to work at the relativistic speeds we talked about earlier. Our kids will be terraforming planets for Science Fair projects. But let’s say we’ve gotten that far.
One theoretical possibility is to build a massive cylinder of infinite length (not merely as long as the universe is wide, but infinitely long). We set it rotating about its long axis at nearly the speed of light, and then play tag with it in very capable vehicles. Certain flight paths around the beast return to the same point in space, but at an earlier time. Voilá, a time machine.
But infinite cylinders require infinite budgets, and that’s not the way science funding seems to be headed. We might not have to build one, though. Cosmologists have postulated that similar things might have been produced naturally in the early universe: linear black holes called “cosmic string,” which Earthly astronomers might be able to detect because circles drawn around them have fewer than 360 degrees. (I told you it was going to get weird.) I’m not going to say any more about infinite cylinders here because a different method is cooler and has an interesting connection to science fiction.
Another writer who famously did his homework was Carl Sagan. For his novel Contact, he wanted a physically plausible way for his heroine to travel to the star Vega and back quickly. He came up with a method that an incredibly advanced alien culture might develop, and mailed a description to Kip Thorne to ask his opinion. Dr. Thorne had a better idea. He shared it with Dr. Sagan, who incorporated it into the book.
Dr. Thorne’s suggestion is known to the initiated as an Einstein-Rosen bridge and to producers and consumers of science fiction as a wormhole. (Check out the Wikipedia article on wormholes for details and pictures, including a formally ray-traced image of a wormhole connecting two places on Earth.) Simply connect the throat of a black hole near Vega with that of another near the Earth, and bingo! We’ve built a shortcut to the stars, and the universe is ours.
But wait, it sounds like we’re talking about faster-than-light travel. What does this have to do with time travel?
Everything. Remember, the central tenet of Einstein’s relativity is that space and time are different aspects of the same fundamental thing. Bend one, and you twist the other.
Dr. Thorne and other theorists have suggested that it might be possible to turn a wormhole into a time machine. Leave one mouth of the tunnel at home, and take the other on a Twin Paradox sortie out into space. The traveling mouth experiences less time than the homebound one. When it returns, you can enter the latter and come out the former in the past. It’s a bona fide time machine. (Physicists use the term “closed timelike curves” when discussing them in print, in an attempt to head off media headlines screaming about scientists inventing time machines.)
The wormhole time machine is limited. It’s hard to adjust the time interval between the two mouths. You can do it only by taking one mouth or the other on a high-speed jaunt. And you can never go back to a time before you built the wormhole, a disappointment to people interested in altering the outcomes of still-earlier elections, sporting events, or armed conflicts. But you could still use it to make a fortune on Wall Street, or assassinate an ancestor and finally put to rest all philosophical posturing about the Grandfather Paradox.
General relativity may allow for the possibility of wormholes, but that doesn’t mean they’re a done deal. There are some construction challenges we don’t yet know how to overcome. First, every normal black hole contains an evil singularity in the center. Anything that crosses the hole’s horizon must fall into that singularity, be disrupted by it, and become one with it. Ouch. Next, there is not an obvious way to coax two black holes to connect with one another. Finally, theorists predict that if two holes are somehow spliced together, the resulting tunnel will pinch itself off before anything could pass through. Considerable intellectual energy has been invested in these topics, though, and there could be a way to solve them.
It might be possible to make two connected and singularity-free black holes out of something besides ordinary mass, which could then counteract the natural tendency of the tunnel to collapse. Theory suggests that this requirement would be met by a substance with negative mass and negative pressure. That’s right: to build a traversable wormhole, we’ll need to use something that weighs less than nothing and is emptier than a perfect vacuum. (Did you not believe me when I said it was going to get weird?) Engineers joke about “unobtainium” for applications that demand materials with unrealistic physical properties, but this stuff takes the cake.