Jill Neimark for Science & Spirit: The more we discover about how the universe works, the weirder it gets! Your book brought home that time and space are not the terra firma we assume. Just before his death, Albert Einstein said: “The distinction between past, present, and future is only a stubbornly persistent illusion.” So why do we experience time as an arrow moving relentlessly forward?
J. Richard Gott: Nobody can explain why time seems to us like watching a movie. Why is it that movie can’t be rewound—even though it’s allowed under the laws of electromagnetism and gravity?
Certainly in the subatomic world, particles called positrons look like they might actually be electrons traveling backward in time. But in the macro world we live in, if you drop a vase and it breaks into many pieces, the fragments of the vase aren’t likely to leap together and reassemble themselves. Our perception of time feels concrete and objective to us.
S&S: Is time travel really possible? Travel in either direction would have enormous implications for life and for our species.
JRG: We’ve already seen time travel to the future, although in a very tiny way. One of Einstein’s great insights was that moving objects age more slowly than stationary ones. Certain subatomic particles, called muons, decay much more slowly in cosmic ray showers, where they’re moving with great speed.
But we, too, can time travel. Cosmo-naut Sergei Avdeyev, who orbited the Earth for 748 days in three space flights, is about one-fiftieth of a second younger than he would have been had he stayed home. In other words, he has time-traveled about one-fiftieth of a second into the future. It’s not much, but with faster rockets, it could be more. So the question is, how much money do we want to spend on this endeavor?
It would be very expensive. I do think in the twenty-first century, we’ll explore time travel to the future, but only in short hops.
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S&S: You said time travel helps us understand how the universe operates and might even show how the universe gave birth to itself. Your book illustrates the relativity of time and space with the image of an astronaut zooming through the universe at eighty percent of the speed of light. He has a clock in his spaceship, and a mirror at the top and bottom of his ship, with a light beam bouncing between them.
If you put a time loop at the beginning of the universe, it would be like having one branch of a tree circle around and grow up to be the trunk. In that way, the universe could be its own mother.
The clock ticks every time light hits a mirror. I’m on Earth watching him, with the same type of clock, with mirrors the same distance apart. We each think we’re living in normal space-time. But if either one of us looks at the other, things get weird. I’m certain his clock is ticking more slowly. For every five years I age here on Earth, he only ages three. What’s going on?
JRG: That light clock is one that Einstein imagined, as a way of showing there is no universal time. First you have to understand that light always travels at the same speed through empty space.
S&S: That alone is strange, isn’t it?
JRG: Very. If you move toward or away from a lightwave it passes you at a constant speed of 300,000 kilometers per second. No matter how fast the astronaut is going, light passes him at the same speed it would if he were on Earth. That’s why he doesn’t think anything unusual is happening.
But when I look at the astronaut’s clock as he’s flying by, I don’t see what he sees. The speed at which he’s moving, relative to me on Earth, changes things. The light beam in his spaceship looks like it’s moving diagonally because he’s flying so fast that by the time that beam goes from the bottom mirror to the top mirror, the top mirror has moved, and so the light has to travel further.
In his view of space-time, everything is consistent and the light is bouncing straight up and down. In my view of space-time, everything is consistent, but the light is bouncing diagonally and taking longer to do so. When we look at each other, we realize that for each of us, time and space are relative. Time travel to the future is made possible by the fact that observers who are moving relative to one another have different ideas of time.
S&S: The concepts sound simple, but the implications are boggling. In the world of relativity, space and time are one big block that already exists. It sounds like a mystical experience: being one with everything, no time, no space.
So what about traveling backward in time? Might someone travel backward and save a loved one from dying?
JRG: Einstein’s theory of curved space and time says it’s possible, but the fact is, this presents us all kinds of paradoxes. If you could go back and kill your young grandmother, then you wouldn’t have been born, and so you couldn’t go back to kill your grandmother. Can the past be changed? If it can’t, is there such a thing as free will? Such paradoxes can be resolved by either the conservative view—that time travelers don’t change the past but were always part of it—or by the more radical many-worlds picture of quantum mechanics—that time travelers can cause new parallel universes to branch off.
Finally, if time travel to the past is possible, why aren’t time travelers from the future already showing up on the White House lawn? The answer: They can’t travel to a time before time machines were built. You can’t use a time machine until it exists.
So, maybe in the year we finally build a time machine, they’ll all start showing up from the future. Paradoxes aside, if you could go faster than the speed of light, you could theoretically go back in time, but that seems impossible.
S&S: So how might we go backward in time?
JRG: According to Einstein’s theory of general relativity, space-time can curve and you might take a shortcut, beating a light beam to its destination, allowing you to travel back into the past. One way we could theoretically do it is to use cosmic strings. Cosmic strings are predicted to be very thin strands of high-density material left over from the early universe. Think of them like spaghetti that is infinitely long or curled up in closed loops. But that spaghetti has so much mass—about 10 million billion tons per centimeter—that it bends and warps space-time. I found a solution where you can actually travel around two moving cosmic strings, and get back in time to greet yourself at the spaceport before you leave. It reminds me of M.C. Escher’s drawing Ascending and Descending, where monks keep climbing a staircase around a monastery and always find themselves right back where they started.
Something even more intriguing came from a work that I published with Li-Xin Li. If you put a time loop at the beginning of the universe, it would be like having one branch of a tree circle around and grow up to be the trunk. In that way, the universe could be its own mother.
Otherwise we have to wonder how something came out of nothing. And how does nothing know the laws of physics? Maybe the universe wasn’t made out of nothing. Maybe it was made out of something, and that something was itself. How could it possibly do that? Through time travel, through a time loop. It even produces a natural explanation for the arrow of time as one moves away from the time loop at the beginning. Einstein’s relativity allows this, but we’ll probably need to have a workable theory of quantum gravity to see whether it’s possible. Time travel impinges on all the deep questions.
Escher, as usual, has a wonderful image, Drawing Hands, to express this. It’s of two hands, with each drawing the other. The idea of the universe as its own mother may be troubling, but maybe the universe should trouble us.
S&S: That idea might trouble theologians. If the universe could create itself, where does God come into the picture?
JRG: Whose picture is it? It’s Escher’s picture. The analogy is that Escher is God. If it weren’t for Escher, there wouldn’t be this picture.
Of course, when we were working on this solution, we had no thoughts about religion or theology. We just solved the equations. But if I take off my scientist’s hat and talk like a regular person, I’ve got to cope like everyone else with religious questions.
I’m a Presbyterian. I believe in God; I always thought that was the humble position to take. I like what Einstein said: “God is subtle but not malicious.” I think if you want to know how the universe started, that’s a legitimate question for physics. But if you want to know why it’s here, then you may have to know—to borrow Stephen Hawking’s phrase—the mind of God.
S&S: If you had one hour to travel in time, in either direction, what would you do and where would you go?
JRG: I’d go forward about 200,000 years to see if we’d survived that long and, if so, what people were up to. Of course, I might have some trouble communicating with humans in the future, because I wouldn’t expect to find anyone speaking English.
S&S: Why 200,000 years?
JRG: Because that’s how long homo sapiens have been around. I’ve done some thinking about time not just in terms of travel or physics, but in relation to how long things last—things like the Berlin Wall or Broadway plays or the human species. In 1993, I published a paper in Nature that applied one of the most famous postulates in science, the Copernican principle, to time.
The Copernican principle is simply the idea that your location in the universe is not special. Most likely your last name falls somewhere in the middle ninety-five percent of the phone book, not right at the beginning or the end, which would be special. And most likely you’re living sometime in the middle ninety-five percent of the length of the human species. Otherwise you’d be in a special position and that’s just less likely.
Using some simple math, I predicted with ninety-five percent confidence that the human race would last at least another 5,100 years, but less than 7.8 million years. Now, that’s a wide range, but an important one. The fate of our own species is supremely important to us. Some people predict we’ll die out in the next hundred years if we aren’t careful; others think we’ll just last indefinitely. Neither is likely. In any case, we’d better not be complacent. The Earth is littered with the bones of extinct species.
S&S: The question would be, is our intelligence special and can it help us last longer? If we’re mammals like all the other mammals, then we don’t have any special chance. But maybe our intelligence is novel and puts us in a different category.
JRG: My estimates of the future longevity of the human species are based entirely on our past longevity as an intelligent species—the only one we know—and make no assumptions that our fate will be similar to that for other species. However, my estimates give us a total longevity (past plus future) quite comparable to that observed for other mammal species, whose average longevity is 2 million years.
Why the coincidence? Well, if we remain confined to Earth, we are subject to the things that routinely cause other mammal species to go extinct. That’s why I am so concerned about the space program. So far, the space program is very brief, and the Copernican principle predicts it will probably go out of business sooner rather than later. And clearly we would increase our chances of surviving if we colonize space.
S&S: In other words, now that we understand that in terms of longevity we’re not special, we had better do something special, and soon.
JRG: Yes. In the short period we’ve been around, we’ve done some remarkable things. We’ve figured out a great deal about the laws of physics and the universe. But the ability to ask questions doesn’t seem to give us any more time. Don’t waste your time, humanity; you have just a little. That is the report from the future.
Other stories in this cover package:
Loop of Faith, by Jill Neimark
Time is Relative, by Lee Smolin
The Universe is a Mystery, by Sir John Polkinghorne