April 21, 2018

# Time travel: What’s holding us back?

Contributed | BDN
By Dana Wilde, BDN Staff

Probably you woke up this morning wondering if it’s possible to travel in time. Or maybe not, I don’t know. But theoretically you could have, so let’s say you did.
Is time travel possible? Well, the answer is yes. Physicists — actual scientists, not science fiction novelists or amateur naturalists or other crackpots — who have investigated the question say that as far as they can tell, nothing in the laws of physics prevents traveling in time.
Several factors complicate it, though. One factor is the physical conditions required for the journey. Another is the problem of chronological paradoxes. In other words, for example, what would happen if you went back in time and killed your grandfather, who then would not have fathered your father, who then would not have fathered you, who then would not have been born and could not go back and kill your grandfather?
But this logical loop aside, theoretically there are ways to travel in time, all involving tremendous masses or tremendous speeds.
Space and time are continuous, meaning they together make up space-time, or the four dimensions we live in. It’s known through the theory of relativity that space-time bends in the presence of physical masses. A baseball bends space-time an imperceptible amount; Earth bends space-time a perceptible amount, which we experience as gravity. Since time is a dimension like space, it’s bending, too, though we don’t seem to notice it.
Now, a large enough gravitational field can bend space-time around so that it folds on itself. An analogy in three-dimensional space is a sheet of paper with two dots marked on it at either end. The dots are far apart, the way two different times are far apart. To get the two dots to touch each other, bend the paper over. Similarly, it’s theoretically possible for space-time to bend on itself so that two distant points in time touch.
The physicists call this theoretical bending of space-time a “closed timelike curve.” The bending could occur in a number of conditions. If the universe was spinning and you traveled around it fast enough, you could arrive at a point in time before you left. Similarly, if the universe was shaped like a cylinder and was rotating, the rotation could drag light through space-time so that it curved along the cylinder and came back to its starting point. If you were moving along with that light, you would return to the point in the past where you started.
Another closed timelike curve could be created when local conditions of gravity warp space-time to the extent that a hole opens leading to another disturbed point — or really, the same disturbed point that has opened into a distant point, analogous to the two dots on the folded sheet of paper. This is called a “transversable wormhole.” It is a sort of tunnel short-cut through space-time.
Such a severe warp would occur in the presence of a “negative energy density,” or antigravity, which, like wormholes, exists theoretically but has never been observed. Now, extreme conditions of gravity are created near neutron stars — which are stars that have collapsed into a ball roughly 10 miles wide — and black holes, which are even further collapsed. Inside a black hole, the laws of physics as we know them break down, and space and time are so warped they have in a sense vanished to our understanding. A wormhole could occur in the vicinity of such gravitation.
But there are major problems with wormhole travel. One is that first you’d have to find the antigravity; another is that you’d have to stabilize the wormhole; and a third is the warping is so intense you’d probably be killed trying to get through. There is also, in theory, a problem of radiation buildup at the entry to a wormhole. In other words, even if you stabilized a wormhole so it didn’t crush you, radiation would travel through the portal with you, and as it came back to the same point in time that it left, it would build up there at the portal and become intense enough to kill you.
The problems with utilizing the spin of the universe to travel in time are that, first of all, the universe does not appear to be spinning, and second, no one has figured out a way to move anywhere near the speed of light.
These are methods of traveling backward in time. Traveling forward is more, well, straightforward. First, we’re all traveling forward in time at the rate of one minute per minute, so to reach the future, just wait a minute. And second, the faster you move, the slower time moves relative to Earth time, so traveling near the speed of light for a while could return you to Earth at a time in the future of the period of time you experienced yourself.
If you could somehow develop the technology to overcome these problems, the physicists tell us, nothing physically prevents time travel. Except for the fact that as far as we know, the past is fixed. Is it possible to change the past and not rearrange the present?
There are at least two possible answers to this question. Stephen Hawking proposed a “chronology protection conjecture,” which holds that an unidentified physical principle in the universe prevents past events from being changed, or prevents time travel altogether to preserve chronological order. Hawking later backed off this hypothesis because the absence of time paradoxes can also be accounted for by the “many worlds” interpretation of quantum physics. That is, the evidence strongly implies that everything that can happen, does happen, and is happening as we speak. If everything happens somewhere, then your killing your grandfather does not cancel out the series of events in which he lives, your father lives, and you live.
Time, after all, is not actually linear but more like an eternal block. What that means for us Earthlings, as one reader recently commented, is hard to wake up to. No matter what the physicists tell us about the dimensional nature of time, in our experience it is still an impervious flow forward, in the same way that, no matter how often we agree that the Earth theoretically revolves around the sun, in our experience every morning the sun still rises. We are just trying to wake up. Or maybe not, I don’t know.

Read more on the nature of time:

“Time, it’s even stranger than you think”
http://www.bangordailynews.com/story/bdn/Physical-lengths-of-time,156407

“Trapped in the fourth dimension”
http://www.bangordailynews.com/story/bdn/Trapped-in-the-fourth-dimension,156940

Amateur Naturalist archive
http://www.bangordailynews.com/topic/95/browse.html

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