## Wednesday, April 6, 2022

### Is Time Travel Possible? Part 1

Traveling into the Future

We can certainly travel into the future, which is what we are doing right now at the rate of 1 minute per minute.  We can even change that rate.  With drugs or alcohol, we can change the perceived rate in which we travel into the future.  But we can also change the actual rate, relative to others, thanks to Einstein’s relativity.  This is called relativistic time dilation and it is caused by two physical phenomena.

The first is relative motion (characterized by Einstein’s “special” relativity).  Imagine you and a friend are wearing really accurate and precise watches.  If your friend is moving toward you or away from you at some speed, her watch will tick slower than yours.  However, the effect in our normal everyday world is very, very tiny.  Take the person’s speed, divide it by the speed of light, then square that result, and that’s the approximate effect.

For example, let’s say your friend is jogging toward you at 3 meters per second (which is almost 7 miles per hour).  Divide that by the speed of light (about 300,000,000 meters per second), and then square that result, and you get 10-16, or 0.0000000000000001.  So for every second your watch ticks, your friend’s watch will tick 100 billionths of a billionth of a second slower.  Imperceptible.  However, if your friend could approach the speed of light relative to you, then her watch would slow down significantly relative to yours.  In fact, if she zoomed away at near the speed of light for long enough, then turned around and returned to Earth, she might have aged only a few months while you aged a few years or longer.

The second phenomenon is gravity (characterized by Einstein’s “general” relativity).  Time slows inside a gravitational potential well.  For example, time ticks slightly slower on the surface of the earth than in outer space, which is why GPS satellites must account for this tiny effect to stay accurate.  The stronger the gravity field, the stronger the time dilation.  A great example was in the movie Interstellar in which the crew lands on a planet closely orbiting a supermassive black hole, leaving one crew member in the mother ship much further from the black hole.  The crew on the planet felt time passing normally, but when they returned to the mother ship, they realized that their hour on the planet had corresponded to many years passing for the crew member on the mother ship.

These things are certainly interesting to think about, but the technology does not exist – and may not ever exist – that will allow humans to experience significant time dilation.  Yes, time dilation is real and we have to account for it in GPS satellites, nuclear reactors, and particle accelerators, but will we ever have the technology to accelerate a human to near the speed of light (relative to another human)?  Probably not.  So while possible in principle, time travel into the future through use of relativistic effects will probably remain science fiction.

Traveling into the Past

The far more interesting question, and the one that most people wonder regarding time travel, is whether it’s possible to travel into the past.  We all want to be Marty McFly (or Doc Brown, depending on your personality), with the freedom to see the past, experience the past, and even change the past.

Lots of reputable physicists, like Nobel Prize winner Kip Thorne (author of Black Holes & Time Warps), claim that the laws of physics allow for time travel into the past, and even suggest fascinating ways of doing so, like stretching open and passing through wormholes.  (Note: wormholes are theoretical predictions and there is exactly zero empirical evidence that they exist.  Even if they could exist, there is no good reason to believe that we could ever create them at will, engineer them to specifications, choose their endpoints in spacetime, stretch them open, or pass matter through them.  They are, purely and simply, science fiction.)

The obvious problem with time travel into the past is the risk of a temporal paradox.  For example, what would happen if you killed your ancestors?  If you did, then you couldn’t be born, which means you couldn’t go back in time to kill your ancestors.  Doesn’t that problem by itself rule out time travel into the past?

Some say that there’s no problem as long as there’s no possibility of actually changing the past, a concept called the self-consistency principle.  For example, maybe you can travel to the past, but as soon as you try to kill one of your ancestors, some mystical force prevents you from following through.  This statement, which comes from the Stanford Encyclopedia of Philosophy, highlights the point: “We should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past.”

So what do you think?  Is it possible to affect, influence, or participate in the past without changing the past?

If time travel into the past was possible, what kinds of things could you do without causing a temporal paradox?

In the next post, I’ll dig into this a little more.