Can you travel in time?Theoretical physicists provide some answers

Can you travel in time?Theoretical physicists provide some answers

Our curiosity about time travel is thousands of years old. Credits: Shutterstock

Time travel regularly appears in popular culture, with countless time travel stories in movies, television and literature. But that’s a surprisingly old idea. Written by Sophocles over 2,500 years ago, King Oedipus of the Greek tragedy can be claimed to be the first travel story.

But is time travel really possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I have found that there are several possible answers to this question, but not all of them are inconsistent.

The simplest answer, if so, is that time travel is impossible because we have already done so. It can be argued that it is forbidden by the laws of physics, such as the second law of thermodynamics and the theory of relativity. There are also technical challenges. It may be possible, but it requires a huge amount of energy.

There is also the issue of the time travel paradox. If free will is an illusion, if there are many worlds, or if we can witness the past but not experience it, we can (tentatively) resolve these. Perhaps time needs to flow linearly, and time travel is impossible just because we can’t control it. Alternatively, time is an illusion and time travel is irrelevant.

The laws of physics

Since Albert Einstein’s theory of relativity (explaining the properties of time, space, and gravity) is our most profound theory of time, we would like to think that time travel is forbidden by the theory of relativity. Unfortunately, one of our colleagues at the Institute of Advanced Studies, Kurt Gödel, invented a universe where not only is time travel possible, but the past and future are inextricably intertwined.

We can actually design a time machine, but most of these (in principle) successful proposals require negative energy or negative mass that does not seem to exist in our universe. increase. If you drop a tennis ball with a negative mass, it will fall up. This discussion is pretty inadequate because it explains why we can’t really time travel just by including another idea that we don’t really understand: the idea of ​​negative energy or mass.

Mathematical physicist Frank Tipler conceptualized a time machine that does not contain negative mass but requires more energy than it exists in the universe.

Time travel also violates the second law of thermodynamics, which states that entropy or randomness must always increase. Time can only move in one direction. That is, the eggs cannot be scrambled. More specifically, by moving to the past, you move from the present (high entropy state) to the past. In the past, lower entropy was needed.

This debate originated from the British cosmologist Arthur Eddington and is at best incomplete. Perhaps it will stop you from traveling to the past, but it says nothing about time travel to the future. In fact, traveling next Thursday is just as difficult as traveling last Thursday.

Solving the paradox

If you can travel freely in time, you will definitely hit the paradox. The most well-known is the “Grandfather Paradox”. You can virtually use the time machine to move past and kill your grandfather before your father becomes pregnant, eliminating the possibility of your own childbirth. Logically, it can’t exist or it can’t exist.

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five, published in 1969, explains how to avoid the grandfather paradox. If there is simply no free will, the grandfather was not killed in the past, so he cannot be killed in the past. Billy Pilgrim, the main character of the novel, can only move to other points on his world line (the timeline in which he exists), but he cannot even move to other points in space-time, so he even thinks about killing his grandfather. I couldn’t.

The “Slaughterhouse-Five” universe is in line with everything we know. The second law of thermodynamics works perfectly within it, with no conflict with the theory of relativity. But it contradicts some of the things we believe in, like free will. You can observe the past as if you were watching a movie, but you cannot interfere with the actions of the people in it.

Is it possible to allow actual changes in the past so that I can return to my grandfather or Hitler to kill him? There are several multiverses that assume that there are many timelines in different universes. This is also an old idea. In Charles Dickens’ Christmas Carol, Ebeneza Scrooge experiences two alternative timelines that lead to shameful death and happiness.

Time is the river

Greek emperor Marcus Aurelius wrote: He will be carried away. “

You can imagine time flowing past every point in the universe, like a river around a rock. However, it is difficult to get the idea accurate. The flow rate is the rate of change. River flow is the amount of water that passes a particular length at a particular time. So if time is a flow, it’s a velocity of 1 second per second, which isn’t a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “chronology protection conjecture” must exist. This is an unknown physical principle that prohibits time travel. The concept of Hawking comes from the idea that you can’t get information from a black hole, so you can’t know what’s going on inside the black hole. But this argument is verbose. Time travel is not possible because time travel is not possible.

Researchers are investigating a more basic theory that time and space “emerge” from something else. This is called quantum gravity, but unfortunately it doesn’t exist yet.

So is time travel possible? Probably not, but I’m not sure.

Time travel may be possible, but only with parallel timelines

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