Can we time travel? A theoretical physicist provides some answers

Time travel is a regular feature in popular culture. One can argue that the Greek tragedy "Oedipus Rex" is the first time travel story.

Is time travel feasible? This is a valid question given the popularity of the concept. There are many possible answers to this question, not all of which are conflicting.

Time travel is not possible because we already do it. It can be argued that it is forbidden by the laws of physics. It may be possible, but it would involve a lot of energy.

There is a question about the concept of time travel.

If free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced, we can resolve time- travel paradoxes. Time travel can't be done because time must flow in a linear manner and we have no control over it, or time is an illusion and time travel is irrelevant.

Our most profound theory of time is Albert Einstein's theory of relativity, which forbids time travel. Kurt Gdel, a colleague from the Institute for Advanced Study, created a universe in which time travel was not just possible, but the past and future were intertwined.

We can design time machines, but most of them need negative energy or negative mass, which doesn't seem to exist in our universe. The tennis ball will fall if it is dropped. The reason we can't time travel in practice only by using negative energy or mass is explained by this argument.

A time machine that does not involve negative mass requires more energy than the universe does.

The second law of thermodynamics says that randomness must always increase. You can't make an egg out of time. By travelling into the past we are going from a high state to a low one.

The English cosmologist Arthur Eddington was the originator of this argument. It doesn't say anything about time travel into the future. It is harder for me to travel to next Thursday than it is to travel to last Thursday.

Resolving paradoxes

If we were able to time travel freely, we would run into paradoxes. One could use a time machine to travel to the past and kill their grandfather before their father's birth in order to eliminate the possibility of their own birth. You can't both exist and not exist at the same time.

The anti-war novel Slaughterhouse-Five describes how to escape the grandfather paradoxes. Since his grandfather was not killed in the past, it is not possible to kill him now. Billy Pilgrim can only travel to other points on his world line, but not to any other point in space-time, so he could not contemplate killing his grandpa.

Everything we know about the universe in Slaughterhouse-Five is correct. There is no conflict with the second law of physics. You can observe the past, like watching a movie, but you can't interfere with people's actions.

Is it possible to modify the past so that we could kill our grandfather or Hitler? There are many theories that say there are many timelines. In Charles Dickens' A Christmas Carol, there are two alternate timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Marcus Aurelius wrote that.

"Time is a river made up of events which happen, and a violent stream, for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away as well."

Time can flow past every point in the universe, like a river. It's hard to make the idea precise. A river's flow is the amount of water that moves in a given time. If time is a flow, it's at the rate of one second per second, which isn't very useful.

There is an as-yet- unknown physical principle that forbids time travel. We can't get information out of a black hole because we can't know what's going on We can't time travel because we can't time travel.

Time and space can appear from something else. This is referred to as quantum gravity but it doesn't exist yet.

Is time travel feasible? We don't know for certain.

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