Why [insult] can't we travel to the past?
If the inability to travel through time was a fundamental part of our world, you'd expect physics to be completely behind this law.

As you can see, we are not completely ignorant of time. We know that time is woven in three dimensions of space and creates a four dimensional texture of the universe. We know that the passage of time is relative. Depending on your frame of reference, you can slowly move into the future. (You just have to get close to the speed of light or get comfortable with a black hole, but these are just minor engineering issues, not physics.)

But as far as we can tell, we can't reverse the flow of time. All the evidence shows that travel to the past is forbidden in our world. Every time we try to build a time machine, a random law enters the universe and takes our hands off the temporary cookie.

But we don't know why. The reasons seem really coincidental. There is nothing fundamental that we can point out, nor is there a law, equation, or concept that definitively explains why time travel is not. This is very disappointing. The universe is clearly telling us something important... we don't know what it is saying.

Go, kill your grandfather

All kinds of philosophical arguments for and against the possibility of travel Take for example the famous "grandfather paradox". Let's say you build a time machine and travel back in time. You find your grandfather and shoot him (I don't know why, but join me). But wait... If your grandfather dies, it means that he cannot be your father's father, that is, you will never exist. So how did you go back in time and do this horrific thing?


Time travel into the past may indeed be allowed, but your actions are limited. Perhaps the past already exists and it is entirely in the rock. What happened simply happened. If you have the ability to travel through time and join the past, then the past must actually encode these actions - nothing new, because it is literally in the past. So you can't kill your grandfather, because you never did, but you can be a stranger making a blind date with your grandmother.

Perhaps, say, there is no time, comrade. Perhaps this is a component of our human consciousness as a way of organizing and synchronizing our sensory input. Perhaps we are imposing a deep and fundamental assumption on a world that does not matter, and so the whole argument is controversial.

This is all part of the legal debate of philosophy. But let's see if physics can make an impact on that. After all, if we could (even in theory) build a time machine, we could solve a lot of our late-night bets.

Can we?

Like closed time curves physicists use a very special language when trying to build time machines: the language of gravity given to us by Albert Pierre himself in the form of general relativity. This is because the language of gravity, as interpreted in GR, is a story of the curvature and warping of spacetime. GR is the theory of motion in our universe and how this motion relates to the four-dimensional fabric of space-time.

In GR, matter tells spacetime how to bend, and spacetime bends tells this matter how to bend. to move.

To determine if we can build a time machine, physicists want to see if it is possible to build space-time - to find a particular, peculiar arrangement of matter - that allows one to go back in time. .


The goal is to find Time Closed Curves, or CTCs.

"Curve" means exactly what you think it does - a path in space and time. Like time means not to cheat - you are not allowed to travel faster than light anywhere. "Closed" means that the curve bumps into itself - imagine you're moving in one direction, always moving forward, and never faster than the speed of light. But at the end of the journey, you realize that you have come to your own past.

This is a time machine. This is the Counter-Terrorism Committee.

The strange thing is the presence of CFCs! Over the decades, we've discovered several solutions of general relativity that make time travel possible: Famous mathematician Kurt G. and Dale (yes, Kurt Gilles Dale) discovered that if the universe is filled with uniform dust. Spinning slowly, you can find paths in this world that revolve in its past. You know wormholes, right? Those shortcuts in space? They can also act as a time machine. The trick is to grab one end of the wormhole and lock it in place. Then grab another one and speed up the light. Keep it at that speed for as long as you like. Now bring this end back to the main part. The verticals of wormholes no longer have synchronous clocks due to the dilation effects of traveling at near-light velocities. Since one end of the past is the other, you can just enter it and travel to the past. Suppose you have an infinitely long cylinder (you can get it at your local home appliance store). Rotate the drum at about the speed of light. If you follow a careful and spiral path around the rotating drum, it will explode far past. Inside a rotating black hole is a very interesting place, where opposing currents of gravitational and centrifugal forces combine to open the throat at the center of the black hole, allowing CTCs. The journey of the James Webb Space Telescope to its final orbit was successful

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