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Requirements for answer:

-hypothetically can work (I don’t care if it’s hard or energy expansive)

  • Made by Mankind

  • Can be used at any time

  • preferably works for going to the past

  • it’s enough if I can send energy (storing a message or similar)

  • Explanation of how it works.

Note: It can be something not proved, it’s enough if modern knowledge doesn’t discard it.

Edit summaries: updated so I stop having conflicting requirements

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Apr 11 at 19:11
  • $\begingroup$ @Draft85 like that is better? Made other update $\endgroup$
    – user84509
    Apr 12 at 0:54
  • $\begingroup$ That should stop the close-votes, hopefully. $\endgroup$ Apr 12 at 0:59
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Many time travel mechanisms in literature have three major problems

  1. A grandfather paradox: go back in time and murder your grandfather, thus change your present so you no longer exist etc...
  2. Energetically implausible: for example, accelerating a body to "beyond the speed of light" so it goes backwards in time is going to require more than to the order of $mc^{2}$ energy. Which is very costly, and also a bit noisy if the process isn't 100% efficient. Other options, such as creating an artificial rotating black hole big enough to 'pass through' have similar issues.
  3. Breaks the laws of physics or uses extreme hand-wavium: for example exceeding speed of light breaks laws of relativity, an Alcubierre drive, exotic matter, negative energy etc are all fairly implausible and hard to explain.

The best plan might be to go with your option that you can only send information back in time. That way can minimize the risk of accidentally stepping on a butterfly, and reduce energy requirements to something that might appear feasible. Then there are a couple of options:

  1. information can be sent back only to an instrument or machine (in which case you cant send data back to before the date that the instrument is manufactured)
  2. alternately information can be sent back to any stable media that has existed in the past (for example - maybe you can etch a message onto the Koh-i-Noor diamond so it is waiting to be discovered after the diamond is dug up)...or directly influence the neurons of a subject (willing or unwilling) so they get 'a message' - maybe that explains certain symptoms of schizophrenia?

As to how you can make things plausible or explain how it works, in practice you will still need a bitt of hand-wavium. In option 1. you machine might be a quantum computer that stores random data in pairs of entangled gravitons - but in the future you can influence the past measurement of one of the graviton's quantum states (cryogenics, low vibration, massive magnetic fields, more costly than the LHC - arm wave, arm wave...) In option 2. you can make nanoscopic black holes for just long enough to send a few high energy gamma rays back in time to etch your diamond. (more arm waves)... Remember, your explanation doesn't need to be plausible, it just has to appear plausible.

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  1. Travel faster than light. This is time travel.

Here is JDlugosz's magnum opus and one of the best explanations I have seen on why FTL means time travel. Are there any ways to allow some form of FTL travel without allowing time travel?

  1. Use an Alcubierre drive to effectively travel faster than light.

An Alcubierre drive is a theoretical device which sidesteps the impossibility of moving faster than light by instead manipulating space. It requires exotic forms of matter and a colossal amount of energy.

https://en.wikipedia.org/wiki/Alcubierre_drive

The Alcubierre metric defines the warp-drive spacetime. It is a Lorentzian manifold that, if interpreted in the context of general relativity, allows a warp bubble to appear in previously flat spacetime and move away at effectively faster-than-light speed. The interior of the bubble is an inertial reference frame and inhabitants experience no proper acceleration. This method of transport does not involve objects in motion at faster-than-light speeds with respect to the contents of the warp bubble; that is, a light beam within the warp bubble would still always move more quickly than the ship. Because objects within the bubble are not moving (locally) more quickly than light, the mathematical formulation of the Alcubierre metric is consistent with the conventional claims of the laws of relativity (namely, that an object with mass cannot attain or exceed the speed of light) and conventional relativistic effects such as time dilation would not apply as they would with conventional motion at near-light speeds.

The end result is that entities within the warp bubble can reach a destination faster than light, without themselves ever actually having travelled faster than light. Alcubierre was aware that a working device of this sort could be used as a time machine. Various protections to causality have been mooted, none very convincing. Maybe a time traveler would find herself in a closed timelike curve cut off from the timeline where she started and doomed to loop. The most convincing to me is that there is some unrealized fatal flaw in the Alcubierre drive that means it could not really be built. Of these reason it seems to me the most likely is that although theoretically possibly, there is no such thing as negative mass which the original Alcubierre drive requires to work. There are other versions of the drive which use other exotic forms of matter which also might not exist.


There is a fair bit on this stack about Alcubierre drives if you choose this method; read up. There is also some to read on the Physics stack.

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If our entire reality is a matrix style simulation, then all you need for time travel is access to the backup and restore utilities.

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  • $\begingroup$ Those same utilities would restore you to a previous state. $\endgroup$ Apr 11 at 3:30
  • $\begingroup$ @SurpriseDog, not necessarily, if you can do partial operations. $\endgroup$
    – Matthew
    Apr 12 at 2:41
  • $\begingroup$ Error: The Universe has detected that the backup is an inconsistent state... shutting down. $\endgroup$ Apr 12 at 2:42
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While technically not time travel the effect is the same.

If the universe is homogeneous and isotropic as well as infinite and uncurved, if you travel an infinite distance at an infinitesimally small amount under the speed of light (so you wont feel the time), eventually you will find an area of the universe that has a matter and energy configuration that is identical to the one you want to go back in time to. This is a variation of Nietzsche's eternal return.

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  • $\begingroup$ Funny, that is also the premise of my atheistic immortality process. In an infinite universe with a finite periodic table and nearly infinite time, the particular arrangement of particles which manifests as my particular consciousness must occur nearly infinite times. As a result, infinite instances of me are and will always exist. Immortality without divinity. $\endgroup$ Apr 11 at 1:55
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Here's an idea: CPT symmetry. It says that our universe has a twin that looks like its mirror image, where all matter is replaced by antimatter, and time flows backwards. If you are able to access said universe without something bad happening to you, you could end up going backwards in time. This is, if you get there by mirroring yourself, flipping your XYZ coordinates, the charge of every last particle of your being, and the way time flows for you. Or by transferring your consciousness to an denizen of the mirror universe. To get to the past-the mirror universe's future-faster, you will need to either travel at relativistic speeds, orbit an supermassive black hole, or both. Once you have traveled far enough into the past, you will need to recreate the steps you took to enter the mirror universe, in order to get back into your birth universe-in its past.

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