I have heard that if there will exist backwards time travel in some reasonable form in the future, presumably they would send the technology back so that we have it earlier. Since we don't have it, presumably we will never discover this technology. Is this true?

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    Speculating about distant future is a rather futile exercise. "Presumably they would send the technology back", based on what? Our avid desire to have it? Some projection of what we would do? We have no idea what "they" will be thinking, or whether "they" will even be interested in time travel were it possible. – Conifold Mar 23 '20 at 3:57
  • @Conifold I think you're right that there needs to be some presumptions in a rigorous response, and that's true for most philosophy. I think we're free to lay down the rules we want to use, and this structure allows our "speculation" to not be futile but rather a philosophical argument. – personjerry Mar 23 '20 at 5:05
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    How do you know they would have sent it back to a time before now? There's also the weird physics of things like Closed timelike curves and the Novikov self-consistency principle. It's not clear if the latter actually applies, or if the former is physically possible; but it leads to interesting possibilities for time travel having to maintain self-consistency. – JMac Mar 23 '20 at 13:37
  • Such a question assumes that the future and the past exist in some real, objective sense. – Nick Mar 23 '20 at 21:31
  • We try to avoid users freely laying down the rules on this site and then having opinion based discussions. This is a Q&A site, not a forum. It is expected that the poster lays down the rules or refers to published philosophical literature that does so. The problem in this case is that it is hard to find a basis for mind reading our distant descendants beyond users' personal opinions. – Conifold Mar 24 '20 at 0:05

I think it is useful to stop thinking about time as a quantity. Time is simply cause-effect relationships. Yes, time slows down in a mathematical way as you approach speed of light, but this can be explained by a slowing of the physics giving rise to cause-effect relationships.

Try this out: time

  • Doctor Who probably isn't the best authority! – CriglCragl Mar 23 '20 at 18:59

We can be sure we don't have a full picture of how time works. In relativity, time is a dimension, and space-time behaves in a way that the dimensions interpenetrate and transform one to the other. In quantum mechanics time is treated as universal and absolute. Thermodynamics gives rise to the arrow of time, and time irreversability - both relativity and quantum mechanics are time-symmetric and events are reversible. Various attempts are being made to approach this problem directly like the purification principle, or through quantum gravity approaches like spin networks in loop quantum gravity.

Our understanding of time travel is founded in general relativity, and we just don't know how gravity affects quantum system because it is such a weak force on the quantum scale. Relativity seems to show gigantic energies are involved, like the Tipler cylinder, or highly entangled blackholes. Or exotic physics like negative energy, which may be limited to the quantum scale. At the energy involved, our descendents concerns would likely be completely unintelligible to us. It is also conjectured the travel couldn't go to a point before the closed time-like curve was made.

So far nothing explicitly forbids time travel. We can't form any strong conclusions either for or against, because we know our physics isn't complete, and it may just involve giant energies, or be limited to quantum scales.

From string theory, it might be possible to travel into parallel dimensions with different conditions. I don't know of any investigations into this.


This is assuming there are people in the future. Maybe backward time travel is possible, but coronavirus will soon mutate into a form that will eradicate mankind before we can develop the technology.

This implicit premise being unwarranted, the conclusion is not sound.

  • As a side note, I also don't believe backward time travel is possible because the past does not exist in any tangible form. Somewhat like you can't go on the other side of the mirror. – armand Mar 24 '20 at 5:44

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