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Time: It is a mathematical dimension to measure the change of state (any motion) of Existence (includes universe/multiverse/entire creation).

In order to travel in Time, one has to change the state of entire existence to the previous/future state which is impossible.

We need to consider our time measurement techniques again. Should we consider time dilation shown on our clocks as actual time dilation?

The laws of existence affect all the physical entities in a different way. All the physical entities have a different effect of gravity and velocity on them depending on their nature. It is not necessary that our biological clock of living beings and other physical phenomena would be affected in the same time measurement scale as it affects the normal clocks and atomic clocks.

Please elaborate and correct me if I am wrong... Thank you

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  • What is the question, exactly? If it is according to the title, then you might find answers here: philosophy.stackexchange.com/questions/8730/… and here: philosophy.stackexchange.com/questions/10743/…
    – user14511
    Jul 26, 2020 at 16:09
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    Technically, time travel is predicted by relativistic physics. Of course, that's forward, not backward. See the twin paradox.
    – J D
    Jul 26, 2020 at 17:21
  • "In order to travel in Time, one has to change the state of entire existence" is fallacious. Time travel (even to the past) need not involve changing the timeline, it may already incorporate temporal loops, see Novikov's self-consistency principle. It is only combination of time travel with some sort of libertarian free will capacity that leads to paradoxes.
    – Conifold
    Jul 26, 2020 at 22:36
  • "In order to travel in Time, one has to change the state of entire existence to the previous/future state which is impossible" That might make sense if you assume the philosophy of time known as presentism, but the alternate philosophy known as eternalism says that objects and events in your past haven't ceased to exist, they exist at a different location in time, so traveling back to them would be more akin to traveling to a different location in space.
    – Hypnosifl
    Jul 27, 2020 at 3:22
  • To quote an answer from this question "Antimatter is in every precise meaningful sense matter moving backward in time". Yet antimatter is a very real thing.
    – Graviton
    Jul 27, 2020 at 12:18

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You are posing two different questions here.

Time travel is considered impossible because it leads directly to a universe where effects precede causes, and things like energy would no longer be conserved. This would be a very big deal.

Relativistic time dilation is real, and it affects all physical processes equally. This includes all biological processes as well, because on the micro scale they consist entirely of physical processes. This principle guarantees that all clocks slow down at high velocities: mechanical clocks, electronic clocks, the growth rates for your hair and fingernails, and your own thought processes.

So, let us perform the following thought experiment. Imagine you are in a speeding spaceship, and that your mental clock is not slowed down at relativistic velocities, while all your mechanical clocks were. Then, by comparing your mental perception of the passage of time with the movement of the hands of that clock, you could tell what your absolute velocity was without looking out the window. This would invalidate special relativity and that entire edifice of knowledge, proven valid in every experimental test thus performed on it, would crumble and fall- but, for the reasons listed above, this will not happen.

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  • Thanks for your answer however as you mentioned that Relativistic time dilation affects all the physical processes equally. I have tried to find if there is any empirical research done to check equal dilation in all clocks. To my best knowledge there is none.
    – Harjeet Singh
    Jul 26, 2020 at 19:21
  • Harjeet, let us perform the following thought experiment. Imagine you are in a speeding spaceship, and that your mental clock is not slowed down at relativistic velocities, while all your mechanical clocks were. Then, by comparing your mental perception of the passage of time with the movement of the hands of that clock, you could tell what your absolute velocity was without looking out the window. This would invalidate special relativity and that entire edifice of knowledge, proven valid in every experimental test thus performed on it, would crumble and fall. That is not likely to happen!
    – niels nielsen
    Jul 26, 2020 at 20:14
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At this point in time, there is no known technology which allows for any human being to travel back in time-(even for a single minute). However, as a former History Instructor, I've learned to be very careful when using words such as, "possibility" and "impossibility". Here are some examples:

In 1902, it would have been viewed as "impossible" to fly human beings several feet above ground for just a few minutes. Yet, by 1903, the Wright Brothers conducted their first airplane flight at Kitty Hawke, North Carolina with great success and essentially, defying the naysayers and ridiculers of their day....the Modern Airplane was born. During the mid 1950's, it was still viewed as an impossibility for human beings to travel beyond the heavens and into space. However, by the late 1950's, the Soviet Union launched Sputnik and by the early 1960's, U.S. Astronaut John Glenn successfully orbited the Earth, thereby leading to the Space Age (and Space Race between the U.S. and the U.S.S.R.). The Internet, as a worldwide public utility, has existed for nearly 30 years-(since the mid 1990's). However, if one remembers the earliest days of the Internet-(circa 1996-1997), it was quite limited in design and global scope, as well as intellectual sophistication. By 1998, 2 Stanford University Engineers transformed the Internet with the founding of Google, allowing human beings to conduct research and acquire knowledge at an unprecedented rate and volume thereby establishing the first true worldwide Digital Library.

And so when looking at the late modern and contemporary history of science and technology, we can see that the impossible, has often become-(and can still become)...possible. The pioneers in Science and Technology have ALWAYS found novel and ingenious ways of advancing and refining the technologies of their time. Does this therefore mean that time travel is automatically possible? No, it does not automatically guarantee that time travel is possible, though simultaneously, it does not automatically make time travel an impossibility either.

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  • According to Wikipedia, the first zeppelin flight took place in 1900, so flight would not have been viewed as impossible in 1902. Also, none of the feats of engineering which you mention here violated any known laws of physics, unlike common conceptions of time travel.
    – Sandejo
    Jan 30, 2022 at 2:51
  • Good point regarding the first zeppelin flight-(I had totally forgotten about it).
    – Alex
    Jan 30, 2022 at 4:19
  • However, the larger theme is not so much about the specific year(s) and the technology of that time; rather, it is a theme which is addressing the value of debunkers when in the presence of naysayers and ridiculers. For example, if I had known about the zeppelin flight of 1900, the theme of my positing would've changed little-(except for the obvious change in chronology by about 2-3 years). I am sure there were naysayers and ridiculers when the first zeppelin was launched in 1900.
    – Alex
    Jan 30, 2022 at 4:22
  • With regard to the space travel and internet examples, they currently do not violate the known laws of physics, though years before space travel and the invention of the internet search bar-(i.e. Google), such inventions would have been seen as exotic, avant-garde and "out of the box"-(belonging to the realm of science fiction or the fantastical). I, for example, lived 20 plus years before Google "came on the scene" during the late 1990's. If someone had told me what Google was able to do within a matter of seconds in the year 1990 or even the year 1995, I would've been skeptical-(though
    – Alex
    Jan 30, 2022 at 4:27
  • not a naysayer or ridiculer). However, there were naysayers and ridiculers during the 1990's who were dismissive of the limitless potential that technology could offer the world-(trust me, these cynical folks did exist 25 plus years ago)....and they existed 65 plus years ago and they existed over 120 years ago.
    – Alex
    Jan 30, 2022 at 4:31
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Wikidedia mentions closed timelile curves, if they exist

would seem to imply at least the theoretical possibility of time travel backwards in time

It seems unlikely. In the absence of a mechanism by which the present moment can coexist with a past moment, we have physically only the present moment.

If a fast satellite in orbit experiences time dilation, it doesn't drift into an alternate physical moment. It is still just in the present. How could it not be?

There is no time-travel if everything is in the present.

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  • Interesting point with regard to the perpetual present. I'm not sure which Pre Socratic Philosopher discussed the perpetual present, though it was a topic that was of significant interest to the Ancient Greek Philosophers.
    – Alex
    Jan 30, 2022 at 20:44
  • As a trained Historian and former History Instructor, I tend to view Time in linear sequence, that is to say, the traditional past, present and future. Admittedly, concepts, such as the perpetual present and for that matter, time travel, defy and certainly challenge my conventional understanding of Time. Nevertheless, the voluminous and complex studying of Time-(in all its knowable forms), will continue to generate historically irresolvable questions, such as a perpetual present, as well as the mechanical ability for humans to travel backward-(or forward) in Time.
    – Alex
    Jan 30, 2022 at 20:48
  • @Alex Certainly in consciousness there is memory of the past and anticipation of the future. Indeed past experience is essential to thinking and living, but physically we're still in the ever-present 'now'.
    – Chris Degnen
    Jan 30, 2022 at 21:12
  • "but physically we're still in the ever-present 'now"....hmmm....interesting. I'll have to ponder that a bit more.
    – Alex
    Jan 30, 2022 at 21:51
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What makes you think it isn't?

Probably the strongest argument against it, is that we don't think we see evidence of macroscopic causality violation. This is basically summarised by the chronology protection conjecture - expect the laws of physics to prevent time travel, because if it were possible we think we'd see far-reaching consequences.

In classical physics, time travel is possible. If time is a dimension just like space, it could be 'folded'. Closed time-like curves involve exotic matter: an infinite tube for the Tipler cylinder, or matter with negative gravity for 'warp drive' style solutions. There is no evidence these exist, but, it's possible eg dark matter is still very mysterious, dark energy seems have opposite gravity to normal, and it hasn't even been proven beyond doubt antimatter doesn't repel ordinary matter gravitationally.

In quantum mechanics, closed time-like curves would violate what we think we know about it being impossible to create entanglement from nothing, and decrease entropy. Given the many decades of work trying to quantise the classical theory of gravity this is now thought very unlikely to happen, instead we expect a truly quantum theory in which space and time are emergent. The Wheeler-DeWitt equation, our best hybrid of quantum & classical gravity, is notably not time-dependent, bolstering that expectation.

There are many proposals about how time and causality work. McTaggart grouped the theories into A & B Series, based on whether tenses are used or not - ie we focus on relative ordering, or state time-location. Presentism, the idea only now is real, and growing-block, the idea the past is certain but the future is not and the present occurs at the junction of those, are both A Series theories. Eternalism, the idea the future already exists and the present is a subjective quirk, is a B Series theory. Broadly speaking B Series theories allow time travel, A Series don't.

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