Researchers at the Hong Kong University of Science and Technology claim to have established that time travel is impossible. More specifically, they claim to have established that an individual photon (the smallest unit of light) may not travel faster than the speed of light (introducing the obvious question of how could it if it is light) and that this precludes anything from going backward in time.


Per the article:


"The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause," the university said.


The findings have been published in the U.S. peer reviewed journal, Physical Review Letters.

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I find it obvious that time travel is impossible. It would take infinite energy to put every photon back in it's original state not to mention every atom in the universe. But that doesn't mean that you can't go to a parallel universe and observe our history through photons that escape.
mmmm a parallel universe with little to no religion would be great to look upon.
You have to place every single atom, subatomic particle, photon back to an initial state or you are not there, you are not at that place in time. These are the things that make up reality. If they are not exactly where they should be you are in a different reality.

Think of it like a flowing river.  To place that river back in time you have to have some way to place every drop of water exactly where it was. I know we can't do it with a river. How can you imagine that we can doit with the entire universe

Some may argue that science cannot prove a negative. Just like it can't prove the non-existence of god(s). How can it prove the non-existence of time travel?


The universe has many surprises left for us to discover. Time travel may be one of them. Possibly through some means other than travelling faster than the speed of light.

Id say time travel is working with physical theory with less dogma attached as opposed to proving non existance of god. Study of existance of time travel I dont think is approched with trying to prove a negative.
Time travel into the future is possible, slowing down time and viewing the past is also theoretically possible... but traveling back in time is not possible.

Impossible, based on what we understand at this time.  I don't think science deals in absolutes... correct?  Just to be nuanced here isn't it better to write there doesn't seem to be any possibility of time travel based on what we know?  


Some interesting responses here. I am not a physicist, so I am not in a position to question the mathematics the Hong Kong scientists used, nor to question the specific methodology. On a conceptual level, there may be some rethinking necessary before one can say with absolute certainty that time travel is impossible. First is the assumption that time travel necessarily involves traveling faster than lightspeed. These, at least on a conceptual level, are not the same things. In Einstein's equations, "t" is positive or an absolute value, but in most quantum equations, "t" is either positive or negative and thus at that level there is not a barrier to traveling along the value one way or another. If we assume that exceeding lightspeed would mean that effect precedes cause, which in this case is purely an informational matter of one set of information going faster than another since photons are the units of information, would this be "time travel"? It is not the same thing as say my returning to 1864 to witness a Civil War battle since the existence of the two events, me and the battle, are so causally removed from each other that chaotic effects would come into play. So, while the Hong Kong team may have proved that effect cannot precede cause, they have not proved the impossibility of time travel. Additionally, I do not see that time travel would involve returning photons to a prior state, what would amount to reversing the universe. This would not be time travel. If everything returned to a prior state, so would the time traveler's brain; the traveler would not remember the "trip" and all would be as though that now future reality had not happened at all. The universe could be reversing and restarting all the time, so to speak, and we would never know it. If we think of time travel in the sense of Wells's Time Traveller or the Doctor and his TARDIS, instead of reversing cause and effect or returning everything to a prior state, the traveler and time machine would be removed from the universe's causal flow and then inserted at some other point in that flow. Whether any ensemble of matter and energy could be removed and inserted in this way is a different matter from either informational sequence or universal rewind. I cannot say whether it is possible or impossible, merely that it is a different issue from what the study discusses.
My understanding is that the person traveling faster than the speed of light is supposed to go back in time not anyone in the original frame of reference. As such, upon returning to the original frame of reference the traveler would have experiences that were incongruous with those who never left it. That is, the traveler's real time observations would be history to the others. Accordingly, the universe would not have to return to an earlier state but, rather, would exist in two states at once.
A good thing about science is that scientists are never satisfied with a conclusion.  They are constantly trying to prove or disprove others' theories...unlike the religious.
Very true.




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