Most discussions about the possibility of “Time Travel” posit that the 4th dimension (time) is similar to the spatial dimensions-i.e. one can reverse along the time dimension, and re-enter the past. But does the past exist in any meaningful sense? I would say no- someone who lived in the 1700’s is dead and his body is dust-his house might still exist, but it is in a state of disintegration-just as entropy increases. So, based upon our experience (of the mundane world) time travel would seem to be impossible-unless the universe is a lot stranger than we can imagine. Is the present the only state that can exist?
It used to exist. It doesn’t exist now, based on the definition of the word “now”.
This is really a semantic question. Much hay was made of it (the assertion that the past doesn’t actually exist) in Zamyatin’s “We” and possibly again in Orwell’s 1984, as an excuse for lying about the past.
In the end, this is a semantic question based on what we mean by verb tenses, which fail miserably when talking about time travel. Even if we could travel back in time, it doesn’t mean that the past exists “now”. It just means that we can travel back in time.
Whether we can travel back in time is, of course, another discussion entirely, but it doesn’t depend on an “existence” argument. The past certainly did exist, and we can either get there, or not. (Unless you’re a solipsist, in which case you can argue that the past might just be illusory.)
Only now exists. The past is over and therefore does not exist. The future has not happened and therefore does not exist. Objects exist which existed in the past when the past was now and objects exist which will still exist when the future is now. This message, when anyone reads it, is an artifact of the past which will exist in the future now, which is your now whilst you read it. 
Of course the past exists now - you see it all the time. In fact that’s all you ever see.
As light travels approximately 186,000 miles per second, if you are looking at an object a mile away, you’re seeing it as it existed 0.00000538 seconds ago. And if you squint and look at the sun, you’re seeing it as it existed eight and a third minutes ago. And we won’t even bring up the Andromeda Nebula (M31).
Sort of interesting that we can never see anything as it exists right now.
or Maybe only the future exists, by the time you say the words “right now” it isn’t anymore.
According to some physicists, the past, future and present all coexist and have always existed, and the passage of time is an illusion. In this view the universe is a static 4 dimensional object where the entire history of the universe already existed when the universe came into being.
The universe is just a very complex equation. The past is still very real in a direction that we are not travelling. We are travelling toward the end of the equation and can’t go back. This is my personal viewpoint. The universe appears to follow laws that can be determined with mathematics. If we knew what the variables are. This is what the particle physics and quantum theory people are looking for, the unknown variables. The Higgs boson is the latest piece of the puzzle to be found. It won’t be the end of the search for unknowns, but it adds a piece to the equation.
There is also the multi-universe theory where every possible combination of events creates a new universe. That all possibilities do occur. This is too much for my mind to digest.
I prefer to think of the past as part of the fabric of the universe, that cannot be removed, or the equation falls apart. So the past must exist in some direction in order for the future of the equation to continue.
It depends on your view of the ontology of time. On the block view - the past and future always exist.
This is something I often enjoy trying to resolve. I never get anywhere with it but it’s fun to contemplate.
I thought the idea of the past simply not existing has largely taken a back seat since relativity. I mean, there are situations in which my past is your future, so the most we can really say is In a given light cone the events of the past no longer exist.
Also it should be noted that an arrow of time hasn’t been found yet at the quantum level. A videotape of individual particles doing their thing would look right played forwards or backwards.
IANA physicist, so expect at least some of this to get slapped down in a moment (or perhaps it has already been slapped down…)
Good point. Anything that has happened at A that could have affected B (light from A has reached B) is in B’s past, and so, would no longer “exist” for B. And vice versa. Therefore, if the past doesn’t “exist”, then “what exits” is different for A than for B. Hmmmm.
But again, this is a waste of time to discuss since it is just a semantic question. The real question is whether any sort of direct communication with or observation of the past or future is possible (other than the obvious that we see light that was emitted earlier, etc.)
The correct usage is "the past existed, the present exists, and the future will exist. If you say anything else, you’re just making a mash of verb tenses. If you try to imply anything about reality from verb tenses, you’re letting the tail wag the dog.
Well, I think the question is: can spacetime curve back on itself? Time is totally unlike the three spatial dimensions-it appears to be non-symmetrical. But perhaps it (Time) appears so strange to us, because of the way our brains work in processing information.
Time is Jenga.
So it goes.
Nice username / post combo.
Jean Claude says the past definitely exists and since he is a Time Cop I believe him! 
I think it might be worth looking at the A-series and B-series philosophies of time. Depending on which is more accurate, time travel will work out differently. So, if the A-series is more accurate, then we have to have a way of reconstructing the past working backwards, but going forward from that is simple, and if the B-series is more accurate, going backward is simple, but if we make a change going forward we now have a Space-Time that isn’t self-consistent. So it doesn’t seem like either theory is consistent to me.
I also think that the general concept of time travel runs into a lot of problems with general relativity. I’m not particularly knowledgeable on this, so hopefully a theoretical physicist can expand. But it’s my understanding that we also have to consider how time travel will appear from different inertial reference frames. Going forward or backward in time seems to assume a privileged or universal timeline that just doesn’t exist in general relativity. So, even if we ignore the grandfather paradox and the nature of time itself from above, we’re still stuck with working out what it means to go forward or backward and how any changes we make will interact with other inertial reference frames.