I just wanted to point out, that I have at least three nitpicks about this article:).
One, and this one is perhaps the more lighthearted of the three, Mr. Adams says that apparently time travelers have never ventured to the past (from the future). Actually, as I recently saw on satellite tv, how do we know this for a fact? I mean, maybe they are here, hidden among us:eek:. Not a big nitpick. But Cecil could have pointed this out too:). (Btw, here is the Star Trek take on the matter.)
Two, Mr. Adams says (or strongly implies, I think) there is no solution to the causality loop problem. Actually there is. The many worlds hypothesis. Maybe you kill your “grandmother”:dubious: in one alternate quantum reality, and maybe she is alive in many others. This still (I at least think) brings up the question of why there aren’t many time travelers from the future among us. But I nevertheless have heard this often offered as a possible solution to the paradox.
And last, Cecil talks of a “Grandmother” paradox. I take he knows that this obvious is more often referred to as the “grandfather” paradox. Again, just a very small nitpick:).
No. Traveling to the past is not possible. There’s no physical evidence that it’s ever happened and no scientific rational for it even being possible.
Traveling into the future is possible. A faster-than-light space ship (or one sufficiently fast) could send people into a future world that they could never have realized otherwise.
You cannot go back in time. You can accelerate time and become a future Rip Van Winkle.
Leaving aside all the cute variations of “I’m time traveling into the future…at 1 second per second!”, isn’t traveling into the future akin to skipping ahead in a book that hasn’t been finished? Unless the traveler and all the traveler uses(including all equipment and energies) are totally divorced from the world in which the traveler wishes travel in, said traveler is “stuck inside the book” and cannot skip forward.
Interesting question. If I were to magically pop out from 2014 to appear one year later in 2015 then there would be a mass/energy deficit of
ONE: The Great Unwashed
+
ONE: The Great Unwashed’s Suitcase with a change of underwear.
That does seem to me to violate energy/mass/(and even momentum) conservation laws.
So if those laws are inviolable, that might really put the kibosh on time-travel: neither going back (because of the Grandparent Paradox), nor going forward (because you’d break the universe).
When you get there, the book (or at least that section of it) is finished. If there’s anything in that scenario that’s “unfinished”, it’s you, in that you haven’t experienced the changes that would have happened to you had you just sat and waited like everyone else. Time travel into the future isn’t difficult at all: It’s qualitatively no different than what happened to Rip Van Winkle.
This is a solution that’s not a solution. If you go back in time to an alternate world, you are no longer in our world and therefore anything you do is irrelevant.
Sexist pig! Why don’t you want to kill your grandmother? She’s every bit as deserving as death as your grandfather. How do you even know for sure that he is your grandfather? Your grandmother was probably cheating on him. You should kill her for that alone!
The fact remains that no one has yet observed an example of cause and effect where the effect preceded the cause, but this doesn’t necessarily mean that such a thing can’t happen. And while we can imagine many scenarios which lead to paradoxes, I have read at least two theories of how this could be resolved. #1 Of all the possible outcomes in a situation, there will always be at least one which avoids the paradox, and what actually happens will be from this small subset. For example, when you try to shoot your grandparents the bullet will miss or the gun will jam. #2 Travel to the past doesn’t take you into your OWN past, but rather takes you to a parallel universe whose past looks identical to yours but whose future is yet to be determined. In that case, killing your grandparents does not create a paradox at all because they aren’t really your grandparents, they are just duplicates. In the parallel universe, they don’t have a grandchild that looks like you but that’s not a paradox, it’s just a detail of how that universe has diverged from the one you came from.
I’ll admit that the evidence for time travelers living among us is rather thin, even less compelling than the evidence for space aliens living among us. But that might not be enough to conclude that time travel is impossible.
FWIW, I’m betting my money on the idea that time travel and FTL travel and time travel are both impossible, or at the very least, insanely difficult and technologically unachievable.
Precisely. If FTL was possible we’d be travelling into the past, not into the future, we’d arrive before we left, so to speak. I hang out on another board at times where a guy claims that he was born 200 years in the future and that time travel will be discovered in the year 2088. That will he says wipe out the AD designation and start over with year one in the new era. I forget what he calls it. No I don’t believe, but do get a kick out of his frequent posts.
My question is - does that break something we are pretty sure is true?
Forward or back in time, you would be breaking the Conservation laws. Skipping forward mean that during that time your mass/energy and momentum are “missing” from the universe - which means at some point it is destroyed (chapter 2) and created (chapter 10). Going back in time increases the mass/energy out of nowhere - even if “you” are still being formed in a supernova somewhere. Is there anything that suggests this is possible other than hand-waving “it’s time travel!” or saying that system is normalized over time?
Then we have the biggest spoil-sport of speculation - entropy. As a state function, couldn’t you break the second law with forward or backwards time travel? Wow, can you imagine that “kick the can down the road” attitude humans seem to be so good at? “Well, in the future they will figure out how to break that pesky 2nd law so we will just package all that entropy up and send it to 'em!” the moment we figure out how…
MIT had a Time Travel Convention a few years back. The theory is that if there were time travelers, they’d go back in time and go to this convention – you only need one Time Travel Convention…
I offered the same objection (immediately after Czarcasm told me off so I wrote quite hastily to pretend I had something to say). The moment I hit send I saw a possible solution or two.
ONE:
2014: TGU disappears being replaced by a mass/energy/entropy-equivalent blob of matter.
2114: TGU appears at the same time that a mass/energy/entropy-equivalent blob of matter disappears.
With an exchange like that there would be no loss/gain of anything.
I’ve not seen this idea in a movie or a book, so I claim it as mine, so nobody steal it.
TWO:
Another possibility is (note, I am not a physicist, so this is merely speculative) that the formation of each our our conservation laws implicitly assumed that time travel is not possible (it would probably have gone *literally *without saying): that is to say that if time travel is possible then these conservation laws are not even applicable.
Arguing against time travel on the basis of conservation laws is no more sensible than arguing against position travel based on conservation laws. If I popped out of existence here and popped into existence downtown, that’d violate conservation of energy… but that doesn’t mean I can’t walk downtown.
Time travel into the future via time dialation isn’t disappearing and then reappearing in the future; it’s experiencing time more slowly than the non-accelerating universe. It’s not like skipping ahead in a book, it’s more like hitting fast forward on a video.
Huh? That makes no sense, you may need to expand on your point or dummy it down for me.
The law states that matter and energy are conserved, during your stroll downtown they are. At no point in time can I *not *define your mass and energy during the walk. I could draw a continuous/smooth graph of those values over time and there would be no undefined points.
If teleportation was possible and it required non-zero time, I’d have the same objection. Since I am unaware of any such teleportation theory, my point seems to still remain. Can you share more detail on this objection?
RaffArundal’s post claimed to be objecting to time travel on grounds of conservation, but it’s not really an objection to time travel, but an objection to teleportation. There’s no reason why time travel would need to involve teleportation, in which case that objection doesn’t apply.