People seem to assume that if time travel is possible, that going a gazillion years back is just as easy as going a few hundred.
If that is not the case, which seems the much more reasonable proposition to me, then if you live very far in time from when time travel is possible/all the rage , your number of visitors will be very low, or even zero if time travel is “hard”.
An analogy would be since we can send probes into space, why arent are probes littering the andromeda galaxy? They aren’t and they probably never will, even though travel there is certainly theorectically possible even today.
Whether there is finite amount of time or infinite doesnt really matter. Those values cancel out. Unless you have a finite amount of civilization existing/time travel possible/time travelers AND an infinite amount of time, in which case you would have a finite number of travelers possibly distributed over an infinite amount of time, in which case the probability of finding a time traveler over any finite time period is zero (or damn close).
Finite only includes “large” numbers if at the very least all the following conditions are true:
Large numbers of people are time traveling.
Large numbers are coming here and now rather than some other there and then.
The time travelers, regardless of their number can ACTUALLY get to this time, even if they want to.
Time travelers would make their presence known and/or interact with the past they are visiting.
And probably some others I havent thought of yet.
All the apparent lack of time travelers knocking at my door tells me is that it isnt trivially easy, which sure a heck doesnt surprise me in the least.
I’ll cheerfully concede that thre may be time travel methods that do not allow for easy time travel back to our time (this thread mentions that the more likely ones only allow for travel back to the initial activation of the machine, which is a very hard limit.
I think it’s safe to say though that time travelers would have an impact on the times they visited - tourists do that. (Presuming actual time travel and not some strange many-universes variant where you’re not actually going physically into your past, anyway.) If nothing else, there is only so much physical space to pack the looky-loos into at any given popular time-travel destination.
And it has been amply noted that if the time travel notion is widely available for long periods of time, then you’d get lots of travelers. If the time machine is only used a half-dozen times before being smashed by a train, well then, not so much.
It stands to reason that the mighty SDMB will exist unto infinity in some form and even in 26,789AD people will still be demanding cites, snarking on the latest pop culture and making lists where the third item is “Hi Opal.” Eventually, a guest researching time travel will come across this thread and decide it would be fascinating to go back in time to when one actually had to use primitive input devices and an external screen (no neural internet! The horror!!) to participate in these odd text discussions.
I propose that this guest should then register as FutureTimeTraveler and post in this thread describing what life is like in their time and how odd life is in ours.
It sounds like I’m standing in a wind tunnel here.
I would dearly love Time Travel in the H.G. Wells/Back To The Future sense to be possible, but realistically I just don’t think it is. Why?
Because history only records one assassination attempt on Hitler that got anywhere near succeeding, and it’s a common meme that the first thing lots of people would do with a Time Machine is go back an shoot Hitler in 1923 while he was on the loo or something like that. That didn’t happen.
Inevitably, you’d also have people trying to save Jesus from being crucified (Come on, you can visualise a bunch of people in Camo armed with M-16s staging a Commando Raid from the future to rescue Him from the Romans. :)), and you’d also have someone else selling AK-47s to the Aztecs, and someone else (me, probably :D) would be arming the British Army fighting the Americans during the War of Independence with Lee-Enfield rifles, Thompson SMGs, and 25pdr Field Guns. You’d have people looking for King Arthur, trying to get their photo taken with Julius Caesar, or simply messing with the heads of early explorers by greeting them on the shores of the New World/somewhere in Darkest Africa/at the North or South Pole, and asking them “What took you so long?”
And that’s without getting into the people suing George Lucas for plagiarising their idea for Star Wars.
I do believe in alternate universes, though. Or at least, I’d like to. The idea (and the possibilities) fascinate me.
Question for all those who believe time travel is possible.
[ul]
[li]Bob invents a time machine at 4pm[/li][li]At 5pm he travels back to 4pm. [/li][LIST]
[li]Now there are two Bobs in the lab at 4pm (the original one and the one that travelled back from 5pm)[/li][/ul]
[li] The two Bobs chat and at 5pm travel back to 4pm. [/li][ul]
[li]Now there are four Bobs in the lab at 4pm [/li][/ul]
[li] …[/li][li] On the N^th iteration there will be 2^N Bobs in the lab at 4pm [/li][ul]
[li]e.g. on the 10^th iteration there will be 1024 Bobs in the lab at 4pm[/li][/ul]
[/LIST]
If the above sequence of events is possible, then where is the mass for all the new Bobs coming from? At some point there will be millions of Bobs in the lab at 4pm. The mass of the universe on that iteration will be higher than the mass of the universe before the time machine was invented. And we can’t say that the “present” is borrowing mass from the future because, among other reasons, if the iterations stop when we have millions of Bobs, the mass of all those Bobs will continue to exist in the future. That is, the mass of the universe will experience a sudden jump at 4pm and stay there forever.
It does seem fairly pointless I admit, only people from the future could use such a machine and then only to travel back to the time/place it was activated.
I don’t know whether it would be possible to traverse the tunnel in both directions. It seems it should be possible but I can’t say for certain. [URL=“This”] is a good book on time travel.
First, there is going to be a physical limit on how many Bobs can fit in the time machine. You also won’t have 1024 Bobs on the 10th iteration…
Bob, having been alone in the lab from 4pm to 5pm steps through and meets himself. Now there are two. at 5, they both step through, but number one is the one who stepped through and became number two in the first place, so once again he becomes number two, and two becomes number three. The 10th time he steps through, there will be 11 Bobs. And if he gets tired of it at that point, every previous Bob will step through, and he will be alone at 5:01pm 11 hours older. So the mass doesn’t stay forever.
I don’t know what it would do to have that extra mass around or where you could say it “came from.”
But this is one reason that some people believe time travel won’t be possible. That is, as you’re bringing your wormhole back from it’s high speed journey, there is a point where time travel becomes possible, i.e. a photon moving through one side can leave the other and arrive back at its starting point before it left. At that point, some say, a stray photon moving in the right direction will do just that, join itself over and over like the feed back loop in a microphone and build up enough energy to destroy the device before you can use it. There could be ways around that though. (Shield it so the photons can’t do that, for instance.)
But, you know how physicists are debating dark matter? All this mass we can’t see? It’s Bob. (the jerk.)
Of course there’s the possibility that a photon goes through over and over building up a 2 trillion photon laser with itself. (photons don’t have the same limit on being in the same place at the same time that Bob does.) But what happens when photon’s 2 through 2 trillion are about to hit the side of the device to destroy it? (they weren’t moving straight through, they were on a trajetory to hit the device,) Doesn’t photon 2 hit the side and cease to have ever been photons 3 through 2 trillion? (oh the paradoxes we conceive.)
The law of conservation of Bobs may come into play.
At some point there will be n Bobs in the room, but as the time comes for each one to enter the time machine to travel into the past one will vanish, eventually only the original Bob will remain (or possibly no Bobs if they all entered the past).
I think it is a little like virtual particles borrowing energy to exist for a short amount of time. Bob does the same thing, eventually all the virtual Bobs will vanish.
I’m much more interested in more mundane examples of time travel…such as a low powered ultra-fast computer that gains its speed not through brute processing power but the ability to send the answers back in time.
I did the math for you. If 1 million people time traveled every year for 5 billion years you’d have 5 quadrillion people able to pop into any-time. If you evenly distribute them across all the days in those 5 billion years you have 2,740/day.
Not a big deal but then who the hell would bother time traveling to January 7, 2009? Nothing particularly interesting happened. It is not special in any way. I suppose you might get some historian whose particular interest was the Israeli/Gaza battle happening now but mostly no one would come.
Think about it this way, if you could travel back in time when would you go to? Some random day in 1339? Or would you go back to get Hitler, see Jesus crucified, watch the first Apollo mission, watch the pyramids get built, watch DaVinci paint the Mona Lisa or a few other handful of possibilities? In the last 2000 years alone there are 730,000 days. I bet you’d be hard pressed to come up with a few hundred you’d want to bother to visit and maybe only a bare handful.
As such our time travelers would tend to cluster at big or interesting events in which case they’d be noticeable (e.g. a few million people appearing to see Jesus would probably have been remarked upon by someone).
Even assuming the law of conservation of mass/energy is strictly absolutely necessarily true*, we can account for this kind of scenario if we understand time travel to involve travel between alternate histories as well. On this kind of account, when Bob2 travels back in time, he thereby creates (and begins to inhabit) a different history than the one he is from. In the original history, Bob1 steps into the time machine and disappears. In the new history, Bob2 spontaneously appears and talks to Bob1. The universe, on this account, consists in the whole complex of alternate histories. And in that complex of histories, mass is conserved. Bob disappears in one, and appears in another.
-FrL-
*I don’t know if it’s got the status of an extremely well-confirmed hypothesis, or of a frame statement without which most physics would fall apart, or what.
Two responses: This could not happen in a singular universe with stable time loops because time only happens once. Either as soon as Bob creates the machine at 4pm, one and only one, future Bob’s shows up, they wait an hour and current Bob goes into the past to become future Bob while future Bob continues on, or no future Bobs show up.
In this case we still have an extra Bob of mass for a short period of time. Where it came from is a good question. I see two possibilities that conserve energy (mass being a state of energy): First, the machine sucks a Bob’s worth of energy from its power source at 4PM and generates a Bob’s worth at 5PM, or the machine itself is capable of containing a region with net negative energy between the time something comes out until it goes back in (which it must in the stable loop scenario).
In the multiple universe version, it could happen and the answer is that the energy comes from the other universes. Each time Bob and company come back, you have a one universe with one Bob’s worth less of energy and one with X*Bob’s worth of extra energy. If you machine had no limit on what it could pull in, you could theoretically pull in enough mass to change an expanding universe into a contracting one.
This whole discussion reminds me of the book The Virgin and the Dinosaur by R. Garcia Y Robertson. In the book you could travel to the past by navigating natural tunnels. You needed both a certain talent and implants to do this. You could retrace your step through the tunnels (the ends of the tunnels aged at the same rate, so after one day in the past, you come back a day after you left). Finding new tunnels to the past was very dangerous, and you couldn’t find new tunnels forward for some reason. A latter story in Analog talked about temporal inertia when a Persian scholar decided he was going to assassinate Alexander the Great (he failed). In the stories time travel was limited both by the inherent danger and difficulty and by a regulated company that controlled it.
I agree with Paleface that time does not exist. I think that time is just a concept invented by humans because we have memory. However, I think the comparison to units of measure like inches isn’t really a good one, because like you say, measurements of object’s physical properties actually do exist.
Like BlinkingDuck said, it seems that for time travel to be possible, all ‘points’ in history/future must ‘exist’ to travel there. To me that would be an infinite number of ‘points’ because what would be the granularity? Is it individual seconds, nanoseconds or something smaller? Is there even a smallest unit of time? With heat measurement you have absolute zero, and with sizes you have things at the atomic level.
Anyway, it seems like a pointless debate because ultimately it just comes down to opinions and feelings. Just felt like throwing in my support for the “no such thing” crowd, there seems to be very few of us.
One cannot travel to a point (in the space-time continnum) that does not exist. Take my great-grandson-he does not exist, and will not exist (unless my daughter marries and has children). So it is foolish to talk of travelling into the future (unless it is a one-way trip). Also with the past:my great-grandfather no longer exists-he is (most likely) in a decomposed state. So, could I visit him in the past? Not likely, unless corpses re-assemble themselves.
Dr. Eddington was right-time’s “arrow” flies in only one direction-and it does NOt reverse course.
Well, as I noted before you CAN travel into the future.
Want to see your great-great-great grandson? Get on a spaceship, fly around REALLY fast (near light speed) for a few weeks and come back. Voila…the future has arrived, you are only a few weeks older and you can meet your grandson (actually if you traveled super close to light speed you could travel to the distant future in seconds of time as it passes for you…go 0.9999999999999999999999951c and you can make a 32,000 light year trip in 3.2 seconds [cite]).
Now, if you dragged one end of a wormhole you created in 2009 along with you then after you say hi to your multi-great grandson you could go back to 2009. (This of course assumes wormholes exist, that you could move one end and that travel through one is possible…all admittedly long shots but hey…there is your time machine to do what you just said could not be done).
At the very least the one way trip to the future is possible today without question. The only problem is getting yourself moving that fast but that is an engineering obstacle.
