Not necessarily.
If you have a finite number of of future time travelers, as well as finite number of time/places in the past for them to visit, then the distribution of time travelers in their past (our “present”) will not be “up to the armpits”.
While I can imagine time travel being possible. I really cant imagine it being something that every tom who’s dick is hairy from the future is spending all their time visiting every possible moment in the past.
If time travel (to the past) is possible, then human history would continually change, as time travellers changed things. also, causality is still one principle that must be observed-results CANNOT precede actions. If this isn’t true, then you can throw out 99% of physics.
ergo, time travel (to the past0 is impossible.
Or perhaps, time is a dimension that we cannot observe directly, and all possible actions occur along a continuum-perhaps that is God’s way of knowing everything.
Well, if you had time travel to any point in the past you’d have ALL future people able to travel back. Not just the 6 or so billion we have today on earth but the 6 billion tomorrow and the day after that and so on. The possible number of travellers from the future = ALL future people till the end of time. While maybe not exactly infinite it is a staggeringly large number (assuming the human race continues to exist in some fashion for the next several billion years).
Impossible. Why? Time doesn’t really exist. Time is nothing more than a unit of measure created by mankind, just as the “inch”. Neither minutes or inches exist.
If time travelers were going back in time and mucking with our history we’d never know it at all. Your history would be changed along with it and all would seem normal even if your whole past just changed 10 seconds ago. Maybe 10 seconds ago you did not exist, someone changed something and now you do. There is no way for us to know one way or another.
As for physics I think when physicists look at their formulas about how the universe works they are time invariant. That is the calculations can be run backwards and forwards just fine. There are no unidirectional formulas in physics that only work one way but not the other. That is why, so far, physicists cannot flatly rule out time travel (at least I think that is the reason).
It should be noted you can sort of time travel today.
If you move very fast time slows down for you. IIRC the cosmonauts on the Mir space station who stayed there for months were 3 seconds or so behind when they left upon returning. Nothing noticeable of course and they are not in our “past” but to them their return put them 3 seconds into the future.
If you got on your spaceship and flew near light speed for awhile a few days may pass for you but upon return you’d find yourself in the future. Of course this only works one way but if you really want to see what the year 2500 looks like you could in theory do so in your lifetime…just travel really, really fast for awhile.
IMO?
NO
You either need an infinite future AND a civilization that lasts that long, or an infinite number of time travelers where anybody can easily travel into the past…AND they pick here and now (roughly speaking) for us to be up our armpits in time travelers.
The fact that we aren’t only tells me “something” isnt infinite and/or there are actual “gasp” practical considerations when it comes to time travel.
Its sorta like the lunites proclaiming that moon-earth travel isnt possible because they aren’t up to their armpits in earthlings.
I agree with this with the possible exception that we are the FIRST generation. In otherwords no other Earth humans (or whatever) has gone before us.
You have to distinguish between time travel strictly within the realm of Earth and time travel between planets, galaxies and such. There may be other civilizations somewhere in the universe but they may be so far we’d never know.
Do the math.
There are 6 billion people on earth today. Say that just a mere 1 million of them choose to time travel on any given day. In 1 year they will make 365 million trips. In the past 2000 years there have been 730,000 days. Make it 10,000 years there have been 3.65 million days. So, in one year 100 people could have visited every day in the last 10,000 years.
Now do that again for next year. And the year after that. And after that one. And so on. In 10 years you would have 1,000 people visiting every day.
Now consider that ALL future people could do this simultaneously. Nevermind extrapolating this for 1 million years. Just 1,000 years of future civilization doing this would put us up to our armpits in future time travelers.
So, you have an infinite number of folks doing something trivially easy and visiting a finite region of time/space. Whoop de doo.
Put some practical considerations into time travel and get rid of an infinity or two (or maybe even only one or maybe even none) and again, you wont be up to your armpits in time travelers.
And THAT even ignores the possibilty that maybe time travelers dont go around with signs on their foreheads in the first place.
Think of it as building a tunnel in time. You could not turn a dial and go to date X, instead you, for example, create a wormhole that instantly transports you from one point to another and nothing else. The theory goes, you move one end at near the speed of light for 50 years (say 26 light years out and back), then when you are done, you could step into it and return to some point close to 50 years before. If you started moving the wormhole in 2020 and the time dilation was 45 years, for example, starting in 2025 you could move back and forth to 2070. Both ends of the wormhole continue to move through time at 1day/day, just like us, so in 2030 you could go to 2075 and so forth.
Another option would be if you could only change the rate you move through time inside some chamber. You could go in and move forward or backward, but you could not go to a point the chamber did not exist, or possible even through a point the camber was occupied.
Not being able to conceptualize something is a human limitation that we are still plagues many wrt quantum mechanics. Temporal theory may some day have as large a conceptual shift.
Pulling from memory, I remember something from a physics popularization (I can’t remember which one) that showed that if you reverse the time direction, matter becomes antimatter. The simple example they used was a electron/positron creation and destruction. This is a common subatomic event that consists of a photon of energy greater than 1.02MeV generating a positron and and electron which exist for a short period and then annihilate together to generate a new photon. According to the book, the same series of events can also be thought of as one particle circling in time, looking like an electron to us as it goes forward, and a positron as it goes back. Photons look identical going either direction, so the two photons can be thought of as acceleration photons.
Also, on the subject of instantaneous transportation to any point in time, which is what we are really talking about here, there are three possible effects
[ol]
[li]Alternate universes, which have been addressed to some extent.[/li][li]Unstable time loops, meaning if you go back and kill your grandfather something will happen to the universe that would erase either just that change or a lot more up to including the universe[/li][li]A stable time loop. If this is the case, nothing you do in the past can make a change on the future because it already happened. If you go back to kill your father you will fail, because you already did. Maybe you can’t do it, maybe you kill the wrong guy, maybe you have a malfunction, but what ever it is already happened.[/li][/ol]
i would also suspect that time travel will be even more limited than interstellar travel. We could have already sent out a ship that will some day make it to Alpha Centuri, but it would have taken a concerted effort of the whole world for the last fifty years. Even if it is possible, it may be so resource intensive that it is almost never used. One proposed method I read required two perfect spheres, each with a mass close to the entire solar system, with opposite infinite charges micrometers apart without touching. Someone draw up an estimate and lets start taking bids!
Jonathan
Imagine this.
Sometime in the future we have 10 billion people on earth. The population stays constant. Time travel becomes possible. Our growth/economy is such that we can send a million people ever year somewhere/sometime and keep a constant 10 billion. People have a finite lifespan.
Our future and society lasts for infinity. The universe never dies. These time travelers can pick anytime to visit (perhaps even future OR past). The whole infinity of “our future” PLUS the xyz billions of year of “our past” that had not yet developed time travel.
My mathematical WAG is that this means that at any particular time (on average) you have one million out of 10 billion time travelers in any particular time, or one outa 10 thousand.
Any mathematicians care to way in at this point?
A good fraction to be sure, but not an ever increasing number due to infinite growth of the future time.
Make the future finite, or the life of our civilization finite, destinations other than earth possible, time travel resource/energy intensive, not a particularly attractive/popular pastime, or times other than “now” much more attractive (or even known for that matter), and time travelers will certainly NOT be over running the local starbucks…
But time does. Just because units of measurement are arbitrary or semi-arbitrary doesn’t mean that what they measure is imaginary. And time is as real as rocks.
Oh yeah, that’s right, I’d read that too now that you remind me of it.
So as soon as someone tries to travel backward in time, they’ll just explode, taking a good chunk of the planet with them.
Let’s not. 
-FrL-
My understanding is that Sci-Fi time travel is theoretically possible, but requires such enormously enormous amounts of energy that it’s impossible in any practical sense.
You have to remember to add up all people over the whole future you are talking about. Assume 10 people in the year 2010 want to come back to today. And 10 people from 2011. And 10 from 2012. To us it they all would pop in simultaneously (or today at least) and we’d see 30 people from just that example.
Now extrapolate that for the whole of the future. Let’s say just 1 million people per year time travel and cap the future at 5 billion years (if you have an infinite future then you have infinite people).
That is 5,000,000,000,000,000 (5 quadrillion) people waiting to go somewhere in time.
Let’s say they can go anywhere in that 5 billion year timeframe. That gives them 1,825,000,000,000 (1.825 trillion) days to choose from. That gives us 2740 people per day from the future.
So in this case we probably would not notice them as in 10 billion people that would be a drop in the bucket. However, I doubt the future people would all evenly spread out. Presumably they’d want to go to better times or more interesting events. So you’d see clustering. Maybe around 9/11 or maybe when Jesus was performing his miracles or maybe to see the pyramids built or maybe to see the first space elevator or when the Martians attacked or whatever.
In any given year there are only a handful of events I would think a time traveler would care to come back and visit. Maybe not even every year. From a perspective of 5 billion years it’d have to have been some pretty big deal to be remembered to come back to. I think clustering would be significant and you’d start noticing those 5 quadrillion people packing in around big events.
Or not. If one postulates the type of time travel where going into the past creates a new alternate universe, then each location gets one time traveller ( or one group if they arrive together ). Instead of a billion time travellers watching Jesus get crucified, you have a billion alternate versions, each with one time traveller watching him get crucified.
You dont get it do you?
If there is an infinite amount of future AND and infinite amount of past/future to visit there will be a FINITE amount of time travelers deposited at any given time if the number of time travelers leaving their particular time at any given is FINITE.
Given that time travel probably requires some energy/mass/resources to pull off, the number of time travelers leaving any particular time for any other time MUST BE FINITE.
Therefore, the number of time travelers existing at some “visited time” must be finite.
The problem with time travel is that it would displace matter.
But since Steven Wright already claims to have moved through time who am I to argue.
So? “Finite” includes some pretty large numbers. How many time travelers need to come back and try to kill every dictator or unpopular world leader before we noticed it happening?
(And there will never be an infinite amount of time to visit - merely a large finite amount of time that gets larger as you go. Not that that’s all that relevent to your point.)