I've heard it postulated that someday we may be able to travel back in time, but that we won't be able to back any farther than before the time machine was invented. I don't get the logic behind that.

I personally don't think backwards time travel will ever be possible, but I don't understand that if it eventually became possible, why we couldn't go back before the time machine was invented.

Is it because you would be able to teach the inhabitants how to make time machines, thereby making it impossible for you to be the first time traveller?

I say a guy talking about this on a BBC program called "Its Only a Theory", but basical the set up is that you need a Time Machine that acts as a "Sender" and one as a "Receiver".

You build your first time Machine in say 2010 and any one with a compatiable machine can come back in time from say 2011 or 2111 for that matter, as there`s somewhere for them to come out into. But you can`t come back further that the date at which the 2010 machine was built in, so 2009 isn`t possible.

It would be like trying to ring somewhere on the telephone that doesn`t have a connection to the exchange.

The most logical thing I've heard about time travel was something Stephen Hawking said: "Time travel is impossible, because if it were possible, there'd be people coming back in time to visit us."
Although, even if he's wrong, I fail to see why they'd bother, and it would take a LOT of energy.

I like Hawking's reasoning, but I don't really view that as proof, since it's very hard to prove something doesn't exist, just that the absence of evidence makes it very unlikely. Who knows, maybe going back in time would take one to a parallel universe and that's why we never meet time travellers.

How can it be illogical? People couldn't travel in cars 'til cars were invented either.

An alternative line of thought on this subject is that we cannot travel backwards in time in this space-time continuum; however, we may be able to one day access another space-time continuum in which time has not advanced as far as it has here. For example, we may be able to access Universe "B" where it is still 1942 and World War 2 is in full swing. What will we be able to do there? At this pont, no one knows. The best bet is nothing: We just sit and passively observe. This theory is the "multiverse" theory, which is an outgrowth of quantum mechanics. In this idea, therea re multiple space-time coninuums existing simultaneously side-by-side, none accessible from any other.

Paradoxes arise from reverse time travel in this continuum. Everyone has, no doubt, heard about the conflict that would arise if you went back in time and killed your grandfather before your dad was born. The multiverse theory gets around that nicely.

As for why anyone would want to travel back in time, the reasons are endless: Who wouldn't want to meet a distant ancestor or historical figure to ask him or her "What were you thinking?" or just to say "Hi". Heck, millions upon millions would give their eyeteeth to go back to 32 A.D. and witness the crucifixion of Jesus Christ, or a few years earlier to see what was REALLY going on before the New Testament was written. What did Jesus REALLY say? Millions more would be equally eager to travel back to Medina in 615 AD to meet Mohammed. Or to go back to the K-T bondary event and witness the death of the dinosaurs firsthand. Or go back 3.8 billion years and get a clue as to the origin of life on this planet (and, by extension, others). The possibilities are endless.

I would personally like to go back to ancient Greece and talk with Socrates, Plato, and Aristotle. But I suspect that it'd be a bit shocking, as would be seeing Jesus for many people. I highly doubt any of them lived up to their posthumous reputations.

There are no physical laws preventing time travel either forward or backwards. There are only philosophical arguments, like the OP's opening question and all the "paradoxes" listed. You can't provide a mathematical formula proving or disproving any of it.

Budichai, you are correct up to a pont, but it depends on whom you ask. There is some small debate about this.

quote:

Originally posted by cijay:
How can it be illogical? People couldn't travel in cars 'til cars were invented either.

That's not exactly true. Think about it this way. Is it not unreasonable to look at a third world country in say 1920 or so, not long after cars were first invented, to find that that someone has imported the first car onto their country? Technically, that country didn't invent a car, but yet, here is someone driving a car a person from a different "Time" brought it over.

It's not a perfect example, but it goes to show that simply because something hasn't been invented doesn't mean it can't appear. Although in terms of time travel, the analogy doesn't work perfectly.

The most likely scenario as to why we haven't seem time travel is that either:
A: The conditions for it to possible currently don't exist or...
B: It isn't possible

However, there is a third possible explanation, that's is perhaps even less likely, or somewhere in-between, and that is that time travelers have appeared in the past, and they have simply threaded lightly. While a bit more "destiny is predetermined" than I like, I have found that possibility that the act of a person going back in time is predetermined. That is, that a time traveler from say the year 3000, even after he traveled through time and reached the present, his history has already recorded the actions that he was about to make in the past. The fact that he was able to travel through time would mean that he hadn't killed his grandfather, or made such a paradox, as perhaps even if he wanted to by some misfortune he would fail. Likely history has already recorded such a failed attempt on his x many of greats grandfather's life (and was known in the time-line of the year 3000). It's an interesting thought, but as well any time travel theory, it can't be proven until it is actually done in practice.

The reason time travel is probably impossible is that it would require everything in the universe to be re-arranged.

Think about it – yesterday everything in the entire universe had a specific 3-dimensional relationship with everything else, including you and every atom in your body. If you were somehow able to move back to yesterday, there are two possibilities:
1. Everything in the universe would have to revert to the same relative relationship it then had, including you and the atoms in your body, which means your brain would have no knowledge of the future.
2. A ‘new you’ would be introduced into a universe where it had never existed. This would constitute an entirely new universe, or timeline, that co-exists with the timeline from which you came, but is different.

What I should have said was what this physicist was talking about utilising was a Wormhole in Space time that travels through an area of gravitational distortion. It would require a lot of exotic mataterials to work, and was at best a highly theortical work. But it was his theory that there was nothing in the Laws of Physiscs that would stop his method of Time Travel from working.


But then again he could just have been watching too many Episodes of Stargate.

What kind of exotic materials does it take to create a wormhole? Are they the kind of materials that do not exist, or are precluded from existing by physical laws, in this universe?

The first time I heard this theory was by TERRANCE MCKENNA.

He also says that the moment the person creates the first time machine, that time machines will just appear everywhere. Because people will come back in time to create more time machines to travel to.


Heres a link to a video where he speaks on it. It's Part 8/15 of a series of videos.



- http://www.youtube.com/watch?v=xybzUhCsqF8



LZ

quote:

Originally posted by Flippons:
The most logical thing I've heard about time travel was something Stephen Hawking said: "Time travel is impossible, because if it were possible, there'd be people coming back in time to visit us."
Although, even if he's wrong, I fail to see why they'd bother, and it would take a LOT of energy.

I'm not entirely convinced on that notion....

I mean if people in the future were smart enough to create a device that could actually manipulate time then maybe they were(will be) smart enough to know how destructive it would be to actually GO back in time....maybe they CAN go back in time but choose not to, or are at the very least stingy on what parts of history they would be willing to tamper with and don't see a reason to tamper with ours....

What is the probability of P=1 given n=500 | p=0?

Personally, I believe time travel is not possible because I have experimented with it and it has failed every time I did it. That is not proof because you can never prove a negative. But when an experiment fails 500 times in a row without a single success it doesn't look good.

My experiment - it starts with "If time travel were possible, then I could time travel if I had a time machine. The time is now 7:00am on 11/7 and I am at home. I remember this time and instruct my future self who has a time machine to travel to this point in the past and pick up my past self. We will both travel to the future where I will get a time machine.

The whole experiment should take a matter of seconds to prove. If my future self appears with a time machine, then time travel is proven. If not, n=501 and p still equals 0.

And once again I have failed to appear.

Again, this is not proof against, but it is very strong evidence. Perhaps one day it will work.

You have to understand, this particular Physicist was not an Applied Physicist but rather a Theortical Physicist. He was thinking in terms of Dark Matter, and it`s unkown properties that would then enable his methodology to work.

But it`s a fairly common thing from what I understand for Applied Physist to curse Theortical ones, who come out with statments like this. I suppose what it amounts to is, if I had some magic beans then I could grow a Giant Beanstalk, in someways.

But I think that it`s the theory of Timetravel that the OP refers to, that in effect you create a network, like the TV show Stargate of Gates, similar except that instead of travelling a shortened distance through Spacetime, they enable you to Travel in Time through Spacetime. But to use that TV shows terminlogy, you can only dial in if there`s a Gate that allows you to do so, so you can`t go earlier than the earliest of the Gates to be built.

It also solves the Steve Hawkin question, there are no Time Travelers as we haven`t built a "Timegate" yet so we haven`t got any.

But look I`m not saying I belive this idea just trying to Explain it, in my own way.

bflynn, that's the dumbest thing I've read today.

everyone is thinking far too linearly.
paradoxes are bah!

we don't have the technology,
it's extremely unlikely in the next few generations or more.

let's say time travel is found possible 14,000 years from now, how would we ever know?
our technology couldn't detect that it had occurred, and visitors probably wouldn't travel to this point,

and even if they did, they're not going to announce themselves.

reminds me of the family guy episode where stewie meets his future self.

that could be happening every day, and no one would ever know.

quote:

Originally posted by Nuclear_man:

It's not a perfect example, but it goes to show that simply because something hasn't been invented doesn't mean it can't appear. Although in terms of time travel, the analogy doesn't work perfectly.

You can say that again! It shows nothing. The moment you put "time" in quotes you blew it. It has nothing to do with "time" but rather "point in history". It's the same all over the world even though time and dates vary. Just because I was born in Scotland on the 7th, doesn't mean I celebrate my birthday in Canada on the 6th just because it was still the 6th here. Nor did I celebrate it on the 8th when I was in NZ. The point in history I was born it was my birthday. The point in history people began riding in cars was the same in Botswana as it was in wherever it was that the first car ride was taken. Just because they didn't HAVE them doesn't mean it didn't exist.