h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

We're all travelling through time, at a rate of approximately one second per second (depending on where the observer is located!)...

We can vary the rate of travel by, for example, standing near a massive object or flying high in a 'plane. If person "A" can travel through time at a slightly faster rate than person "B" (say that "B" is in the vicinity of a strong gravitational field, for example) then it can be said that person "B" is travelling backwards in time *relative to person "A"*...

So, the principle has been established in both directions. The only problem is that it would take a huge mass or a expenditure of energy (probably the equivalent of that mass) to scale the effect up to anything that could be considered of any "practical" value.

I believe there's an interesting paper on the subject entitled "Rotating Cylinders and Global Causality Violation" by Frank Tipler. I haven't found a copy online, but I'll post a link when I do.

"In an infinite universe, anything, even H2G2, is possible."

I remember I saw something about orbiting an infinitely long 1-dimensional black hole to travel through time... or something like that.

It seems to me that time travel will never be invented - If it will be, why haven't any future people traveled back in time to today?

If someone came up to you in the street and said 'I'm from the future', would you believe them? Would anyone? I'm betting they'd be more likely to end up in a psychiatric ward than in the pages of New Scientist!

But then what do I know? I'm only a squirrel.

The problem with the relative time thing is that you cannot travel backwards into your own past. You can travel forward much slower than everyone else which would mean you flitting round like a humming bee while everyone else moves as if through treacle. So, with current understanding of time travelling backwards isn't possible. Also, to avoid the paradox problem, even if you can travel backwards you could never travel back into your own personal past. You may slip to another dimension/reality (as believed by Einstein hmself, so it can't be that daft) but, if free will is a reality, then you can't travel into your own past as you remember it when you were not there.

If we assume however that when you travelled back into your own past, you deliberately chose not to interact with yourself, then that argument seems to me not to apply.

That means you could only manipulate events indirectly, but it wouldn't stop you manipulating them, it would only mean that the manipulation would have to remain unknown to our past self, providing that it was unknown to them at the time.

I need painkillers for this headache.

That would seem to be an advocation of the strong Anthropic ("intelligent universe") principle, where the rules governing soace and time would have to prevent you interacting with your past self, causing the universe to prevent you in some (probably improbable) way if you tried. Pass the aspirin.

Exactly why I reckon you couldn't travel into the past, without being in an alternate dimension, Sliders stylee. Otherwsie it would be very difficult not to change something you knew about. For instance if you ent back to the dawn of time, say just after the dinosaurs. All those little furry creatures you see about could well be an ancestor and I don't see how the universe could make sure you didn't kill any important ones.

It could drop a tree on you.

I remember reading something about time travel, which said that if a time machine could be built it would only be able to take you back within the time machines own timeline. For example if I built a time machine today, I could use it tomorrow to travel back to today. I would not be able to travel back any further because the time machine would not exist.

Hmm, sounds like a good argument for time-travelling in your very oldest clothes ...

We currently know diddly-squat about Time Travel, just as we know little about Space Travel. Give it 200 years and all the current theories will probably have been added-to out of recognition or dismissed.

I think travelling through time would be seriously mentally disorientating. It may even have physical repercussions. I'm writing a screenplay at the moment for a friend where Time Travel causes physical sickness.

I would like to travel back to the times of the Musketeers. Assuming it was possible, but then wouldn`t everybody want to leave their time and go to the time of their choice. What if everybody does this. Will our time cease to exist? will we be forever in a continuous loop of going forwards and backwards? are we in that loop? did the world get destroyed in the future and is human kind living in the past? Does the world exist anymore? Do I exist? why am I here?

* Takes a couple asprin and chants 42 over and over.


whoa got away with myself there.
Elle

Okay, if Physics says we can't travel backwards in time, does that exclude taking everything *except us* forwards in time, thus achieving the same effect...?

Dammit, that was intended to be facetious, but I just realised a serious point - that would tie in with the theory that you could never travel back before the invention of your time machine... That would be because the machine would be effectively making everything else travel *forward* (relative to you) and you couldn't do that before the machine existed.

very interesting, but woulden`t going forwards be harder because it hasn`t happend yet? Or is it easier for that very same reason...

Argggg
Elle

It's already been proved that we can vary the rate at which we move forwards! (see other threads - I'm too to explain it at the moment...)

No I understand that we can move forwards but not significantly. Like years ahead. Unless we move really slow for a long time...

It is indeed a bad argument to suggest that time travel isn't possible because there aren't travellers currently. We can easily imagine time machine which involve a booth which de-stabilises time, so you'd need a booth to travel back to, but once there was one, people could travel back to it.

Okay. Here's how you build a time machine:
You get two blackholes, which are very close to each other, grabs each other them and pull them apart about a few million miles. The gravity well would be so intense that you'd move between them instantly - in relativity, instant movement is time travel. But this machine wouldn't get you back to a time before it was created, it would only allow time travel from the entrance to the point at which the exit was created.

What are we going to manipulate these black holes with? Seems to me the only thing big enough to control a black hole is another black hole ... which makes this problem infinitely reductive. Or something.