You have to read this, it's brilliant :

http://www.dailymail.co.uk/home/moslive ... chine.html

Very cool article.

I think the most interesting bit is the last paragraph about travelling long distances in an apparently shorter amount of time due to the time dilation.

Makes a practical application of time travel.

Not really. The point about travel at relativistic velocity is it only dilates time from the perception of the people travelling. This means you can travel longer distance because effectively your life is passing more slowly.

Look at it this way - say the average lifespan of a human is 100 years. if a human being is travelling at a speed that doesn't cause much dilation (say 1% of the speed of light), they can travel (100*365*24*671 million)*.01 miles (the speed of light is roughly 671 million miles per hour). The total of that is 5.8 million million miles (give or take).

So we up the speed a bit - say to 10% of the speed of light. Normally, they would then be able to travel 58 million million miles, yes? 10 times the speed, 10 times the distance. But say at 10% of the speed of light time slows down 50%, that means they get to travel for twice as long because they age half as quickly, so they in fact go 116 million million miles in the same time, as far as everyone else is concerned (which is the really really weird bit). The numbers aren't exactly right - the calculations for how much time dilates as you get close to C are not really the stuff for message boards - but the example fits the overall logic.

Course this is a stupid example as you'd have to be born the day you set off and die the day you arrive, which makes the journey kind of pointless. However, the example works in terms of the ageing of the people involved over a set distance - if you say they only have to travel 5.8 million million miles at 10% of c, it essentially would seem to them that it has taken half as long to travel as they thought it would when they set off (but it would still have taken as long for everyone else).

The key point is this : relativistic travel seems to take less time, but only for the people travelling.

The proper application of time travel in terms of actual space travel is fairly obvious - if you know it's going to take you 200 years to get to where you're going and you can do time travel, you go back in time 200 years and set off then, so it appears you travel the distance instantaneously.

Which is exactly the point I was making.

If you take a distance (say 99 light years) and you travel there at 99% the speed of light then by all counts it would take you 100 years to get there.

However, on the space ship that is travelling that distance, time would be dilated by a factor of 52 (taking the numbers from the article) to each year that passes to the outside world would only be experienced as a week to the people on the inside of the space ship.

Thus they would get to their destination and only be 2 years older than when they set off.

Of course this example doesn't account for the time spent actually getting to that speed.

Like I said, it's the only really practical part of the whole time travel thing. But as yet it is still impractical due to the fact that a spaceship that travels at that speed would be impossible to make.

I also know that relativistic time is only dilated for the people travelling. I realise I didn't exactly specify every parameter of what I was thinking about but I studied maths at university and physics is an interest of mine.


To coin a phrase... "Not really". In the first part of the article Stephen Hawking talks about the fact that travelling back in time would cause paradoxes in radiation which would instantaneously destroy any worm hole that you managed to set up to put you back in time in the first place and thus stop you from travelling back in time.

It looks, for now, that time travel is a one way ticket to the future.

Which is no use at all, really, is it? The future will get here soon enough anyway.

Jon

True true.

Hmm... speaking of the whole relativistic time dilation effect.

If a ship is travelling away from earth at 99% the speed of light and so in the ship they are experiencing a factor 52 reduction in the speed of time. Then they send a communication back to earth...

What would the communication look like on earth?

Would it be slower than normal? Would it look completely normal?

The communication would indeed appear slowed down and on a lower frequency. Much like Red shift. The exact same is true for communications from Earth to the ship.

One of my favourite facts is that you can reach relativistic speeds quite quickly (a few months) by accelerating at one G. If you accelerate until half way and then decelerate the rest, you have "artificial gravity" for the entire trip.

A less favourite fact is the amount of energy required to get anywhere interesting

LOL! If you were to accelerate at 1g for 1 year you would end up travelling 1.032c

Although this isn't entirely true as it doesn't take relativity into consideration.

An interesting read, but like Jon, I'm sceptical about the practical applications. If the perception of time is relativistic and given the huge distances we're talking, it would mean next to nothing to arrive at a distant world in "the future". As it stands, the way we measure time in our daily lives is almost entirely arbitrary. Of course, physicists could tell you "when" you were by the position of celestial objects relative to where they are now, but unless your journey to the future ends at Earth, you would have no way of knowing when you were.

That's true, but if the object was to explore a distant star's planet system then the goal is a where not a when.

If a star system is at a distance such that it would take 100 years to get there and 100 years to get back then you could start a mission now and in 200 years it would return with a crew that was only say 20 years older than when they set off.

I wouldn't want to be one of that crew though as it would mean everyone on earth who I know now would be long dead.

But theoretically it could happen.

But don't you have to travel faster than the speed of light to travel through time?

And wouldn't you have to have a mass of zero in order to do so?

No, time dilation increases relative to speed (even travelling in a car at 30mph means time is travelling slower to you relative to the person standing on the side of the road only the effect at those speeds are negligible).

Travelling at the speed of light would effectively mean that time would stand still relative to everything else.

Travelling at 0.99c dilates time so that 1 year to us is experienced as 1 week to the thing travelling at 0.99c. (according to the article)

I saw lost in space too!

I think the article makes it almost clear that it’s not possible to travel back in time and stop the illegal immigrants coming here before they do.

Yes bit what if the immigrants have got a time machine and travel back in time to before the "indigenous population"?