time travel

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So it wasn't incorrect? How close you were to the speed of light would have a huge impact on the time in years, so both 3000 and 59000 could be possible :smile:

Yes, both are possible! :smile:

I agree with that, just there is a big difference between the passing of 1000 years or so and 59'000 years or so :tongue:

when you said 'close to the speed of light' I presumed you meant I tiny fraction away from the speed of light :smile:

Original post by a.partridge

Didn't he say that more time would have passed on earth? He's not wrong.

If you accelerate away from the earth and return, you are younger than your twin.

No, he was quite correct in saying that more time would have passed on Earth! :smile:

I was just saying the the approximation on how much time would have elapsed on Earth was inaccurate :smile:

Reply 21

freakymonkey

Physics nerd answer time!

- Really, no. One would either need to travel faster than the speed of light (violating all sorts of physical laws), or travel anticlockwise around an infinitely long cosmic string (which, according to string theory, is a kind of relic of the big bang).
- Experience-wise, possibly. Computing powers may get so powerful over the next however-many years that one could simulate an entire world, including the surroundings you encountered on the day you regret.

Buzzkill I know, but the physics of time-travel is still pretty interesting.

Reply 22

Fires

Original post by CharlieBoardman

Yes - I also believe this to be true.

Time travel is actually an impossibility. For time travel to be possible, we would need to find a way to travel at or faster than the speed of light. The goal we would be trying to achieve would to catch up to or 'take over' the beam of light in essence. However, this is impossible. Just for an example, however absurd it sounds, just imagine this: you put a torch on the ground. You switch the torch on, such that a beam of light fires out of the torch. Now, just imagine that we figured out a way to run at the speed of light. So, you would imagine than we are running along side the beam of light yes? Well actually, we wouldn't be. We would be moving close to 300'000'000 m/s (speed of light) relative to the Earth, but, the thing is with light is that it is a constant. Even though we would be running at the speed of light, light would be moving at 300'000'000 m/s relative to us. So it is still moving away from us at 300'000'00 m/s. So we could never catch up to it!

I've always found this aspect of Einstein hard to bear. It's so flippin counter-intuitive!

I do think a simpler explanation though remains that time is essentially a human illusion - what we actually have are processes, starting right down at the Quantum world of subatomic events and then right up to the macro world we inhabit and the planets, stars and galaxies, energy and matter are interacting in various processes. Some of those processes move along in regular ways and mark out what we call "time", such as our ageing process. However, we are only observing the entropic flows of energy and events. The things that have happened, have happened and we can never, ever, have that precise set of energy/matter patterns again. They are "over" and we have a "memory" of some of them, but the memory is just a recorded illusion, like watching a TV programme of someone who has since died, they are simply not around any more and never will be recoverable.

Sorry.

Reply 23

miser

When will it exist is perhaps the wrong question...

Reply 24

Rubgish

Original post by Fires

Little problem that you will need vast amounts of energy as the closer you approach the speed of light, the greater the mass - therefore for your space shuttle, on current theories you will require more energy than the Sun outputs in its entire existence to get it up to 90% the speed of light.

Also Wikipedia probably won't be around then to correct.

Erm, 90% of the speed of light gives you requiring a total energy to be only ~ 3.16 times rest mass of the rocket. Thats easily possible, and no-where near the energy output of the sun.

Reply 25

dknt

Original post by CharlieBoardman

Yes, both are possible! :smile:

I agree with that, just there is a big difference between the passing of 1000 years or so and 59'000 years or so :tongue:

when you said 'close to the speed of light' I presumed you meant I tiny fraction away from the speed of light :smile:

Well a "tiny fraction" is in itself ambiguous. Just a few differences in decimal places and the time dilation varies by a lot.

Reply 26

SubAtomic

You might be interested in reading this and reading about spacetime in general.

Reply 27

a.partridge

Original post by Fires

to be fair, these kind of special relativity questions are the easiest and most intuitive relativity questions you can get. The derivation is very easy to follow so might be worth looking up if you don't have confidence in it - it only requires basic algebra and simple assumptions.

I don't entirely see how the independence of the speed of light from the reference frame rules out time travel as he has suggested though. It may be ruled out by other things but not by this.

p.s the twin paradox is not really a special relativity question (well the paradox part is but not the reason that causes the age difference, so arguing about it in these terms isn't going to get you anywhere!)

(edited 11 years ago)

Reply 28

Ryan_94

Time travelling will never exist. I can confirm this because, if time travelling ever will exist, then the people of the future would have come to their past - which is our present - and told the people of their past - which is us - that time travelling will exist.

Reply 29

a.partridge

Original post by Ryan_94

maybe they decided to avoid travelling back on time to earths past to stop some kind of butterfly effect killing themselves off eh?

Reply 30

Fires

Original post by Rubgish

Erm, 90% of the speed of light gives you requiring a total energy to be only ~ 3.16 times rest mass of the rocket. Thats easily possible, and no-where near the energy output of the sun.

Seriously? I thought it was far more than that, maybe I was thinking of 99% the speed of light, but when you say 3.16 times the rest mass of the rocket, are you talking about total mass>energy conversion?

A physicist here calculates the 90% figure as follows:
http://www.giantitp.com/forums/archive/index.php?t-77600.html

So per kg travelling at 0.9c, we need:
400g antimatter
10kg hydrogen fusion
200kg uranium fission
5 million tonnes coal
5 million tonnes TNT

Reply 31

freakymonkey

Original post by Ryan_94

Unless it is physically impossible to travel back in time! This has been proposed as a way of keeping causality the way is was either 'created' at the time or predetermined (this is what you'd have to believe if religious). In the same way that gravity (and my beer belly, but mainly gravity :P) prevent me from flying, however much I want to fly, the same happens with causality. This may be relevant, I didn't read it through entirely -http://en.wikipedia.org/wiki/Chronology_protection_conjecture

Reply 32

a.partridge

Original post by Fires

I make it 2.29 times rest mass.

you can work out the 'gamma factor' for different speeds yourself with

gamma = (1- (v/c)^2)^-0.5

mass doesn't really increase when you go faster... just the gamma factor... it gives the appearance of being more massive

edit 99% C comes out as ~7.1 rest mass

(edited 11 years ago)

Reply 33

CharlieBoardman

Original post by dknt

Well a "tiny fraction" is in itself ambiguous. Just a few differences in decimal places and the time dilation varies by a lot.

Yes, I agree that it was quite ambiguous. But would a couple of changed decimal places really change the amount of time which has elapsed this dramatically?

Original post by a.partridge

mass doesn't really increase when you go faster... just the gamma factor... it gives the appearance of being more massive

Don't objects shrink as they travel faster? According to Einstein anyway..

Reply 34

Rubgish

Original post by Fires

10kg of hydrogen fusion is nothing, the suns mass is decreasing by several million kilograms a second due to hydrogen fusing into helium :smile:

I'm talking about relativistic momentum, you can say an objects total energy is its Kintetic Energy + Rest mass. So basically, do 1 divided by the square root of 1 - (v/c)^2 and you know how many times rest mass the total energy is.

for 0.99c, you are still only talking about 7 times rest mass.

For protons accelerated in the LHC at 7 TeV, the rest mass of a proton is 938 MeV, meaning that you are looking at 7462 times rest mass, or something like 0.99999998c

(edited 11 years ago)

Reply 35

Dirac Spinor

If you accept that spacetime can have it's topology locally changed then a wormhole could send you back in time.

Reply 36

ThatPerson

Original post by Superstar6318

I am in deep regret over one day, trivial as it sounds, I need to go back

Will time travel ever exist?

Relatively speaking, you could travel forward in time faster. Time Dilation means that as you approach the speed of light time moves slower relatively to you.

So if you you have person A and B; person B remains on earth, person A goes around and flies in his spaceship at the speed of light. Relative to person B, time moves slower for person A. This means that when Person A lands back in earth, he would have perceived less time than person B perceived so technically he would've traveled to the future. It's impossible to travel back in time I think, apart from something random I read a while ago about wormholes.

Reply 37

kittyb99

I was actually reading this article the other day!

http://www.dailymail.co.uk/home/moslive/article-1269288/STEPHEN-HAWKING-How-build-time-machine.html

Very interesting indeed :smile:

Reply 38

dknt

Original post by CharlieBoardman

Yes, I agree that it was quite ambiguous. But would a couple of changed decimal places really change the amount of time which has elapsed this dramatically?

Don't objects shrink as they travel faster? According to Einstein anyway..

It changes by quite a bit. If you travel at 0.9c, time passes ~2 times faster relative to you. At 0.99c its ~7 times, 0.999c is ~22 and 0.9999c is ~70.

And yes, sort of. That's length contraction. If something is moving, relative to you, then the object appears to be smaller. Relative to that object however, it remains the same size and its you that's smaller.

When he said more massive, I think he was referring to its energy, not physical size.