# Light speed and time travel

1. Dec 14, 2011

### c0ke

Hi, I'm not sure if this is the right sub-forum because i know very little about this kind of stuff :/ But something has been bugging me for a while and it's probably very stupid but like i said, I know very little :)

So, from what i understand if we can travel close to the speed of light we will be travelling in to the future? So this leaves me with a question, why doesn't light itself travel in to the future? They say it could take so and so millions of years for a light from a distant star to reach us, but sinse it's travelling so fast how come it doesn't reach us sooner, by travelling in to the future sinse it's, well.. light speed :/

Another question, some day we'll probably have some kind of space craft that will go quicker than light - we have to if we want to explore other galaxies within a humans life span, but i heard that if you travel FASTER than light, you go back in time, how will this affect getting to other galaxies? it's almost like rowing backwards to get down a river :/

I'm not very good with explaining this kind of stuff & english isnt my mother language so sorry if it's a bit messy :)

thanks!

2. Dec 14, 2011

I too, am quite new to physics, but i believe i have a rough answer to your question,

I think, you're missing a small point, which is that, we don't so much, travel to the future, but instead, our time, relative to those not moving slows down, what this means, is that someone close to the speed of light, will age slower, than someone standing still.

If you consider special relativity, you can build up in image of space-time, where everything travels at the same speed (3x10^8 m/s or so) and everthing divides that speed between the two factors of distance within space-time, being, ofcourse, space and time. Light it self then, does not "experience" time. It doesn't travel across time, but instead used all its spacetime quota on travelling through space.

As far as a spaceship travelling faster than the speed of light, and moving backwards in time. Unfortunately, another concept, which i believe also makes up special relativity, is that as we increase our speed (or kinetic energy), our mass will also increase (described by Einsteins most famous equation). What this means to us, is that the faster we get, the more mass we have, and as we approach the speed of light, we become so massive, that AT the speed of light, our mass is infinite. Now, to pass this point, we would need an infinite amount of energy, which is quite hard to come by. So traveling backwards in time, is still a long stretch.

Just to branch off, it's worth considering tachyons, which ALWAYS travel faster than the speed of light, and it would take infinite energy to slow the down to sub-luminal speeds.

Hope that helps, and i look forward to being corrected by someone significantly more intelligent than me!

3. Dec 14, 2011

### joris_pixie

tachyons are theoreticly ;) Probably can't and don't excist

But ofcourse never say never

also to answer some questions :)
light does go at the speed of light ;) and it takes so long simply 'cause the stars are so far away !

For example as you should know our solar system is very very very small if you compare it to the universe.
But already light takes about 8 mins to travel from the sun to earth !
so that means that the sun is at a distance 8 x 60 x 300 000km !

Ofcourse if you did realise this and you are wondering why it doesn't "time travel". It's like the previous guy said :)
Special relativity says that "time goes slower when you are going fast"

Imagine you have a clock in your hand and you are running very fast around the world.
People will see your clock ticking more slowly and you will see people their clock going faster then yours. For example for you 5 seconds will have passed on your clock. But for the other people on earth a day will have passed.

It's the same way for light. For light from our perspective it's like light his clock has stopped. But our clock on earth is still ticking while it travels to earth.

Well it's quite confusing ;) but relativity is sometimes just weird

Last edited: Dec 14, 2011
4. Dec 15, 2011

### c0ke

what affect does going faster than light have on the clock? does the clock tick backwards? :p

5. Dec 15, 2011

Theoretically, yes, although, as explained, this is simply impossible.

6. Dec 15, 2011

### PAllen

If you travel close to the speed of light you are simply aging slower from the point of view of stars etc. who 'see' you moving fast. You can, in principle, travel to distant stars without aging much. Rather than traveling into the future, it is probably better to think of 'suspended animation' applied to everything on your rocket (clocks, all physical processes). There are probably insurmountable problems achieving even this, in practice (impossible fuel requirements, even assuming such things as 'antimatter engines'; impossible shield requirements - a speck of dust hitting your craft would deliver the energy of a nuclear bomb to your craft).

As for traveling faster than light, nothing much scientific can be said about it. There are some extreme scenarios in general relativity allowing for faster than light travel (traversible wormholes, Alcubierre drive), but they involve such far fetched conditions (enormous amounts of negative energy) and properties that they don't really allow one to say anything about 'what would faster than light travel be like if it could be achieved'. The only thing you can say is that actual faster than light travel is so far outside of known physics, it cannot be addressed as a scientific question.

As for tachyons (for which extensive searches have turned up no evidence), if they exist, and could be generated 'at will', they would allow communication from the future to the past (look up tachyonic antitelephone). If certain limitations on their creation exist, such communication can be avoided. They would not be relevant to transportation - a tachyon always travels faster than light, and can never slow down, any more than matter can reach or exceed the speed of light. You cannot change matter to be tachyons, then back to matter.

Here is a discussion of the fuel issues:

http://www.desy.de/user/projects/Physics/Relativity/SR/rocket.html

The shielding issues are a direct consequence of the relativistic kinetic energy and doppler formulas.

7. Dec 16, 2011

### tiny-tim

welcome to pf!

hi c0ke! welcome to pf!
no

loosely speaking, it ticks sideways!

to be precise, the time dilation of a clock moving at speed v (the rate of that clock divided by the rate of a stationary clock) is √(1 - v2/c2)

for v = 0, that's 1 (obviously, the two clocks have the same rate)

for 0 < v < c, it's less than 1, and approaching 0

for v = c, it's 0​

and for v > c, it's imaginary!

… there simply is no mathematical justification for the standard science-fiction rule that moving faster than light makes you age backwards

(the time dilation of a moving clock is the same as the slowing-down of ageing of a person)

the meaning of this is that if you and i both have clocks, and if your speed relative to me is v, then:

if v < c, then we can adjust our clocks so that my measurement of the rate of your clock is the same as your measurement of the rate of my clock

in particular, if (before adjusting the clocks) i regard your clock as running at a rate A, and you regard my clock as running at a rate B, then AB = 1 - v2/c2, which is positive, and we can easily adjust one or both clocks so that the rates are the same

but if v > c, that's not possible … AB has to be negative! … if i regard your clock as running forwards, then you must regard my clock as running backward … we cannot adjust one or both clocks so that the rates are the same, or even so that they are in the same direction

8. Dec 17, 2011

### Staff: Mentor

You travel into the future just sitting in place too.

9. Mar 25, 2012

### mrfrosty

Hi there, new to this group. My understanding is that light has a speed which if I can remember right is 2.997925 x 10 to the power of 8 m/s. It takes light 8 1/2 minutes to travel 93 million miles(distance from the sun to earth) this means that light travels 93 million miles in 8 1/2 minutes.
With this in mind, and I will come back to it, it has been proven that a small time difference can occur with speed. I believe the experiment was with an aircraft and on returning to the starting point the time was different, any hows I think there have been numerous experiments documenting this fact. Now if the equation can be determined which slows time down, surely the faster the speed, the greater the variable will be regarding time. If you could determine the amount of difference between times at certain speeds, and then work out how much the speed of light would differ the time difference, you have a starting point.
The problem I see is that time can never be stopped, but could theoretically be slowed down to such a degree that it would seem that time is no longer passing. You can sort of work it out, in my mind, like this. If you double the speed at which light travels, theoretically, you halve the time light takes to reach another point, ie, double speed of light, it then only takes 4 1/4 minutes to reach earth from sun and so on, eventually the light being instantaneous no matter what the distance, this point theoretically being the speed of time. The only barrier I see goes back to a different theory which is that if you halve the distance you take towards a wall each step you take, you will not actually reach the wall, but the distances would become so small, down to the size of atoms and beyond, that it would appear that the goal has been reached. The problem as I see it, is that there is no way of being able to prove the theory practically, but it would be able to be worked out theoretically. In conclusion, I believe that time cannot be stopped, but only slowed down to a degree to which it appears to be stopped, and the speed in which you would need to travel would be unnattainable.

Last edited: Mar 25, 2012
10. Mar 25, 2012

### phinds

You misunderstand. Time dilation is something that a stationary observer (relative to a moving object) observes in the moving object. The OBJECT does NOT observe anything out of the ordinary.

The equations that show the degree of time dilation relative to your speed as a percentage of light speed are quite well known.

11. Mar 25, 2012

### mrfrosty

Surely then the amount of time dilation can be made greater or lesser dependant on the speed, so the object that dilates time does not see any difference within itself, but sees the difference on objects not within its dilation, therefore, you mkust be able to reach a point where dilation is so great that it appears that the outside objects are at a standstill.

12. Mar 25, 2012

### PAllen

Well, 'outside' is pretty ambiguous. If you see objects moving at extremely near c relative to you, it will appear that all physical processes on them are at a virtual standstill. However, they will see your processes at virtual standstill as well. You only get mutually agreed difference in accumulated time if two observers separate and come back together. Then, for example, if one of them never accelerated, and the other did (in order to turn around and rejoin the first), then the one that turned around will have aged less.

13. Mar 26, 2012

### phinds

I'm not clear on what you mean by this but just to be sure you understand, the moving object does not experience time dilation. Period. It is not a function of speed.

14. Mar 26, 2012

### PAllen

Well, for what one observer determines about various particles, it is *only* a question of speed of those particles per that observer.

15. Mar 26, 2012

### phinds

I don't follow. Can you expand a bit, please.

16. Mar 26, 2012

### Capngarrett

I think this has already been handled brilliantly but I'll try to offer my own more semantically-orientated explanation.

It's hard to tell how a beam of light experiences the flow of time if you were put into it as an observer, but it is known that at the speed of light c time is effectively frozen.

According to modern theories, nothing can travel faster than lightspeed. As you accelerate towards that speed, the closer you get to becoming frozen in time - you are slowing down in time - but from your perspective time flows naturally and the effect of this is that your observed surroundings speed up (even though you are moving towards it, the beam of light still travels away at c from your perspective!) The thing is, an object of mass cannot reach c, but it can try and get as close as it can.
It is because of this that light always travels at c from any perspective; whether you are stationary, moving, accelerating, decelerating - time and space will always dilate to compensate and ensure that the rule (light always travels at c) is not broken.

This is why a lightbeam can only travel one lightyear per year. Travelling into the future only means the slowing down of your state as you move, there is no jumping. Theoretically, if you were to travel fast enough in a spacecraft that can almost hit lightspeed, you can travel for millions of years and reach distant galaxies while - inside the craft - you experience time normally and you could reach them within your natural lifespan. The universe outside your spaceship is literally whizzing by in time to make sure that a beam of light travelling ahead of you still manages to get away at the speed c.

I hope this helps build your insight to this wonderful theory :)

17. Mar 26, 2012

### phinds

No, it's not "hard", it's impossible. A photon doesn't HAVE a reference frame, so the question of how light experiences time or velocity is meaningless.

You do not travel for millions of years, you travel for a few years. Earthbound observers, could they live long enough and see far enough, would SEE you traveling for millions of years but that is meaningless to you.

18. Mar 26, 2012

### PAllen

Given any observer studying a bunch of particles, e.g. with identical decay rates at rest. Nothing but their speed (not direction, not acceleration) determines how slow their observed decay rate is compared to their rest rate, for that observer.

It is true the moving particle (or and observer moving with it) experiences no time dilation. However, for every observer, the the time dilation observed for moving objects depends only on their speed (in SR; let's avoid GR for now). This is what is normally meant by time dilation. The dependence only on speed is the 'clock hypothesis', only it is more of experimental fact as well as a prediction of SR.

19. Mar 26, 2012

### phinds

Yes, that makes perfect sense. Thanks. So the hypothetical earthbound observer would see particles on the near-light-speed vehicle decay at phenomenally slow rates and the oberver on the ship would see nothing strange.

20. Mar 26, 2012

### PAllen

Exactly.

21. Mar 27, 2012

### Capngarrett

1. I said 'if you were put into it as an observer' as an abstract thought which - I thought - clarified that I was talking loosely.

2. I know - I explained that in the very same sentence! The OP stated that English was not their mother-tongue, so I was trying not to convolute the explanation by adding a reference point every few words.

I'm sure that without a pedantic, critical approach, my post is followed easily enough. Even with these points made I still see no problem with it, but thank you anyway for the input.

22. Mar 27, 2012

### nitsuj

lol I'll take a shot at the impossible then.

Time & length are defined by c. So as Capngarrett puts it "It's hard to tell how a beam of light experiences the flow of time if you were put into it as an observer," is a situation where time & length cannot be defined. they're null. Or again, as Capngarrett puts it "time is effectively frozen". As you put it; an FoR cannot be defined.

Oh wait you said "A photon doesn't HAVE a reference frame.." that's wrong Phinds, a reference frame is not something one can have. It may confuse people into thinking "I will just apply this FoR to this photon, and there it is; an FoR for a photon." It should be said as "A reference frame cannot be defined for a photon (by a photon)." )

This is meaningless from a purely physical perspective.
It's really neat from a "what is time in SR" perspective.

for the second issue you take with Capngarrett...
Actually it's both, he travels for millions of years & for only a few years. Both are observed physical realities. This isn't even similar to the first comment where you had a point.

It is not at all meaningless to the traveling observer, let alone to the one imagining this scenario. The twin paradox has been resolved. Why do you find such a cool fact of physics as meaningless? Is time meaningless to you?

Lastly Capngarrett's post was pretty accurate. It is very interesting you missed the most obvious erroneous comment he/she made that isn't semantics but is about the mechanics.

"As you accelerate towards that speed, the closer you get to becoming frozen in time - you are slowing down in time - but from your perspective time flows naturally and the effect of this is that your observed surroundings speed up"

Can you spot it phinds? Given the accuracy of the rest of Capngarrett's post, I'd bet he/she could correct it to be a more accurate statement of SR, in other words it was merely an oversight.

I hear ya Capngarrett (post #21), posting on PF will condition you to perform the nearly impossible task of writing comments that are semantically pleasing for the layman on through to the most annal "physicist". (don't get me wrong, in summation it's what makes the site awesome, just remember not every member "corrects" other posters for the right reasons & will sacrifice the message of SR in the process)

Last edited: Mar 27, 2012
23. Mar 27, 2012

### Capngarrett

Ouch, you really skewered me there! By observed surroundings I should have stated 'the universe through the window', not the interior of the craft.
Is that what you meant? Or am I still wrong?

I also disagree with the 'meaningless' nature of percieved time from an outside perspective. Considering this is a forum of the theories of relativity, I would have thought that observed effects from a different POV would have been paramount to the scenario!

24. Mar 27, 2012

### PAllen

No, if your rocket is inertial (you don't feel acceleration), and stars and planets outside are moving by at near lightspeed, they will appear to have clocks, radioactive decay, aging, etc. going very slow compared to you - by exactly the same amount as your time rate appears slow to them. This effect is completely symmetric as long as motion is inertial. [There can be asymmetries if one or both observers accelerate].

Last edited: Mar 27, 2012
25. Mar 27, 2012

### nitsuj

Dude I was sticking up for you. I discredited phinds cold response of "corrections", and then pointed out that the only error in your post was a small one & probably just an oversight. (just like phinds oversight of the mechanics error) I guess it wasn't, but still; it wasn't a "skewering" was it? Sorry if it was, it wasn't my intention to "skewer" you.