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Author Topic: Relativity question
trollface
The Bills of St. Mary's


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Okay, this is probably down to my incomplete/incorrect understanding of relativity, but a thought occurred to me today, and it doesn't seem to be going away.

Right, according to the theory of relativity (the Special Theory, IIRC), if something is moving at less than the speed of light, then it travels forwards in time. If it's moving faster than the speed of light, then it travels backwards in time. And, if it's moving at the speed of light, then time has stopped for it. Certainly, I know that the first has been proven, with fast planes and very accurate clocks.

However, speed is distance divided by time. Meters per second, usually in physics. So how can something travelling at the speed of light have a speed at all?

What I mean is that if something is travelling at the speed of light, then time has stopped for it. So, how can it have a speed, which involves a measurement of time?

There's only two ways I can really think about this. Either I'm wrong in that something travelling at the speed of light has no time passing (although I'm sure about things travelling both slower and faster), or light actually stays still and everything else moves relative to it. Which seems wrong because you can have photons moving in opposite directions.

The third possibility is that it doesn't matter that the light has no time for it, as the speed is measured relative to the observer, and the time passing for the observer is the important factor. But I don't think that that's really true. Because the point is that light-speed is an absolute value. It has certain properties, which are intrinsic to it being light speed. In fact, IIRC, the Special Theory states that the speed of light is the same for all observers, no matter their own motion.

So, how can it have a speed if, for it, no time passes for it to travel any distance in?

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Michael Cole
Deck the Malls


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quote:
Originally posted by trollface:
Okay, this is probably down to my incomplete/incorrect understanding of relativity, but a thought occurred to me today, and it doesn't seem to be going away.

I'm not an expert by any stretch of the imagination, but I'll give it a go.
quote:

Right, according to the theory of relativity (the Special Theory, IIRC), if something is moving at less than the speed of light, then it travels forwards in time. If it's moving faster than the speed of light, then it travels backwards in time.

Nothing can move faster than the speed of light, by definition. Thus nothing can ever move backwards in time, unless it is with the use of wormholes, and even then it is moving forwards with respect to itself, but backwards wrt an observer. All time must move forwards, it is just that it moves at different rates according to an observer, depending on the speed that the object is travelling.
quote:

And, if it's moving at the speed of light, then time has stopped for it.

Only for it as respect to an observer obsering it. If the observer is the one travelling at the speed of light, then time would appear to be moving at the same rate that it always has. It is relative to the observer.

Note that this is theoretical, as no object with mass can obtain the speed of light, and as we would assume that an observer has mass, that ain't gonna happen.
quote:

Certainly, I know that the first has been proven, with fast planes and very accurate clocks.

However, speed is distance divided by time. Meters per second, usually in physics. So how can something travelling at the speed of light have a speed at all?

What I mean is that if something is travelling at the speed of light, then time has stopped for it. So, how can it have a speed, which involves a measurement of time?

Because time hasn't stopped for it observing itself, it has only stopped for an outside observer observing it. It is relative.
quote:

There's only two ways I can really think about this. Either I'm wrong in that something travelling at the speed of light has no time passing (although I'm sure about things travelling both slower and faster),

It certainly does have time passing, its just that for someone observing it from the outside, time doesn't appear to be passing.
quote:

or light actually stays still and everything else moves relative to it. Which seems wrong because you can have photons moving in opposite directions.

The third possibility is that it doesn't matter that the light has no time for it, as the speed is measured relative to the observer, and the time passing for the observer is the important factor. But I don't think that that's really true.

Actually, it is.
quote:

Because the point is that light-speed is an absolute value. It has certain properties, which are intrinsic to it being light speed. In fact, IIRC, the Special Theory states that the speed of light is the same for all observers, no matter their own motion.

Yep, the speed doesn't change at all.
quote:

So, how can it have a speed if, for it, no time passes for it to travel any distance in?

Because for it, time does pass.

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Q. What's the difference between a Computer saleman and a Used Car Salesman?
A. The Used Car Salesman knows when he is lying.

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Michael Cole
Deck the Malls


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Actually, let me try to make it easier. You and someone else are travelling at certain speeds. You are travelling faster than he is.

Now when you look at your watch, it is still ticking along at the same rate. But when you look over at his, it seems to be ticking over at a much faster rate than yours.

Similarly, when he looks at his watch, his watch (according to him) is ticking over at a normal rate, but when he looks at yours, yours is running slow.

Now if you then both started travelling at the same speed, both watches would start ticking at the same rate, but yours would be behind his. He has actually aged more than you.

Does this help?

[Edit]
Which of course means that, contrary to the Road Safety campaigns, if you want to live longer, you have to drive everywhere really, really fast...

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Q. What's the difference between a Computer saleman and a Used Car Salesman?
A. The Used Car Salesman knows when he is lying.

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Troberg
Angels Wii Have Heard on High


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I can sum it up in one simple statement:

The theory of relativity is like an embarassing erection. The more you think about it, the harder it gets.

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/Troberg

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Dropbear
Angels from the Realms so Glurgy


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I thought it was like an urgently desired erection - the more you think about it the less tangible it becomes.

Dropbear

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" The villagers had said justice had been done, and she'd lost patience and told them to go home, then, and pray to whatever gods they believed in that it was never done to them. -- (Terry Pratchett)

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trollface
The Bills of St. Mary's


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quote:
Originally posted by Michael Cole:
[QB]Nothing can move faster than the speed of light, by definition.

I know for a fact that that's wrong. Nothing can accelerate beyond the speed of light, but something created moving faster than the speed of light will continue to move beyond the speed of light.

Tachyons are nothing but theory, but there's nothing to say that they don't or can't exist.

quote:
Only for it as respect to an observer obsering it. If the observer is the one travelling at the speed of light, then time would appear to be moving at the same rate that it always has. It is relative to the observer.
But even that can't be true, and it still doesn't invalidate my observation.

If time passed normally for photons, but was still for the observer of photons, then how could light have a speed? It would appear to move instantaneously to the observer because, while the light was moving, time would stand still for the observer.

How could you measure the speed? You still need a component of time to make a measurement of speed and for you, the observer, there would be no time in which to measure the speed.

quote:
Because time hasn't stopped for it observing itself, it has only stopped for an outside observer observing it. It is relative.
But then the only observer who could measure light speed would have to be travelling at light speed. And then you still couldn't measure distance, because the rest of the universe would seem to be travelling at light speed relative to you.

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seriously , everyone on here , just trys to give someone crap about something they do !! , its shitting me to tears.

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Hans Off
Markdown, the Herald Angels Sing


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I know there are some published thought experiments that show you what you would perceive if you could sit on a photon and travel at the speed of light....

I've not got time now, but IIRC they are easily findable with a quick google.

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Richard W
Ding Dong! Merrily on High Definition TV


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I think the trick is that distances are also contracted if you're travelling at or close to the speed of light. So if you were travelling at the speed of light, although time would appear to be passing normally for you, it would take zero time (from your point of view) to reach your destination, because the apparent distance there would have contracted to zero.

So, as far as an observer travelling at the speed of light could see, they would be going at c = d / t = 0 / 0, which is undefined, as you say.

Even an observer travelling at the speed of light would see photons overtaking them at the speed of light, though... that is, if they perceived any time passing at all.

The actual point of the theory of relativity is that the speed of light, c, is constant in all frames of reference, and so no matter how fast you're going, photons will always overtake you at speed c. Rather than appearing to move quicker or more slowly relative to you, the length contraction changes their frequency, hence redshifts and blueshifts.

Equations for time dilation. It's easier to do the maths than it is to think about it...!

Not sure if I've actually explained this correctly or not. Rather than take my word for it, what you want is this book:

Mr Tompkins in Paperback by George Gamow.

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trollface
The Bills of St. Mary's


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You're right, Richard, in that I'd forgotten that distance also contracts. However, I've found that while I have no problem with relativity at sub or superluminal speeds, it's the actual speed of light itself that I can't quite grasp.

Maybe I've found a gap in the equations which, if I can solve them, will make me rich and famous? That'd be nice. I'll start booking the rehab now.

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seriously , everyone on here , just trys to give someone crap about something they do !! , its shitting me to tears.

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Griffin at the Maul
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Personally, I believe that the speed of light will be a lot like the speed of sound. We will never actually travel AT it, but we may very well (eventually) travel faster than it. And since it is basically impossible (due to shockwaves and aerodynamics, etc) to maintain the speed of sound perfectly, the speed of laight should be the same.

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Dreams of Thinking Machines
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quote:
Originally posted by Griffin 2020:
Personally, I believe that the speed of light will be a lot like the speed of sound. We will never actually travel AT it, but we may very well (eventually) travel faster than it. And since it is basically impossible (due to shockwaves and aerodynamics, etc) to maintain the speed of sound perfectly, the speed of laight should be the same.

Humanity could break the speed of sound for a good 500 years before the Bell rocket plane flew. Sure, we used bullets and musket balls, but we knew that we could break the speed of sound theoretically and actually. But this isn't the case with light. We can't even make protons travel faster than the speed of light. If we do find a way to travel faster than the speed of light it will probably be through cheating and using something like hyperspace. Anyway, back to the topic at hand. It would be very odd to travel at the speed of light. I imagine that if you were traveling at the speed of light and you looked backwards that you wouldn't actually see anything, or you'd see a still image, because no light would actually hit your eye. I also wonder what would happen to all the thermal energy your body radiates...would it be instantly stripped from your body or what?

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Obi Wan: "Only a Sith deals in absolutes!"
Anakin: "Um, isn't your last statement an absolute?"

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Nick Theodorakis
We Three Blings


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quote:
Originally posted by Hans Off:
I know there are some published thought experiments that show you what you would perceive if you could sit on a photon and travel at the speed of light....

I've not got time now, but IIRC they are easily findable with a quick google.

That was the gedankenexperiment that led Einstein to special relativity, at least according to legend. He realized that a "standing wave" of light is incompatible Maxwell's equations.

As to the moving at the speed of light: it's really a moot question. Only massless particles such as photons can move at the speed of light. It's not clear what "time" means to a photon, so the question is essentially meaningless.

The Usenet Physics Faq has a good section on relativity, btw.

Nick

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trollface
The Bills of St. Mary's


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quote:
Originally posted by Nick Theodorakis:
As to the moving at the speed of light: it's really a moot question.

And there was me asking for practical reasons...

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seriously , everyone on here , just trys to give someone crap about something they do !! , its shitting me to tears.

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Hans Off
Markdown, the Herald Angels Sing


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YOMANK

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"British English speakers point to Americans adding more syllables so that they can make even more noise without actually saying anything." Llewtrah


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Richard W
Ding Dong! Merrily on High Definition TV


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Well, Nick does have a point - I thought of saying something about the mass increase too, and that this could only ever apply to massless particles, but I couldn't put it as concisely as he did.

(Edit to clarify) A massive object travelling at the speed of light would have a mass of infinity. It's not just "moot" because it's unlikely to ever happen in practise; it's theoretically impossible for a massive object to accelerate to the speed of light because the force required to accelerate it increases towards infinity as well.

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Arriah
The First USA Noel


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quote:
Originally posted by trollface:
In fact, IIRC, the Special Theory states that the speed of light is the same for all observers, no matter their own motion.

/nitpick
The speed of light is the same for all observers in inertial reference frames. If a reference frame is accelerating, special relitivity doesn't apply.

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Conforming meant that everyone liked you except yourself
Rebecca

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trollface
The Bills of St. Mary's


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The thing is that I didn't ask about something with mass travelling at the speed of light, I asked about something travelling at the speed of light. Which is something that some things do, indeed, do. Light springs to mind.

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seriously , everyone on here , just trys to give someone crap about something they do !! , its shitting me to tears.

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Richard W
Ding Dong! Merrily on High Definition TV


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True, I thought that you'd asked about what an observer would perceive, but you didn't.

You might be able to see it by considering the tendency to zero of the time and the distance. When they both are zero, then the speed is undefined from "on" the photon. But mathematically it's dealt with by gradually increasing v nearer and nearer to c until it's infinitesimally different. At that point, the distance and time are both infinitesimally small but non-zero so you can get a meaningful result by dividing the two.

I'm not sure whether that's an adequate reply or not because I do see what you mean. But Nick is still right in that you seem to be talking about "time" as "perception of time" rather than "mathematical time in the equations". Mathematically it might be counterintuitive but I don't think there's a problem - a lot of maths and theoretical physics cuts out singularities by similar means.

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