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# What does it mean to say that time stops at the speed of light? watch

1. Time stopping at the speed of light is essentially a consequence of special relativity.

If you set a limit on the maximum speed obtainable in he universe, then you have to figure out what happens to something that reaches that speed or attempts to exceed it.

A simple way to see it is this:

Acceleration is define in

If you have a force, you have acceleration, and it means you will get faster.

But we know there's a limit, which is the speed of light.

So, if you can theoretically reach the speed of light, you would have reached the maximum speed. And then even if you attempt to accelerate yourself by any amount, you cannot increase speed.

This means your change in speed is zero, even though a force is being applied. Or in other words:

Clearly this is absurd because it shows which is a strong indicator that the equations needed to be updated to the special relativistic form.

But, from looking at this you can at least see that the acceleration needs to come to zero.

Hence

which means

If dt is infinite, it means time goes on 'forever' in any given moment, hence time stops.

(This also shows that it's impossible to reach the speed of light)

(Also, this is a crude explanation that any proper physicist would probably scoff at. It's kind of hard to explain such a thing in simple terms!)
2. (Original post by Pessimisterious)
Time stopping at the speed of light is essentially a consequence of special relativity.

If you set a limit on the maximum speed obtainable in he universe, then you have to figure out what happens to something that reaches that speed or attempts to exceed it.

A simple way to see it is this:

Acceleration is define in

If you have a force, you have acceleration, and it means you will get faster.

But we know there's a limit, which is the speed of light.

So, if you can theoretically reach the speed of light, you would have reached the maximum speed. And then even if you attempt to accelerate yourself by any amount, you cannot increase speed.

This means your change in speed is zero, even though a force is being applied. Or in other words:

Clearly this is absurd because it shows which is a strong indicator that the equations needed to be updated to the special relativistic form.

But, from looking at this you can at least see that the acceleration needs to come to zero.

Hence

which means

If dt is infinite, it means time goes on 'forever' in any given moment, hence time stops.

(This also shows that it's impossible to reach the speed of light)

(Also, this is a crude explanation that any proper physicist would probably scoff at. It's kind of hard to explain such a thing in simple terms!)
It's fine, I appreciate it, I do have one follow up question though.

What I don't understand is if a blachole can pull light into it, it must pull objects into it faster than light travels... Like my treadmill example, if you were on a treadmill running at the speed of light and the treadmill still managed to pull you backwards until you fell off the belt, would that not mean the belt is moving faster than the light and as such any object on that belt (belt being space) would also travel faster than light?

Feel free to explain it in layman's terms.
3. (Original post by AishaGirl)
It's fine, I appreciate it, I do have one follow up question though.

What I don't understand is if a blachole can pull light into it, it must pull objects into it faster than light travels... Like my treadmill example, if you were on a treadmill running at the speed of light and the treadmill still managed to pull you backwards until you fell off the belt, would that not mean the belt is moving faster than the light and as such any object on that belt (belt being space) would also travel faster than light?

Feel free to explain it in layman's terms.
Heh well this where the general relativity stuff comes in, which I haven't actually studied. It's a specialist area which a physics student can actually never study in their whole course, if they choose to take a different route. I hated relativity from the start so I've barely touched on it since 1st year.

I think the thing about black holes only goes to illustrate how gravity effects everything in the universe, including light. Gravity can't be described as a conveyor belt because it's still as mysterious as light itself. It isn't a solid object that you can touch. I'd be willing to suggest gravity and light are one and the same in many ways - manifestations of pure energy.

That's about as much as I can say on the matter though because I haven't studied general relativity!
4. (Original post by Pessimisterious)
Heh well this where the general relativity stuff comes in, which I haven't actually studied. It's a specialist area which a physics student can actually never study in their whole course, if they choose to take a different route. I hated relativity from the start so I've barely touched on it since 1st year.

I think the thing about black holes only goes to illustrate how gravity effects everything in the universe, including light. Gravity can't be described as a conveyor belt because it's still as mysterious as light itself. It isn't a solid object that you can touch. I'd be willing to suggest gravity and light are one and the same in many ways - manifestations of pure energy.

That's about as much as I can say on the matter though because I haven't studied general relativity!
ok that's fine thanks
5. (Original post by AishaGirl)
It's fine, I appreciate it, I do have one follow up question though.

What I don't understand is if a blachole can pull light into it, it must pull objects into it faster than light travels... Like my treadmill example, if you were on a treadmill running at the speed of light and the treadmill still managed to pull you backwards until you fell off the belt, would that not mean the belt is moving faster than the light and as such any object on that belt (belt being space) would also travel faster than light?

Feel free to explain it in layman's terms.
Thinking about black holes in terms of Euclidean geometry is where I think the problem lies.

We need to think in terms of Minkowski manifolds where space and time (space-time) are interwoven to describe events and not simply by geometric co-ordinates independent of time.

In the absence of mass, the Euclidean model works for non-relativistic speeds where objects travel in a straight line. The presence of mass warps space-time such that object motion is described by curves in Euclidean space. i.e. satellites orbit and photons are deflected around stars etc.

The mass of a black hole warps space-time to the extent that anything within the Schwarzschild radius (event horizon) is constrained to orbit but never escape. The photons do not fall back into the black hole, they are postulated to spread out around the horizon. (Do not confuse this with Hawking radiation).

This model preserves Special Relativity for both observer and observed both inside and outside the Schwarzschild radius.

The singularity describing infinite mass and zero space is a mathematical convenience. In truth, because no information leaves the event horizon, we cannot know what happens within the Scwarzschild radius and are forever consigned to theorising.
6. (Original post by uberteknik)
Thinking about black holes in terms of Euclidean geometry is where I think the problem lies.

We need to think in terms of Minkowski manifolds where space and time (space-time) are interwoven to describe events and not simply by geometric co-ordinates independent of time.

In the absence of mass, the Euclidean model works for non-relativistic speeds where objects travel in a straight line. The presence of mass warps space-time such that object motion is described by curves in Euclidean space. i.e. satellites orbit and photons are deflected around stars etc.

The mass of a black hole warps space-time to the extent that anything within the Schwarzschild radius (event horizon) is constrained to orbit but never escape. The photons do not fall back into the black hole, they are postulated to spread out around the horizon. (Do not confuse this with Hawking radiation).

This model preserves Special Relativity for both observer and observed both inside and outside the Schwarzschild radius.

The singularity describing infinite mass and zero space is a mathematical convenience. In truth, because no information leaves the event horizon, we cannot know what happens within the Scwarzschild radius and are forever consigned to theorising.
OK so this makes sense kind of, would it be more accurate then to say that light is being bent to such a degree that it cannot escape? As demonstrated by gravitational lensing the light is being bent, but not enough to make it loop around the galaxy forever?

Sorry is that does not make sense. If space can bend, warp and fold does this not make it materialist? Space must be a physical object if it has physical properties?
7. (Original post by AishaGirl)
OK so this makes sense kind of, would it be more accurate then to say that light is being bent to such a degree that it cannot escape? As demonstrated by gravitational lensing the light is being bent, but not enough to make it loop around the galaxy forever?
Yeah, that kind of sums it up. Light will be deflected around massive objects, but only something as massive as a black hole will cause it to smear and loop so to speak.

If space can bend, warp and fold does this not make it materialist? Space must be a physical object if it has physical properties?
"According to General Relativity, space is endowed with physical qualities; in this sense, therefore, there exists an ether.

Space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."

Albert Einstein. 5th May 1920.

I don't think science understands the nature of space-time anywhere near well enough to be able to make a statement bold enough to suggest causality. In the same way, we don't know why the universe exists or the matter within it. All we know is how it behaves (to an extent) and have tracked it's beginning back to a single event.

Modelling behaviour is only a tool used to describe the observed nature of something. It still does not give an answer to 'what' causes it to behave that way.

A bit like the whether the chicken or the egg came first. Until the theory of evolution was postulated, the two were bound in an infinite loop with no answer.
8. (Original post by uberteknik)
we don't know why the universe exists or the matter within it.
I know the answer but this is not the topic for such discussion.

Thanks for the replies.
9. I'm no physicist, but from the little I do know about relativity, I'm pretty sure that one's own frame of reference is always constant (as in you experience time moving normally) regardless of the speed you're going at.
10. (Original post by Plantagenet Crown)
I'm no physicist, but from the little I do know about relativity, I'm pretty sure that one's own frame of reference is always constant (as in you experience time moving normally) regardless of the speed you're going at.
Yes but when you're travelling at c there is no time for you to "experience" anything. You need time to experience something. If there is no time you either experience everything instantly or nothing forever.

Not sure which is correct.
11. (Original post by AishaGirl)
I know the answer but this is not the topic for such discussion.

Thanks for the replies.
My pleasure even though I know we are at opposite ends of the spectrum so to speak!

Best and sincerest good wishes.

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