# The Naked Scientists Forum

### Author Topic: Speed of light  (Read 9206 times)

#### turnipsock

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##### Speed of light
« on: 30/01/2008 10:48:09 »
Reading through these pages, it strikes me that we are going nowhere until we can travel faster than the speed of light. Therefore, we should maybe all work on cracking that one first.

What actually limits the speed of light to 3x109m/s?

For instance, if light (traveling at the speed of light) was heading towards a black hole, it would be subject to an acceleration caused by the gravitational pull of the black hole, so woudn't it speed?
« Last Edit: 30/01/2008 11:52:50 by turnipsock »

#### DoctorBeaver

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##### Speed of light
« Reply #1 on: 30/01/2008 14:39:38 »
Accelerating a mass takes energy. At ordinary speeds, that energy doesn't really amount to a great deal. At relativistic speeds, however, the amount of energy required escalates enormously and to actually reach light speed would take infinite energy.

Photons are light, so light can only travel as fast as photons. In the case of a photon being accelerated towards a black hole, you have to take time dilation into account so that the speed of the photon still cannot exceed 3x109m/s.

What actually limits the speed of light is an unknown. There are fundamental values in the universe that have to be exactly what they are for the universe to be how it is and, hence, for it to have evolved just so in order for us to be here to observe it.

It is possible that there are domains where the speed of light is indeed different - maybe faster, maybe slower - but we could never observe them.

#### another_someone

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##### Speed of light
« Reply #2 on: 30/01/2008 15:27:30 »
What actually limits the speed of light to 3x109m/s?

Photons are light, so light can only travel as fast as photons. In the case of a photon being accelerated towards a black hole, you have to take time dilation into account so that the speed of the photon still cannot exceed 3x109m/s.

Minor detail (which I assume everbody knew, but had finger trouble when typing) - it is 3x108, and not 3x109m/s.

The other thing to bear in mind is that speed is relative (hence why it is the theory of relativity), so it is not how fast something is going, but how fast it is going relative to something else.

This is important because it invalidates ideas such as how fast is this thing going.  What matters is how fast does it appear to be going to an observer, which is different for different observers - but no observers will ever see the object travelling faster than the speed of light.

This is important when you ask how fast an object is falling into a black hole.  When you look at something falling into a black hole, the image you have of it will get more and more stretched out in time, so it will appear to be travelling slower than it might do if you did not have this stretching of them timescale of the image; but when it gets to the point where that simply wont do it any longer, then the object will disappear below the event horizon (i.e. it may well be travelling faster than light, but you don't see it any more, so it still abides by the rule that it can never be seen to travel faster than the speed of light).

#### DoctorBeaver

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##### Speed of light
« Reply #3 on: 30/01/2008 15:37:22 »
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Minor detail (which I assume everbody knew, but had finger trouble when typing) - it is 3x108, and not 3x109m/s.

oops  [:I]

I just copied & pasted without reading it properly.

I deliberately didn't mention speed relative to the observer as I knew you were lurking and I decided to leave that to you

#### lightarrow

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##### Speed of light
« Reply #4 on: 30/01/2008 18:12:17 »
Reading through these pages, it strikes me that we are going nowhere until we can travel faster than the speed of light.
In which sense? If you take a starship and accelerate to very near light's speed, you can travel along all the visible universe in a few seconds.
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Therefore, we should maybe all work on cracking that one first.

What actually limits the speed of light to 3x109m/s?

For instance, if light (traveling at the speed of light) was heading towards a black hole, it would be subject to an acceleration caused by the gravitational pull of the black hole, so woudn't it speed?
It wouldn't be subject to acceleration and it wouldn't speed up. Light falling down a gravitational field acquires energy (gravitational blue-shift) but the speed is the same.

#### Soul Surfer

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##### Speed of light
« Reply #5 on: 30/01/2008 20:46:44 »
Light arrow you should stress that it is in theory only a few seconds of your time because you will take billions of years in the rest of the universe's time.

Also assuming that you don't want to stress yorself with more than a g or two it will take you quite a long time to get up to that soert of speed and to slow down again.

The terrible truth is that human beings can and will never be able to leave the solar system unless they build totally self sufficient ark ships that take many generations to reach the nearest stars.  Mind you if we do that and build more ark ships it is quite possible to populate the whole galaxy in a few million years.

#### another_someone

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##### Speed of light
« Reply #6 on: 30/01/2008 21:22:40 »
Light arrow you should stress that it is in theory only a few seconds of your time because you will take billions of years in the rest of the universe's time.

In non relativistic terms, could we not approach the speed of light in about a year accelerating at 1g.

Taking into account relativity, clearly, we will still be short of the speed of light, and any outside observer would see our rate of acceleration decrease even as we believe we maintain a constant acceleration; but by that time, our own sense of time would be slowing down, so we would be ageing less, so we would still be covering the original distance with ever less of our own experience of time.

#### turnipsock

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##### Speed of light
« Reply #7 on: 30/01/2008 23:59:59 »
oops, sorry I got c wrong.

Interestingly, the fastest speed a human has traveled is only 11,080m/s which is a long way short of 300,000,000m/s. It was attained during the Apollo 10 mission.
« Last Edit: 31/01/2008 00:04:13 by turnipsock »

#### lightarrow

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##### Speed of light
« Reply #8 on: 31/01/2008 08:02:35 »
Light arrow you should stress that it is in theory only a few seconds of your time because you will take billions of years in the rest of the universe's time.
Yes, for this reson I wrote "In which sense?" And I didn't want to be more explicative because the OP wrote that he had already read about it in the previous threads of this forums.
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Also assuming that you don't want to stress yorself with more than a g or two it will take you quite a long time to get up to that soert of speed and to slow down again.
I don't think we will not be able to sustain higher accelerations in the future; I mean I'm confident that technology and physiology will be able to find something about this problem and increase, maybe tenfold, the sustainable acceleration. Instead, the problem that time on earth will have advanced very much when the traveller comes back, is presumably impossible to solve (or at least I don't think it could be solved in the near future).
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The terrible truth is that human beings can and will never be able to leave the solar system unless they build totally self sufficient ark ships that take many generations to reach the nearest stars.  Mind you if we do that and build more ark ships it is quite possible to populate the whole galaxy in a few million years.

#### Judge

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##### Speed of light
« Reply #9 on: 02/02/2008 08:05:15 »
This is not really an answer to your question but it relates. If we ever broke the light speed barrier, in theory, it would take less energy to accelerate and would actualy take additional energy to slow down. It would take an infinite amount of energy to deccelerate to the speed of light.

#### another_someone

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##### Speed of light
« Reply #10 on: 02/02/2008 14:06:04 »
This is not really an answer to your question but it relates. If we ever broke the light speed barrier, in theory, it would take less energy to accelerate and would actualy take additional energy to slow down. It would take an infinite amount of energy to deccelerate to the speed of light.

There is a theoretical group of particles known as tachyons which are speculated to posses this property - but the trouble is, since tachyons and ordinary matter cannot interact, even if tachyons did exist, we could never know they existed (which is as good as saying that for all practical purposes they do not exist).

#### DoctorBeaver

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##### Speed of light
« Reply #11 on: 02/02/2008 16:04:10 »
This is not really an answer to your question but it relates. If we ever broke the light speed barrier, in theory, it would take less energy to accelerate and would actualy take additional energy to slow down. It would take an infinite amount of energy to deccelerate to the speed of light.

There is a theoretical group of particles known as tachyons which are speculated to posses this property - but the trouble is, since tachyons and ordinary matter cannot interact, even if tachyons did exist, we could never know they existed (which is as good as saying that for all practical purposes they do not exist).

I thought theories that predicted/required tachyons had been largely debunked.

#### another_someone

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##### Speed of light
« Reply #12 on: 02/02/2008 16:29:35 »
Requiring tachyons is one thing, excluding the possibility of their existence is another.

I was trying to work out in my mind, if a tachyon existed, what would this inverse relationship between energy and velocity actually mean.  It would imply that a tachyon would have least energy when it was travelling at infinite velocity - but what in practice does infinite velocity mean?  It means that an object is simultaneously in more than one place (in fact, that it is probably simultaneously everywhere).  This would simply mean that the object cannot be localised - i.e. that you would require to add energy to a tachyon, not to accelerate it, but to localise it.  How would this relate to the difficulty we have in localising particles on a quantum scale?

I am not suggesting this as any new theory, just as a train of thought.

#### DoctorBeaver

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##### Speed of light
« Reply #13 on: 02/02/2008 16:35:53 »
It does not necessarily imply that the tachyon is everywhere simultaneously, just that it is everywhere on its own worldline simultaneously. But isn't that true of any object that travels at c? Would an observer travelling at c not see the entire life of the universe in the blink of an eye, so to speak?

#### another_someone

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##### Speed of light
« Reply #14 on: 02/02/2008 16:44:44 »
It does not necessarily imply that the tachyon is everywhere simultaneously, just that it is everywhere on its own worldline simultaneously.

No, this is not so.  If a tachyon is travelling ant infinite velocity, then it is everywhere on the timeline which observes it as travelling at infinite velocity.  It is perfectly reasonable to say that on other timelines it is not travelling at infinite velocity, but we are talking about an outside observer who does observe it (if the term 'observe' has any meaning in this context) as travelling at infinite velocity.  To that observer (not necessarily to the tachyon itself), the tachyon would appear non-localised.

#### DoctorBeaver

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##### Speed of light
« Reply #15 on: 02/02/2008 17:09:26 »
I'm not suggesting that it would appear localised. But consider this...

Take the 3 points of a triangle as being A, B & C. A tachyon travels from A to B at infinite velocity. It would only be observed as being everywhere on that line simultaneously. It would never be observed at point C, or in the centre of the triangle, or outside it etc.

#### another_someone

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##### Speed of light
« Reply #16 on: 02/02/2008 17:37:45 »
I'm not suggesting that it would appear localised. But consider this...

Take the 3 points of a triangle as being A, B & C. A tachyon travels from A to B at infinite velocity. It would only be observed as being everywhere on that line simultaneously. It would never be observed at point C, or in the centre of the triangle, or outside it etc.

Yes, but if the tachyon requires energy to slow down, and an infinite amount of energy just to slow down to the speed of light (let alone the energy to get any slower); so having travelled at infinite speed from A to B, what does it do when it gets to position B?

Beyond that, one has to ask, if a tachyon cannot travel slower than the speed of light, then is this limited to one dimension, or must this be in all dimensions or no dimensions?  Does it mean that an object can be travelling faster than the speed of light in one dimension but stationary in a perpendicular direction, or would it have to be travelling faster than the speed of light in all directions?

#### DoctorBeaver

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##### Speed of light
« Reply #17 on: 02/02/2008 20:12:20 »
I'm not suggesting that it would appear localised. But consider this...

Take the 3 points of a triangle as being A, B & C. A tachyon travels from A to B at infinite velocity. It would only be observed as being everywhere on that line simultaneously. It would never be observed at point C, or in the centre of the triangle, or outside it etc.

Yes, but if the tachyon requires energy to slow down, and an infinite amount of energy just to slow down to the speed of light (let alone the energy to get any slower); so having travelled at infinite speed from A to B, what does it do when it gets to position B?

Beyond that, one has to ask, if a tachyon cannot travel slower than the speed of light, then is this limited to one dimension, or must this be in all dimensions or no dimensions?  Does it mean that an object can be travelling faster than the speed of light in one dimension but stationary in a perpendicular direction, or would it have to be travelling faster than the speed of light in all directions?

I wasn't saying it stopped at B. Beyond B it would just continue on its merry little way.

I don't see any reason why it should not be stationary in a higher dimension. In 3 dimensions it is perfectly OK for an object to travel on the x axis, but be stationary on the y and z axes.

#### another_someone

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##### Speed of light
« Reply #18 on: 02/02/2008 20:50:07 »
I wasn't saying it stopped at B. Beyond B it would just continue on its merry little way.

So at least in one dimension it is an infinite line, where it cannot be localised anywhere along the line.

I don't see any reason why it should not be stationary in a higher dimension. In 3 dimensions it is perfectly OK for an object to travel on the x axis, but be stationary on the y and z axes.

It is an interesting possibility, because such a scenario would lead to some interesting (and possibly testable) possibilities.

Since it is assumed that a particle that is superluminal can never travel at subluminal speeds, and visa versa; and that superluminal particles have imaginary mass while subluminal particles have real mass, so we are now speculating about a particle that has imaginary mass in one dimension and real mass in another dimension, and that this situation is inviolate (i.e. the dimension in which the particle has real mass can never become imaginary mass, and visa versa).

By comparison, conceiving of a particle that has imaginary mass in all dimensions is fairly straight forward (particularly if you cannot localise that mass), but how would a hybrid particle which contains both real and imaginary mass vectors behave?  Could energy be transferred between the real and imaginary mass vectors of a particle?  In fact, one of the very novel parts of this scenario is just that mass must be treated as a vector quantity, whereas we tend to think of mass as a scalar quantity.
« Last Edit: 02/02/2008 20:52:19 by another_someone »

#### DoctorBeaver

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##### Speed of light
« Reply #19 on: 02/02/2008 21:12:29 »
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So at least in one dimension it is an infinite line, where it cannot be localised anywhere along the line.

Not necessarily infinite. It could be a geodesic in a closed spacetime.

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In fact, one of the very novel parts of this scenario is just that mass must be treated as a vector quantity, whereas we tend to think of mass as a scalar quantity.

May I mention multi-dimensional angular momentum?  [:o)]

What if it is our dimensions in which it is stationary and a higher dimension (or dimensions) in which it is superluminal. I don't know if the maths allow that.
« Last Edit: 02/02/2008 21:16:25 by DoctorBeaver »

#### McQueen

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##### Speed of light
« Reply #20 on: 07/02/2008 00:04:46 »
The speed of light is  3x 108 m/s.

#### The Naked Scientists Forum

##### Speed of light
« Reply #20 on: 07/02/2008 00:04:46 »