[Johann Mahne]

Johann Mahne  asked the Naked Scientists:
   
Hi Chris,

If the "metric expansion" of space exeeds the speed of light itself, then does the statement "The speed of light is the same to all observers" still hold true?

What do you think?

[Phractality]

Most cosmologists think in terms of "comoving coordinates", in which the increasing distance of distant galaxies is not considered to be a "velocity". Velocity is measured relative to the coordinates, and the coordinate, themselves, are expanding.

The coordinates are like a grid of measuring chains, with new chain links being added all the time. For every 4 x 10^17 links, one new link is added every second. Observers who are comoving with eachother are stationary relative to the chain links near them. Light has the same speed everywhere relative to the chain links, so all observers perceive light moving at the same speed.

You could define a different kind of coordinate system, consisting of a grid of ideal, massless, unstretchable chain links, with no new links being added. In such a coordinates system, only one point (the origin) would be stationary relative to a set of comoving objects. Links far from the origin would be accelerating relative to comoving space; if they had mass, they would pull away from the origin. The expansion of space, in such a system, would be equivalent to a gravity hill centered on the origin, wherever the origin happens to be. The speed of light would still be the same everywhere by definition. But an observer stationary relative to the coordinates would see comoving meter sticks near him length contracted, while a comoving observer would see the chain links near him lenght contracted. Each observer would see light moving faster in terms of the other observer's meter sticks, but at the same constant speed in terms of his own meter sticks.

[MikeS]

Yes.

In simple terms, speed is distance divided by time.  Both distance and time (the 'length' of a meter and a second) are variable and they vary in such manner that the speed of light always remains constant.

[simplified]

waitedavid137   Posted: May 6 2011, 06:14 AM
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QUOTE (dimazin @ May 5 2011, 07:12 PM)
What is "slowing of light"? Is it slowing of light speed in your understanding?

It means that with respect to the coordinates appropriate for an observer far from any gravitational sources the speed of light near a mass like a star for example as calculated by those coordinates is less than c."
         see link: http://www.physforum.com/index.php?showtopic=29063&st=15
         

[yor_on]

Johan, you are asking one of the most important questions I know. Why is the speed of light a constant. When I see people answer this question using relativity 'proving' that it has to be a constant exemplifying it by 'frames of reference' counting on the differences, I see people that stared themselves blind on their professors explanations. The truth is both simpler, and more confusing. When Einstein created the theory of relativity he built it on Maxwell's electromagnetism, well not really, he really built it on his imagination, but he anchored it in Maxwell.

What I think he did was to look at light, and the experiments we already had, and from that draw one simple conclusion. That light to its nature is invariant, of one same 'speed', no matter if its source was moving or standing 'still', as relative some arbitrary point. And furthermore the same would be true for the 'sink' (eye). Motion had nothing to do with that 'relative constant speed'. From that simple conclusion you will get 'time dilations' and 'Lorentz contractions' when you start to introduce 'frames of reference'. And I think that he must have used the experiments proving that light didn't care for earths relative motion to prove it to himself.

It's a question that cuts to the marrow of physics, which hasn't been answered by any 'theory' yet. We know that light is a constant, we can see that the room it 'propagates' in deforms and distorts depending on observer. We know that there are no exact same 'frames of reference', even though people labor under the apprehension that we can use such definitions, rigidly speaking they just don't exist. Einstein left us a fantastic beautiful build, but it's time to look at the structure I think, and try to answer the real question.

Why is light always a constant 'c'.
It's worth thinking of.

[simplified]

Speed of light is a constant to local observer.Speed of light is not a constant to external observer. []

[yor_on]

Simplified, that doesn't make sense to me?
Where do you get that notion from?

[simplified]

Simplified, that doesn't make sense to me?
Where do you get that notion from?

The gravitational delay of light speaks about my concept. And you should understand  that far gravitation does not reduce my coordinates,but slows motion of that local light! [8D]

[yor_on]

There is a distorted room time in a gravitational field that makes my head hurt if that is what you was thinking of? And as seen from outside that gravity you see one thing (spacecraft hanging at EV) and inside it you see something else (space 'growing') but none of the observer will see light getting a new 'speed', neither when they measure their own frame of reference, or as they measure the 'opposite' frame of reference.

The light always come at you at 'c'. If it didn't you could assume that light coming from a black holes EV should be 'slower' but it isn't. And that we should see on Earth measuring the incoming light from the rest of the universe. Only if you accept that light nowhere has a different speed will Einstein make sense to you

The physics we have make some assumptions.

Space is homogeneous and isotropic, meaning that it looks and behaves the same everywhere, also defining that Space is expected to give us the same 'physics outcomes' everywhere. SpaceTime is also expected to be symmetric under a time translation, meaning that the experiment you do today will be valid tomorrow, as well as having been valid yesterday. We also assume that the equations of physics still will work under a motion. All of those are reasonable ideas. If you succeed to prove it otherwise you would introduce a 'magic universe' where physics would differ depending on your position in some coordinate system relative some other point. That's also why light is expected to be a constant, having a unvarying speed in a vacuum.

the other way becomes the magical kingdom, where different areas is presumed to have different lightspeeds, also invalidating the fact that three observers uniformly moving can observe the same point in space, seeing three 'time dilations' there. In Einsteins universe this is possible, in a universe where all points is expected to be regulated by some 'magical lock down' aka, for example gravity, that shouldn't be possible.

[simplified]

The light always come at you at 'c'. If it didn't you could assume that light coming from a black holes EV should be 'slower' but it isn't. And that we should see on Earth measuring the incoming light from the rest of the universe. Only if you accept that light nowhere has a different speed will Einstein make sense to you



[/quote] []Even traveling light from Sun has delay about 53 microseconds!

[MikeS]

Why is light always a constant 'c'.
It's worth thinking of.

Why does light have a very fast but finite speed?  We know that a gravity well does affect light.  If ‘time were a constant’ then it would slow the speed of light down but as time is variable, the speed of light remains constant and time dilates.

Everywhere within the universe there is a ‘gravitational field’ therefore time is being dilated everywhere.  In other words the local time dilation is due to the strength of gravity in that locale. Hypothetically, if you remove the gravitational field then a photon can travel at it’s natural speed which is instantaneous. 

I could elaborate on this but then it would be new physics.

See Time 1 http://www.thenakedscientists.com/forum/index.php?topic=39214.0;topicseen
Time 2 http://www.thenakedscientists.com/forum/index.php?topic=39218.0
Gravity and time http://www.thenakedscientists.com/forum/index.php?topic=39216.0

[imatfaal]

Mike - the way gravitational time dilation is calculated at present is that in a region with a low graviational potential time will pass more slowly - we measure this as a ratio of coordinate time (there are differences between this and gravitational field strength which you are relying upon).  Coordinate time is the measured time between events for a higher ticking clock observer at an arbitrarily high graviatational potential ie an arbitrarily large distance.  By definition light travels at c in this arbitrarily high-ticking region. 

To postulate a background gravitational potential that is everywhere, needs more theory, testability, and reason than an extrapolation of gravitational time dilation. The reason you need to do more than extrapolate is that time dilation due to gravitational potential goes to a limit of coordinate time and arbitrarily high gravitational potential but not infinite speed of light. 

[yor_on]

What are you quoting simplistic? Feel free to give me the link to what you mean there
Are you thinking of group and phase velocity, or is it something else you mean?

For you locally measuring the speed of light it will always be at 'c'. There are some tricks in where you by using clocks in a accelerating frame can define light as having different speeds depending on direction, but those are created by ignoring the way the space distorts and gravitates under a acceleration. And even there when measuring the lightspeed directly you will find it to be at 'c' as I understands it. So, ignoring conceptual frames of reference, going for what you actually measure firsthandedly there will be no measurement giving you any other speed than 'c'. That all speeds as such are arbitrarily defined doesn't change this. It is a constant.

[MikeS]

Mike - the way gravitational time dilation is calculated at present is that in a region with a low graviational potential time will pass more slowly - we measure this as a ratio of coordinate time (there are differences between this and gravitational field strength which you are relying upon).  Coordinate time is the measured time between events for a higher ticking clock observer at an arbitrarily high graviatational potential ie an arbitrarily large distance.  By definition light travels at c in this arbitrarily high-ticking region. 

To postulate a background gravitational potential that is everywhere, needs more theory, testability, and reason than an extrapolation of gravitational time dilation. The reason you need to do more than extrapolate is that time dilation due to gravitational potential goes to a limit of coordinate time and arbitrarily high gravitational potential but not infinite speed of light. 

Surely, it must follow if gravity from a massive object dilates time in its locality then the average background gravity within the universe must have an effect upon time.  This must be self evident.

This is just the average background gravity of the universe (far from a source of gravity).

The universe could be defined as a gravitationally bound system that contains the aggregate of all existing matter, energy, and space. I am not postulating anything new here the universe does have a background gravitational 'field' (for lack of a better word).  

I don't know as I understand this sentence correctly.  It may be that you are using 'dilation' in a confusing manner where 'contraction' would be more appropriate?  Ultimate time dilation is found at the event horizon of a black hole. And yes, this is a limit to coordinate time.  If this black hole was 'the black hole at the end of the universe' then time and space would have been 'squeezed' out of existence.  I don't see any of this as being 'new physics' and therefore, does not need more theory and testability.

That the universe has a background gravitational 'field' is self evident.
Gravitational time dilation has been experimentally proven.
Therefore it must follow (my quote)
"Everywhere within the universe there is a ‘gravitational field’ therefore time is being dilated everywhere.  In other words the local time dilation is due to the strength of gravity in that locale".
The only extrapolation that I have made is to add (my quote)
"Hypothetically, if you remove the gravitational field then a photon can travel at it’s natural speed which is instantaneous".
It must follow that if gravity dilates time then the lack of gravity must contract time.  The ultimate contraction of time being infinite contraction.  If that is so then it follows that the speed of light would in a 'sense' be infinite but still obeying the idea that the speed of light is constant.
I must just add this as it puts the above into context.  For a photon to travel instantaneous it would have to be outside of a gravitational 'field' which means outside the universe but that would also put it outside of time itself.

Can it be experimentally proven that a photon travels instantaneously outside of a gravitational field?  Probably not as we can not divorce the experiment from the all encompassing effects of gravity.
However, the opposite experiment can and has been done and proved that gravity does dilate time.
Is this not essentially the same experiment?
It is generally accepted (but not proven) that time stands still at the event horizon of a black hole.
It takes no more stretch of the imagination to accept that in a gravity free environment a photon travels instantaneous.

[MikeS]

Quote from my last post
Can it be experimentally proven that a photon travels instantaneously outside of a gravitational field?  Probably not as we can not divorce the experiment from the all encompassing effects of gravity.

This does however, fit in with quantum mechanics idea of energy of the vacuum.

If energy can spontaneously appear from the vacuum it is doing so outside of time.  In which case it must be instantaneous.  Therefore, it follows that outside of a gravitational field a photon travels instantaneous.

[yor_on]

Mike, your ideas are not mainstream. Neither are they backed up by any evidence. Place them in New Theories, not here.

[JP]

This does however, fit in with quantum mechanics idea of energy of the vacuum.

If energy can spontaneously appear from the vacuum it is doing so outside of time.  In which case it must be instantaneous.  Therefore, it follows that outside of a gravitational field a photon travels instantaneous.

This does not fit with quantum mechanics or any other mainstream theory.  If it's your own theory, please state it as such and don't mislead people into thinking it's mainstream.  If it's your own theory, please keep it to the New Theories section, as yor_on suggested.

[MikeS]

I have taken your advice and opened this in under new theories.  'What is the speed of light outside of a gravitational field?'

[Ken Hughes]

Johann,

My understanding of "The metric expansion of space" is of an abstract mathematical phenomena and not necessarily of a real event. Certainly there is no velocity involved since all velocity in our universe is measured against space itself, or at least some reference frame within space or space-time. Space, may or may not "expand", although I don't see how "expansion" is an appropriate word or idea within our universe.
I guess this has been answered differently and more traditionally with the co moving reference frame argument.

What I wanted to ask was a basic question about relativity connected to this thread.
Is the "expansion" of space derived from GR or simply implied from our redshift observations and, do we believe it is a real effect or just an abstract mathematical tool which gives us the right answers when using relativity theory?

[yor_on]

No Ken, relative motion is always defined relative objects moving, not relative space. Although it will be the 'space' metrics that change, when defining those objects moving relative each other. And the metric expansion of space is related to the way all those objects seems to be accelerating from everyone else, as confirmed through the red shift and brightness of distant supernovae. But it could be wrong of course, there might be some other explanation that we haven't realized yet. Still, it's no weirder than we existing at all