0 Velocity?

[Saint]

If you are in a set of inertial frames the Lorentz transform/velocity addition applies and none of the speed will exceed c. If you want to see anything with c>v you have to be in a situation where GR applies and have space expanding.

 

Does this imply that light gets a "free ride" on the universal expansion wave, so to speak? And is there any proof that space itself is expanding, or is that simply a way of rationalizing speeds greater than c with relativity?

[swansont]

Does this imply that light gets a "free ride" on the universal expansion wave, so to speak? And is there any proof that space itself is expanding, or is that simply a way of rationalizing speeds greater than c with relativity?

 

The redshift of galaxies (part of it, anyway) indicates motion away from us. Further galaxies generally move faster.

[Spyman]

Does this imply that light gets a "free ride" on the universal expansion wave, so to speak? And is there any proof that space itself is expanding, or is that simply a way of rationalizing speeds greater than c with relativity?

I would say it's more like the photons have to swim against the stream. If the star emitting the light is distant enough, it will be outside our "event horizon" and the photons will be "pushed" backwards faster than they can "swim" and thus will never reach us.

[Saint]

The redshift of galaxies (part of it, anyway) indicates motion away from us. Further galaxies generally move faster.

 

But the redshift doesn't require that "space" expand, only that the galaxies are moving through space, correct?

 

As far as redshift, or blueshift is concerned let me ask another question. Let's say a star puts out a beam of light (simplified for the example) at some frequency, let's call it X. This would mean that an stationary observer wrt the star would see X wave peaks per second. Now, when the observer moves toward the star, he will start to see more (X+) wave peaks per second. Effectively shrinking the wavelength - blueshift. The star did not start putting out a different frequency, but the observer is seeing a different frequency. He is moving throught the light beam at a higher rate, and therefore encountering more wave peaks per second. Does this not suggest that the observer is has increased his rate of travel wrt to the light beam?

[Spyman]

But the redshift doesn't require that "space" expand, only that the galaxies are moving through space, correct?

That is correct as long as the galaxies doesn't exceed the speed of light, which current redshifts imply.

 

Here is a link to "Space expanding FTL": http://www.scienceforums.net/forums/showthread.php?t=9693

 

Quote from a small part of a good post by Martin in that thread:

it is not unusual for astronomers to observe galaxies which were receding away from us faster than the speed of light at the time when they emitted the light which is now reaching us

[swansont]

But the redshift doesn't require that "space" expand' date=' only that the galaxies are moving through space, correct?

[/quote']

 

You're right; I was fixed on the motion >c part and didn't answer the whole question. The expansion evidence includes the microwave background.

[Saint]

That is correct as long as the galaxies doesn't exceed the speed of light' date=' which current redshifts imply.

 

Here is a link to "Space expanding FTL": http://www.scienceforums.net/forums/showthread.php?t=9693

 

Quote from a small part of a good post by Martin in that thread:[/quote']

 

Thanks for the link. The problem I have with the idea that space is expanding is that know one seems to know what space is. How can you possibly say that space is expanding, or that a galaxy is not moving THROUGH space, if you can't define space to begin with? This goes back to my question to Swansont. If there is no definition of space, the only reason to posit that it is expanding is to rationalize the apparent observations that there are things in the universe moving away from each other at FTL speed with the ideas put forth in GR.

 

Or to simplify the question - if there were no limit on the speed of physcial objects in the universe, would there be reason to propose that "space" itself is expanding?

[Johnny5]

Or to simplify the question - if there were no limit on the speed of physcial objects in the universe' date=' would there be reason to propose that "space" itself is expanding?[/quote']

 

There is no speed limit on it.

 

Regards

[Spyman]

Or to simplify the question - if there were no limit on the speed of physcial objects in the universe, would there be reason to propose that "space" itself is expanding?

Well there are more reasons which also all are part of Big Bang theory.

(Without expansion i guess BB would fall...)

 

I don't know all of them but these Two should be the most important:

 

1. Why are all far away galaxies moving away from us ?

(If it was just normal speed through space it should be more like 50/50 against/away.)

 

2. Why does galaxies further away move away faster than those closer ?

(Again if it was just normal speed their speed should be randomly distribiuted.)

 

Here is another link to a thread I started: http://www.scienceforums.net/forums/showthread.php?t=9729&page=1&pp=20

[swansont]

There is no speed limit on it.

 

And the evidence to support this is...?

[Saint]

And the evidence to support this is...?

 

Swansont - I think that the evidence is clearly there to support the idea that physical objects can, and do, move away from each other at speeds greater than c. At least if the doppler shift rule is to be trusted.

 

The burden of proof should really fall on the claim that those objects are not moving "through space". Or, that space itself is expanding. At least that's my take.

 

One thing - I don't really understand how the expansion of space itself could result in any doppler shift. It may take longer for light to reach us if it's swimming upstream, as described by Spyman, but if light isn't otherwise affected by space (that is, space is not the "aether"), it shouldn't result in any doppler shift.

[Spyman]

One thing - I don't really understand how the expansion of space itself could result in any doppler shift. It may take longer for light to reach us if it's swimming upstream, as described by Spyman, but if light isn't otherwise affected by space (that is, space is not the "aether"), it shouldn't result in any doppler shift.

Consider a long part of space with a light beam going through it, if that part of space gets streached will the light wave inside break up in small pieces or also be streached ?

[Saint]

Consider a long part of space with a light beam going through it, if that part of space gets streached will the light wave inside break up in small pieces or also be streached ?

 

That's why I inserted the comment about how light is not supposed to interact with "space". If it did, space would become the ellusive aether, wouldn't it?

[Saint]

Consider a long part of space with a light beam going through it, if that part of space gets streached will the light wave inside break up in small pieces or also be streached ?

 

Let me further clarify - I don't think there is a mechanism by which "space" can pull a light beam apart. That would require some frictional force wouldn't it? I've never heard of light interacting with space in that manner.

[swansont]

Swansont - I think that the evidence is clearly there to support the idea that physical objects can, and do, move away from each other at speeds greater than c. At least if the doppler shift rule is to be trusted.

 

But that is not the same as limit on the speed of physcial objects though the difference is perhaps subtle and may be only due to lack of proper definition.

 

I take "limit on the speed of a physical object" to be a local, flat-spacetime restriction. An object may be receding from us at superluminal speeds, but a local observer (to that object) would never see any such behavior. Conversely, we would never observe such behavior locally.

[Spyman]

Let me further clarify - I don't think there is a mechanism by which "space" can pull a light beam apart. That would require some frictional force wouldn't it? I've never heard of light interacting with space in that manner.

That's funny, I have "hunted" this very same "friction" in my thread, (link in post #34), without even consider that part of it. If You haven't read it You really should. I reacted on the part that expansion brings matter with it without friction.

[Saint]

But that is not the same as limit on the speed of physcial objects though the difference is perhaps subtle and may be only due to lack of proper definition.

 

I take "limit on the speed of a physical object" to be a local' date=' flat-spacetime restriction. An object may be receding from us at superluminal speeds, but a local observer (to that object) would never see any such behavior. Conversely, we would never observe such behavior locally.[/quote']

 

I'm confused here - if we (on earth) see a distant galaxy, redshifted to the degree that would require a superluminal speed of separation, I see that as strongly suggesting that the earth and that galaxy (both physical objects) are moving away from one another at superluminal speeds.

 

The question then is: are they separating FTL in space, or is space itself expanding FTL?

[swansont]

I'm confused here - if we (on earth) see a distant galaxy' date=' redshifted to the degree that would require a superluminal speed of separation, I see that as strongly suggesting that the earth and that galaxy (both physical objects) are moving away from one another at superluminal speeds.

 

The question then is: are they separating FTL in space, or is space itself expanding FTL?[/quote']

 

We can do local experiments that confirm that the objects could not be moving faster than c if space were not expanding. So when superluminal speeds are observed, it's evidence in favor of expanding space.

[geistkiesel]

But that is not the same as limit on the speed of physcial objects though the difference is perhaps subtle and may be only due to lack of proper definition.

 

I take "limit on the speed of a physical object" to be a local' date=' flat-spacetime restriction. An object may be receding from us at superluminal speeds, but a local observer (to that object) would never see any such behavior. Conversely, we would never observe such behavior locally.[/quote']

Why would we not be able to observe the radiation from objects moving at superluminal speeds from, us? Assume that such a supermoving object were one light year from us when it radiated a photon pulse. What would stop the light from reaching us? Is not there a postulate of light that assures us that the motion of light is independent of the motion of the source of the light?

 

Geistkiesel

[swansont]

Why would we not be able to observe the radiation from objects moving at superluminal speeds from' date=' us? Assume that such a supermoving object were one light year from us when it radiated a photon pulse. What would stop the light from reaching us? Is not there a postulate of light that assures us that the motion of light is independent of the motion of the source of the light?

 

Geistkiesel[/indent']

 

In inertial space, yes. But AFAIK expanding space does not qualify.

[geistkiesel]

In inertial space, yes. But AFAIK expanding space does not qualify.

Is there some experimental result that confirms the statement that in expanding space emitted light would forever be excluded from reaching any arbitrary point in space?

 

I suppose I should ask: Is there any experimental results establishing the fact of "expanding space"? If so where? This in itself would assign mechanical properties to space would it not? Then what are some, or any, mechanical attributes of space: density, for instance, in kg/cm^3?

 

Geistkiesel

[J.C.MacSwell]

Is there some experimental result that confirms the statement that in expanding space emitted light would forever be excluded from reaching any arbitrary point in space?

 

I suppose I should ask: Is there any experimental results establishing the fact of "expanding space"? If so where? This in itself would assign mechanical properties to space would it not? Then what are some' date=' or any, mechanical attributes of space: density, for instance, in kg/cm^3?

 

Geistkiesel[/indent']

 

The experiments were done some time ago past Andromeda and we're expecting results soon. Any Millenium now.

[swansont]

Is there some experimental result that confirms the statement that in expanding space emitted light would forever be excluded from reaching any arbitrary point in space?

 

I suppose I should ask: Is there any experimental results establishing the fact of "expanding space"? If so where? This in itself would assign mechanical properties to space would it not? Then what are some' date=' or any, mechanical attributes of space: density, for instance, in kg/cm^3?

 

Geistkiesel[/indent']

 

Since you couldn't observe such light, by definition, there really can;t be direct evidence for it. But AFAIK it's consistent with the rest if the observations and with the theory.

 

Recessional velocities and the cosmic microwave background are evidence. Recessional velocities were observed, the CMB was predicted and then measured.

[J.C.MacSwell]

Since you couldn't observe such light' date=' by definition, there really can;t be direct evidence for it. But AFAIK it's consistent with the rest if the observations and with the theory.

 

Recessional velocities and the cosmic microwave background are evidence. Recessional velocities were observed, the [b']CMB was predicted [/b]and then measured.

 

And from what I understand (or misunderstand) this was not predicted by steady state models, in fact I think it was predicted to not be there.

 

Why would steady state space have no temperature/CMBR?

[bascule]

The CMB was predicted and then measured.

 

The CMB was predicted, but before the group that predicted it had the opportunity to finish constructing the equipment they needed to observe it, it was discovered by accident by AT&T when they launched the first microwave communication satellites and noticed an annoying crackling in the background. They hired Penzias and Wilson to locate the source of the problem, which they did using the Horn Antenna at the Crawford Hill Bell Labs, and the rest is history.