Understanding the Speed of Space in Relativity

[us40]

Hello, we know that according to special relativity nothing in the universe go faster than speed of light,but space can.Now according to E=mc^2 entity with mass can not reach speed of light and photon is mass less with mass=0.So it can reach speed of light so its meaning that space contain negative mass so it has speed greater than speed of light,and why special relativity does not put any constrain on speed of space.Thanks in advance.

 

[ZapperZ]

Hello, we know that according to special relativity nothing in the universe go faster than speed of light,but space can.

Please provide a source that says this.

Zz.

 

[us40]

http://en.wikipedia.org/wiki/Expanding_universe

In it metric expansion paragraphand

http://curious.astro.cornell.edu/question.php?number=575

 

[BruceW]

In general relativity, two distant objects can move away from each other at greater than the speed of light. (I think that is what you mean).

 

[BruceW]

In special relativity, (and also general relativity at short distances), objects with mass are always moving at less than the speed of light relative to each other. This is really an experimental fact, as far as I know. (although, you could use it as a postulate of special relativity if you wanted, I guess).

 

[us40]

@BruceW

Yes space is expanding greater than speed of light currently and after some (Billions) years we can not see speed of light from nearby galaxy clusters.So can we say that space contain negative mass so it posses speed greater than light?

 

[ZapperZ]

Hello, we know that according to special relativity nothing in the universe go faster than speed of light,but space can.Now according to E=mc^2 entity with mass can not reach speed of light and photon is mass less with mass=0.So it can reach speed of light so its meaning that space contain negative mass so it has speed greater than speed of light,and why special relativity does not put any constrain on speed of space.Thanks in advance.

http://en.wikipedia.org/wiki/Expanding_universe

In it metric expansion paragraph


and

http://curious.astro.cornell.edu/question.php?number=575

Then there are a few problems with your conclusion here that space "contain negative mass".

1. Why should something moving faster than c has negative mass? You can actually solve the SR equation for v>c and get, say, imaginary mass, not negative mass.

2. We actually know what negative mass do! In solid state physics, negative effective mass occurs often (i.e. in holes). They do not more faster than c.

Space isn't an object. It is a concept in which we frame location or position.

Zz.

 

[BruceW]

Yes space is expanding greater than speed of light currently and after some (Billions) years we can not see speed of light from nearby galaxy clusters.So can we say that space contain negative mass so it posses speed greater than light?

'not really' is my answer.

edit: I think this is a case of putting two and two together and getting five. don't be disheartened, it's good to keep thinking.

 

[phinds]

us40, your problem is that you do not understand the difference between MOVING at some rate and RECEDING at some rate. Look up "metric expansion". Space is not MOVING at > c, objects IN space are receding from each other faster than c, which is not at all the same thing.

 

[my2cts]

us40, your problem is that you do not understand the difference between MOVING at some rate and RECEDING at some rate. Look up "metric expansion". Space is not MOVING at > c, objects IN space are receding from each other faster than c, which is not at all the same thing.

I also do not understand this difference. Please enlighten me.

Thanks

 

[dauto]

It's very simple. Suppose space expanded and doubled in size after 1 billion year. An object that was 1 lightyear away before will be 2 lightyears away afterward, and receded at the rate of 1 lightyear per billion year. Another object that was at 1 billion lightyear away before is 2 billion lightyears away afterwards. That is, it recedes at a rate of 1 billion lightyear per billion year = c!. An object that was more than a billion year away receded faster than the speed of light. How is that possible. That's possible because we are talking about comoving coordinates in a cosmological scale. That is not you garden variety distance between objects.

 

[my2cts]

It's very simple. Suppose space expanded and doubled in size after 1 billion year. An object that was 1 lightyear away before will be 2 lightyears away afterward, and receded at the rate of 1 lightyear per billion year. Another object that was at 1 billion lightyear away before is 2 billion lightyears away afterwards. That is, it recedes at a rate of 1 billion lightyear per billion year = c!. An object that was more than a billion year away receded faster than the speed of light. How is that possible. That's possible because we are talking about comoving coordinates in a cosmological scale. That is not you garden variety distance between objects.

So the object at 1 billion ly moves at v=c away from us. An object further away would move faster than c relative to us. That contradicts SR.

 

[BruceW]

In general relativity, two distant objects can move away from each other at greater than the speed of light. (I think that is what you mean).

this.

 

[my2cts]

this.

So SR appears to be not applicable or so.
I have another question. At what scale of space does the expansion take effect?
I do not believe a hydrogen atom or the solar system are expanding. What about the local cluster of galaxies? Is it expanding or is the expansive motion thermalized by mutual exchange of momentum by gravitation?

 

[phinds]

us40 and my2cts, Google "metric expansion" for a discussion.

 

[Drakkith]

So SR appears to be not applicable or so.
I have another question. At what scale of space does the expansion take effect?
I do not believe a hydrogen atom or the solar system are expanding. What about the local cluster of galaxies? Is it expanding or is the expansive motion thermalized by mutual exchange of momentum by gravitation?

Any objects bound together through gravity or one of the other fundamental forces of nature do not expand/recede. Gravity finally falls off enough for expansion to occur somewhere around the Supercluster level, which is on the scale of millions of light-years. Clusters and superclusters of galaxies are the largest "structures" that can stay together. Past that, things are receding from each other.

 

[my2cts]

Following phinds' advise I read on wikipedia, the first google hit:
"metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere"
The distance between two non-moving objects can increase, even with v>c.
That is not understandable.

 

[my2cts]

Any objects bound together through gravity or one of the other fundamental forces of nature do not expand/recede. Gravity finally falls off enough for expansion to occur somewhere around the Supercluster level, which is on the scale of millions of light-years. Clusters and superclusters of galaxies are the largest "structures" that can stay together. Past that, things are receding from each other.

That makes more sense. But how could Hubble have discovered the expansion if this is true?

 

[Drakkith]

That makes more sense. But how could Hubble have discovered the expansion if this is true?

We can measure the redshift of spectral lines in distant supernovas to determine that they are receding from us. It turns out that the further away a supernova was, the more redshifted its spectrum was. Lots of observations and math have led us to believe that this redshit is the result of the expansion of the universe causing objects to recede from us.

I'd elaborate, but I have to head to class.

 

[phinds]

Following phinds' advise I read on wikipedia, the first google hit:
"metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere"
The distance between two non-moving objects can increase, even with v>c.
That is not understandable.

I assume you mean YOU do not understand it. Since many of us do understand it, perhaps you should give it some more thought.

 

[my2cts]

I mean a cluster of galaxies interacts at a timescale of millions of years, which should short enough to wipe out the effect of a big bang having occurred more than 10 billion years ago.

 

[my2cts]

I assume you mean YOU do not understand it. Since many of us do understand it, perhaps you should give it some more thought.

If you understand it so well, and thanks for distorting my typeface, then you are the perfect individual to explain this once and for all to the rest of us, thoughtless, people.

 

[my2cts]

I assume you mean YOU do not understand it. Since many of us do understand it, perhaps you should give it some more thought.

POerhaps you should also take a look at http://en.wikipedia.org/wiki/Hubble_constant#Redshift_velocity
The Fizeau-Doppler formula tells me that z goes to infinity when v goes to c.
No v>c needed.
How's that for "giving it some more thought".

 

[BruceW]

Following phinds' advise I read on wikipedia, the first google hit:
"metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere"
The distance between two non-moving objects can increase, even with v>c.
That is not understandable.

hehe, this is the problem when you want to write a short article for the layperson, without using any specific terminology.

 

[my2cts]

hehe, this is the problem when you want to write a short article for the layperson, without using any specific terminology.

Please elaborate what the problem is.

 

[BruceW]

to get a good understanding, a longer explanation is required, and specific terminology for the subject must be used. relativity is an awesome subject, I think you will enjoy learning about. I'm still learning too, I've only just started learning about general relativity really.

 

[my2cts]

to get a good understanding, a longer explanation is required, and specific terminology for the subject must be used. relativity is an awesome subject, I think you will enjoy learning about. I'm still learning too, I've only just started learning about general relativity really.

You are assuming that I am a lay person, but I am not. A good place to study GRT is Landau&Lifshitz. Down to earth, if that can apply to GRT!
Do not forget to read about the Fizeau-Doppler formula, as it resolves the v>c paradox.

 

[BruceW]

that uses the equation for the Doppler effect in a flat spacetime. at very large distances, you also need to take into account general relativistic expansion of the universe.

edit: I think this page will be the most helpful: http://en.wikipedia.org/wiki/Comoving_distance

 

[my2cts]

that uses the equation for the Doppler effect in a flat spacetime. at very large distances, you also need to take into account general relativistic expansion of the universe.

edit: I think this page will be the most helpful: http://en.wikipedia.org/wiki/Comoving_distance

On this page light itself can go faster than c. Goodbye special relativity.

 

[BruceW]

yeah, it gets pretty crazy, once we lose the assumptions of inertial reference frames. one thing that is guaranteed, is that locally, light moves at c in vacuum.

 

[ZapperZ]

On this page light itself can go faster than c. Goodbye special relativity.

If you think so, then you'd better stop flying and using your GPS, among other things.

This thread has gone quite off-topic. Please look at the OP.

Zz.

 

[Tenunbow]

Following phinds' advise I read on wikipedia, the first google hit:
"metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere"
The distance between two non-moving objects can increase, even with v>c.
That is not understandable.

This is where thought experiments become invaluable, the easiest way to explain is to imagine an expanding soap bubble, if you blow a bubble inside that first bubble it will attach itself to the original, now imagine blowing billions of bubbles inside the original. After that, what you end up with is the original outside expanding bubble with trillions of interconnected bubbles inside. Now imagine that each interconnected surface has a number of objects attached to them. Every object will be moving away from the other objects as each surface expands, even though the objects themselves are not moving, but the expansion of everything is being driven by the original expanding bubble. Now change the bubbles to space-time and the objects to galaxies and you have the answer to how the distance can increase between non-moving objects.

 

[my2cts]

If you think so, then you'd better stop flying and using your GPS, among other things.

This thread has gone quite off-topic. Please look at the OP.

Zz.

Actually I do not think so but I am quoting from a page pointed out in a post.
And I believe this thread is still on topic.

 

[ZapperZ]

Hello, we know that according to special relativity nothing in the universe go faster than speed of light,but space can.Now according to E=mc^2 entity with mass can not reach speed of light and photon is mass less with mass=0.So it can reach speed of light so its meaning that space contain negative mass so it has speed greater than speed of light,and why special relativity does not put any constrain on speed of space.Thanks in advance.

Actually I do not think so but I am quoting from a page pointed out in a post.
And I believe this thread is still on topic.

Read the OP again, there is a severe misunderstanding here. This isn't just a topic about the expansion of space. If it is, it belongs in the astrophysics forum. Rather, it is the assertion that the apparent super luminal expansion of space is DUE to a "negative mass" of space! It is VERY specific!

Zz.

 

[Drakkith]

I mean a cluster of galaxies interacts at a timescale of millions of years, which should short enough to wipe out the effect of a big bang having occurred more than 10 billion years ago.

I'm not quite sure what you mean here. What effects are being wiped out?

 

[Tenunbow]

Hello, we know that according to special relativity nothing in the universe go faster than speed of light,but space can.Now according to E=mc^2 entity with mass can not reach speed of light and photon is mass less with mass=0.So it can reach speed of light so its meaning that space contain negative mass so it has speed greater than speed of light,and why special relativity does not put any constrain on speed of space.Thanks in advance.

The simple answer is that a negative mass would have the same constraints as a positive mass, neither of which can travel faster than c.
The difference is that a positive mass will move with the force applied and the negative mass will move opposite to the force applied.

 

[my2cts]

I'm not quite sure what you mean here. What effects are being wiped out?

I mean that thermalisation would wipe out the memory of a specific state of motion at a certain timescale. That timescale should follow from dynamics.

 

[my2cts]

Read the OP again, there is a severe misunderstanding here. This isn't just a topic about the expansion of space. If it is, it belongs in the astrophysics forum. Rather, it is the assertion that the apparent super luminal expansion of space is DUE to a "negative mass" of space! It is VERY specific!

Zz.

I agree. But this thread is at least partly about superluminal expansion.

 

[dauto]

So the object at 1 billion ly moves at v=c away from us. An object further away would move faster than c relative to us. That contradicts SR.

No it doesn't. You are missing a very very very important point here. The distance we're are talking about here is the comoving distance. That means the distance is not measured the same way it would usually be measured in special relativity and any receding rate cannot be directly used to find how fast an object is moving with respect to another. Note that that is possible to do even within Special relativity. There is no need to use General relativity.

 

[my2cts]

No it doesn't. You are missing a very very very important point here. The distance we're are talking about here is the comoving distance. That means the distance is not measured the same way it would usually be measured in special relativity and any receding rate cannot be directly used to find how fast an object is moving with respect to another. Note that that is possible to do even within Special relativity. There is no need to use General relativity.

I am not missing the point. I use Fizeau-Doppler. GRT tells me that the rate of change of distance is not related to velocity and frankly I do not know what to think of that.

 

[BruceW]

I agree with 2cts, that we only get objects moving at relative velocities greater than c because we are using general relativity. I can't think of any situation in special relativity where two objects would have relative velocity which is greater than c...

edit: I guess it depends on where you draw the line between special relativity and general relativity...?

 

[us40]

The simple answer is that a negative mass would have the same constraints as a positive mass, neither of which can travel faster than c.
The difference is that a positive mass will move with the force applied and the negative mass will move opposite to the force applied.

This link may help to understand my view:

http://www.livescience.com/38533-photons-may-emit-faster-than-light-particles.html

 

[BruceW]

It essentially is saying that if photons were not massless, then other objects could move faster than photons. Yep, but that's not very interesting in my opinion. c would still be the upper speed limit, and relativity would still work the same, but photons would be massive particles instead.

 

[ZapperZ]

This link may help to understand my view:

http://www.livescience.com/38533-photons-may-emit-faster-than-light-particles.html

You need to be very careful here. The paper in question (not this news report) is similar to the investigation on the UPPER LIMIT of photon mass. It does NOT imply that it has a mass. The same goes with this particular study. It is saying that if a photon has a mass, then it must have a lifetime and decay to other particles. So they estimated the LOWER LIMIT of the decay lifetime of a photon. It doesn't mean that they have actually found it! Furthermore, look at the lifetime that they computed, and compare that to the age of the universe!

a visible wavelength photon in our reference frame would be stable for 10^18 years or more

There's a lot of "If's" here and there's a lot of "sensationalistic noise" that you have to overcome to get the actual physics.

Zz.

 

[us40]

It essentially is saying that if photons were not massless, then other objects could move faster than photons. Yep, but that's not very interesting in my opinion. c would still be the upper speed limit, and relativity would still work the same, but photons would be massive particles instead.

What I trying to say is...
Photon is quantum of light.Now in loop quantum gravity(candidate theory for quantum gravity) physicist create quantum of space.(http://en.wikipedia.org/wiki/Quantum_loop_gravity).
Now say that quantum of space spaceon(sorry for wired name). Now according to article I have referenced in previous post if particle is lighter than photon it can go with greater velocity.So spaceon can go faster than speed of light.If photon has positive mass spaceon has mass somewhat lesser than photon and if photon has zero mass mass of spaceon is negative.That is my point.

 

[ZapperZ]

What I trying to say is...
Photon is quantum of light.Now in loop quantum gravity(candidate theory for quantum gravity) physicist create quantum of space.(http://en.wikipedia.org/wiki/Quantum_loop_gravity).
Now say that quantum of space spaceon(sorry for wired name). Now according to article I have referenced in previous post if particle is lighter than photon it can go with greater velocity.So spaceon can go faster than speed of light.If photon has positive mass spaceon has mass somewhat lesser than photon and if photon has zero mass mass of spaceon is negative.That is my point.

But what exactly is the physics that allows you to say that if a particle has v>c, it mass must be negative?

Look at tachyons, for example. One can arrive as the mass of these hypothetical particles quite directly from SR. See, for example:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html

Look at the criteria for M^2 to be negative. That is why I said earlier about mass being "imaginary" in this case. Nowhere in here is the requirement that mass has to be "negative" for this particle to have v>c!

Again, to repeat, we know and already have particles with negative mass. They have the opposite behavior as regular particles in a gravitational field, etc. But they certainly do not have superluminal behavior.

Zz.

 

[BruceW]

Now say that quantum of space spaceon(sorry for wired name). Now according to article I have referenced in previous post if particle is lighter than photon it can go with greater velocity.So spaceon can go faster than speed of light.If photon has positive mass spaceon has mass somewhat lesser than photon and if photon has zero mass mass of spaceon is negative.That is my point.

The article is saying if the photon has mass, then another particle might go with greater velocity. This is possible because this other particle could still have velocity less than c, but greater than the speed of the photon. The main thing is that massless particles move at c, and particles with mass move at less than c.

A (hypothetical) particle which travels at greater than c is called a tachyon, and would have imaginary mass, as ZapperZ was saying. Also, about the 'spaceon' I'm not sure what you mean. I don't know anything really about quantum gravity. But I think in string theory, a 'graviton' is predicted (which is a massless particle). But from what I read in the wikipedia page on quantum loop gravity, there is no particle like the graviton which is fundamental to quantum loop gravity.

 

[TurtleMeister]

Again, to repeat, we know and already have particles with negative mass. They have the opposite behavior as regular particles in a gravitational field, etc. But they certainly do not have superluminal behavior.

Zz.

underline by me

Sorry to butt in, but I would like to read more about this. Do you have a link or source?

 

[Dale]

What I trying to say is...
Photon is quantum of light.Now in loop quantum gravity(candidate theory for quantum gravity) physicist create quantum of space.(http://en.wikipedia.org/wiki/Quantum_loop_gravity).
Now say that quantum of space spaceon(sorry for wired name). Now according to article I have referenced in previous post if particle is lighter than photon it can go with greater velocity.So spaceon can go faster than speed of light.If photon has positive mass spaceon has mass somewhat lesser than photon and if photon has zero mass mass of spaceon is negative.That is my point.

OK, this is just getting weirdly speculative. Even in theories where the volume operator is discrete that doesn't mean that there are spaceons that travel anywhere. Please stop just making stuff up.

Also, your question wrt negative mass has been answered. To recap: the article you cited was talking about the possibility of photons having some non-zero (positive) mass. If that were true then photons would not travel at c and particles with (positive) mass could travel faster than photons but still slower than c. Hypothetical particles which would go faster than c are called tachyons and would have imaginary mass, not negative mass.

Thread closed.