# Understanding the Speed of Space in Relativity

• us40
Please explain.So the object at 1...billion lightyears away receded faster than the speed of light. Please explain.f

#### 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.

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.

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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).

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).

@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?

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 [Broken]

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.

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us40 said:
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?

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.

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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.

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

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.

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.

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.

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?

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

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.

"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.

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?

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.

"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.

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 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.

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".

"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.

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.

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.

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!

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.

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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.

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

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.

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.

Zz.

"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.

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

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.

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.

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?