[BlackHole]

Special relativity sais that no physical signal (or no energy) can pass through space faster than light. This means that a peak can move faster than light speed (Four billion km/h) but the total energy of the pulse does not. Therefore Einstein's relativity is preserved and causality is not violated. Signals also get weaker and more distorted the faster they go, so in theory no useful information can get transmitted at faster-than-light speeds.

But if so why is it said (within the context of general relativity which generalizes special relativity) that space itself can expand 50 times the speed of light? Is it possible that the distance between two objects is so great that the distance between them expands faster than the speed of light (locally)?

[Syd]

Noone can give You the answer untill we will know what space is. If it's some form of energy it can't go faster than c. But if its made of sth we don't even how to name yet, who knows...

[Martin]

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...

But if so why is it said (within the context of general relativity which generalizes special relativity) that space itself can expand 50 times the speed of light? Is it possible that the distance between two objects is so great that the distance between them expands faster than the speed of light (locally)?

the expansion of space doesnt enter into special rel (space in special rel is unable to expand, which makes special rel only an approximation good locally but not over long distances)
so the special theory does not impose any restriction on expansion

when space expands NO SIGNAL IS BEING SENT it just means that the distance is increasing between stationary objects that are already a long ways apart

the Hubble law says recession speed of objects (at the present time) is proportional to their present disance from us, and the ratio of (present) speed to distance is called the "Hubble parameter" H₀

(some people call H₀ the Hubble "constant" but it is not constant over time so astronomers are gradually getting away from calling it that. it confuses people to call something a constant that is always changing).

Lots of the stuff which astronomers observe is currently more than 13.8 billion lightyears from us and is therefore receding faster than light. When something that distant emitted the light we are now getting from it, it would have been considerably closer but that is another issue.

the first hurdle to get over is that the measured value of H₀
is 71 km/s per Megaparsec, which translates to c at 13.8 billion LY.

that means that something whose distance from us is half 13.8, or
6.9 billion LY, is receding at c/2 (half the speed of light)
and something whose distance from us is twice 13.8, or
27.6 billion LY, is receding at 2c (twice the speed of light)

there are more questions to ask about this and more conceptual hurdles to get over. other people will probably jump in here and say more. keep asking!

If you want to read something about this there is a good popular article by Lineweaver and Davis in the March 2005 SciAm

[Johnny5]

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that means that something whose distance from us is half 13.8, or
6.9 billion LY, is receding at c/2 (half the speed of light)
and something whose distance from us is twice 13.8, or
27.6 billion LY, is receding at 2c (twice the speed of light)

Just out of curiousity, if the conclusion is that something is receding from us at twice the speed of light, doesn't that alone falsify SR?

We are in one place in the universe, and something else is simultaneously somewhere far far away, a distance D as measured by our trusty rest ruler.

If that thing is receding from us FTL, then that falsifies SR does it not?

I am aware that the light we see now from it, in our local region of space, was emitted eons ago in the past, but I presume that someone who knows mathematics, correctly extrapolated its current speed relative to us.

Unless they didn't do that. So help me out here Martin.

[[Tycho?]]

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Just out of curiousity, if the conclusion is that something is receding from us at twice the speed of light, doesn't that alone falsify SR?

We are in one place in the universe, and something else is simultaneously somewhere far far away, a distance D as measured by our trusty rest ruler.

If that thing is receding from us FTL, then that falsifies SR does it not?

I am aware that the light we see now from it, in our local region of space, was emitted eons ago in the past, but I presume that someone who knows mathematics, correctly extrapolated its current speed relative to us.

Unless they didn't do that. So help me out here Martin.

Look it up on google. I called Martin on that exact same thing months ago, but he proved me wrong.

[Johnny5]

'[Tycho?] said:

Look it up on google. I called Martin on that exact same thing months ago, but he proved me wrong.

Well if you understood that proof, tell it to me. I am also checking google right now.

Well I found this:

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According to the Hubble Law, two galaxies which are a distant D apart are moving away from each other at a speed HD where H is Hubble's constant. In that case two galaxies which are a distance greater than c/H apart are moving away from each other faster than the speed of light. This is quite correct. The distance between two objects can be increasing faster than light because of the expansion of the universe. However, it is meaningless to say that the universe is expanding faster than light because the rate of the expansion is measured by Hubble's constant alone which does not even have the units of speed.

The italicized part indicates the reasoning. The reasoning is wrong.

[Martin]

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Just out of curiousity, if the conclusion is that something is receding from us at twice the speed of light, doesn't that alone falsify SR?
...

Here is a link to the SFN astronomy reference thread
http://www.sciencefo...2965#post142965

Here are links to some stuff from the March 2005 SciAm


http://www.sciam.com...B7F0147_p40.gif
Can galaxies recede faster than light?

http://www.sciam.com...B7F0147_p42.gif
Can we see galaxies receding faster than light?

The above two links are to illustrated sidebars on the main article
called "Misconceptions about the Big Bang", by Charles Lineweaver
and Tamara Davis
http://www.sciam.com...52383414B7F0147

[Martin]

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

this means that the light, even tho aimed in our direction, initially got farther away from us (as if swept backwards by the expansion of space) but it made it to us nevertheless

I guess you could say this is the normal story because this would have happened with the light from any object that is now being observed with redshift z > 2 (actually even z=1.7 is already enough)

and galaxies are observed much farther away than z = 2, a bunch have been seen out to z = 6 and beyond. Volumewise, most of the observable universe is z > 2. So in that sense it is USUAL or typical for the light now being received to have begun life being swept back and getting farther from us.

the reason that light emitted from something receding FTL can eventually reach us is that the hubble parameter used to be so much greater. The expansion has slowed. So light that is initially swept back away from us can eventually begin to approach us. One of the SciAm pictures in those links tries to explain this visually.

A good way to begin to understand the basics of modern cosmology is to play around with one of the online calculators that will convert redshift into distance. An astronomy professor named Siobahn Morgan has a good one
so does Ned Wright, who teaches cosmology at UCLA, and he has a good FAQ.

Here are links to the calculators that I posted earlier in the SFN astronomy reference thread
http://www.sciencefo...56565#post56565

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Here are two good online cosmology calculators

Siobahn Morgan's
http://www.earth.uni...ogy/cosmos.html

and Ned Wright's
http://www.astro.ucl.../CosmoCalc.html

homepages for Morgan
http://www.earth.uni.edu/smm.html
and Wright
http://www.astro.ucl...ight/intro.html

To use Siobahn's calculator put Lambda = 0.73
Omega = 0.27
H = 71 (or leave her default value of H = 70, nearly the same)
those are the dark energy and the matter densities as fractions of rho crit,
and H is the present value of the Hubble parameter

then put in any redshift z,
like z =1 or 3 or 10 and it will tell you how far away the thing
was when it emitted the light we are now getting from it
and how far away it is now
and how fast it was receding then
and how fast it is receding now, at the present moment

[Martin]

vast reaches of space (and the galaxies contained there) are receding from us at speeds greater than c
and this does not contradict special relativity

nothing ever catches up with and passes a photon
no signal can overtake light
no little green men can drive their saucers past earth at superlight speeds

that is not what we are talking
all we are talking about is that some distances between regions of space are expanding FTL

nothing is traveling thru space FTL, no signal is being sent, no ship is sailing thru space FTL
nothing is being done that is forbidden by special rel.

recession speed is different, it is the rate that distances expand.
Special rel DOES NOT EVEN KNOW about expansion of space because the model of space in special rel is a rigid construct called "Minkowski space" that CANNOT EVEN EXPAND. It is not stretchy like the space in General Rel.
So special rel is not contradicted because it is irrelevant. It does not apply. the expansion of space is outside the special rel department.

[BlackHole]

Syd said:

Noone can give You the answer untill we will know what space is. If it's some form of energy it can't go faster than c. But if its made of sth we don't even how to name yet, who knows...

It's called 'dark energy' but we don't know what it is. Future work in particle physics and space missions to come might finally provide an answer. This is how the expanding universe looks according to the standard big bang model:

Martin said:

the expansion of space doesnt enter into special rel (space in special rel is unable to expand, which makes special rel only an approximation good locally but not over long distances) so the special theory does not impose any restriction on expansion

Special relativity does not apply to recession velocity so we are left with general relativity and the cosmological constant.

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when space expands NO SIGNAL IS BEING SENT it just means that the distance is increasing between stationary objects that are already a long ways apart. The Hubble law says recession speed of objects (at the present time) is proportional to their present disance from us, and the ratio of (present) speed to distance is called the "Hubble parameter" H0 (some people call H0 the Hubble "constant" but it is not constant over time so astronomers are gradually getting away from calling it that. it confuses people to call something a constant that is always changing).

Lots of the stuff which astronomers observe is currently more than 13.8 billion lightyears from us and is therefore receding faster than light. When something that distant emitted the light we are now getting from it, it would have been considerably closer but that is another issue.

the first hurdle to get over is that the measured value of H0
is 71 km/s per Megaparsec, which translates to c at 13.8 billion LY.

that means that something whose distance from us is half 13.8, or
6.9 billion LY, is receding at c/2 (half the speed of light)
and something whose distance from us is twice 13.8, or
27.6 billion LY, is receding at 2c (twice the speed of light)

there are more questions to ask about this and more conceptual hurdles to get over. other people will probably jump in here and say more. keep asking!

If you want to read something about this there is a good popular article by Lineweaver and Davis in the March 2005 SciAm

Thanks for the explanation Martin.

If the cosmos expands itself, new space is created. This means that before the big bang there was no space because the big bang was the explosion of space. Therefore if the galaxies are stationary and not moving through space, then the next question is: What is space?

Johnny5 said:

The italicized part indicates the reasoning. The reasoning is wrong.

In this case i think the reasoning in ok because space is really expanding itself but the galaxies are not moving through space. Strange but that's all it is... :confused:

[Johnny5]

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recession speed is different, it is the rate that distances expand.
Special rel DOES NOT EVEN KNOW about expansion of space because the model of space in special rel is a rigid construct called "Minkowski space" that CANNOT EVEN EXPAND. It is not stretchy like the space in General Rel.
So special rel is not contradicted because it is irrelevant. It does not apply. the expansion of space is outside the special rel department.

I have heard that SR is a special case of GR.

Here it sounds like you are saying that SR and GR are mutually exclusive, since in SR space cannot stretch, and in GR space can stretch.

Also, what do you mean when you say that space can stretch, I'd really like to know.

Kind regards

[Martin]

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... then the next question is: What is space?

...

I think that is a good question to be asking at the present time because in searching for a quantum theory of gravity people may be arriving at an improved notion of space (or of space and time)

I dont expect people will ever get a final answer to such a hard question, but our model of space does gradually evolve and get more realistic

newton 1680 space was like an absolutely rigid framework, a big piece of 3D graph paper that never changed and was the stage where things could move around

special rel 1905 space (called minkowski space) was also rigid, unaffected by anything, but it had tricky new rules of perspective that made the speed of light look the same to all observers------it was like newton space but with the transformations between moving observers fudged slightly.

general rel 1915 space was dynamic, stretchable, flexible, able to expand and contract. Its geometry could interact with matter. Geometry (distances, angles) was no longer merely a stage, but instead became part of the action.

However in 1925 we got a quantum theory of matter according to which the placement of matter was uncertain. Since space was in close contact with matter---each affecting the other----you would expect the geometric observables, like angle, length, area, volume to ALSO be quantum mechanical. You would expect spatial relations to be uncertain also, since matter was.

In the 1930s, top physicists, like Dirac, began to search for a quantum theory of spacetime geometry (since our understanding of gravity is based on spacetime geometry, the grav. field is essentially how space is shaped, this is also called "quantum theory of gravity").
So far no quantum theory of spacetime geometry (no "quantum gravity" theory) has reached the point of making unambiguous testable predictions.

there is a lot of loose talk about this but the fact is that there are theories under construction but no mature test-ready theory.

when there is such a theory it will probably change our ideas about the nature of space, about how space interacts with matter at microscopic scale, about what happens to space and matter when a star collapses (i.e. how we think about conditions at the center of a black hole), about the conditions of the big bang and the causes of inflation.

that is, when we get a better theory of space (a "quantum theory of gravity") it will change some basic models of nature in fundamental ways.

Any scientific theory must predict the outcomes of future experiments or else it is meaningless. the quantum theories of spacetime that are being developed should be tested before being given credibility.
The first possible test is by the GLAST satellite in 2007. Lee Smolin has bet his brand of QG on GLAST observing a slight deviation in the speed of very high energy gamma rays. this is pretty audacious. if the effect is observed it will not automatically prove his QG is right, but if it is not observed then it will refute his Loop type QG (i.e. shoot it down). that is traditionally how science is played. Einstein published GR in 1915 and it predicted a certain bending angle, which Eddington looked for in 1919. he found it. if he had not found it that would have shot GR down.

I hope that GLAST is not cut from the NASA budget because it is the only
scheduled experiment that will test one of the candidate QG theories.

Anyway, your question is "what is space" and that is a fundamental question and the answer has changed over the centuries. Maybe we are now at a time in history when the answer will get a little better.

Smolin had a popular article in the January 2004 Scientific American called "Atoms of Space and Time".

[BlackHole]

It's because general relativity generalizes special relativity. Special relativity is a "special" case because it deals with a special case of costant motion in a straight line (without any gravitational effects). Also according to the hot big bang model, space must expand with time.

[Johnny5]

BlackHole said:

Also according to the hot big bang model, space must expand with time.

Does space have inertia?
Can I push off the vacuum?

[Martin]

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I have heard that SR is a special case of GR.

SR is a local approximation. Like a piece of paper can be a very good local approximation to a rubber balloonskin.
the approximation will be very good as long as the balloonskin is not too curved, and as long as the patch you are trying to approximate is not too large, and as long as the balloon is not expanding or contracting too fast.

so SR can be a very good local and temporary approximation to reality.
Like a sailor's chart of the local waters. but you need to use judgement about how far to trust it. SR has limited applicability

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Also, what do you mean when you say that space can stretch, I'd really like to know.

in GR (and in the simplified version of GR called the Friedmann model used by cosmologists) the geometry of spacetime is represented by a METRIC which is a distance function that can tell you stuff like areas and angles and distances between two points and what is a geodesic path etc.

for space to expand all that has to happen is for the metric to be time-dependent, so that the distance between two points increases with time.

If you pick up a cosmology book and look at the Friedmann metric (also called FRW, Friedmann Robertson Walker metric) you will see the time dependence,

there is a scalefactor written a(t) or R(t) and it is an increasing function (arbitrarily taken to be equal 1 at present time) and it is plugged into the metric in such a way that distances increase in proportion to this scalefactor.

there are technical issues like knowing what you mean by a frame being at rest with respect to the cosmic microwave background, or at rest with respect to the hubble flow (the expansion process). so cosmologists have to deal with some issues in defining the FRW metric,

but the basic idea is a very simple one, that distances just increase

(you dont have to imagine anything pushing atoms of space apart or weird stuff like that, you dont have to imagine space is a material made of anything, it is much simpler: distances simply increase according to a certain formula, the FRW metric, and the model fits the data)

((dark energy acceleration is just a detail, the Friedmann model, with expansion goes back to around 1922 and has been fitting the data since
hubble in 1930s, dark energy was just a slight improvement in fit that happened in 1998. it is exciting but the main expansion model is much older))

[Johnny5]

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but the basic idea is a very simple one, that distances just increase

(you dont have to imagine anything pushing atoms of space apart or weird stuff like that, you dont have to imagine space is a material made of anything, it is much simpler: distances simply increase according to a certain formula, the FRW metric, and the model fits the data)

This was well said. Wasn't I just looking at the Friedman metric the other day, in another post? There was a link to a formula.

[BlackHole]

Martin said:

I think that is a good question to be asking at the present time because in searching for a quantum theory of gravity people may be arriving at an improved notion of space (or of space and time)

It's so true. We know that our space is not empty. According to QFT there is energy in the vacuum. With energy comes pressure, since pressure is just the work done to change the volume of a box. In this case the pressure is negative, since increasing the volume of the box increases the energy. Therefore if space-time has a positive energy, the expansion will accelerate. The present model predicts that the present universe should also be filled with neutrinos, fundamental particles with no mass or electric charge.

Quote

I dont expect people will ever get a final answer to such a hard question, but our model of space does gradually evolve and get more realistic

Another very hard question is what hapanned at and before Planck time (in the singularity).

Quote

newton 1680 space was like an absolutely rigid framework, a big piece of 3D graph paper that never changed and was the stage where things could move around

special rel 1905 space (called minkowski space) was also rigid, unaffected by anything, but it had tricky new rules of perspective that made the speed of light look the same to all observers------it was like newton space but with the transformations between moving observers fudged slightly.

general rel 1915 space was dynamic, stretchable, flexible, able to expand and contract. Its geometry could interact with matter. Geometry (distances, angles) was no longer merely a stage, but instead became part of the action.

In GR space is dynamic but is it possible for anything physical to move in space-time?

Quote

However in 1925 we got a quantum theory of matter according to which the placement of matter was uncertain.

QFT (QED) and Heisenberg uncertainty principle. QCD was developed only after 1983.

Quote

Since space was in close contact with matter---each affecting the other----you would expect the geometric observables, like angle, length, area, volume to ALSO be quantum mechanical. You would expect spatial relations to be uncertain also, since matter was.

In the 1930s, top physicists, like Dirac, began to search for a quantum theory of spacetime geometry (since our understanding of gravity is based on spacetime geometry, the grav. field is essentially how space is shaped, this is also called "quantum theory of gravity").

So the gravitational field is basically the curvature of space-time?

Quote

So far no quantum theory of spacetime geometry (no "quantum gravity" theory) has reached the point of making unambiguous testable predictions.

there is a lot of loose talk about this but the fact is that there are theories under construction but no mature test-ready theory.

There is string theory but it is currently untestable.

Quote

when there is such a theory it will probably change our ideas about the nature of space, about how space interacts with matter at microscopic scale, about what happens to space and matter when a star collapses (i.e. how we think about conditions at the center of a black hole), about the conditions of the big bang and the causes of inflation.

True but we have yet to determine what prevailed before the big bang.

Quote

that is, when we get a better theory of space (a "quantum theory of gravity") it will change some basic models of nature in fundamental ways.

Any scientific theory must predict the outcomes of future experiments or else it is meaningless. the quantum theories of spacetime that are being developed should be tested before being given credibility.
The first possible test is by the GLAST satellite in 2007. Lee Smolin has bet his brand of QG on GLAST observing a slight deviation in the speed of very high energy gamma rays. this is pretty audacious. if the effect is observed it will not automatically prove his QG is right, but if it is not observed then it will refute his Loop type QG (i.e. shoot it down). that is traditionally how science is played. Einstein published GR in 1915 and it predicted a certain bending angle, which Eddington looked for in 1919. he found it. if he had not found it that would have shot GR down.

If LISA will not detect gravitational radiation (also called ripples in space-time), would it shoot down GR?

Quote

I hope that GLAST is not cut from the NASA budget because it is the only scheduled experiment that will test one of the candidate QG theories.

Anyway, your question is "what is space" and that is a fundamental question and the answer has changed over the centuries. Maybe we are now at a time in history when the answer will get a little better.

Smolin had a popular article in the January 2004 Scientific American called "Atoms of Space and Time".

What is space and time are the two fundamental questions. Maybe in the future time will not be relevant, only energy will.

[Johnny5]

BlackHole said:

Another very hard question is what hapanned at and before Planck time (in the singularity).

Exactly how does one compute the Planck time (sometimes I see the factor of 8pi in it, other times no), and more importantly, how does one interpret it?

Thank you

[Martin]

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If LISA will not detect gravitational radiation (also called ripples in space-time), would it shoot down GR?
...

I believe so. but I am not an expert so you must get confirmation from
tom mattson, swansont, severian etc.
I know that GR predicts waves and that one can estimate the energy of the waves that would come from a collapsing star, or the merger of two neutron stars, and I believe that the current detectors have been designed to be sensitive enough to detect waves of the expected energy.
so if waves are, in fact, NOT seen then something would be seriously wrong. either with the instrument, or the calculations, or the theory.
If practical error could be ruled out it would seem to me to refute GR. I do not think this will happen though.

[Johnny5]

What is LISA? What is a gravitational wave?

When I ask what is a gravitational wave, I guess I want to know what is waving (what medium does the wave travel in), what induces the wave, what is the wavespeed measured relative to, what governs the amplitude of the wave, and its frequency.

Are gravitons, and gravity waves mutually exclusive?