Difference between expansion of space and objects just moving apart?

[ThomasT]

I read that the expansion of our universe is the expansion of space itself. Apparently, the phrase, expansion of space itself, is meant to characterize the observed displacements of very large scale cosmological structures as an artifact of the isotropic stretching or expanding or some unknown physical structure called "space" (thus continually creating new space), and to distinguish this sort of intrinsic expansion from an isotropic stretching or expansion of large scale cosmological structures into an already existing space.

I imagine that the very large scale isotropic expansion (and therefore also the very large scale structure of the universe) is due to kinetic energy imparted during some sort of (extrinsic) Big Bang event and not a function of (intrinsic) wave interaction, and that at smaller scales it is wave interaction intrinsic to our universe (in the forms of gravity, electricity, magnetism, weak force, strong force, etc.) that is the structural determinant.

My question is, if the ontological status of "space" is unknown, then how is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?

 

[kmarinas86]

I read that the expansion of our universe is the expansion of space itself. Apparently, the phrase, expansion of space itself, is meant to characterize the observed displacements of very large scale cosmological structures as an artifact of the isotropic stretching or expanding or some unknown physical structure called "space" (thus continually creating new space), and to distinguish this sort of intrinsic expansion from an isotropic stretching or expansion of large scale cosmological structures into an already existing space.

I imagine that the very large scale isotropic expansion (and therefore also the very large scale structure of the universe) is due to kinetic energy imparted during some sort of (extrinsic) Big Bang event and not a function of (intrinsic) wave interaction, and that at smaller scales it is wave interaction intrinsic to our universe (in the forms of gravity, electricity, magnetism, weak force, strong force, etc.) that is the structural determinant.

My question is, if the ontological status of "space" is unknown, then how is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?

Question: Is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?

 

[ThomasT]

Question: Is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?

I don't know, but many apparently knowledgeable people speak as though it is known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium. That is, they speak as though the apparent large scale expansion is intrinsic (ie., due to wave interaction) rather than extrinsic (ie., due to kinetic energy imparted via Big Bang).
Is there any way to tell the difference from the available data?

 

[marcus]

...My question is, if the ontological status of "space" is unknown, then how is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?

You need to provide some concrete evidence like this: a link to some professional writing by a professional cosmologist where he or she asserts that space is expanding.

Then we can look at what they actually said and see what is going on. Kmarinas has already challenged you to do this.

It is not enough to say "many people talk as if they think such and such" because we don't know who they are or what they actually said that gave you the impression.

Maybe they were just talking popularization in the mass media, from which nothing can be deduced. In pop sci talk people necessarily use figurative expressions because the audience is unfamiliar with the basic math.

What would be interesting is if you could find us a link to some professional communication between experts which makes this kind of assertion----and backs up your allegation.

===========================
for my own part, I never say space expands.
I am a big fan of mainstream cosmology--the standard LCDM model. I follow research and scan for new papers of interest on a regular basis. I don't recall anybody saying space expands but maybe I just didnt notice, or forgot.

I believe in certain things---I accept certain premises as valid. I can tell you the main ones if you are interested.

A. the CMB (cosmic microwave background)

B. the concept of being approximately at rest with respect to CMB----i.e. temperature nearly the same in all directions

C. merely as a mathematical construct---observers in other galaxies also at CMB rest, like us. Our situation is not exceptional, in that respect. However I don't suppose that conscious being actually exist They may or may not.
I simply think of the universe as full of such observers so I can consider how things would look from their standpoint as well.

D. the construction of the present moment. all those observers for whom the CMB is the same temperature as it is, now, for us belong to the present moment. this construction is approximate. it gives an idea of an approximate universal now.

E. the approximate validity of Hubble Law, from the standpoint of every contemporary observer at CMB rest. It is a kind of Copernican assumption, we arent exceptional. this law as I have always seen it stated depends implicitly on a universal present moment. it says the present distance to a remote object is increasing (at this moment) at a rate which is proportional to the present distance. In other words Hubble Law deals with where an object is now. So it has implicitly built in some of what I was talking about in points A thru D, or the equivalent.

Maybe one of the professionals or grad students will find something wrong with what I just said, Thomas, which is fine with me. It would be interesting . Or maybe you have some question or disagreement. I have to go now but will check in in a few hours or tomorrow.

 

[Wallace]

As marcus explain, 'the expansion of space' is a phrase used mainly in the populisation of cosmology, rather than 'at the coal face' as such. It certainly doesn't describe a physical theory, i.e. the idea that space is a viscous medium dragging galaxies apart. It is nothing more than a metaphor.

 

[jonmtkisco]

I see it frequently STRONGLY IMPLIED in professional cosmology and GR books and papers, as a sort of postulate of General Relativity, that the increase in physical distance between galaxies is a mandatory precursor to any increase in the volume of space "as a whole". In other words, that the empty vacuum we see between galaxies MUST RESULT FROM the movement of galaxies away from each other; and that yes, in effect, the galaxies are NOT moving through a pre-existing volume (finite or infinite) of empty vacuum.

Conversely, I am not aware of ANY body of mainstream cosmology/GR books or technical papers which say that it is the mainstream view that galaxies are merely moving through pre-existing empty vacuum, and that if the scale of the matter/radiation universe is finite, then a vast expanse of empty vacuum is believed to exist beyond the outer "edge" of the matter/radiation universe (assuming it's not infinite).

There seem to be a variety of reasonable objections to the idea that the movement of galaxies enables new space to "well up" or otherwise come newly into existence. Here's one objection to which I have never heard a satisfactory response:

If the physical "spreading out" of matter and radiation cause new volume of space to exist, then surely it is not the spreading out of the galaxies themselves that performs this task. Presumably in every quasi-local frame everywhere in the universe, relativistic particles are "spreading apart" more rapidly than any galaxies or other non-relativistic matter are. In a quasi-local frame galaxies are constrained to recede from each other at non-relativistic velocities. Relativistic particles, such as the CMB photons, the pre-CMB neutrinos, or even the light and cosmic rays emitted by galaxies must be "spreading apart" at a dramatically faster pace. Therefore, if the movement of matter/radiation causes new volume of space to exist, an insignificantly tiny percentage (maybe none) of that space we see today was created by the movement of galaxies; the galaxies were preceded "everywhere" by relativistic particles. The galaxies are merely "backfilling" a small fraction of the volume already "created by" the movement of relativistic particles.

So the simple question is, why can't we observe the enormous volume of space created by the spreading apart of these pioneering relativistic particles? If this new volume of space were "welling up everywhere" between galaxies, at relativistic "recession velocities", then either (a) galaxies in every quasi-local frame would be forced to recede from each other at relativistic recession velocities (obviously this isn't the case), or (b) the newly created volume of empty vacuum would be "flowing past" the slow moving galaxies at relativistic speeds. The latter picture seems absurd, e.g., where is the "flowing" space going? Even if it were true, it would mean that as galaxies recede from each other at locally pokey speeds, they are in fact moving into pre-existing space, they are not causing new space to be created. Because the new space must have arrived before the galaxies did.

EDIT: It seems unreasonable for a number of reasons to draw a distinction, that the movement of "massive" particles causes new space to be created, while movement of "zero rest mass" particles does not. For example, most cosmic rays are protons, so they are indeed massive particles even if they are spreading apart at very high (but not entirely relativistic) velocities.

In sum, the more one pursues this whole line of argument, the more absurd it seems. Why shouldn't we just adopt the seemingly reasonable proposition that, regardless of whether the universe is finite or infinite, the increasing volumes of space between galaxies already existed SOMEWHERE before the galaxies moved into that space?

The emphasis of the mainstream approach strikes me as being based on little more substance than the old paradox: if a tree falls in the forest and no one hears it, was there a sound?

Jon

 

[yuiop]

You need to provide some concrete evidence like this: a link to some professional writing by a professional cosmologist where he or she asserts that space is expanding.
Then we can look at what they actually said and see what is going on. Kmarinas has already challenged you to do this.

It is not enough to say "many people talk as if they think such and such" because we don't know who they are or what they actually said that gave you the impression.

Maybe they were just talking popularization in the mass media, from which nothing can be deduced. In pop sci talk people necessarily use figurative expressions because the audience is unfamiliar with the basic math.

What would be interesting is if you could find us a link to some professional communication between experts which makes this kind of assertion----and backs up your allegation...

I think sketchtrack really puts his finger on the matter in his post here https://www.physicsforums.com/showthread.php?t=242647&page=10 where he says "I have always been told the universe is expanding faster than C. Isn't the expanding space rather than moving galaxies thing just to satisfy the hypothesis that mass cannot move faster than C?"

This is backed up by the John Baez Physics FAQ http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#13 on the question of examples of apparent superluminal motion
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"Expansion of the Universe
According to Hubble's Law, two galaxies that are a distance D apart are moving away from each other at a speed HD, where H is Hubble's constant. So this interpretation of Hubble's Law implies that two galaxies separated by a distance greater than c/H must be moving away from each other faster than the speed of light. Actually, the modern viewpoint describes this situation differently: general relativity takes the galaxies as being at rest relative to one another, while the space between them is expanding. In that sense, the galaxies are not moving away from each other faster than the speed of light; they are not moving away from each other at all! This change of viewpoint is not arbitrary; rather, it's in accord with the different but very fruitful view of the universe that general relativity provides. So the distance between two objects can be increasing faster than light because of the expansion of the universe, but this does not mean, in fact, that their relative speed is faster than light.
-----------------------------------------------------------------

Now as far as I know John Baez is not a crank but I am not sure if he qualifies as a "professional writer or cosmologist" in your eyes as I have not researched his credentials. A search in the scholar section of google using the keywords "space itself is expanding"+universe pulled up 23 hits as listed here: http://scholar.google.co.uk/scholar...s+expanding"+universe&as_ylo=2003&btnG=Search The reader is invited to decide for him/herself if any of the papers in the list qualify as peer reviewed papers published in a respectable journal as defined by PF rules.


The point is that questions about "why superluminal recession velocities do not contradict Special Relativity" is often explained away by suggesting that receding galaxies are at rest with the local space in the large scale "expanding space". If the expanding space is a myth as you suggest then an alternative explanation is required for why superluminal recession velocities do not violate Special Relativity. It is not good enough to say SR is not violated because space itself is expanding and have a subclause (but space is not expanding) in the small print. That is having your cake and eating it

 

[George Jones]

If the expanding space is a myth as you suggest then an alternative explanation is required for why superluminal recession velocities do not violate Special Relativity. It is not good enough to say SR is not violated because space itself is expanding and have a subclause (but space is not expanding) in the small print. That is having your cake and eating it

Why should special relativity have anything to say about this situation?

 

[hellfire]

You need to provide some concrete evidence like this: a link to some professional writing by a professional cosmologist where he or she asserts that space is expanding.

Charles Lineweaver does so in "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe". I think he is authoritative enough and this is a very important paper on this topic. Personally I would prefer "expansion of the universe" (that Lineweaver uses also), because one may change coordinates to conformal time and space to have a space-time expanding instead of an expanding space only, but in the usual time coordinate space expands and the usage of the term is justified. I do not see why this should be a term that should not be used.

 

[ThomasT]

Thanks to all repliers for your comments, suggestions, links, references, etc.

 

[marcus]

Thanks to all repliers for your comments, suggestions, links, references, etc.

You are welcome, Thomas! I hope the links and discussion have resolved the issue for you. As a reminder of what we are supposed to be talking about, here is your first post.

I read that the expansion of our universe is the expansion of space itself. Apparently, the phrase, expansion of space itself, is meant to characterize the observed displacements of very large scale cosmological structures as an artifact of the isotropic stretching or expanding or some unknown physical structure called "space" (thus continually creating new space),..

This sounds to me as if you were imagining space as something physical in the sense of substance or material---that would require being created so that the amount of it could grow. I don't think that is what cosmologists have in mind. So I asked to see some casewhere a professional cosmologist talking to colleagues (not to popular media) said "space expands"---because we can look at a definite example in context and see what is going on. What is actually meant by the phrase.

What resulted is instructive. Hellfire came up with an example: a 2003 article by Lineweaver and Davis. They use the phrase "expansion of space" and it is clear from context that it is just a shorthand for increase of proper distance in the conventional model.

Here is the article which Hellfire pointed to:
http://arxiv.org/abs/astro-ph/0310808

I think you can be reassured that there is no implication here of an "unknown physical structure called space" which is stretching or swelling up or growing new substance. Not in the sense of a physical entity more of which would need to be created as it grew. I will quote a large piece of the context so you can see from context what they mean. I'm pretty sure you will agree that all they mean is simply that a bunch of distances are increasing.
==quote==

3.1 Misconception #1: Recession velocities cannot exceed the speed of light

A common misconception is that the expansion of the Universe cannot be faster than the speed of light. Since Hubble’s law predicts superluminal recession at large distances (D > c/H) it is sometimes stated that Hubble’s law needs special relativistic corrections when the recession velocity approaches the speed of light [App. B: 6–7]. However, it is well-accepted that general relativity, not special relativity, is necessary to describe cosmological observations. Supernovae surveys calculating cosmological parameters, galaxy-redshift surveys and cosmic microwave background anisotropy tests, all use general relativity to explain their observations. When observables are calculated using special relativity, contradictions with observations quickly arise (Section 4). Moreover, we know there is no contradiction with special relativity when faster than light motion occurs outside the observer’s inertial frame. General relativity was specifically derived to be able to predict motion when global inertial frames were not available. Galaxies that are receding from us superluminally are at rest locally (their peculiar velocity, v_pec = 0) and motion in their local inertial frames remains well described by special relativity. They are in no sense catching up with photons (v_pec = c). Rather, the galaxies and the photons are both receding from us at recession velocities greater than the speed of light.

In special relativity, redshifts arise directly from velocities. It was this idea that led Hubble in 1929 to convert the redshifts of the “nebulae” he observed into velocities, and predict the expansion of the universe with the linear velocity-distance law that now bears his name. The general relativistic interpretation of the expansion interprets cosmological redshifts as an indication of velocity since the proper distance between comoving objects increases. However, the velocity is due to the rate of expansion of space, not movement through space, and therefore cannot be calculated with the special relativistic Doppler shift formula...
==endquote==

Now we've got a definite example of someone using the phrase, let's see if there is any confusion about what they are talking about. Or if anybody has questions.

Other people may disagree but the way it looks to me is that the main trouble in the original post is with the interpretation ("Apparently the phrase... is meant to characterize...") Judging from the example that Hellfire turned up, the phrase is not meant to characterize what you say.

 

[yuiop]

Why should special relativity have anything to say about this situation?

Special Relativity has something to say because if a distant galaxy is moving through space it should be subject to time dilation. Modern cosmology does not include SR time dilation so therefore modern cosmology does not consider distant galaxies to be moving through space but to be moving with space.

In SR an object is constrained to move at less than c relative to the observer while in GR the velocity of objects are constrained to move at c relative to the local space and that includes velocities that are greater than c relative to a distant observer. For example if a distant galaxy that is locally at rest with the CMB but moving at 5c relative to us on Earth, then any object or photon local to that galaxy is constrained to move at 4c<v<6c relative to us on Earth. Some might argue that motion is constrained to +/- c relative to other local objects but the fact is that objects the same distance as the our galaxy from Earth are constrained to 4c<v<6c even if they are in a void away from any other objects. That suggests that the constraint on velocity is the local space and not local objects or distant observers.

A further complication is this. If a value of zero for Omega(mass) and a value of zero for Omega(Lambda) is plugged into the FLRW metric then the answers are very different from those of SR even though those values suggest no mass or cosmological constant. Shouldn't SR apply to universe with no mass or gravity or cosmological constant? What exactly is dragging a photon outwards in the early stages of its voyage in a universe with no gravity or cosmological constant?

...Lineweaver states: " Galaxies that are receding from us superluminally are at rest locally."

Distant galaxies are at rest locally with respect to what? I could guess they mean that the galaxy is at rest with the local CMB but I am sure the CMB has no physical effect on the motion of the galaxy and the CMB is not an aether that objects move with respect to. So my final guess as to what the galaxy is at rest with respect to is the local space and the local space is not at rest with our local space. This gives a physical quality to space. The local space near the galaxy is moving relative to our local space and presumably the space inbetween is expanding.

...
In special relativity, redshifts arise directly from velocities. It was this idea that led Hubble in 1929 to convert the redshifts of the “nebulae” he observed into velocities, and predict the expansion of the universe with the linear velocity-distance law that now bears his name. The general relativistic interpretation of the expansion interprets cosmological redshifts as an indication of velocity since the proper distance between comoving objects increases. However, the velocity is due to the rate of expansion of space, not movement through space, and therefore cannot be calculated with the special relativistic Doppler shift formula...
==endquote==

So there we have it. A professional cosmologist saying "the velocity is due to the rate of expansion of space, not movement through space". That clearly implies that the velocity of a receding galaxy is due to it being dragged along by the expansion of space and that the velocity of a receding galaxy is not due to its own momentum taking it through space. He also suggested that the galaxies are at rest locally with the space. In other words the distant galaxies are stationary objects embedded in space and it appears to be moving relative to us because the space it is embedded in is moving relative to us. This is not just a philosophical difference in interpretation because the maths is different. If the object is embedded in comoving space then there is no SR time dilation in the calculations. Modern cosmology does not include time dilation in the redshift calculations so there is no motion relative to space.

 

[marcus]

...A professional cosmologist saying "the velocity is due to the rate of expansion of space, not movement through space". That clearly implies that the velocity of a receding galaxy is due to it being dragged along by the expansion of space and that the velocity of a receding galaxy is not due to its own momentum taking it through space. ...

You have your own way of thinking about it.
It is probably useless to try to communicate my point of view to you but I'll make a brief try.
For me distances exist. But space has no physical objective reality (several einstein quotes to this effect).

For me, the Hubble Law describes the regular increase of distances. (on average largescale pattern) To call that "expansion of space" is merely figurative, a metaphor, a figure of speech----as I said before a shorthand expression. Professionals occasionally use it, but context tells us what they actually mean.

I try not to say "expansion of space" because it gives noobs the impression that space is a THING with some kind of objective physical reality. The mistaken impression that, as you said, space is so substantial that it can drag stuff along .

That is a mistake: it is not a material. It is not a thing. It cannot expand because it does not exist. It cannot drag galaxies around (as you suggested) because it does not exist. What exists are events and relations between events, such as distance. These are things which we can observe.
Distances can increase.

but I see no reason why you shouldn't talk to your heart's content about space as if it were a material and capable of expanding and dragging. You use whatever turns of phrase you like. what I am saying is that I try not to use that phrase, even as a shorthand or metaphor, because it seems to mislead people. that is my personal choice.

 

[hellfire]

I find your view very reasonable marcus. But I thnik that one can also interpret it in another way: space or spacetime is a metric, which in turn represents distances, and this turns "space expands" into "distances expand" that should be more inline with your view.

 

[George Jones]

A further complication is this. If a value of zero for Omega(mass) and a value of zero for Omega(Lambda) is plugged into the FLRW metric then the answers are very different from those of SR even though those values suggest no mass or cosmological constant. Shouldn't SR apply to universe with no mass or gravity or cosmological constant?

No, the answers are the same as SR. It just that in the no mass no cosmological constant scenario, standard cosmological coordinates reduce to unusual (but valid) coodinates for a patch of Minkowski spacetime. See the last few posts in the thread

https://www.physicsforums.com/showthread.php?p=1757333#post1757333.

 

[oldman]

... It is nothing more than a metaphor.

A metaphor is 'a figure of speech in which a phrase is used to stand for something else '. So says my dictionary. The issue here seems to be that in this case the nature of the 'something else' confuses many clever folk. You have company, Thomas T. It incudes popularisers (say Brian Greene) and, as pointed out in this thread, John Baez and Charles Lineweaver, not to speak of others in this thread.

I'm not very clever, and have been severely confused for quite a while by what is written about the ongoing global change that seems to be a feature of our universe. I think that Wallace's advice is sensible. I take it to be: assume that the 'expansion of space' is a metaphor for change that is difficult to comprehend, and leave it at that. But some folk, like Marcus here, have well-founded opinions that go beyond accepting a metaphor. And I also have a bit more to say about the 'something else':

Expansion is a familiar phenomenon that can be directly measured with laboratory apparatus or radar on a local scale; examples are the expansion of a heated rod or a gas, the expansion of the Earth-Moon axis (3.3 cm/yr) and the just-discovered contraction (negative expansion) of the planet Mercury. Sadly, we cannot so directly establish that the distant parts of the universe are 'expanding' away from us, or receding kinematically .

What cosmologists have done is to invent a description of how the universe behaved in the past, as if time ran backwards so that expansion becomes contraction. This quantitative invented description is firmly based on the red-shift, as described by GR. It suggests that the universe was once hot and dense, and that relic radiation from this era must exist. Because the CMB does exist it is widely accepted that charge in the universe is indeed something like 'the expansion of space'. There is of course a mountain of supporting evidence for this view.

But claiming that space itself does expand, just like a familiar local-scale phenomenon, is going too far. It is rather like saying that space is curved, which is a metaphor that (esoterically) stands for the shape of geodesics in the spacetime of GR. But the metaphor does not stand for the shape of sufficiently rigid measuring rods. Such rods do not have to partake fully of the distortions of spacetime mandated by gravity, since their shapes are ruled by much stronger interactions of nature.

The smoke and mirrors here is that if such distortion is uniform expansion, the requirement of rigidity falls away. This is why the metaphor of "space expanding' is flawed.

But I agree that it's about the best we can do, Wallace. Perhaps one should leave the mysteries to the philosophers.

 

[yuiop]

Expansion is a familiar phenomenon that can be directly measured with laboratory apparatus or radar on a local scale; examples are the expansion of a heated rod or a gas, the expansion of the Earth-Moon axis (3.3 cm/yr) and the just-discovered contraction (negative expansion) of the planet Mercury.

You raise an interesting point here. How do we distinguish between the moon moving away from us at 3.3cm/yr and the moon being stationary while the distance between the moon and Earth increases (expands) at 3.3cm/yr? At the end of the day is it all just about semantics?

Sadly, we cannot so directly establish that the distant parts of the universe are 'expanding' away from us, or receding kinematically...

There are some reasons cosmologists prefer the expanding space rather the receding kinematically picture. One is that the luminosity distance of some objects appears to be as much 47GLyrs while the age of the universe is around 13.7 Gyrs. This does not seem compatible with a model of static space where recession velocities would be limited to the speed of light. However it should be noted that redshift does not directly tell us recession velocities. You can only calculate a recession velocity when you have first chosen a model. It is probably not widely recognised that Special Relativity predicts a dimming of luminosity that makes objects to appear to be much further away by a factor of (1+z)^2 than their proper distance at the time of emmision bringing distances more in line with comoving expanding space model. I personally believe that if you add in a large inflation factor at the beginning, you can dispose of the expanding space and accelerating expansion but I am still working on the maths to prove that.

 

[jonmtkisco]

Hi kev and oldman,
The inability to distinguish which of two fundamentally opposed theories is valid and which is invalid cannot reasonably be described as semantics or philosophy. If one theory is valid, the other is invalid; they can't both be valid at the same time, and one of them is fundamentally wrong. No philosophy or careful wording can change that.

Our present inability to rule out either of them is frustrating for us, but by no means is it the end of the story. If we're still doing astronomy and physics in future decades, eventually we should expect to develop the technological and analytical capability to rule one of them out.

If the Hubble flow of galaxies causes new vacuum space to "well up" between galaxies, then don't we need to inquire into the physical mechanism which allows that to occur?

At what scale of granularity does this "new" vacuum insert itself between the existing vacuum and particles? Above or below the Planck scale? Why doesn't the "new" vacuum appear between gravitationally or electromagnetically bound particles that periodically move slightly apart, thereby preventing them from subsequent returning to their former, closer spacing?

At the instant "new" space is created, can we categorically exclude the possibility that any adjacent electromagnetic radiation wave will physically extend into the "new" space -- and if so, doesn't this require a modification of the Heisenberg uncertainty principle?

Must we conclude that only Hubble motion causes expansion of space, while peculiar motion does not? If so, then what physical aspect is fundamentally different as between Hubble motion and peculiar motion? Do the same concepts of momentum and inertia apply to both kinds of motion? Do we try to answer this question through quantum mechanics? Or do we content ourselves with the knowledge that GR spacetime geometry precisely describes the difference mathematically, and just leave the physical causation of the difference to forever remain a metaphysical mystery?

Jon

 

[marcus]

Who says that new vacuum space wells up between galaxies?

As I understand the Hubble flow, nothing new is created. It simply refers to distances changing in a certain way.

If the Hubble flow of galaxies causes new vacuum space to "well up" between galaxies, then don't we need to inquire into the physical mechanism which allows that to occur?

At what scale of granularity does this "new" vacuum insert itself between the existing vacuum and particles? Above or below the Planck scale? Why doesn't the "new" vacuum appear between gravitationally or electromagnetically bound particles, thereby "pushing them" apart?

...

It seems to me that you are thinking of space as a thing. Involved in a physical process of new vacuum inserting itself. Vacuum pushing vacuum apart.

I don't understand the role of the quote-marks. Who is being quoted? None of this makes sense to me.

At the instant "new" space is created, can we categorically exclude the possibility that any adjacent electromagnetic radiation wave will physically extend into the "new" space -- and if so, doesn't this require a modification of the Heisenberg uncertainty principle?

Who says that new space is created? Why attribute reality to something where there is no evidence of it?

All we have in the Hubble flow is a case of distances changing between stationary objects. General relativity tells us this is natural and to expect it. Change is just something that distances do (in accordance with the field equation, initial conditions, and the influence of matter).

Einstein got over this hangup already in 1915 when he said the principle of general covariance deprives space and time of the last shred of physical reality.

I wish you could apply your considerable rhetorical skills to helping others get over the (materialising of space) hangup, rather than making it worse.

 

[jonmtkisco]

Hi Marcus,
The terminology about space "welling up" is from one of your and my favorite https://www.physicsforums.com/archive/index.php/t-175900.html", on this subject, "Expanding Space: the Root of all Evil?" 7/07 by Francis, Barnes, James & Lewis"

Remembering that the FRW metric describes a homogeneous universe filled with a fluid of uniform density, and assuming that test observers can measure their velocity with respect to that fluid, we can now describe the formal statement of the phenomenon we refer to as expanding space:

The distance between observers at rest with respect to the cosmic fluid increases with time.

Since two bodies, both at rest with respect to the fluid defining the FRW metric, find the distance between them has increased after a certain time interval, it seems sensible to suggest that there is more space between them than there was previously. It may be misleading to suggest that the space that was there stretched itself as the universe expanded. Perhaps a better description, in simple terms, is to suggest that more space appeared, or ‘welled up’ between the two observers, however this is a largely semantic distinction.

They go on to conclude:

This description of the cosmic expansion should be considered a teaching and conceptual aid, rather than a physical theory with an attendant clutch of physical predictions. We have demonstrated the power of this pragmatic conceptualisation in guiding understanding of the universe, particularly in avoiding the traps into which we can be lead without rigorous recourse to general relativity.

I have no problem with the ambiguity of the conclusion, since we don't know the answer, but I think it's worth focusing on what physical mechanism the reference to new space "welling up" is intended to invoke. If the answer is that none was actually intended, fine, but then why don't we just explain the FLRW model as "a cloud of self-gravitational gas and matter expanding through space as a result of something which simultaneously imparted a range of initial momenta to its constitutent particles?" Why the mystical, metaphysical references to new space being created? I fail to see how such references are helpful to either amateurs or professionals if indeed they are intended to be gratuitous.

I'm trying to avoid coming across as merely disagreeable. But at the end of the day, space itself either is, or is not, expanding physically. We can help clear away a lot of hocus-pocus pixie dust from the general understanding of cosmology if we simply describe both alternative physical theories, and explain what the physical implications are of each. On the other hand, in my opinion, using the concept of expanding space as a teaching tool in some contexts while denying any reliance on it in other contexts is confusing at best, and disingenuous at worst. And saying that "it doesn't matter" fundamentally defies my sense of scientific curiosity.

However, I can't disagree that of late this has been the most "politically correct" course.

Jon

 

[marcus]

Hi Marcus,
The terminology about space "welling up" is from one of your and my favorite https://www.physicsforums.com/archive/index.php/t-175900.html", on this subject, "Expanding Space: the Root of all Evil?" 7/07 by Francis, Barnes, James & Lewis"

...I fail to see how such references are helpful to either amateurs or professionals if indeed they are intended to be gratuitous.

You have to take that up with Francis Barnes James & Lewis. Its a matter of explanatory wording, almost style you could say, and ideas of what will be helpful to non-math readers.

Your problems are more interesting, frankly, and I think they stem from a false dichotomy you constantly present and may unalterably believe (I can't tell.)
That is, you present an either-or choice which doesn't reflect the true range of possibility.

But at the end of the day, space itself either is, or is not, expanding physically...

So far we don't have an ontology of space. So statements like that involve speculative premise---which is all right with me: I am all for speculation as long as it is conscious and advertised as such.

The people who are farthest along with a working ontology are the Utrecht group and you would do well to read their July SciAm article carefully---also look at their illustrations of fractals on page 48.

A fractal is self-similar. It has no atoms. It can adopt any size without apparent change.
=============

In the regular man-in-street imagination, if something gets larger then either it is getting more atoms stuffed in, or it is somehow stretching. It is intuitive to think of the structure getting new units pumped in, new material, or the same amount of material becoming more tenuous and stretched out.

But there are plenty of space-filling mathematical objects which aren't like that.

Loll's group was not looking for fractal structure. They just set up a kind of minimalist computer simulation of quantum spacetime and did lots of runs and studied the results. And they found hints of fractal structure down at very small scale (as the dimension of spacetime goes down from 4 to around 2, it gets more and more fractally).
Surprising finding. Just hints of what might be, of course. No conclusions yet.

Have to go

 

[yuiop]

...
Einstein got over this hangup already in 1915 when he said the principle of general covariance deprives space and time of the last shred of physical reality...

I don't think we should let go of a concept of physical reality so readily. That makes life too easy for people trying to sell vehicles that run on water only, or machines that produce endless free electricty from vacuum energy.

 

[marcus]

sounds like you have it backwards, Kev

I don't think we should let go of a concept of physical reality so readily. That makes life too easy for people trying to sell vehicles that run on water only, or machines that produce endless free electricty from vacuum energy.

I don't understand, Kev. In what way do you think Einstein was mistaken? I think he was exceptionally rigorous in how he thought about physical reality. He kept track of who was doing the observing, and how they measured. He wasn't apt to be fooled by words and abstractions, into reasoning with unobservable and unmeasurable quantitities.

So he figured out that spacetime points were phony. They had no physical reality.
He struggled with this for about three years 1912-1915 even while in a race with David Hilbert to discover Gen Rel. It was a forced mathematical conclusion----from the general covariance of the theory.

A lot has been written about this. Basically it arises from Einstein keeping his knives sharp. Keeping his ideas well-defined.

Nowadays everybody and his brother is realizing that space and time are probably not fundamental but are emergent: that they are not hardrock foundation physical reality, but are epiphenomena that show up at our human scale. Even David Gross says this is likely to be the case.

Modes of perception, ways we conceptualize, not the fundamental degrees of freedom that nature uses.

Einstein saw this first, 90 years before people like David Gross started saying it at every speech they make.

The idea of what is physically real has to be treated rigorously----be clear as you can about what is and what isn't. If you think it was Einstein who was mistaken about this, and not you, then you probably have the shoe on the wrong foot.

 

[jonmtkisco]

Hi Marcus,
I respect your many contributions to this Forum, which I think have been particularly helpful.

But suggesting that one is a "crackpot" for being reluctant to abandon any attachment to physical causality is more than a bit beyond the pale.

Can we please lower the rhetoric level and get back to studying observation, theory and logic.

Thank you,

Jon

 

[marcus]

Jon, thanks for the respect and the kind words about contributions. I think Kev was way off base in associating what Einstein was saying with people who promise to run your car on water, or generate free electricity. But I don't include you in that. My message to you is post #21 above and I hope you read it. It is some friendly advice about something you could read that might help you avoid getting into a false dichotomy.

Don't misunderstand me though. I am not suggesting that you BELIEVE the Utrecht new model.
But reading the July SciAm article on the emergence of smooth 4D spacetime (as quantum average, at macro scale) from a 2D (or 1.9-2.1) fractal microscopic substrate---reading that would help you avoid overly simple reasoning about space.

At this point several new mathematical models of what underlies space time and matter are emerging. The Utrecht model is only one. it is leading partly because they have been running computer simulations for several years now. creating small universes and studying them, walking around inside, gathering stats on their geometry.

The name of the game now is not to BELIEVE one model or another, but to avoid simplistic reasoning based on a narrow view of the options.

I have the link to the SciAm article in my signature, if you haven't read it yet.

 

[marcus]

some Einstein quotes

https://www.physicsforums.com/showthread.php?p=1386960#post1386960
the thread is called "Empty space is something?"
We've been over this ground before, you see :smile"
George Jones has some interesting quotes from Einstein, Feynman,...

I put in these

“Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität. ..."

“Thereby time and space lose the last vestige of physical reality”.

(To try to paraphrase,...space does not have physical existence, but is more like a bunch of relationships between events)

In case anyone wants an online source, see page 43 of this pdf at a University of Minnesota website
www.tc.umn.edu/~janss011/pdf%20files/Besso-memo.pdf[/URL]

==quote from the source material==
...In the introduction of the paper on the perihelion motion presented on 18 November 1915, Einstein wrote about the assumption of general covariance “[b]by which time and space are robbed of the last trace of objective reality[/b]” (“durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt werden,[/color]” Einstein 1915b, 831). In a letter to Schlick, he again wrote about general covariance that
“[b]thereby time and space lose the last vestige of physical reality[/b]” (“Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität.[/color]” Einstein to Moritz Schlick, 14 December 1915 [CPAE 8, Doc. 165]).
==endquote==

Both quotes are from Nov-Dec 1915, one being from a paper on perihelion motion. and the other from a letter to Moritz Schlick a few weeks later.

Ultimately the point is not who said it---we arent going on authority---but that it is based on a mathematical theorem (the socalled Hole Theorem) proven in the Gen Rel context. And Einstein did say it forcefully and clearly.

 

[oldman]

... Or do we content ourselves with the knowledge that GR spacetime geometry precisely describes the difference mathematically, and just leave the physical causation of the difference to forever remain a metaphysical mystery?
Jon

Not forever, I hope.

The metaphysical mystery --- what I have called smoke and mirrors --- is inherent in the very nature of GR. General Relativity is nothing more than a description of how gravity behaves. Despite it's mathematical sophistication and satisfying concordance with observation it tells us precisely zip about the mechanism by which mass/energy distorts space. GR doesn't 'explain' gravity any more than Newtonian physics does. Talk of how space 'wells up' or 'stretches' as the universe changes is just idle chatter, rather than an explanation of physical causation. We poor ignoramuses can't even define 'space' properly yet! We're in a bad way here!

The trouble with GR is that it describes phenomena that are quite alien to our everyday mesoscopic experiences. No wonder that a century after it was invented we're still arguing about how to interpret it in terms of everyday concepts, like expansion. In fact, we can't do this.

This is why it's better to accept Wallace's advice, and treat 'expanding space' as a metaphor for change we don't yet understand. Until we figure out how mass/energy (whatever this is) distorts space (whatever this is) we'll just have to put up with Nature's smoke and mirrors.

If that's not taking a philosophical view, I don't know what is.

 

[marcus]

The metaphysical mystery --- what I have called smoke and mirrors --- is inherent in the very nature of GR. General Relativity is nothing more than a description of how gravity behaves. Despite it's mathematical sophistication and satisfying concordance with observation it tells us precisely zip about the mechanism by which mass/energy distorts space. GR doesn't 'explain' gravity any more than Newtonian physics does. Talk of how space 'wells up' or 'stretches' as the universe changes is just idle chatter, rather than an explanation of physical causation. We poor ignoramuses can't even define 'space' properly yet! We're in a bad way here!
...

Good summary! Couldn't agree more.

Also I liked your tongue-in-cheek definition earlier---space is what I can swing a cat in.

The research where we can expect a mechanism by which mass/energy distorts space to appear is quantum gravity.
That is where some interesting ideas of what the fundamental degrees of freedom underlying space time and matter have been showing up. In a model where geometry and matter are both fundamentally the same thng, and arise from the same elementary dynamical description, then one will be able to act on the other.

I don't subscibe to any metaphysical mystery. It's clear. If you stay within Gen Rel, then spacetime doesn't exist, all there is is the gravitational field---which is to say the metric---which is to say distances.
And distances can be expected to change. Nobody ever said they wouldn't.

To get out of the classical Gen Rel context, you need a quantum version of General Relativity, and a successful quantum gravity has to answer that basic question of joining geometry and matter at the root and seeing how one can distort the other. should also get rid of the classical singularities and explain the cosmological constant.

Oldman, did you happen to read the Ambjorn Loll article in this month's SciAm? I'd be interested to know your reaction.
I think there has been some progress on the quantum gravity front.
the link is in my signature.

 

[oldman]

... How do we distinguish between the moon moving away from us at 3.3cm/yr and the moon being stationary while the distance between the moon and Earth increases (expands) at 3.3cm/yr? At the end of the day is it all just about semantics?

In this case 'expansion' is not quite appropriate. My apologies. It's the inter-centre-of-mass distance that increases, and this is measured with radar. I like the operational approach, and therefore believe that specifying the measurement method is all one need say. Then semantics becomes irrelevant?

...the luminosity distance of some objects appears to be as much 47GLyrs while the age of the universe is around 13.7 Gyrs... redshift does not directly tell us recession velocities. It is probably not widely recognised that Special Relativity predicts a dimming of luminosity that makes objects to appear to be much further away by a factor of (1+z)^2 than their proper distance at the time of emmision bringing distances more in line with comoving expanding space model..

Thanks, Kev. I didn't know this. But isn't there a problem with a model which uses SR, applicable only when spacetime is flat, to a universe where the accepted FLRW model has flat space sections but a curved spacetime?

 

[yuiop]

I don't understand, Kev. In what way do you think Einstein was mistaken? I think he was exceptionally rigorous in how he thought about physical reality. He kept track of who was doing the observing, and how they measured. He wasn't apt to be fooled by words and abstractions, into reasoning with unobservable and unmeasurable quantitities.

When Descartes said "I think, therefore I am" he highlighted the difficulty of proving the physical reality of anything including your own existence. So when we discuss physics we start with the assumption that we exist and build on the patterns we observe to build our own picture of physical reality. Now, I was not stating Einstein was wrong, but too loose an interpretation of "there is no physical reality" could encourage others to create over fanciful theories of physics. I will try to explain a working definition of "physical reality" in as much as we can declare anything to be "real".

In one description of the equivalence principle, Einstein said something like this. Imagine you turn the engines of your windowless rocket on and start accelerating. You can feel the acceleration and you can measure the acceleration with onboard accelerometers. Can you be absolutely sure your velocity is changing? The answer is no! A gravitational field may have sprung up behind you and your blasting engines are merely holding you in place preventing you being sucked into the gravitational body that has suddenly appeared behind you. Now if we only do this once, then it is just possible that what you observe can be explained by coincidence. But let us say you repeat the experiment a hundred times or a thousand times in different directions with different degrees of acceleration, you still would not be able to prove a new gravitational field sprang up each time you used your engines, but if you wanted to have some vestige of physical reality you could reason that it statistically unlikely or highly improbable that the coincidently appearing gravitational fields are the explanation and that it is more plausable that it is the engines causing the acceleration and that your velocity is actually changing. If the rocket had windows, an assumed view of physical reality would note that there is no physical gravitational body visible to explain the coincidental gravitational field and that each time you accelerate all the stars move in the opposite direction. You could reason that is more plausible that it is the rocket that accelerates each time you fire the rocket engines rather than the entire universe accelerating in the opposite direction. You could not prove that the it not the universe accelerating, but if you wanted to retain a remant of physical reality you could assume the accelerating universe is a less plausible explanation. Similar reasoning could be applied when you look up at the night sky and turn your head rapidly to one side. Is it more plausible that your head turned or that your head stayed still and the whole universe and your body rotated in the opposite direction?

Here is another example. Assume we have two hollow tubes of equal length, sealed with glass ends. One has a vacuum inside and the other has some sort of transparent medium inside. By carefully timing how long it takes a light signal to travel through the tubes we note it takes longer for a light signal to pass through the tube containing the transparent medium. There are two possible explanations for this observation. One is that the light travels slower in the medium than in a vacuum and the other is that the speed of light is constant in any medium and that the tube with medium inside it has "more distance" inside it than the tube with a vacuum inside it. The slower light speed explanation is more plausible.

Here is a more dramatic example. Take a look these two diagrams http://www.astro.ucla.edu/~wright/cosmo200.gif and http://www.astro.ucla.edu/~wright/cosmo215.gif by Ned Wright. The first shows galaxies moving away from a small area we think of as the Big Bang, while the second diagram shows the same model in comoving coordinates. Note that is a simple coordinate transformation and they are indentical mathematically in their predictions, but the comoving coordinates show that galaxies stay at a constant distance from us over all time and mathematically you can not prove that all galaxies are not stationary with respect to us. Whether you want to think of galaxies as receding or stationary comes down to your own personal interpretation of physical reality because the maths in either set of coordinates predicts the same thing. It is surprising that the Anti-Big-Bangers have not latched on to comoving coordinates because it mathematically proves what they want to believe!
The two diagrams are from Ned Wright's cosmology webpage: http://www.astro.ucla.edu/~wright/cosmo_03.htm.


Basically, what I am getting at, is that we should encourage a best endevour to retain some form of finger hold on physical reality by resisting new physics until all possible plausable explanations based on our current experience have been exhausted. That is all I meant by not rejecting a notion of physical reality too readily.

As for Einstein, I would readily agree that he was one of the most intelligent people that ever lived, but he was not infallible. His belief that the universe is static was proved to be wrong (although you could argue that the universe IS static in comoving coordinates ) and it is largely agreed that Einstein was defeated at Copehagen over the CPR paradox and the issue of "whether God plays dice with the universe".

 

[yuiop]

In this case 'expansion' is not quite appropriate. My apologies. It's the inter-centre-of-mass distance that increases, and this is measured with radar. I like the operational approach, and therefore believe that specifying the measurement method is all one need say. Then semantics becomes irrelevant?

Hi Oldman, I did not mean to imply that the space between the Earth and the Moon is really expanding. I just thought it would interesting if we could apply the principles that make us absolutely certain that the Moon is moving away from us, rather than space expanding option, to distant galaxies. One obvious difficulty is that because of the distances involved, radar ranging is not a viable option for distant galaxies.

Thanks, Kev. I didn't know this. But isn't there a problem with a model which uses SR, applicable only when spacetime is flat, to a universe where the accepted FLRW model has flat space sections but a curved spacetime?

Yes, there is a problem with SR where curvature is involved. My suggestion is to take the GR/FLWR model with superluminal velocities and do a coordinate transformation to a system with sub-luminal velocities. I think at the very least it would be illuminating

Ned Wright shows the transformation from FLWR coordinates in this diagram http://www.astro.ucla.edu/~wright/omega0.gif to Specail Relativity coordinates in this diagram http://www.astro.ucla.edu/~wright/omega0sr.gif The example transformation that he does if for a universe without gravity and am sure the transformation for a universe with graivty would much more difficult mathematicaly but not impossible.

Ref: http://www.astro.ucla.edu/~wright/cosmo_02.htm

 

[George Jones]

Basically, what I am getting at, is that we should encourage a best endevour to retain some form of finger hold on physical reality by resisting new physics until all possible plausable explanations based on our current experience have been exhausted.

So, are you saying that we should resist using relativity?

 

[yuiop]

So, are you saying that we should resist using relativity?

No, I count relativity as old physics. The maths of relativity is correct. The physical interpretations are probably philosophical.

 

[oldman]

...the Ambjorn Loll article in this month's SciAm? I'd be interested to know your reaction.
I think there has been some progress on the quantum gravity front.

Thanks for the link. I've now skimmed through the article, which is certainly thought-provoking. I know little more of quantum gravity than the names of proposed approaches, so the Sci. Am. level is well suited to my level of ignorance.

In suggesting that the number of dimensions may change with scale, from 2 to 4, the authors seem to imply that some 'dimensions' emerge? unfold? or are born? out of quantum chaos. If so, it seems possible that the relation between our time and space dimensions, as measured by the ratio of their GR metric coefficients, could change. I've often wondered if this ratio is eternal. If it were to change...

Perhaps Loll et al.'s wish for predictions of predictable consequences for their ideas have long ago been granted.

 

[marcus]

...
In suggesting that the number of dimensions may change with scale, from 2 to 4, the authors seem to imply that some 'dimensions' emerge? unfold? or are born? out of quantum chaos...

that is right. the triangulations approach was stalled for a number of years in the 1990s because they would put simplexes in the computer expecting to get a 4D spacetime out---intending them to swarm together making a 4D spacetime according to some simple programmed rules of assembly---and the observed dimensionality would either blow up or be degenerate-----they'd all clump together like sardines or they would feather out.

in 1998 they changed the rules of flocking (self assembly with one's neighbors) slightly and found the trouble was cured in 2D simulations, and gradually built up to 3D (I think in 2001) and finally 4D in 2004. It was a real triumph when they got dimensionality of about 4.02 to emerge. We heard about it from John Baez. the APS ran an article in Focus by Adrian Cho.

the way that 4.02 was measured was simply to compare radius to volume. R is the number of steps you take out from a point, stepping from simplex to simplex. V is the number of simplexes you can encounter by taking R steps. If V goes experimentally as the 4.02 power of R, then that is the dimensionality.

and it is just a statistical average. the assumption is that more computer runs and larger samples would make it come closer to 4.0. but that was their 2004 result.

I'd like to talk more but I promised my wife I woud help in the kitchen with a stew she is preparing. Have to go.
Anyway, their new result of December 2007----the emergence of deSitter space---is important.

 

[marcus]

I'm back, the stew is simmering in the oven. But it is too late to continue the other post so I'll start a new one.

...
In suggesting that the number of dimensions may change with scale, from 2 to 4, the authors seem to imply that some 'dimensions' emerge? unfold? or are born? out of quantum chaos...

=============
My attitude about the Utrecht model quantum universe is that it is one example out of a handful of quantum geometry models---one which has reached the break-out stage.
One where they have reached the stage of running computer simulations of the universe and having spacetime emerge as an epiphenomenon.

the appearance of classic smoothness (satisfying Einstein eqn.) arising from microscopic quantum roughness and confusion

spacetime as a path integral---average of all different crazy ways of getting from this spatial geometry to that. maybe 4D spacetime doesn't exist, maybe it is always just a path from this space-state to that one.

So my attitude is to watch for other approaches, like spinfoams, to get to a stage where they have a path integral and where they can run their models in computer and get smooth classic geometry to emerge as a largescale average.

And I want to see if there are OTHER approaches that, when they get to this stage, also predict fractal microstructure and a decline in dimensionality at small scale.

there have been hints of this kind of development. Freidel just posted a paper obtaining a path-integral for spinfoams (making the spinfoams approach look more like the Utrecht model---evolutionary convergence) and two papers were delivered about this at the international QG conference last week in UK.
Martin Reuter at University of Mainz has an approach which actually preceded the Utrecht people in finding hints of spacetime dimension around 2 down at very small scale, hints of microscopic fractal structure. His is yet a different approach (not spinfoams, not simplex) which in other respects is perhaps not as satisfactory, but at least confirms this point coming from a different direction.
These are just straws in the wind.

In suggesting that the number of dimensions may change with scale, from 2 to 4, the authors seem to imply that some 'dimensions' emerge? unfold? or are born? out of quantum chaos. If so, it seems possible that the relation between our time and space dimensions, as measured by the ratio of their GR metric coefficients, could change. I've often wondered if this ratio is eternal. If it were to change...

If I understand you mean an evolving speed of light. In their model they do have a shape parameter which is the ratio of the timelike edges to the spacelike edges, of the simplex. the simplex does not need to be equilateral. All simplexes are identical. they have sometimes played around with this shape ratio parameter, but it stays constant for all simplexes for the duration of the computer run. as far as I know it has always been treated as a constant. I don't know the very latest.

It doesn't matter what my hunch is, of course, because I could so easily be wrong. My hunch is that a variable speed of light will never come out of the Utrecht model. the model is too simple. As the SciAm article says, it is hard to imagine any more minimalist way to treat quantum gravity. minimal paraphernalia, minimal assumptions, simple rules.

it might be for some other computer modeling approach farther down the road to try a variable speed of light, not this approach----but I could easily be mistaken.

 

[sketchtrack]

If a galaxy can have a recession speed greater than C, then why could not a space ship? If a spaceship accelerates to .8 C relative to earth, and then stops accelerating, it will no longer be accelerating, but it will still be moving away from Earth at .8 C although it will now be at rest in its own inertial frame. What is to stop it from then later accelerating another .8 C and then stop accelerating again coming to rest in its inertial frame, but now moving away from Earth at 1.6 C?

 

[George Jones]

If a galaxy can have a recession speed greater than c, then why could not a space ship?

In Friedmann-Robertson-Walker universes, the the rate of change of proper distance between a spaceship and a planet with respect to cosmological time can sometimes be greater than c when the proper distance between them is greater than zero, but this doesn't violate any fundamental principle of relativity.

Suppose you and I are both on planet A, and planet B is accessible to A by rocket. Suppose you set off for B in a rocket. At the same moment that you set sail, I can fire a laser in a direction such that the laser light reaches B before you do. In this sense, rockets alway move more slowly than light speed.

There is another sense in which the speed of light is a speed limit. Suppose that you are deep in space. At any particular time, there may be other astronauts, as well light beams, coincident with you. If you measure the speed of any astronaut that is coincident with you, you find that this speed is less than the speed of any light pulse that is coincident with you.

 

[marcus]

If a galaxy can have a recession speed greater than C, then why could not a space ship?

It easily could have a recession speed >c. Nobody said it couldn't, that I know of.
All Special Rel implies is that the spaceship could never catch up with, and pass, a photon.

Have you read Lineweaver and Davis, "Expanding Confusion"? I used to recommend that paper to PF people a lot back in 2004-2005. I see it is still getting recommended. Basic paper.
there is also the SciAm article by the same authors. March 2005.. so important to read the basic stuff.

If a spaceship accelerates to .8 C relative to earth, and then stops accelerating, it will no longer be accelerating, but it will still be moving away from Earth at .8 C although it will now be at rest in its own inertial frame. What is to stop it from then later accelerating another .8 C

Why should it start the engines a second time? Why waste fuel? If it just WAITS long enough it will eventually be receding with a recession speed of c, or 1.5c, or 2c, or whatever.
But it will never catch up with a photon.

=================
EDIT: hi George! didn't see your response when I was writing this.

 

[sketchtrack]

It easily could have a recession speed >c. Nobody said it couldn't, that I know of.
All Special Rel implies is that the spaceship could never catch up with, and pass, a photon.

Have you read Lineweaver and Davis, "Expanding Confusion"? I used to recommend that paper to PF people a lot back in 2004-2005. I see it is still getting recommended. Basic paper.
there is also the SciAm article by the same authors. March 2005.. so important to read the basic stuff.

If a spaceship accelerates to .8 C relative to earth, and then stops accelerating, it will no longer be accelerating, but it will still be moving away from Earth at .8 C although it will now be at rest in its own inertial frame. What is to stop it from then later accelerating another .8 C

Why should it start the engines a second time? Why waste fuel? If it just WAITS long enough it will eventually be receding with a recession speed of c, or 1.5c, or 2c, or whatever.
But it will never catch up with a photon.

It is just that I always hear people saying that aliens could never make it here from distant galaxies because they are so far away, and that a spaceship cannot move faster than light.

Say I am racing a photon to a destination which is an infinite distance away, and we start at the same time same place, I am constantly accelerating, how will I never pass the photon?

 

[marcus]

It is just that I always hear people saying that aliens could never make it here from distant galaxies because they are so far away, and that a spaceship cannot move faster than light.

Say I am racing a photon to a destination which is an infinite distance away, and we start at the same time same place, I am constantly accelerating, how will I never pass the photon?

Because of special rel.

Here is the Lineweaver Davis SciAm article. You need to read this.
http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf

there are many PF threads about your confusion about recession speed (recession speed needs to be treated different from speed in your local frame) here is one possible thread tht might help
https://www.physicsforums.com/showthread.php?t=212573

 

[sketchtrack]

Wouldn't it require that the recession speed of light from the starting location depends on the recession speed of me. How would it work if there was another contender who accelerated slightly faster than me? If you measure distance from participant and starting line over time according to an observer at the starting line, then at some time I would be further so long as light never moves away from the observer faster than C.

I guess this is where we step through the looking glass.

 

[marcus]

...as light never moves away from the observer faster than C.

whoever told you that? I certainly never said that. don't know where you would have gotten that idea.

most of the light in the world that happens to be moving away from us is, in fact, doing so much faster than C

because it has its local velocity of C plus its recession speed (associated with its distance from us)

 

[sketchtrack]

because it has its local velocity of C plus its recession speed (associated with its distance from us)

Could you elaborate? What do you mean by local velocity? What other way do you measure velocity other than a function of Distance and time? Is light not traveling faster than C, and therefor has a velocity greater than C?

 

[marcus]

Is light not traveling faster than C, and therefor has a velocity greater than C?

A couple of posts back I suggested you look at this thread

here is one possible thread tht might help
https://www.physicsforums.com/showthread.php?t=212573

did you have a look, sketchtrack? Did you find anything that was helpful?

You ask is the light not traveling faster than C----if you combine both the local velocity and the recession speed together. Of course it is. Most of the photons in the universe that happen to be aimed away from us ARE going away with combined speed greater than c. often much greater.

what's the problem? I assume you know that the space of special relativity is primarily useful as a local approximation.
Special rel rules apply locally, like when one thing is catching up to something else and passing it and both are in the same local frame. have to go.
try reading something and figure out by yourself

the reference to how to get "Expanding Confusion" is on page 45 of the Lineweaver SciAm article. the link to that is in my sig if you need it. on page 45 it saysExpanding Confusion: Common Misconceptions of Cosmological
Horizons and the Superluminal Expansion of the Universe. Tamara M.
Davis and Charles H. Lineweaver in Publications of the Astronomical
Society of Australia, Vol. 21, No. 1, pages 97–109; February 2004.
astro-ph/0310808
==========
when you see something like astro-ph/0310808, change it to http//arxiv.org/abs/astro-ph/0310808
and put that in the browser window---when the page comes up click on PDF.
everything on arxiv is a free download

====================

OK I'm back. Here's something (you should read Lineweaver and Davis "Expanding confusion" for yourself but here's something I can add):
there is a sweet easy way to keep track of these motions that a lot of people use (not all but many or most) which uses the CMB as an anchor. Using the CMB as a reference gives you a clear way of defining distances and analysing overall change in distance into a local component and a longrange recession component.

It's not all that complicated. I can try to outline it to you, if you want. maybe in a separate thread. Let me know if you are interested.

 

[yuiop]

whoever told you that? I certainly never said that. don't know where you would have gotten that idea.

most of the light in the world that happens to be moving away from us is, in fact, doing so much faster than C

because it has its local velocity of C plus its recession speed (associated with its distance from us)

Lets say we observe a galaxy with redshift z=2 and launch a projectile in its direction at 0.1c. The equivalent SR velocity of the galaxy is v/c=0.8c and in SR terms it seems the projectile would never catch up with the galaxy, but in comoving terms the projectile would have its own velocity plus the local recession speed (just like the photon) and would eventually catch up with the distant galaxy because the projectile is moving at 0.1c relative to the local CMB while the galaxy it is heading towards (and all the galaxies it passes on the way) are at rest with the CMB. Does that seem valid? At no time does the rocket exceed the local velocity of light. It is clear that there are different predictions here between the SR interpretation and FWR/GR comoving interpretation.

 

[oldman]

My attitude about the Utrecht model quantum universe is ....These are just straws in the wind.

Thanks for illuminating the present state of affairs for me. I guess one must just be patient and keep a watching brief for real progress --- with a sharp eye --- which is what you seem to be doing .

If I understand you mean an evolving speed of light...

it might be for some other computer modeling approach farther down the road to try a variable speed of light, not this approach

No -- I can't bring myself to accept VSL theories --- I find myself clinging to the belief that physics is observed to be too invariant across time and space for this to be a fruitful approach. But I do sometimes wonder if it is the time dimension that is as invariant as we suppose it to be, letting the space scale factor cause all change --- altering expansion and starting, sustaining and stopping exponential inflation. Seems to me to be dangerously conservative to blame everything on varying space dimensions.

 

[ThomasT]

I hope the links and discussion have resolved the issue for you.

I think the discussion has resolved that my interpretation of what is meant by the "expansion of space" is not what is meant by mainstream cosmologists when they speak of the expansion of space -- and that what they mean by the expansion of space is simply that the distances between large scale cosmological structures are increasing.
Is this correct?

On the other hand, the issue of what's actually happening in reality (with respect to the nature of empty space, etc.) seems to me to be maybe an unresolvable problem. I'm not sure how I want to think about this yet. But the discussion has been very helpful. I hope it continues, and that some other threads branch from it. There's enough "food for thought" to keep me busy for quite a long while I think.

My own intuitive speculation with regard to the ontology of empty intergalactic space is that it's composed of all sorts of wave activity involving media of unknown structure. The problem is to fashion some sort of model that assumes hierarchical media/wave interaction that can eventually yield some testable hypotheses. I don't understand GR, but I think of the idea of "curved space" as a simplification of the wave mechanics that would more precisely describe what's actually happening regarding the gravitational behavior of cosmological structures. That is, gravitational attraction isn't really due to curved space, but is, rather, due to wave interactions within and across various media. The center of a gravitational field would be the area circumscribing the most intense wave activity of the field.

Anyway, I gather that when mainstream cosmologists speak of "space", they're simply referring to the "metric" of some theory, and not the sort of physical cauldron of wave activity that I imagine it to be.

 

[marcus]

I think the discussion has resolved that my interpretation of what is meant by the "expansion of space" is not what is meant by mainstream cosmologists when they speak of the expansion of space -- and that what they mean by the expansion of space is simply that the distances between large scale cosmological structures are increasing.
Is this correct?

I believe so

the shorthand figure of speech refers to somewhat more----to the regular pattern of increase distance expressed in Hubble Law.
that on average and at large scale the rate of increase is proportional to the distance itself----the ratio being the Hubble parameter during whatever time period is being considered.

everything in that pattern is anchored to a particular idea of now. the distance to the galaxy where it is now at this instant, the rate of increase at this instant, and that idea of now of course requires an idea of all observers being at rest, in this case with respect to the CMB

you need that in order to define the idea of distance occurring in the law----to make any meaningful statement about distance one has to be very clear about what it means operationally (there are different ideas of distance)

this is the proper distance at the present moment, or whatever moment is being considered

but that is a technicality which there is rarely time to go into in a casual conversation

so the main thing is that when one says space expands it is a shorthand for saying that Hubble Law applies---that rate of distance increase is proportional to distance at large scale

it is actually an empirical fact. Hubble Law can be seen to work! so we arent just talking theory and models here.

On the other hand, the issue of what's actually happening in reality (with respect to the nature of empty space, etc.) seems to me to be maybe an unresolvable problem. I'm not sure how I want to think about this yet. ...

that is accurate and perceptive. it is actually a question of ONTOLOGY---what is the underlying reality from which appearances arise.

Naturally it would not be something one would be apt to state in the English language. Mathematics is normally found to describe things and then the math is popularized with metaphors and analogies. And we are right now in a revolution or transition between the CLASSICAL spacetime ontology of vintage 1915 Gen Rel and a new QUANTUM ontology where there is a deeper mathematics underlying the older version, from which the older emerges at large scale.

the fundamental descriptors (traditionally called degrees of freedom) at the micro level have not been established, but for a sample of what they might look like see my favorite SciAm article in my sig. the picture of spacetime at micro might be rough chaotic fluctuating geometry in which matter and geometry are intimately involved in their very existence with each other, the macro picture that arises from that rough chaotic picture is by contrast smooth. the SciAm article uses the analogy of snow. the flakes are feathery and fractally, but the snowdrifts and ski-slopes are smooth and rounded

But the discussion has been very helpful. I hope it continues, and that some other threads branch from it. There's enough "food for thought" to keep me busy for quite a long while I think...

Anyway, I gather that when mainstream cosmologists speak of "space", they're simply referring to the "metric" of some theory, and not the sort of physical cauldron of wave activity that I imagine it to be.

You are right. And that is the old CLASSICAL ontology. All that 1915 Gen Rel gives us is the metric. That is, it gives us the gravitational field itself (the field is essentially the metric up to an equivalence). And in classical Gen Rel there is no physical reality to points of spacetime. Events exist, like the collision of two particles, and geometric relations between exist.

And Gen Rel is still the established theory of spacetime geometry! It is our best and almost only theory of gravity! So naturally when people talk they are often referring to the classical ontology based on Gen Rel. What else can they use without risking speculation? So naturally they have to say that spacetime has no physical existence---as Einstein already pointed out.

But there is a more modern quantum ontology that people are working on, and you are already speculating yourself about what it could be. You present ideas in your post.

There is a new book scheduled to come out in March 2009 by Cambridge University Press called Approaches to Quantum Gravity---Towards a New Understanding of Space, Time, and Matter. and one of the early chapters is by GERARD 'T HOOFT. It will be interesting to see what he has to say. The chapter is called The Fundamental Nature of Space and Time.

't Hooft leads the theoretical physics institute at Utrecht. He is senior to the researchers in the Utrecht group responsible for the SciAm article I mentioned, but though not part of the group he is aware of their research. He is a highly original thinker and I am looking forward to what he has to say. In fact the whole book, which includes chapters by over a dozen other top people, should be interesting. I think we are on a kind of threshold as regards the ontology of spacetime and matter.

But that is just my speculation. We can't ever know for certain what the future of fundamental physics research holds.

 

[ThomasT]

Thanks again marcus et al.