Is spacetime a "substance" or not?

[Kuznetzova]

This is more or less what I have already said: Galactic red shift is believed to be caused by the Doppler Effect. It is the same phenomenon that makes the pitch of a fire engine siren rise and fall as it rushes towards and then away from you. When a fire engine goes dee, if it is approaching, by the time it goes dah, it will be a bit closer, so the dah doesn't have so far to travel. The result is that the sound waves are bunched up, they arrive at you more frequently; this increase in frequency is interpreted as a rise in pitch. Exactly the opposite happens if the fire engine is receding.
The same effect occurs with light, but in this instance the decrease in frequency moves everything towards the longer, redder end of the spectrum. The difference is that sound waves are known to move through a substance, the ones we hear travel through air generally. Light can be thought of as a stream of photons, but assuming the frequency of emission is constant, the same thing happens.

I mostly agree with the facts listed here. However, your explanation would result in a constant decay of photon frequencies. This is not what we see in the sky. Instead, the farther away a galaxy is, the more red-shifted its light appears to us. So the data shows, farther away = receding faster.

At this point I don't mean to talk authoritatively. We will have to start this perplexing discussion. The distant galaxies emitted that light centuries, nay, tens of millions of years ago. The light we see so red shifted is not where those galaxies are located "right now" (whatever that means). This is something you take for granted when talking about ambulances racing by and the dah's emitted from their sirens. The ambulance is still mostly where it is, in terms of time. The ambulance is not ten million light years off course from the sound. With galaxies, they really are.

I got into this topic pretty hardcore with a physics teacher in high school. I won't say any more about this until I see your response to the above.

[tillingborn]

Ah, Kuznetzova, splendid! Perhaps we can have a civilised discussion.

...your explanation would result in a constant decay of photon frequencies. This is not what we see in the sky. Instead, the farther away a galaxy is, the more red-shifted its light appears to us. So the data shows, farther away = receding faster.

Indeed. I was attempting to explain galactic red shift rather than cosmological expansion. They're related, of course, but not the same. For the latter, stick the fire engine on the surface of a balloon and blow it up.

At this point I don't mean to talk authoritatively.

I wouldn't, it is the evidence you present which is or isn't authoritative, not how you talk.

We will have to start this perplexing discussion.

Yah.

The distant galaxies emitted that light centuries, nay, tens of millions of years ago. The light we see so red shifted is not where those galaxies are located "right now" (whatever that means). This is something you take for granted when talking about ambulances racing by and the dah's emitted from their sirens. The ambulance is still mostly where it is, in terms of time. The ambulance is not ten million light years off course from the sound. With galaxies, they really are.

Apart from scale, I can't see what the difference is; by the time we hear it, the ambulance (actually I said fire engine) is no longer where the sound came from, neither spatially nor temporally.

I got into this topic pretty hardcore with a physics teacher in high school. I won't say any more about this until I see your response to the above.

A high school physics teacher, eh? I'm agog.

[jackles]

I think spacetime must be a condition of energys presents in void.so void reaction to energy is to warp its self in to space and time dimensions to accomadate energy .energy would as it where abstract timespace from void. Regs jackles.

[tillingborn]

For anyone who doesn't understand why distant galaxies are moving away faster, this might help.

[jackles]

I like that diagram.but the universe is only expanding from the point of view of an observer inside the event.or from a position in the sizelike event.

[Ginkgo]

I like that diagram.but the universe is only expanding from thw point of view of an observer inside the event.or from a position in the sizelike event.

Imagine a balloon before it is blown up. If you were to put those spots on a balloon and then blow the balloon up each spot would be moving away from every other spot the more you inflated the balloon. There is no centre on the surface of the balloon that can be identified.

This is not exactly how it works but is is a good analogy.

[Kuznetzova]

That's two posts now with a balloon analogy. Are you guys suggesting that space is expanding?

Would not that entail that space is a substance?

(and/or explain how space expands without space being a substance).

[tillingborn]

Dear kuznetzova,
Please refer to my 'Blimey! I'm an aether theorist.' thread. viewtopic.php?f=16&t=11180 Could I also take this opportunity to remind you of my frequently repeated contention that the most likely source of the phenomena that give the impression of a universe made of some stuff, is some stuff that the universe is made of.
Regards
tillingborn

[tillingborn]

ps. Space, in strict positivist reckoning is only, the separation of objects. The galaxies are receding (Oh yes they are Godfree) hence the space between them is expanding.
This is another of mine that argues that 'spacetime' is a substance viewtopic.php?f=12&t=9856 , so's this: viewtopic.php?f=12&t=9140

[Kuznetzova]

I'd like to see some other responses from other people outside of tillingborn plugging his own independent research.

From interactions with physicists in both a high school and university setting, I have collected the following "impressions" (lets call them).

A photon emitted by more distance galaxies is redshifted more as that photon has been in flight longer. While in flight through intergalactic space, the photon is successively accumulating little expansions of space. A photon that has been in flight longer will be more redshifted when it arrives.

What is written above in red font face is corroborated by after-class conversations with several physicists in both the high school and university setting. That they don't know each other, work for different institutions, yet both told me the same thing, should give any reader pause.

What should give the reader even more pause, is that nothing in the above description makes any concession or relation to the galaxy being in motion. Nothing there says anything about that. The redshift is the result of successive accumulations of expanding space, and has nothing at all to do with the relative motion of the galaxy which originated the photon in question. The galaxy could be in relative motion, even very high relative motion, or it could be stationary. And in both cases, its distance when the photon was emitted will be the deciding factor in the redshift-- (not its motion).

A number of corollaries pop out of this. First, it is completely wrong to make an analogy between the doppler affect and galactic redshift. Doppler affect assumes the siren is moving through air that is relatively stationary to both siren and listener. Redshift, as described above, is literally the photon itself building up successive little expansions while in flight. Who or what emitted that photon, or what that emitter was doing when it emitted it, are null issues.

Anyone on this forum who would claim that galactic redshift is a separable phenomenon from the expansion of the universe must be in a state of utter confusion. What I have presented above , is the mainstream/academic/textbook understanding of both redshift and cosmic expansion.

So what I think it happening on this forum is that first, tillingborn is utterly confused. Secondarily, Godfree is convinced that scientists simply believe in the expansion of space for some religious reason, and who then go collect facts to support their beliefs. The brutal truth is that redshift itself is the evidence that led scientists to conclude the universe expands.

I may be wrong. Scientists may be wrong. All of my high school and university professors could be wrong. And that's fine.
But before we claim that they are wrong, we must first understand precisely what their actual claim is.

[Arising_uk]

... the photon is successively accumulating little expansions of space. A photon that has been in flight longer will be more redshifted when it arrives. ...

I have little concern about such issues but what does this, "successively accumulating little expansions of space" mean?
p.s.
Does this, http://www.fnal.gov/pub/inquiring/quest ... hift1.html, aid this discussion at all?

[tillingborn]

The galaxy could be in relative motion, even very high relative motion, or it could be stationary. And in both cases, its distance when the photon was emitted will be the deciding factor in the redshift-- (not its motion).

What I have presented above , is the mainstream/academic/textbook understanding of both redshift and cosmic expansion.

Not according to my independent research, nor indeed ArisingUK's. Is there a mainstream academic you can quote, or a textbook or web page that includes the phrase "successively accumulating little expansions of space"?

Anyway, back to the chase:

That's two posts now with a balloon analogy. Are you guys suggesting that space is expanding?

Well, I can't speak for anyone else, but yes, I am suggesting that space is expanding. (Strictly speaking I believe the most compelling explanation for the observed red shift of distant galaxies is the Doppler effect caused by their recession.)

Would not that entail that space is a substance?
(and/or explain how space expands without space being a substance)

Well, logically if in the proposition, 'Space is expanding.' the term 'Space' has a referent, then yes it would, but:

Space, in strict positivist reckoning is only, the separation of objects.

Describing 'space' as a substance is ontology, metaphysics, and therefore not strictly physics, although as my Blimey! I'm an aether theorist thread (available at viewtopic.php?f=16&t=11180 folks!) demonstrates, Einstein, and (according to R. Loughlin) most physicists believe space is a substance.

[Kuznetzova]

Describing 'space' as a substance is ontology, metaphysics, and therefore not strictly physics,

I completely agree with this.
On the other hand, we can make some concessions at a baseline level of distinctions. In this case space, as a "substance" would mean it contains internal properties independent of any massive objects residing within it. That might seem superficially to be at odds with General Relativity, however we can imagine a situation in which a single photon is traveling in deep intergalactic space. In that case, the photon will be moving along a path given by the geodesic of that space. That is to say, the geodesic is an "independent entity" as an aspect of the space, rather than being determined by the photon's existence.

Einstein himself wrote an article about this in the year 1950.

The procedure just described overcomes a deficiency in the foundations of mechanics which had already been noticed by Newton and was criticized by Leibnitz and, two centuries later, by Mach: Inertia resists acceleration, but acceleration relative to what? Within the frame of classical mechanics the only answer is: Inertia resists acceleration relative to space. This is a physical property of space-space acts on objects, but objects do not act on space. Such is probably the deeper meaning of Newton's assertion spatium est absolutum (space is absolute). But the idea disturbed some, in particular Leibnitz, who did not ascribe an independent existence to space but considered it merely a property of "things" (contiguity of phYSical objects). Had his justified doubts won out at that time, it hardly would have been a boon to physics, for the empirical and theoretical foundations necessary to follow up his idea were not available in the 17th century. According to general relativity, the concept of space detached from any physical content does not exist.

[Ginkgo]

That's two posts now with a balloon analogy. Are you guys suggesting that space is expanding?

Would not that entail that space is a substance?

(and/or explain how space expands without space being a substance).

The balloon analogy is a way of demonstrating that there is no centre of the universe, so there is no privileged position in relation to this event. This is of course assuming there was a Big Bang.

As far as an expanding universe is concerned I guess it is important to know exactly what we mean by a substance. That is to say, the difference (if any) between a physical substance and a philosophical substance. As far as space and time are concerned I don't think it is possible to avoid the ontological implications.

[tillingborn]

In this case space, as a "substance" would mean it contains internal properties independent of any massive objects residing within it. That might seem superficially to be at odds with General Relativity, however we can imagine a situation in which a single photon is traveling in deep intergalactic space. In that case, the photon will be moving along a path given by the geodesic of that space. That is to say, the geodesic is an "independent entity" as an aspect of the space, rather than being determined by the photon's existence.

According to General Relativity, mass creates the topology/shape of space. Space, or spacetime, may have properties, but if they independently affected any geodesics we should expect to find gravitational anomalies. Instead, as you yourself have pointed out, what we discover is that calculating the geodesics according to the mass present works extremely well. In other words, space might have properties, but as far as we can tell, in the absence of mass, it is completely flat.

As far as an expanding universe is concerned I guess it is important to know exactly what we mean by a substance. That is to say, the difference (if any) between a physical substance and a philosophical substance.

Depends on your philosophy. If you are a realist, you would claim that a philosophical substance is something that actually exists. In the current context, both Kuznetzova and myself are claiming that the universe is made of some stuff.

As far as space and time are concerned I don't think it is possible to avoid the ontological implications.

The problem, as I see it, is that calling this stuff spacetime loads it with the very ontological implications I would much rather avoid. I think the concept of 4D Minkowski spacetime is a useful tool for mathematics, but I don't think it follows that either space or time exist independently of stuff; which is what I understand Einstein to be saying in the quote provided by Kuznetzova:

According to general relativity, the concept of space detached from any physical content does not exist.