What is Spacetime?

To be succinct about it, this is word salad garbage.

 

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Pincho, this line of reasoning has been raised before. I believe that it is as misguided now as it was then.., for the following reason.

"Aether, and the theory of Relativity", was the title and subject of a lecture Einstein gave at the University of Leyden in 1920. That was just 5 years after the general theory of relativity was first published and only 15 years after his paper introducing the special theory of relativity. They did not have the Internet back then and while SR may have begun to work its way into universities at graduate levels, perhaps even upper undergraduate levels, GR was a new and revolutionary theory. His audience for the most part had been educated from the perspective of the aether model of Maxwell, Lorentz and their predecessors.

Einstein was giving an address that connected GR to what they had been taught. Near the end of that same address he says,

From the bold portions of the above quote it seems evident that Einstein in that address was using "space" and "ether" interchangeably, not to imply that space was ether, instead to imply that everything they had been taught in the ether model of the day could be explained as an interaction of matter and space, by the theories of special and general relativity.

It is almost always important when reading historic accounts like the above address, to keep in mind the context and general understanding that was prevalent at the time.

I don't believe that Einstein believed or promoted space as ether. He just used that as an analogy to introduce a revolutionary idea.

 

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I use them interchangeably too, because they are interchangeable. That's why they both belong in this thread. Einstein never figured out what the Aether was, and he never figured out what space time was. So he had no rules on whether they could be interchangeable or not. All he had was a general theory, and general is not totally formulated. It is an essence of a theory, a grasp at understanding the invisible. If you want to get into Einstein's head, you need to study the Aether first. Then get a complete grasp at what the Aether would need to be to create both space time, and photon waves, and fail the Michael Morley experiment.

Nobody should be turned away from the Aether when asking about space time. They should be pointed towards it, and then told to use it as a history study. It was my most important study, and it should be other peoples study.

 

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I disagree in a number of respects. I was about to post a reply to a number of earlier posts when I found your above post had already made the boards.

Though the reply i had intended to post did involve the ether vs spacetime, I do not now believe that discussion is appropriate for this thread. There is too much potential to derail the topic.

Perhaps another time in a thread more suited for that discussion.

 

Search arXiv for papers with aether in the title and read Contemporary Ideas in the wiki article on aether theories:

Robert B. Laughlin, Nobel Laureate in Physics, endowed chair in physics, Stanford University, had this to say about ether in contemporary theoretical physics:

“It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed . . .

The word 'ether' has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. . . . Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry.

It turns out that such matter exists. About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo.”

The important thing to remember is that aether is essentially just a dirty word for space, whilst spacetime isn't the same as space. Instead spacetime is a mathematical "space". I think it's best to set gravity aside for a moment and focus on Minkowski spacetime. Objects move through space, but there's no motion through Minkowski spacetime. It's "all-time view" of space and motion through it along with the resulting events.

 

Farsights opinion on this is not correct. An aether, or a fluid that exists everywhere is spacetime is absolutely NOT the same as spacetime itself. Just learn the tiniest by of GR and that much is patently obvious.

 

What is Red shift?

 

Most of his (Farsight's) post was a quote from Laughlin (and Wiki).

My original reason for saying that it was perhaps not best for this topic is that it does stray some from the main stream opinion. That does not mean that there is not accepted scientific research and experiment to support the general concept.

Still I have no personal objection to a discussion, of the ether as space or vs space.., and though I would agree that there has been no ether theory proposed that is fully consistent with GR, QM and experience, ether theories are still being explored and do present some interesting potential.

Continuing to characterize the or an ether, simply as a fluid in space, starts the discussion from a restricted perspective. Certainly the luminiferous aether of Maxwell, Lorentz and their time is not consistent with what we have come to understand of the universe today. It is a safe bet to say that an ether along those lines does not exist. That does not preclude the existence of an ether of any description.

 

A change in the perceived wavelength of light from receding objects:
http://en.wikipedia.org/wiki/Redshift
Objects moving away from us have their spectrum shifted toward the red end and objects moving towards us have it shifted to the blue end.
Think of the Doppler effect, but applied to light rather than sound.
(Plus the expansion of space itself tends to cause Redshift).

 

Harping on about an aether when you're actually talking about something else is pretty disingenuous don't you think? The things that are being called an aether have a perfectly valid, and generally experimentally tested descriptions from real scientists, sorry, the mainstream.

 

So then this is a fair subject of discussion here?

Provided at least some logic and reason is included.

 

true in the thinking that Aether has no consequence

but what Pincho is saying is that Aether has a consequence and was not included in either theory , Special and General . which Aether was not . this is true

 

To expand a bit on James R's post, The general theory of relativity is the most general geometric theory of gravity it is possible to write down in 4 dimensions. That is, the Einstein Hilbert action \(S \sim \int d^4 x \sqrt{-g} \left(R + 2 \Lambda \right)\) cannot be non trivially added to unless we consider the theory in more than 4 dimensions.

 

Huh? Just read what I said again. Let me repeat it, with a little bolding:

I certainly didn't say that an aether that exists everywhere is spacetime. And you certainly won't find me talking about space or aether as a fluid. Now shape up, and pay attention.

 

You need to pay attention:

 

I understand the consensus theory of expanding space-time. Supposedly, dark energy expands space-time. The question is what is the mechanism? I claim that dark energy reds shifts energy without needing to expand space-time. This is done via the potential difference between mass/energy and dark energy. What is the mechanism to support the claim that dark energy expands space-time. Without a mechanism how can we call that science?

 

I'm not allowed to post my theory in here, but I have a mechanism, I'm not sure what mechanism science uses.

 

I believe that was originally directed at me. No offense taken, but at the time to have gone further into my intent would have been a sidetrack from the consensus view.

You are correct and yet there is some limitation based in convention, past meaning and use, of the word "space" in the context of some of the more recent astronomical observations and experimental proofs of some aspects of GR.

Using the word "ether", carries its own issues of past associations, but if it is narrowly defined in a manner that makes it indistinguishable from "space", some of those conceptual limitations change and with that change there is an expanded potential for the exploration of the interaction between space and matter that, I find intriguing.

It is ashamed that words all on their own come to carry so much past baggage that one word can change the way intelligent people view, understand and react to, the larger context of a discussion.

I don't know where this discussion really belongs. I agree that most of the ether models one might run across are extremely flawed. (In most cases that is a generous assessment.) That does not change the fact that "space" carries its own baggage that may limit our ability to integrate some of what we are beginning to discover about the world in which we live.

 

Sorry, you are wrong. Dark energy is theorized to be the cause of the ACCELERATION of the expansion.

Yes, I believe you said dark energy sucks the energy out of energy. This is hardly a mechanism and does not really make any sense anyway.

How can there be a 'potential difference'? What does that mean?

That is a rather limiting view of science. What is the mechanism that causes matter to warp space? We don't know, do we? I would still say it is science, wouldn't you?