Is spacetime a physical entity?

[Naty1]

I have maintained here several times , amid howls of objection from a few, that spacetime IS something more than a mathematical construct or that at least we should maintain an open mind about that possibility. I have been thinking WHY I have that view and a significant reason is the presence of vacuum energy and virtual particles. Seems like those are theoretical entities which manifest only via spacetime. Is that true or can other entities produce vacuum energy via quantum fluctuations and virtual particles? And do we even have solid experimental evidence for vacuum energy and virtual particles?

Wikipedia sez on vacuum energy:

Its effects can be observed in various phenomena (such as spontaneous emission, the Casimir effect, the van der Waals bonds, or the Lamb shift),

In Lamb shift, wikipedia says, for example:

This particular difference is a one-loop effect of quantum electrodynamics, and can be interpreted as the influence of virtual photons that have been emitted and re-absorbed by the atom.

http://en.wikipedia.org/wiki/Lamb_shift


I never heard of van der Waals bonds before and a quick scan in Wiki did not cause me to jump out of my chair in excitement...but since DARPA is experimenting something must be going on...

Any consensus on those "observations"? as evidence? I'm somehwat familiar only with the Casimir effect and don't really think it shows observational "proof" of vacuum energy, but I do take it to be a signpost in that direction. Ditto for Lambshift.

Maybe we should think of spacetime as a feature derived from vacuum energy, quantum jitters?

 

[turbo]

Naty1, I highly recommend "The Philosophy of Vacuum" by Simon Saunders and Harvey Brown. In it are essays by Einstein, Penrose, Sciama and many others on the nature of "empty" space. It's an expensive book, though I found a "used, like-new" copy through Amazon for $40. I could have borrowed it from a library, but wanted to be able to take my time reading, comparing, re-reading, so I needed my own copy. Plus, I tend to make lots of margin-notes, which is not a good thing to do to library books.

 

[Naty1]

cheapest used is now $50 on Amazon...so I wait!..I could easily go broke buying great books at such prices... I highlight so reselling is not likely...

 

[apeiron]

Hi Naty1

Who are these people calling spacetime a mathematical construct? They may be making merely a modelling point (hey, we chucked away the old style ether) but most physicists seem happy with new versions of the ether. It is only that - until recently - it was not cool to call it an ether.

For example, just the other week I was reviewing Wilczek's Lightness of Being. Not a great book as it skips over the technical detail of his thinking (which would be really interesting). But he talks feely about the vacuum as a collection of condensates. And he calls his view ether on steroids.

"No ether" was for a long time a slogan to sell relativity as a radical break from what went before. But new ideas of material reality have arisen to take the old ether's place. Scientists, like nature, still abhor a vacuum!

Seriously though, there is a problem with taking a "substance" view of the void. That is too strong a metaphysical commitment. The void is just as much a "form". Which some could take as being the "its just a mathematical construction" aspect.

So what I am saying here is that the vacuum can be described in both substantial terms and formal terms and we can still be talking about the same thing really. Therefore no need to get hung up on religious either/or issues. It is more about peoples' conceptual and modelling preferences. And the fullest view would include both angles.

The same if we asked the similar kind of question, is the vacuum a structure or a process? Does it exist or does it persist? Is it stasis or is it flux? You can get a sense of how both the substantial approach and the formal approach can be two views of the same animal.

 

[Naty1]

Apeiron: I believe those who think in strictly mathematical construct terms may be coming from relativity within the standard model...accompanying comments suggest to me anything beyond the standard model is suspect...

 

[Hurkyl]

accompanying comments suggest to me anything beyond the standard model is suspect

Of course -- the standard model is standard because it fits the totality of the data we have better than other models. If some other model was a better fit, then that would become the standard model.

 

[Vanadium 50]

Let's not go tossing away the Standard Model just yet.

First, the question of what is a "physical entity" is for the philosophers. Is a force? What about a potential? (There have been long arguments among philosophers of science as to whether forces or potentials are more "real"). A field? Electric charge? A photon? A phonon?

Physicists build mathematical models of nature. Some are good, some are less good. Some have a wide range of validity, some a narrow one. Some last for centuries, other much less. Determining what is "real" is philosophy.

Second, you seem to want to be merging the question of fields, which are functions which have values at all places and times, with spacetime itself.

Third, Wikipedia is not the most reliable source on these things - many of the articles on the "cool" parts of physics seem to be written by science fiction fanboys. I wouldn't trust it as my only source.

Fourth, it's very useful to be able to do the calculations yourself, and I would certainly want to do this before trying to overthrow everyone else's physics. For example, one can derive the Casimir force by looking at properties of conductors in the situation where <E(t)>=0 but <E²(t)> is not. If you do it this way, it's clear that you are talking about fields and materials, and properties of the vacuum hardly enter into it.

 

[Naty1]

Vanadium posts:

Second, you seem to want to be merging the question of fields, which are functions which have values at all places and times, with spacetime itself.

Perhaps because you can't have the former without the latter. If no other physical entity allows vacuum energy and accompanying fields to perpetuate, that suggests to me at the very least, an AMAZING coincidence. And coincidences do exist, of course, but they often point to underlying relationships. You see, positively, no relationship between vacuum energy and spacetime??... yet I'll bet you do see the relationship between space and time.

If there is no theoretical nor experimental evidence for that presently, so be it. But if there is any of either, would that not be fascinating?? one small step for man..etc.etc...

If you do it this way, it's clear that you are talking about fields and materials, and properties of the vacuum hardly enter into it.

Well Brian Greene,for one, begs to differ with your viewpoint. (FABRIC OF THE COSMOS,PG332) According to Greene, the first tests confirming Casimir's calculations were by another Dutch physicst, Marcus Spaarnay and increasingly precise experiments confimed findings by among others in 1997 by Steve Lamaoreau, then at University of Washington, to with 5%. Greene unequivocally attributes Casimir force to vacuum fluctuations. I did not even go so far in my post.

Third, Wikipedia is not the most reliable source on these things

Here we go again! A silly observation...As I have posted a number of times here in forums, if you have sources you trust which conflict, quote them and post...otherwise such comments add no value. I LIKE to reference Wikipedia, when I already have similar explanations from world renowned physicsts, because it gives all readers here an opportunity to get some background if desired. Wikipedia gets trashed too often on this forum from a few people because those few have different, unconfirmed, beliefs.

Also, I hope you'll note I did NOT take Wikipedia at face value on it's assertions for vacuum energy experimental confirmation...I am specifically asking if others have confirming sources.

 

[Naty1]

Oh I missed this "gem" :

Fourth, it's very useful to be able to do the calculations yourself, and I would certainly want to do this before trying to overthrow everyone else's physics.

Whoa!, another irrational unnecessary comment. I never even hinted at "overthrow" of anything...quite the opposite,in fact, if you read my post.

I was attempting to ask two questions, perhaps not clearly: (a) Anybody know if vacuum energy has been confirmed experimentally, (b) Anybody know if anyone has theoretically tied vacuum energy to spactime. And please note I did post this in "beyond the standard model".

One vague reply to (b) would be the big bang...but it's still a speculation at this point, as far as I know, that vacuum energy fluctuations spawned spacetime and everything else in our universe.

 

[apeiron]

(b) Anybody know if anyone has theoretically tied vacuum energy to spactime. And please note I did post this in "beyond the standard model".

One vague reply to (b) would be the big bang...but it's still a speculation at this point, as far as I know, that vacuum energy fluctuations spawned spacetime and everything else in our universe.

By spacetime, do you mean "that which is modeled by GR", while vacuum energy is "that which is modeled by QED/QCD"? Therefore the theoretical connection would be a successful theory of quantum gravity?

I don't really see the general difficulty of identifying spacetime with the vacuum energy otherwise. What am I missing?

 

[Naty1]

By spacetime, do you mean "that which is modeled by GR", while vacuum energy is "that which is modeled by QED/QCD"? Therefore the theoretical connection would be a successful theory of quantum gravity?

that would be ok, but I did not want to be too specific. In reading a bit about quantum gravity I have yet to see such stated as an objective or something included in an emerging theory.

I don't really see the general difficulty of identifying spacetime with the vacuum energy otherwise. What am I missing?

my perspective,too; hence this post.

 

[Vanadium 50]

Well Brian Greene,for one, begs to differ with your viewpoint.

Baloney. Utter rubbish. What Brian Green has done is write a popularization. A popularization is, by definition, not complete and often not entirely accurate. That's not it's job.

The fact that you think that because you read a popularization that you somehow know more about the Casimir effect than someone who has actually studied quantum mechanics and has done the calculations is flabbergasting.

 

[Naty1]

Vanadium posts:

The fact that you think that because you read a popularization that you somehow know more about the Casimir effect than someone who has actually studied quantum mechanics and has done the calculations is flabbergasting.

Not as flabbergasting as your post, however, which is negative, accusatory,and unhelpful.

 

[Vanadium 50]

And true. Don't forget true.

You get one kind of knowledge by studying something in depth, to the point where you can do the calculations yourself. You get another kind of knowledge by reading popularizations - books written by people with the first kind of knowledge to give a taste of what they know. You're, in effect, arguing that the second kind of knowledge is better than the first.

Let's look at the specifics - I made a point that one could derive the Casimir effect another way entirely. It's usually not done this way, particularly when being introduced, but one can do this. Your point was that this is invalid based not on the fact that you know from first hand experience that this is invalid, not because Brian Greene explicitly says it's invalid, but because in a popularization he chooses not to explain things that particular way. That's simply nonsense.

 

[moniker2]

i don't know if these papers help with your spacetime work.http://www.theresonanceproject.org/pdf/torque_paper.pdf"

http://arxiv.org/abs/physics/0306074"

 

[apeiron]

For example, one can derive the Casimir force by looking at properties of conductors in the situation where <E(t)>=0 but <E2(t)> is not. If you do it this way, it's clear that you are talking about fields and materials, and properties of the vacuum hardly enter into it.

Hi Vanadium - can you give a bit more to explain what you mean here? Is this a technicolor superconductor approach along the lines of Wilzcek or something quite different. I could not recognise the reference from your terse remark.

And are you just making a modelling point - that there are multiple ways of accounting for a phenomenon. Or are you also arguing that the popular description of Casimir forces are actually likely to be wrong and misleading in invoking the image of vacuum fluctuations?

I've seen Greene's account in Scientific American and many other popular sources explaining things in terms of virtual fluctations of the vacuum so thought it was uncontroversial.

For example, here is another description. Is this faulty in your view?

http://physicsworld.com/cws/article/print/9747

"Understanding the Casimir force
Although the Casimir force seems completely counterintuitive, it is actually well understood. In the old days of classical mechanics the idea of a vacuum was simple. The vacuum was what remained if you emptied a container of all its particles and lowered the temperature down to absolute zero. The arrival of quantum mechanics, however, completely changed our notion of a vacuum. All fields - in particular electromagnetic fields - have fluctuations. In other words at any given moment their actual value varies around a constant, mean value. Even a perfect vacuum at absolute zero has fluctuating fields known as "vacuum fluctuations", the mean energy of which corresponds to half the energy of a photon.

However, vacuum fluctuations are not some abstraction of a physicist's mind. They have observable consequences that can be directly visualized in experiments on a microscopic scale. For example, an atom in an excited state will not remain there infinitely long, but will return to its ground state by spontaneously emitting a photon. This phenomenon is a consequence of vacuum fluctuations. Imagine trying to hold a pencil upright on the end of your finger. It will stay there if your hand is perfectly stable and nothing perturbs the equilibrium. But the slightest perturbation will make the pencil fall into a more stable equilibrium position. Similarly, vacuum fluctuations cause an excited atom to fall into its ground state.

The Casimir force is the most famous mechanical effect of vacuum fluctuations. Consider the gap between two plane mirrors as a cavity. All electromagnetic fields have a characteristic "spectrum" containing many different frequencies. In a free vacuum all of the frequencies are of equal importance. But inside a cavity, where the field is reflected back and forth between the mirrors, the situation is different. The field is amplified if integer multiples of half a wavelength can fit exactly inside the cavity. This wavelength corresponds to a "cavity resonance". At other wavelengths, in contrast, the field is suppressed. Vacuum fluctuations are suppressed or enhanced depending on whether their frequency corresponds to a cavity resonance or not.

An important physical quantity when discussing the Casimir force is the "field radiation pressure". Every field - even the vacuum field - carries energy. As all electromagnetic fields can propagate in space they also exert pressure on surfaces, just as a flowing river pushes on a floodgate. This radiation pressure increases with the energy - and hence the frequency - of the electromagnetic field. At a cavity-resonance frequency the radiation pressure inside the cavity is stronger than outside and the mirrors are therefore pushed apart. Out of resonance, in contrast, the radiation pressure inside the cavity is smaller than outside and the mirrors are drawn towards each other.

It turns out that, on balance, the attractive components have a slightly stronger impact than the repulsive ones. For two perfect, plane, parallel mirrors the Casimir force is therefore attractive and the mirrors are pulled together. The force, F, is proportional to the cross-sectional area, A, of the mirrors and increases 16-fold every time the distance, d, between the mirrors is halved: F ~ A/d4. Apart from these geometrical quantities the force depends only on fundamental values - Planck's constant and the speed of light.

While the Casimir force is too small to be observed for mirrors that are several metres apart, it can be measured if the mirrors are within microns of each other. For example, two mirrors with an area of 1 cm2 separated by a distance of 1 µm have an attractive Casimir force of about 10-7 N - roughly the weight of a water droplet that is half a millimetre in diameter. Although this force might appear small, at distances below a micrometre the Casimir force becomes the strongest force between two neutral objects. Indeed at separations of 10 nm - about a hundred times the typical size of an atom - the Casimir effect produces the equivalent of 1 atmosphere of pressure."

 

[Naty1]

Vanadium posted:

you seem to want to be merging the question of fields, which are functions which have values at all places and times, with spacetime itself.

Can we "merge" them?? Interesting idea, but not mine...

My basic question(s):

Seems like those are theoretical entities which manifest only via spacetime. Is that true or can other entities produce vacuum energy via quantum fluctuations and virtual particles? And do we even have solid experimental evidence for vacuum energy and virtual particles?

So, I am inquiring not "wanting" anything... except perhaps some insights into my question(s). Vanadium's statement is a potentially interesting one but I don't really understand it because my question was about vacuum energy and virtual particles and I'm unsure if the "fields" with "values at all places and all times fits"...describes those entities.

I also asked:

Maybe we should think of spacetime as a feature derived from vacuum energy, quantum jitters?

 

[Naty1]

Moniker: Thanks for post #15...they belong in this thread and I'll post them there:

Structure of Minkowski Space,
https://www.physicsforums.com/showthread.php?p=2312916#post2312916

I just skimmed a portion...looks interesting...

 

[Naty1]

From (2003) http://arxiv.org/abs/physics/0306074...

there are, if I understand the author, some bizarre statements here...

If I did not know the cold war was over, the USSR disbanded, I'd think this might be "disinformation"...


Page 2:

The indissoluble association of time and space takes on special importance in the light of the concept of physical field, which was called by Einstein the most important discovery in physics after the times of Newton. According to this concept, the occurrence in space of a force field means that space turns into a physical environment, which is capable to interact directly with other bodies and gains, thus, physical properties, becoming an active participant of physical processes. In view of the fact that space and time are indissolubly related to each other, the presence of a force field in some area of space must necessarily result in the appearance of physical properties of time caused by the motion of body in this area.
Thus, from the synthesis of the notion of space-time and of the idea of physical field it
follows with necessity that the course of time in a given region of space should depend on physical processes in this region, i.e. time, as well as space, should have physical properties.

and

The idea about the existence of the physical properties of time belongs to N. Kozyrev [9].
By introducing into mechanics an additional parameter taking into account the directivity of the course of time, Kozyrev has formulated causal (asymmetrical) mechanics from which it follows that time has physical properties. According to the results of theoretical and experimental investigations conducted by Kozyrev and his followers [9-13], events can proceed not only in time, but also with the help of time, information being transmitted not through force fields, but via a temporal channel, and the transfer of information happens instantaneously. The appearance of additional forces, associated with the physical properties of time and capable to fulfill work, testifies that time can serve as a power source.

The conclusion section on page 18 might also be of interest.

 

[Vanadium 50]

And are you just making a modelling point - that there are multiple ways of accounting for a phenomenon. Or are you also arguing that the popular description of Casimir forces are actually likely to be wrong and misleading in invoking the image of vacuum fluctuations?

I'm directly making a modeling point. You can describe the physical phenomenon coming from several directions. Indirectly, I am making the point that one shouldn't take any single model too seriously, because the same phenomenon can be described other ways.

 

[atyy]

I'm directly making a modeling point. You can describe the physical phenomenon coming from several directions. Indirectly, I am making the point that one shouldn't take any single model too seriously, because the same phenomenon can be described other ways.

Was this what you had in mind?
http://arxiv.org/abs/hep-th/0503158

 

[apeiron]

I'm directly making a modeling point. You can describe the physical phenomenon coming from several directions. Indirectly, I am making the point that one shouldn't take any single model too seriously, because the same phenomenon can be described other ways.

I agree it is vital to keep modelling and ontology distinct, but also still to do work at both intellectual levels - the theories and the meta-theories. And Naty1 seemed to be asking a meta-theory question rather than disputing anyone's theory.

So for example, there seems an obvious problem of realism when it comes to the virtual particle pairs imagery at the base of vacuum fluctuation stories. Two matched particles just happen to pop out at the same time and swiftly annihilate. This mechanism keeps the books balanced but is there anything that forces the vacuum to yield a virtual positron and electron, rather than a positron now, an electron some other time?

It is like the BCS electron pairs in superconductivity, a modelling trick that seems unreal but works.

So the question - for those like myself interested in what may be the reality beyond the models - is whether the virtual particle modelling device seems just so hokus to professionals in the field that it calls into question any fluctuating vacuum image. Something else would be a better imagining of the situation? Scrap that idea because it is nowhere near what could work?

 

[Vanadium 50]

Was this what you had in mind?
http://arxiv.org/abs/hep-th/0503158

Not exactly, but Bob and I have discussed this in the past. He's coming at it from a slightly different perspective than I did, but of course, the point that he's making is the same as the one I'm making, just via a slightly different example: the Casimir effect tells you less than you think about the nature of spacetime and the vacuum.

 

[aro]

Regarding vacuum and virtual particles:

Since Einstein put forward the possibility of converting energy into praticles and vice versa (providing all relevant conservation laws are obeyed) the old concept of 'billiard ball'-like particles became insufficient. There had to be a construct that permeated spacetime and could facilitate the emergence or destruction of particles: a field.
Such a field has vibrational modes that can be excited or not. These excitations are 'particles', with wavelike properties. The different modes are distinguished by the quantum numbers of the particles they give rise to. For every type of particle there is a separate field, different fields may be coupled meaning that those particles feel each other.
If no vibrational modes are excited (locally) then the field is in it's ground state, which we call the vacuum. So a vacuum is associated with a specific field, and there are many.

Now from QM we have the Uncertainty Principle, a consequence of the fact that the best description of the world that we have is a statistical one (which has bothered many people ever since it was conceived). This allows for the possibility of the excitation of a field mode, even at a point in space that is isolated. And there is this statement in physics saying that: 'What is not forbidden will happen'. So the vacuum state, in the statistical description will not actually contain zero particles. But it's expectation value, which is the observable in this case, will be zero.
These particles, which in our description emerge temporarily, are the virtual particles. This is just a name, but as you can see they are a consequence of two very well established principles which we take to be true unless proven otherwise. The Casimir effect is just one of many experimental proofs of their existence. Besides this effect every accurate calculation of a scattering amplitude should account for virtual particles, thus providing a huge amount of experimental evidence for this concept.

Of course another question now remains: what is the 'reality' of a field? Honestly I cannot answer that, so if anyone can I would be grateful (and surprised). Perhaps a field is our version of the old Aether, which had to explain the propagation of light in vacuum... time will tell.

So to come back to some previous questions I saw here: Vacuum is associated with quantum fields, which are a very useful tool for describing Standard Model physics. In this case we take space-time for granted and the fields are embedded in it.
In the theory of gravity, which we cannot describe with QFT, there is a more intricate connection with space-time but the concept of vacuum and virtual particles is of no use here.

This is my view, and maybe it shows lack of vision and imagination but I think this is more or less what we know now. For deeper understanding I think we should wait for what the new, developing, theories will bring.