Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is spacetime a physical entity?

  1. Aug 7, 2009 #1
    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:
    In Lamb shift, wikipedia says, for example:
    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?
     
    Last edited: Aug 8, 2009
  2. jcsd
  3. Aug 7, 2009 #2

    turbo

    User Avatar
    Gold Member

    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.
     
  4. Aug 7, 2009 #3
    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....
     
  5. Aug 7, 2009 #4

    apeiron

    User Avatar
    Gold Member

    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.
     
  6. Aug 8, 2009 #5
    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....
     
  7. Aug 8, 2009 #6

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
     
  8. Aug 8, 2009 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  9. Aug 9, 2009 #8
    Vanadium posts:

    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...


    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.

    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.
     
    Last edited: Aug 9, 2009
  10. Aug 9, 2009 #9
    Oh I missed this "gem" :

    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 vaccum 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.
     
    Last edited: Aug 9, 2009
  11. Aug 9, 2009 #10

    apeiron

    User Avatar
    Gold Member

    By spacetime, do you mean "that which is modelled by GR", while vacuum energy is "that which is modelled 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?
     
  12. Aug 10, 2009 #11
    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.

    my perspective,too; hence this post.
     
  13. Aug 12, 2009 #12

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  14. Aug 18, 2009 #13
    Vanadium posts:

    Not as flabbergasting as your post, however, which is negative, accusatory,and unhelpful.
     
  15. Aug 19, 2009 #14

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  16. Aug 19, 2009 #15
    Last edited by a moderator: May 4, 2017
  17. Aug 19, 2009 #16

    apeiron

    User Avatar
    Gold Member

    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."
     
    Last edited by a moderator: Apr 24, 2017
  18. Aug 19, 2009 #17
    Vanadium posted:
    Can we "merge" them?? Interesting idea, but not mine...

    My basic question(s):
    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:
     
    Last edited: Aug 19, 2009
  19. Aug 19, 2009 #18
  20. Aug 19, 2009 #19
    From (2003) http://arxiv.org/abs/physics/0306074.........

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

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


    Page 2:
    and
    The conclusion section on page 18 might also be of interest.
     
  21. Aug 19, 2009 #20

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is spacetime a physical entity?
  1. Quantum Spacetime (Replies: 26)

  2. Spacetime equivariance (Replies: 1)

  3. Witten on spacetime (Replies: 11)

Loading...