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Can spacetime exist without energy?

  1. Jul 21, 2009 #1
    As I understand it a vacuum is space that doesn't contain matter, however is it possible to have space that is a vacuum that also doesn't have any energy?
  2. jcsd
  3. Jul 21, 2009 #2


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    No, there is a non-zero vacuum energy in empty space.
  4. Jul 21, 2009 #3
    But if there is matter, there is energy and vice versa, no ?
  5. Jul 21, 2009 #4


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    hmm... well I guess that could depend on the exact definition of "matter," but my understanding is that you can have energy without matter. For instance, photons: they definitely have energy, but I don't know that anyone would consider them matter.

    So if there is matter, there is energy, but not vice-versa.
  6. Jul 21, 2009 #5
    In a "vacuum", as has been pointed out, there is a non-zero vaccuum energy. In addition, because of Heisenberg's Uncertainty, depending on what you're doing/looking at it can be said that there are virtual particle pairs being created and annihilated constantly thus one could say there is also mass.
  7. Jul 21, 2009 #6
    So if space is always filled with energy (photons) does that imply that em waves are waves in a sea of waves, or would that imply interference patterns?
  8. Jul 21, 2009 #7
    The visualization of vacuum energy as consisting of photons is inherintly incorrect. In addition, I believe a photon would be treated as an excitation and thus, conceptually, I'd find it difficult to say that there are non-virtual photons in a vacuum
  9. Jul 21, 2009 #8
    I wasn't trying to express a photon as a particle (still struggling with what a photon is), rather I was trying to say it was a form of energy. In order for you to better understand how I understand em waves so far I will use a comparison. (what i'm about to say isn't neceassarily a statement of fact unless I do actually understand it) I do not consider a photon a "particle" that travels through a medium, just as I do not consider the energy that moves a wave across the ocean a particle (henceforth coined a "water photon" ). Rather the photon, wether an em photon or a water photon, is a unit of energy which doesn't require mass. The way this is comeing together in my head and what it is I am trying to understand is that if space (including vacuums) inherently contains energy (em waves) then to me the energy's medium in a vacum is other em waves.
  10. Jul 21, 2009 #9
    There is nothing wrong with thinking of a photon like a particle since it is one. The notion of an EM wave (i.e. mutually supporting oscillations of a perpinduclar electric and magnetic field) is a CLASSICAL one, not a quantum one. A photon IS a spin 1 boson, which is a particle. Furthemore a photon is no more a "form" of energy than an electron is.

    It is interesting you should bring up the analogy of water waves because all oscillatory phenomena (be it water waves, sound waves, spin waves, magnetic moment waves) can be quantized and treated like what is called a quasi-particle and quasi-particles behave just like particles, they can be created and destroyed, they can interact like fermions or bosons (or in fact, for quasi particles there are other options like anyons). This similarity is certainly reflected in their naming: sound waves propogating in a lattice can be quantized and called PHONONS, spin waves are called SPINONS, magnetic moment waves are called MAGNONS and so on. There are actually some theories of QM/GUT that suggest that elementary particles such as electrons and photons are simply quasi-particles of some special medium. The best example of this off the top of my head is Wen's String-Net theory (which is in no way related to String Theory, despite the similarity in name)
  11. Jul 21, 2009 #10
    I'd imagine you find my responses quite impenetrable and not very helpful. The fact of the matter is is that an understanding of quantum simply cannot be conveyed in a qualitative manner of analogy and diagrams. It is a very complex theory expressed mathematically. It's frustrating but you really can't get anywhere with "understanding" quantum unless you actually learn your calculus, learn your linear algebra and differential equations and crack open a quantum textbook. Now i'm not suggesting you do that (but maybe you want to, I don't know) I'm just saying I'd set your bar a little lower in terms of how much understanding you can expect to get from these sort of qualitative analogy based discussions.
  12. Jul 21, 2009 #11
    quite silly..no one "understands" quantum mechanics...and of course it could be very well understood qualitatively if it's nature was clearly understood...humans just aren't wired that way...

    space generally has three types of energy: vacuum energy, which averages zero and from which particle/antiparticles emanate; non zero vacuum energy also called the cosmological constant also called dark energy; and electromagnetic and gravitational waves coming from outside the volume under consideration...
  13. Jul 21, 2009 #12
    If you can explain the aharonov-bohm effect, or the how's and why's (not just the what) of the uncertainty principle (the actual one, originating from non-commuting operators, not vague descriptions of looking for an electron with a photon) or spin statistics then you are a far better educator than I and I concede the point.
  14. Jul 21, 2009 #13
    Also, I've always strongly dislike this notion that we don't "understand" quantum physics. We understand the physics just fine, it's the metaphysics that people are talking about but have we ever understood the metaphysics of our models? When we though the world was classical did we know WHY the universe obeyed Newton's Laws? Did we know WHY gravity behaved as a spherically symmetrical force with proportionality constant G? No. But you didn't hear people going around saying "well no one UNDERSTANDS newton's laws, we just know how to use them". How is musings on WHY the position and momentum operators don't commute any different? In our model they do and that model seems to correctly model reality to our best experimental precision.
  15. Jul 21, 2009 #14
    Well when you say it like that physics doesn't seem as fun, I think that I am in danger of becoming a metaphysicist :surprised. Though I do have an analytiacal mind and have gone through a calc book on my own time, so the math isn't so much of a problem for me. Though that was some time ago so I am a bit rusty in that regard. To get back on track all space contains energy yes? What your telling me is that I need to crack open some quantum learning materials so that you can talk to me in the language of math instead of English so that you can further articulate your arguments to me in a way that is possible for you.

    Do you suggest that I not ask questions on this form until I have a better understanding?
  16. Jul 21, 2009 #15
    I'm certainly not saying that. Scientific curiosity is a very healthy thing and it is certainly possible to convey the wonder of a lot of quantum oddities through qualitative discussion but to get to a place where one could really understand (in as much as one can "understand" QM) would take the better part of a decade of dedicated study. I suppose i'm really just saying I'd manage your expectations. Whenever you compare general relativity to a bowling ball distorting rubber or visualize the quantum vacuum as a "foam" of fluctuating particles and energy you're going to lose most of the potential understanding and the analogy is also going to throw off your intuition on a lot of things. It simply can't be helped. Just realize that quite a bit is getting 'lost in translation', so to speak.

    As for metaphysics, yes. It's not really the pervue of physics to ask questions like WHY the universe is the way it is (although, sometimes there is actual incite to be found by asking the question, like WHY do particles follow Newton's laws? Because particle's take paths of extremal action). However at a certain point you just reach a barrier with why's and you simply have to say "that's just the way it is".

    In general, these kind of questions are philosophical in nature and unfortunately in practice the vast majority of "philosophers of science" end up being an annoying nuisance then of any benefit to understanding. On that note I'm going to post a link to Louis CK's "Why" bit:
    Last edited by a moderator: Sep 25, 2014
  17. Jul 21, 2009 #16
    sorry maverick I am missing out on the joke cause i'm running ubuntu (linux) and my comp setup isn't able at this moment to play flash/youtube videos. As for the 10 year bit, I got lots of time and the more I learn the more I want to.
  18. Jul 21, 2009 #17
    I suppose I can make an attempt at at least conveying some of the difficulty in answering these questions. Imagine superposing two wavefunctions/light EM waves/whatever that are cos(wt) and -cos(wt). The result is a net oscillation of zero. Now in quantum mechanics if we have an area of space that has a zero net probability function (i.e. the wavefunction is zero in that area, i.e. a vacuum) it is not only physically but metaphysically impossible to say that there are zero particles their vs. two particles whose wavefunctions exactly cancel, vs. 4 particles whose wavefunctions exactly cancel or a billion particles whose wavefunctions exactly cancel you just can't tell. But it's not a situation of the form "this is unknowable we're screwed" it's a situation of the form "it doesn't make a lick of difference, if we measure the net wavefunction in that region it'll be zero, this presents no problem for us in terms of prediction". Often a wavefunction, in general, is a complicated creature and we often deal with it by taking a fourier transform (I would encourage you to look for a little java applet demonstrating what this is) and represent it as a sum of basis functions. So is there one particle there who have this crazy wave function? Or is their an infinite number of particles their whose summed wavefunction make this total wavefunction? *shrug* the math'll work out no matter what mental image you have in your head.

    So I hope I'm conveying a sense of what it is very difficult to say "there is something there or there isn't something there". When we quantize space and essentially slap a quantum oscillator on each "grid point" those oscillators have a ground state energy that is not zero. Where there is energy there is Heisenberg's uncertainty which basically says that as long as the uncertainty in the energy of a system times the uncertainty in the time of measuerment are greater than some value it's all allowed. Which means when you ask "was their an electron-positron their" you can get an answer like "depends how long we take to measure". And if that seems to make no sense well I'd say the problem is the visualization/expectations of particles being these distinct pee like things shooting around and interacting with each other. At the end of the day you could say there's a quantized probability amplitude (or in the language of second quantization a certain energy assigned to) each "grid point". Any attempt to say, well that amplitude was caused by three electrons with the following wavefunctions vs. 6 billion electrons with the following wavefunction is not wrong, it's flat out meaningless.
    Last edited: Jul 21, 2009
  19. Jul 21, 2009 #18
    In conclusion, we can predict experimental results helluva accurately with QM but it's helluva weird.
  20. Jul 22, 2009 #19
    Even with virtual pariticles there would be no NET mass.
  21. Jul 22, 2009 #20

    Well that's no really true. Energy is mass (not to mention the fact that there's no such thing as negative mass, in quantum anyways so if you have a particle pair it's going to add 2m the mass m of 1 particle and take away 2mc^2+2KE from the energy, where KE is the kinetic energy that one of the particle starts with).
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