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

Empty space is something?

  1. Jul 23, 2007 #1
    I have a question.

    If empty space is, according to relativity theory, curved, and it is expanding (ie. it has properties) then logically, empty space must be made from "something" right?

    And this something could be interacted with?
  2. jcsd
  3. Jul 23, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    It's made from space.
    Not necessarily.

    100years ago people thought that 'something' was needed for waves to travel through - like water waves need water, light waves must need some kind of physical space to travel through ( called the aether)
    This isn't true - light can travel through empty space without having to interact with it.
    Last edited: Jul 24, 2007
  4. Jul 23, 2007 #3


    User Avatar
    Gold Member

    Einstein believed that "empty" space was indeed "something". In his 1920 book on General and Special relativity, he insisted that there could be no gravitational lensing of light unless the velocity of propagation of light through the vacuum varied with location, couching this refraction in terms of classical optics. He also stated in his 1920 Leiden address and later in his 1924 essay "On the Ether" that space was conditioned in its variable properties by the ponderable matter embedded in it. These are not concepts that were well-received in his time, nor are these concepts taught today, though they were central to his life-long pursuit of explaining the connection between space, matter, gravitation and electromagnetism. Pursue them at your peril, if you wish to work professionally in physics or astronomy. BB has subsumed GR to the extent that it is sacrosanct and unquestionable. Roger Penrose says in his popular lectures that QFT and GR will both have to change before they can be resolved. IMO, GR is going to take the biggest hit by far.
  5. Jul 23, 2007 #4


    User Avatar

    Well it's almost never 'empty' according to
    quantum mechanics.

    Your question seems a bit circular, though. Yes, it's
    space. Space by definition can be interacted with in
    that it has properties of dimension, contents wrt.
    fields, temperatures, energies, particles. Its geometry
    is effected by Hubble type expansion as well as by
    metric distortions due to enegry / mass / gravity waves.

    It can distort immensely e.g. look at what happens in
    string theory at very small scales of space. Also look at
    what happens near singularities etc.

    So almost "everything" we can see / conceive of in the
    universe has a significant effect upon space by
    its distortion or filling with fields.
  6. Jul 23, 2007 #5
    Ah, right. This reminds me to mention that the "vacuum of space" is really actually more interesting than "nothing," take a look at Wikipedia:

    The energy of space that isn't zero.

    - Bryan
  7. Jul 24, 2007 #6
    I understand that space is never really empty as such, but I am talking about just that....empty space!

    Is is not logical to assume that if this "something" can expand, well it must be something? Some kind of "fabric" perhaps?
  8. Jul 24, 2007 #7


    User Avatar
    Science Advisor
    Homework Helper

    Yes space must be something = space.
    It can have properties as defined by General Relativity, but not necessarily material properties like temperature, strength mass etc.
    It's a bit like saying time exists so time must have fabric like properties.

    Saying it's 'not logical' or 'not intuitive' when describing either quantum mechanics or the structure of space-time doesn't really make sense.
    You have intuition about physical objects with a certain size and temperature around you - you can't always extend that understanding to the rest of the universe.

    ps _ I left out quantum vacuum stuff - classical space is complicated enough!
  9. Jul 24, 2007 #8
    This is one of the most intriguing questions in Physics. Following the principle of Occam's razor, the theory of space and time has been devised to eliminate any mention of a medium. That formalism works, as long as we stay within the confines of relativity. But what if we turn Occam's razor around and ask the question: Does relativity exclude the possibility of a medium?

    As we attempt to explain dark energy or vacuum energy, or even gravity itself, we may find that we need to put something back in. But so far we've been able to avoid it.
  10. Jul 24, 2007 #9


    User Avatar
    Gold Member

    An honest appraisal of the Casimir effect requires us to acknowledge a basic quality of the vacuum. The vacuum is suffused with zero-point fluctuations, and if we put a pair of polished plates close enough to suppress these fluctuations at wavelengths larger than the separation, the pressure exerted outside the gap is larger than that exerted withing the gap, forcing the plates together. The reality of this effect has been confirmed repeatedly, with bounding surfaces of various geometries. Even "empty" space has properties that can be modified by the presence of embedded matter.
  11. Jul 24, 2007 #10
    You may find interesting this paper

    R. L. Jaffe "The Casimir effect and the quantum vacuum" http://www.arxiv.org/hep-th/0503158 [Broken], Phys.Rev. D72 (2005) 021301

    which directly challenges your statement that the Casimir effect is a direct evidence of zero-point vacuum fluctuations.

    Last edited by a moderator: May 3, 2017
  12. Jul 24, 2007 #11
    No - empty space is not a something in relativity.
  13. Jul 24, 2007 #12


    User Avatar
    Gold Member

    Einstein in his 1920 book on relativity, his 1920 Leiden address, and his 1924 essay "On the Ether" disagreed with your viewpoint. I don't think I'd bet against him.
  14. Jul 24, 2007 #13
    In 1939, Einstein wrote that black holes cannot exist (Einstein, A. 1939, AnMat, 40, 922) and in 1936 said that there can be no gravitational waves (http://www.physicstoday.org/vol-58/iss-9/p43.html [Broken]). Appealing to authority is not the way to have a discussion.

    As for the nature of spacetime, see the recent contributions;

    Expanding Space: the Root of all Evil?
    Authors: Matthew J. Francis, Luke A. Barnes, J. Berian James, Geraint F. Lewis
    (Submitted on 3 Jul 2007)

    Abstract: While it remains the staple of virtually all cosmological teaching, the concept of expanding space in explaining the increasing separation of galaxies has recently come under fire as a dangerous idea whose application leads to the development of confusion and the establishment of misconceptions. In this paper, we develop a notion of expanding space that is completely valid as a framework for the description of the evolution of the universe and whose application allows an intuitive understanding of the influence of universal expansion. We also demonstrate how arguments against the concept in general have failed thus far, as they imbue expanding space with physical properties not consistent with the expectations of general relativity.


    Coordinate Confusion in Conformal Cosmology
    Authors: Geraint F. Lewis, Matthew J. Francis, Luke A. Barnes, J. Berian James
    (Submitted on 13 Jul 2007)

    Abstract: A straight-forward interpretation of standard Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies is that objects move apart due to the expansion of space, and that sufficiently distant galaxies must be receding at velocities exceeding the speed of light. Recently, however, it has been suggested that a simple transformation into conformal coordinates can remove superluminal recession velocities, and hence the concept of the expansion of space should be abandoned. This work demonstrates that such conformal transformations do not eliminate superluminal recession velocities for open or flat matter-only FRLW cosmologies, and all possess superluminal expansion. Hence, the attack on the concept of the expansion of space based on this is poorly founded. This work concludes by emphasizing that the expansion of space is perfectly valid in the general relativistic framework, however, asking the question of whether space really expands is a futile exercise.


    A direct consequence of the expansion of space?
    Authors: Michal Chodorowski (Copernicus Center)
    (Submitted on 19 Oct 2006 (v1), last revised 27 Mar 2007 (this version, v3))

    Abstract: Consider radar ranging of a distant galaxy in a Friedman-Lemaitre cosmological model. In this model the comoving coordinate of the galaxy is constant, hence the equations of null geodesics for photons travelling to the distant galaxy and back imply the following equation:
    \int_{t_e}^{t_r} dt/a(t) = \int_{t_r}^{t_o} dt/a(t).
    Here, t_e, t_r and t_o are respectively the times of emission, reflection and observation of the reflected photons, and a(t) is the scale factor. Since the universe is expanding, a(t) is a monotonically increasing function, so the return travel time, t_o - t_r, must be greater than the forward travel time, t_r - t_e. Clearly, space expands, and on their way back, the photons must travel a longer distance! The present paper explains why this argument for the expansion of space is wrong. We argue that, unlike the expansion of the cosmic substratum, the expansion of space is unobservable. We therefore propose to apply to it -- just like to the ether -- Ockham's razor.
    Last edited by a moderator: May 3, 2017
  15. Jul 24, 2007 #14


    User Avatar
    Gold Member

    You are welcome to scatter-shot citations as you wish. the mathematical models that glommed onto Einstein's formulations of SR and GR bothered him greatly because he needed to develop a mechanical model for the emergence of gravitation, inertia and EM effects. He spent his whole life trying to bridge this gap.
    Last edited by a moderator: May 3, 2017
  16. Jul 24, 2007 #15
    This is not a "scatter-shot" of citations, but part of a recent set of ongoing discussions on the nature of spacetime. As noted in these paper, in relativity spacetime is not a thing - it's a mathematical construct
  17. Jul 24, 2007 #16


    User Avatar
    Gold Member

    Spacetime is not a mathematical construct. If you choose to model it in these terms, you can see it this way, but spacetime will exist regardless of the nature of your interpretation.

    If you think that your mathematical model of spacetime has an objective reality that is somehow superior to the real deal, you have adopted a problem of perception that pervades cosmology of the 19th century. A mathematical model that predicts the behavior of gravitationally-bound bodies does not explain the mechanics of that behavior. We should start teaching epistemology in grade schools so that over-educated graduates with poorly-developed reasoning capabilities cannot clog our processes.
  18. Jul 24, 2007 #17

    Great question! To answer in short, if we only use Einstein's theory of relativity (both General and Special), empty space by itself isn't really anything. Rather, we deal with a metric, which is a mathematical tool which is used to describe the distribution of matter and energy in an empty space. The result is a sort of 3D stretchable grid. The expansion is due to the distance between objects changing. Empty space itself, in Einstein's equations, isn't made out of anything.

    I'm a little disappointed by answers given in this thread, especially given the high quality of answers given in a discussion on this exact same topic a few weeks ago. The question here is about how "empty space" can expand, specifically as envisioned by Einstein's theory of relativity.

    Despite some of the answers offered, quantum fields filling space are not part of this picture. Einstein knew little of them when he developed relativity, and his equations do not take them into account.

    The person here who best understands the question is Cusp. I thank him for his references to recent papers on the topic, which wipe away the lingusitic confusion and wordplay which have bedeviled this field for decades.

    I suggest that people read the recent discussion: How can "empty space" expand? (Reality behind the GR equations.)
    Every person who answers there properly understands the question, and is aware of the linguistic confusion that far too often surrounds this issue.


  19. Jul 24, 2007 #18


    User Avatar
    Gold Member

    For those who think that "empty space" is a null background against which all the interplay of our universe is worked out: here is a great place to explain how that happens. Please drop in.
  20. Jul 24, 2007 #19
    Just like the wavefunction and the magnetic field, spacetime is a construct. You can never devise a test (within relativity) to *prove* its existance. If you ant to think this implies existance, then be prepared for it to vanish in a future model of the universe.
  21. Jul 24, 2007 #20
    Nature abhors a space-time continuum.
  22. Jul 24, 2007 #21


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Joza, your question was answered correctly by Cusp early in the thread. All I can suggest is that you try to understand his answer. In your original post you said

    and then Cusp said
    Joza you are wrong in your beginnng assumption. the popularizers LIED to you and gave you the idea that in Gen Rel the continuum has properties.

    What has properities, in Gen Rel, is a collection of distances called the "gravitational field". The properties reside in the distances and in the relations among them.

    Einstein said that the points of spacetime do not have even a shred of physical reality. Some nice quotes. sound especially pungent in German. I will look them up if you are interested in his exact words.

    More exactly what has properties is an equivalence class of metrics under diffeomorphism but that is more of a technicality. It works to think of the gravitational field as represented by a metric (a distance function). Where you just THROW AWAY any idea of an underlying continuum you might have used to get the definitions going to start with.

    So all you have left is a web of RELATIONSHIPS between events.

    I think Cusp was saying this, I didnt read all his posts. Anyway that is the right idea.

    The metric, that web of relationships, is what has to be a solution of the Einstein Equation. That is what Gen Rel is----an equation that all metrics have to satisfy.

    What you have to do to quantize Gen Rel is quantize that web of relationships---quantize the metric and its relation to matter.

    There is still no "space" in the picture. There never is. and never was. They just LIED TO YOU and told you that because it is too hard to explain the more abstract relational theory.

    And I do that too. I say SPACE EXPANDS when what I really mean is that certain DISTANCES are expanding in a certain metric which is a solution to the basic eqn.

    I have to go, back tomorrow or whenever.
  23. Jul 25, 2007 #22
    The properties of Space are Sui Generis - we customarily think of reality in terms of condensed matter - particles - but closer examination reveals they are not real chunks of material - rather waves, radiation and energy in some not well understood combinations. But Space is different. As Hawking said "Empty Space isn't Empty"

    Einstein concluded that matter concentrations condition space - he rejected Mock's principle in favor a view that related the Inertial properties of matter to the local properties of space. But the notion of a substantive space is by no means confined to Einstein - Dirac, Bell and others voiced similar sentiments.

    For those who claim space is not something substantive, there is much to explain.... the 377 ohm impedance of free space, its permittivity, permeability, the instantaneous inertial reaction of masses acted upon by forces, to name but a few. Why should the velocity of em radiation be determined by the properties uo and eo? How are de Brogie waves communicated except by some spatial property? How does one justify the interpretation of rotating space around black holes, or the Robertson red shift as a stretching of space? The list of things that can be most easily explained by attributing properties to space is numerous - i suspect, however, all the features that make space a mystery can be rationalized by other theories - but i wonder how many of these these alternatives are themselves based upon questionable foundations.
  24. Jul 25, 2007 #23


    User Avatar
    Gold Member

    Another thing to pay attention to is that in GR, space and time aren't fundamental. What is fundamental are events, we use space and time as a way to measure the interval between events. But it isn't always straightforward to calculate the interval using measurements of space and time. Next to a massive object for example the calculations can be quite difficult. But we don't have to say that space and time are "real" anymore than latitude and longitude are "real", they're just quantities that we can easily measure and make predictions using those measurements.
    But this all depends on your definition of "real". If you mean that they can interact with other things than as I said, this doesn't have to be true (though it doesn't mean that it isn't!). If you mean that they influence our predictions than they're definitely "real" - as our predictions are usually expressed in terms of space and time.
  25. Jul 25, 2007 #24

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Lots of folks, including Einstein near the end of his life, (have) speculate(d) that spacetime can't be modeled by a continuum.

    "One can give good reasons why reality cannot at all be represented by a continuous field. From the quantum phenomena it appears to follow with certainty that a finite system of finite energy can be completely described by a finite set of numbers (quantum numbers), This does not seem to be in accordance with a continuum theory, and must lead to an attempt to find a purely algebraic theory for the description of reality. But nobody knows how to obtain the basis of such a theory." (Albert Einstein, 1954)

    "On th other hand, I believe that the theory that space is continuous is wrong, because we get these infinities and other difficulties, and we are left with questions on what determines the size of all the particles. I rather suapect that the simple ideas of geometry, extended down into infinitely small space, are wrong. Here, of course, I am only making a hole, and not telling you what to substitute." (Richard Feynman, 1964)

    "My own view is that ultimately physical laws should find their most natural expression in terms of essentially combinatorial principles, that is to say, in terms of finite processes such as counting or other basically simple manipulative procedures. Thus, in accordance with such a view, should emerge some form of discrete or combinatorial spacetime." (Roger Penrose, 1972)
  26. Jul 25, 2007 #25


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    George, Im glad to see the 1954 quote from Einstein, and also the ones you chose from Feynman and Penrose.
    Einstein was probably various sides of the fence about space at different times. I'm suspect you may know these 1915 quotes but will post them anyway partly because I like the way it comes across in German

    “Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität. ..."

    “Thereby time and space lose the last vestige of physical reality”.

    (To try to paraphrase, I guess you could say space does not have physical existence, but is more like a bunch of relationships between events)

    In case anyone wants an online source, see page 43 of this pdf at a University of Minnesota website
    www.tc.umn.edu/~janss011/pdf%20files/Besso-memo.pdf [Broken]

    ...In the introduction of the paper on the perihelion motion presented on 18 November 1915, Einstein wrote about the assumption of general covariance “by which time and space are robbed of the last trace of objective reality” (“durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt werden,” Einstein 1915b, 831). In a letter to Schlick, he again wrote about general covariance that
    thereby time and space lose the last vestige of physical reality” (“Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität.” Einstein to Moritz Schlick, 14 December 1915 [CPAE 8, Doc. 165]).

    To sum up, both quotes are from Nov-Dec 1915, one being from a paper on perihelion motion. and the other from a letter to Moritz Schlick a few weeks later.
    Last edited by a moderator: May 3, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook