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What is vacuum?

  1. May 20, 2009 #1
    Okay, here’s the concept.
    Is it just “empty space” or something else?
    But since relativity; we know that space cannot, and does not exist without time,
    And it’s a continuum "spacetime" rather than space and time.
    So does it means that vacuum is just “empty spacetime”

    There arises a logical contradiction.

    Vacuum should be “empty spacetime” as its concept started after big bang.
    We say there was nothing before big bang; we don’t say there was vacuum before big bang.

    (But wait! “The nothing” is what we say vacuum. So does that mean there was vacuum before big bang?)

    So, according to this concept vacuum should be just "empty spacetime".

    let's take the other one.

    If vacuum were to be “empty spacetime”, and then let’s say we have a region of spacetime, where there is “nothing”. Let’s pass a ray of light form this region, the regions is still in vacuum, as we know that light can travel thru vacuum. But spacetime is no more “empty” as we’ve got an event “the passage of light ray from that region” that is in occurrence. So, as spacetime is no emptier any more but vacuum is still there, this implies that empty spacetime cannot be a vacuum.

    IS not this a contradictory result?

    Also if vacuum is none of these, then what else?
  2. jcsd
  3. May 20, 2009 #2


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    You're thinking waaaay too much about this. A vacuum is space empty of matter. Period. Time is a dimension (like space) and has nothing to do with the question of what a vacuum is. Saying a vacuum should be devoid of time is like saying it should be devoid of length: meaningless.
    Last edited: May 20, 2009
  4. May 20, 2009 #3


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    I like your idea about before the big bang we say there was nothing, but in a vacuum there is nothing, so how does a vacuum differ from before the universe began?

    From that definition the vacuum definitely does exist as SOMETHING-it does have an energy associated with it, possibly a structure too.

    I think we need to be careful we don't stray off into metaphysics talking about the state of the vacuum before the universe began, however.
  5. May 20, 2009 #4


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    There is a difference between an empty set, and a set of {0}. If you can understand something elementary in set theory, why not that?

  6. May 20, 2009 #5
    A vacuum is the absence of matter, not space.
  7. May 20, 2009 #6
    I disagree with this. In vacuum there is not "nothing", there is you and me and more generaly all we know. Just consider the proportion between volumes occupied by elementary particles (where the energy-matter is concentrated) and the whole volume of the universe. After that please tell me if the today universe really differs from "vacuum" (classical sense) except in some specific regions.
    Otherwise for theorist in Quantum domain there is not only one unique empty volume(the vacuum) but a complicated structure of quantum states attached with each configuration in that empty volume (the vacuo).
  8. Aug 25, 2009 #7
    If a vacuum is so easy to understand then why do we have "Pure Vacuums" versus "Perfect Vacuums"? And, to my mind, a "Pure Vacuums" contains nothing...not matter, not energy, not matter/energy...just nothing. Can we discuss "existence" with such a thing?
  9. Aug 25, 2009 #8
    there is also a difference between an artificial creation such as Mathmatics and the real world of matter and energy. To say that an empty set is different from a set of [0] and then comparing that to the "Pure Vacuum" of the real world is comparing apples and oranges. My guess is that Mathmatics and Physics will surely fail us in determing the "physicality" of a "Pure Vacuum" because of the infinite smallness of the volumes involved...infinite smallness and technology just don't go together.
  10. Aug 25, 2009 #9
    Actually, I think this is one of the most important concepts that Physics needs to address. "Pure Vacuum" and what it means. For example, can you have motion in this Universe without "Pure Vacuum" to move into. I think Physics needs to come to grips with the concept of absolute nothing ..."Pure Vacuum" otherwise the Big Bang Theory slides into the world of Religion.
  11. Aug 25, 2009 #10


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    Google for "The Philosophy of Vacuum" by Simon Saunders and Harvey Brown. Amazon will give you a preview that covers the introduction, and the full text of the first two chapters - by Einstein and Penrose. It's a wonderful book, and as I go through it, I have to keep going back to previous materials to make connections, contrast approaches, etc.

    It's an expensive book, but I found a like-new copy on Amazon for $40. I could have gotten it through an inter-library loan, but a book like this needs highlights and margin notes, and that's not good for library books.
  12. Aug 31, 2009 #11
    The "the Philosophy of Vacuum" book attacks vacuum from the wrong end. It should have included discussions of vacuum beginning from "Pure Vacuum" which is the concept of nothingness. Instead it discusses vacuum from a "what does it entail" point of view. This leads to endless discussion of what is in a vacuum rather than whether or not "pure vacuum" can or could exist.
  13. Aug 31, 2009 #12


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    The book is well-crafted and addresses the concept of vacuum from a number of viewpoints. It is dedicated to the nature of space. We cannot imagine space in our universe without the various fields (gravitational and EM at a minimum) that suffuse it, including matter, no matter how diffuse. Dedicating a book to "pure vacuum" would be an exercise in navel-staring.
  14. Sep 1, 2009 #13
  15. Sep 1, 2009 #14


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    No, it is not a concept physics needs to deal with- it was dealt with long ago. Yes, of course, you can have motion without "Pure Vacuum" to move into! I suggest that you learn some physics. The question of "vacuum" is irrelevant to "big bang theory".

    Actually your "otherwise the Big Bang Theory slides into the world of Religion" suggests you need to learn about physics and religion!
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