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Spacetime - it warps, it curves, but can't expand?

  1. Sep 9, 2007 #1
    Spacetime -- it warps, it curves, but can't expand??

    I have a problem understanding this. The general consensus of respected posters in cosmology is that space (and I assume spacetime) is nothing, therefore it cannot expand. Distances just increase. On the other hand, when it comes to gravity and GR, spacetime is no longer nothing -- it warps and curves and holds giant bodies in place. These seem like powerful and real physical properties, yet when it comes to the expansion of the universe it is apparently heretical to suggest that spacetime can have properties which can contribute to or cause the expansion. Is there a layman's explanation for why we can assign powerful properties to spacetime in one theory and dismiss the possibility that it has any effect in another theory?
  2. jcsd
  3. Sep 9, 2007 #2


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    Um, what general consensus is that? What you describe doesn't sound like mainstream cosmology to me.
  4. Sep 9, 2007 #3
    Hi Russ,

    These, for example:

    MeJennifer: "Expansion of space" is a completely wrong terminology.
    It implies that space is some sort of a substance that can expand and contract.

    oldman: nonsense that is written about "expanding space"

    Marcus: I want to steer clear of thinking of "units of space" or anything suggesting that space is a medium or a substance.

    there is NO problem of "where extra space comes from" because space is not a material substance-----it is just the distances between things a web of geometric relations----you DONT HAVE TO MAKE MORE.

    and Wallace: the 'expansion' (I agree that's an awful word to describe the effect)

    That seems like a general consensus to me that space does not expand.
  5. Sep 9, 2007 #4
    It sounds more like they are disagreeing with the idea that space is an object. Space is nothing. You are talking about the expansion of the universe? I guess the distance between objects is increasing but space cannot increase as it isn't anything.
  6. Sep 9, 2007 #5


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    Space is not "nothing" in general relativity, it's a manifold with dynamical properties (curvature) of its own. But marcus is right that it's not a substance so that the "amount of space" is not something that should be conserved like energy, there's no need to worry about where new space "comes from". Still, I don't think that means it's wrong to talk about the expansion of space, have a look at this thread along with this paper posted on that thread for further discussion.
  7. Sep 9, 2007 #6
    There is no dichotomy; in such examples cosmologists also use general relativity theory. Now in both these fields, it is very convenient language to say that "spacetime itself" warps, stretches, curves, expands, etc. However, the one big downside of such colourful and imaginative language is that non-experts often mistakenly get the inappropriate impression that "spacetime itself" should be thought of as a "thing" (like in ether theories). Whereas, in all cases, the language is really just intended as shorthand for long boring statements like "the distance between inertial test masses has a nonzero second derivative" or "circumference to diameter ratios are scale dependent".
  8. Sep 9, 2007 #7
    The fact that space is not material does not mean it does not have properties - it has a characteristic impediance, a permeability, a permittivity, and as near as I can extrapolate Einstein's several comments about space, it is in some manner responsible for inertial affects.
  9. Sep 9, 2007 #8
    I would like to take a moment to thank you and all the experts in this forum for taking the time to try to make physics understandable to laymen and other interested parties like me. You all are a wealth of information.

    I read the paper you cited and found it interesting that it quotes Harrison, whose book on cosmology I have read. I found Harrison's explanation of expanding space as a solution to superluminal velocities very satisfying and intuitive, though there certainly appears to be differences of opinion. He for one clearly states that space expands, whereas others state it is just a construct. I suppose that both approaches reach the same conclusions and predictions to some extent, and this may be more important than the methodology one prefers to think in terms of.
  10. Sep 9, 2007 #9


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    Yes, all the different positions are still assuming the same physical theory--general relativity--so there's no difference in predictions, it's just a question of how one prefers to conceptualize the mathematics involved.
  11. Sep 18, 2007 #10
    It seems worthwile to also consider the quantum perspective on vacuum. As far as I can tell, the quantum folks think vacuum is a real "thing" that has measurable characteristics. Of course their calculations so far produce a vacuum energy which is 120 orders of magnitude larger than the cosmological constant. At least the Casimir effect is a well documented feature of vacuum space. Here are some points from Wikipedia:

    "In quantum mechanics, the vacuum is defined as the state (i.e. solution to the equations of the theory) with the lowest energy. To first approximation, this is simply a state with no particles, hence the name.

    "However, even an ideal vacuum, thought of as the complete absence of anything, will not in practice remain empty. One reason is that the walls of a vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. If this soup of photons is in thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. Another reason that perfect vacuum is impossible is the Heisenberg uncertainty principle which states that no particle can ever have an exact position. Each atom exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Even the space between molecules is not a perfect vacuum.

    "More fundamentally, quantum mechanics predicts that vacuum energy will be different from its naive, classical value. The quantum correction to the energy is called the zero-point energy and consists of energies of virtual particles that have a brief existence. This is called vacuum fluctuation. Vacuum fluctuations may also be related to the so-called cosmological constant in cosmology. The best evidence for vacuum fluctuations is the Casimir effect and the Lamb shift.[8]

    "In quantum field theory and string theory, the term "vacuum" is used to represent the ground state in the Hilbert space, that is, the state with the lowest possible energy. In free (non-interacting) quantum field theories, this state is analogous to the ground state of a quantum harmonic oscillator. If the theory is obtained by quantization of a classical theory, each stationary point of the energy in the configuration space gives rise to a single vacuum.

    "Not only is the Casimir effect easily and accurately measured in specially designed nanoscale devices, but it increasingly needs to be taken into account in the design and manufacturing processes of small devices. It can exert significant forces and stress on nanoscale devices, causing them to bend, twist, stick and break.

    "Other experimental evidence includes spontaneous emissions of light (photons) by atoms and nuclei, observed Lamb shift of positions of energy levels of atoms, anomalous value of electron's gyromagnetic ratio, etc."
  12. Sep 24, 2007 #11
    However then how do you explain gravity? It is the force that conserves space, bringing particles together so that they occupy less space.
  13. Sep 24, 2007 #12
    Hi Denton,

    Gravity works in two ways. First, it curves local space, causing an apparent force on nearby masses. In effect, it pulls nearby masses closer THROUGH space. This is the force we normally associate with gravity.

    Gravity (supposedly) also counteracts the effect of the "momentum of spatial expansion". In doing so, gravity can slow down the expansion rate, stop it, or even cause space to contract. However, this effect is much less intense (and noticeable) than the first way gravity works as described above.

    Space definitely is not "conserved", no matter how you look at it.

  14. Sep 24, 2007 #13


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    This is an imaginative (and erroneous) definition of conservation.
  15. Sep 24, 2007 #14


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    As DaveC426913 says, you're misunderstanding "conservation", in physics it means a quantity which doesn't change over time--for example, "conservation of energy" means that the total energy of all the parts of a system will always stay constant, provided the system isn't receiving energy from outside its boundaries, or losing energy to the outside.
  16. Sep 24, 2007 #15
    Why don't we think of time dilating as the reason that space is expanding? Thinking of space as the distance we measure via the photon and its motion relative to us upon detection, and time as being the distance that it has moved, as near as we can tell today, since it was emitted. Time is not anything, as a dimensionless point we think of it as nothing more than a way to keep track of events. Is my thinking of time as both something and nothing or if you will a dimensionless point, always dilating not just as Planck’s lowest common denominator for reality, but also as the leading edge of our visible universe wrong?
  17. Sep 25, 2007 #16


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    There are a couple of different sorts of time in relativity. The sort of time that one measures with a clock is not a dimensionless point, but an interval. Just as you measure the length of a space-like curve with a ruler, you measure the time interval along a time-like curve with a clock.

    Another sort of time is a number that one assigns to events. In this case, it would be correct to say that the events are dimensionless points, but the time itself isn't a point, it's a number. This would be generally called "coordinate time".

    The curve traced out by a pointlike object as it moves through time is called a worldline, and as the name implies, it's one dimensional. Since a worldline is a time-like curve, the interval between any two points on a worldline is a time interval, the sort that I mentioned eariler that can be measured with a clock.
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