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Four forces or five?

  1. Nov 9, 2011 #1
    I read an article the other day by a physicist who said there are only four forces and for the first time in my life it struck me as incomplete. Is not the force which is causing the universe to expand and which is speeding up, not at least a candidate for a fifth force? Since we know so little about it, can it be ruled out?
     
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  3. Nov 9, 2011 #2

    DaveC426913

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    It may be force but there's no reason yet to think it is a fundamental force.

    The question isn't can it be ruled out; the question is: is there sufficient cause to rule it in. i.e we don't invent new entities until the old ones are proven inadequate.
     
  4. Nov 9, 2011 #3

    BillSaltLake

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    I always thought that quantum pressure (the effect that prevents two odd-half-spin particles from occupying the same location if they are in the same quantum state) should be called a fifth force, or at least a fourth force if electroweak is a single force. Quantum pressure prevents certain high-density stars from collapsing into black holes. However, it can't be called the "cause" of expansion, unless some very circuitous reasoning is applied.
     
  5. Nov 9, 2011 #4
    This 'cause' is supposedly said to be dark energy. And as we all know, we are still in the dark about it.

    Sigh.

    Anyways, what makes a particular form of interaction eligible to be called a fundamental force? Should it be unexplainable using conventional theories and the other 4 forces?
     
  6. Nov 10, 2011 #5

    Chalnoth

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    You don't need a fifth force for this. It's just gravity.

    There are a number of extremely careful and detailed searches for additional forces. So far they all have come up empty.
     
  7. Nov 10, 2011 #6
    Since it appears to do the opposite of what gravity does, how can it be gravity? Not scoffing, just asking. What steps of empirical observation, therorizing, conception, reasoning, math, or whatever could be conceivably be used to demonstrate that it is gravity? Again, I am not scoffing, but sincerely interested in the reply.
     
  8. Nov 10, 2011 #7

    Chalnoth

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    In General Relativity, the gravitational force depends not only upon energy density (in Newtonian mechanics, this is just the mass), but also upon pressure. With normal matter, which usually has negligible pressure compared to its mass energy, General Relativity describes gravity as we expect it to behave. If instead you have matter that has significant positive pressure, such as electromagnetic radiation (light), then it tends to gravitate and respond to gravity more strongly than we would naively expect (this is observed by the deflection of light waves by gravitational lensing being twice what we would expect from Newtonian gravity).

    But dark energy has negative pressure. A very large negative pressure. So it has the opposite effect: gravity actually causes dark energy to want to push away from itself. This causes the accelerated expansion.
     
  9. Nov 10, 2011 #8
    Chalnoth, what pressure do you mean? Are you sayng that a mass which emits alot of EM radiation experiences a stronger gravitational attraction to another mass?

    How does this lead to negative gravitational force and a possible explanation for dark energy expansion? (which is itself an increase in the measurement of distance between two very distant gravitationally unbound bodies)
     
    Last edited: Nov 10, 2011
  10. Nov 10, 2011 #9

    Chalnoth

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    Well, with radiation, if you have a box full of radiation (this happens naturally if you just heat the box up), then that radiation will exert a pressure on the walls of the box equal to one third its energy density.

    By contrast, if you fill a box with gas, though it will exert some pressure, that pressure will be completely and utterly negligible compared to its energy density (because the energy density includes its mass energy).

    No. I'm saying that the radiation itself exerts a gravitational force. This is particularly important in the very early universe, when most of the energy density was in radiation. At that time, when our universe was dominated by radiation, the extra gravitational attraction due to the pressure of the radiation caused the expansion to slow even more rapidly than it did later, when the dominant energy was in matter.

    Individual objects, by contrast, can never emit enough radiation to compete with their mass energy, just because mass energy is so tremendously large.

    Well, you can calculate it explicitly in General Relativity. Positive pressure = stronger attraction, expansion slows down faster. Negative pressure = weaker attraction, potentially even repulsion, causing the expansion to either slow down more slowly or speed up.
     
  11. Nov 10, 2011 #10
    No. I'm saying that the radiation itself exerts a gravitational force.

    Thanks Chalnoth, I didnt know this. Does this mean that radiation (photons) has mass? Is this the same mass pressure that would make a solar sail work?

    I still cant see how you could ever get negative pressure or negative gravity from this.
     
    Last edited: Nov 10, 2011
  12. Nov 10, 2011 #11

    Chalnoth

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    No, not at all. In General Relativity, gravity doesn't respond to mass. It responds to energy density, momentum density, pressure, and shear (twisting forces).

    That energy density includes mass energy, which completely dominates all of the other components for normal matter (except in extreme conditions, such as neutron stars).

    Well, some forms of matter are weird. The cosmological constant can be seen as a sort of matter with pressure that is equal to its energy density, only negative. A large number of quantum mechanical fields that are relatively easy to write down on paper (though we have no evidence for) also have similar negative pressure.
     
  13. Nov 10, 2011 #12

    BillSaltLake

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    Is there a short explanation of negative pressure somewhere? Otherwise I'll write one here. (I.e., photons exert pos. pressure so as a piston expands, they lose energy, and the total # of photons remains same. In contrast, dark energy, if it is other than just a Λ term, exerts neg. pressure, so energy per particle increases w/ expansion, required to keep dark energy density a constant.)
     
  14. Nov 10, 2011 #13
    Chalnoth is then an accepted explanation for dark energy? I thought it was not possible at present to explain dark energy.
     
  15. Nov 10, 2011 #14

    DaveC426913

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    No, Chalnoth is then an accepted explanation for loud, vulgar eructations and noxious fumes but he is not an explanation for dark energy. :biggrin:
     
  16. Nov 10, 2011 #15

    Chalnoth

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    Well, there are many possible explanations. The simplest explanation that agrees with the current data is the cosmological constant, and as a result it is the leading explanation. But so far the evidence is far, far too sparse for anybody to say with any degree of confidence that the cosmological constant is the explanation.

    So it's not so much that we can't explain it, but rather the problem is that the evidence is far too sparse at the moment to say which of the many possible ideas is the correct one (if any).
     
  17. Nov 10, 2011 #16
    Dave lol! I forgot to add "this"
     
  18. Nov 10, 2011 #17
    One of these days I am going to have to find out what GR is really all about. I did SR and QM (but apparently no one understands that anyways). The past 30 years I have been involved with RF so I forgot most of what I knew about the rest of Physics.
     
  19. Nov 10, 2011 #18

    phyzguy

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    This page has a nice explanation of why the vacuum has negative pressure. The mechanism is easily understood, it is the magnitude that we have difficullty explaining.

    http://www.astro.ucla.edu/~wright/cosmo_constant.html
     
  20. Nov 10, 2011 #19
    In the piston analogy I thought it was the pressure on the outside from randomly moving air molecules pushing the piston back in?
     
  21. Nov 10, 2011 #20

    Chalnoth

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    In this analogy, consider the piston to be in a vacuum.
     
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