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I Anti-gravitons effect on spacetime

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  1. Aug 10, 2015 #1
    So, I'm still a bit new to the realm of physics but I was discussing theorized particles with my professor the other day and came to the conclusion, if gravitons do exist, could anti-gravitons be an explanation for why the universe is expanding? I was reading around and came across a similar topic saying that for this particle to have an anti-gravity property, it would basically need negative energy/mass. Being that the Higs Boson is the particle that provides mass, could there possibly be an anti-Higs particle that could be a solution to this idea of needing negative energy? Therefore having an opposite effect of gravity? Or could it be possible under certain conditions, to have an inverse curvature of space time, similar to the curvature produced by matter interacting with space time to produce a similar effect? I understand this is all very theoretical considering gravitons have not been discovered, but nonetheless its an entertaining idea.
     
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  3. Aug 10, 2015 #2

    Drakkith

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    Gravitons are their own antiparticles. In other words, there is no such thing as an anti-graviton. There are only gravitons. The same is true for the higgs boson.
     
  4. Aug 10, 2015 #3
    So what makes them their own anti-particles? I've heard that about photons as well but I've never had it explained
     
  5. Aug 10, 2015 #4

    Drakkith

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    I'm not certain, but my understanding is that every particle (normal or anti) has various quantum numbers assigned to it that describe different properties, such as charge, lepton number, and spin (isospin?). Each particle-antiparticle pair will have opposite numbers for each property. So an electron has a charge of -1, isospin of -1/2 and lepton number of +1, while a positron would have a charge of +1, isospin of +1/2, and lepton number of -1.

    Photons, gravitons, and other particles which are their own antiparticles usually have a zero for all of these quantum numbers, so it's impossible to have an antiparticle since there is no opposite number to zero.

    Someone correct me if I'm mistaken.
     
  6. Aug 10, 2015 #5

    Chronos

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    The simple answer is the graviton, if it exists, must be a boson, With the exception of the W boson, all known bosons are their own antiparticles. The W boson is unique among bosons because it carries a charge. An antigraviton could exist if the graviton has a charge, but, this is highly unlikely. Empty space is chargeless which means the graviton, which would act across empty space between galaxies, must be chargeless.
     
  7. Aug 10, 2015 #6
    Thanks guys, I appreciate the quick responses. I learned a bit more out of this post than I expected.
     
  8. Aug 10, 2015 #7
    Is there any particular reason the W boson is the only boson which carries a charge?
     
  9. Aug 10, 2015 #8

    Chronos

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    The emission of a W boson can either increase or decrease the charge and spin of a particle by 1 unit. As such, the W boson can change a down quark into an up quark and vice versa. . In order for a neutron to decay into a proton one of its down quarks must be converted to an up quark and an electron neutrino and W boson are emitted to carry off the charge difference. The W boson then promptly decays into an electron and electron antineutrino. Thus the net charge of the decayed neutron is conserved. Why the charge must be conserved is unknown.
     
  10. Jan 18, 2017 #9
    As far as I know, gravitons have not actually been detected, only hypothesised. So how can you you state that they are their own antiparticle and there is no such thing as an anti-graviton? Science requires repeatable experimental results to be acceptable as proof, not just hearsay.
     
  11. Jan 18, 2017 #10

    Vanadium 50

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    We use words to express a concept. The word "graviton" and "anti-graviton" express the same concept. (And since we don't need two words to describe the same thing, we only use the first)
     
  12. Jan 18, 2017 #11

    Drakkith

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    The graviton is a hypothetical particle, but in order to fit current observations it must have certain properties. From wikipedia's article on the graviton:

    If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson. A spin-2 particle is also known as a tensor boson, compared to a spin-0 scalar boson and spin-1 vector boson. The spin follows from the fact that the source of gravitation is the stress–energy tensor, a second-order tensor (compared to electromagnetism's spin-1 photon, the source of which is the four-current, a first-order tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way that gravitational interactions do. As the graviton is hypothetical, its discovery would unite quantum theory with gravity.[4] This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton.


    In addition, the graviton must be its own antiparticle.

    Of course, these are unverified predictions of science. It's always possible that the graviton is very different than what we've predicted. However, any meaningful talk of what the graviton's properties are should be given in the context of current scientific predictions, whether verified or not. At least here at PF. Just remember that when we say things "must" have certain properties, we are talking about current scientific predictions and not claiming any absolute knowledge about the graviton.
     
  13. Jan 18, 2017 #12
    Let's just say hypothetically that you are right and the anti-graviton was a thing as this is just theoretical. To add on to this hypothesis I'd like to suggest that perhaps anti-gravitons could perform similarly to with antimatter the way that regular matter performs with regular gravitons. In other words I'm proposing the idea that antimatter would repel regular matter and perhaps attract other antimatter. How one would go about testing the gravitational influence of the few atoms of antimatter you could collect is beyond me, however it's still a cool theroy in my opinion.
     
  14. Jan 18, 2017 #13

    Drakkith

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    While it hasn't been verified yet, antimatter is believed to act identically to normal matter when it comes to gravity. There have been some theories where antimatter is repulsive under gravity with normal matter, so if you want to discuss that situation then please keep it within the realm of one of those theories and keep personal speculation out of the discussion.
     
  15. Jan 18, 2017 #14

    julian

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    Negative mass is odd as:

    "This would be a very bizarre situation because such a body would exert antigravity: if brought close to a normal body, it would repel the normal body, while the normal body would attract it, and the pair would accelerate itself away."

    from the book " The new physics" edited by Paul Davies.

    What I call the `Benny Hill effect'.
     
    Last edited: Jan 18, 2017
  16. Jan 30, 2017 #15
    This is wrong. Decay of d into u emits only W-, without any additional neutrinos. The rest is correct:

    The charge must be conserved because electromagnetic U(1) gauge group is not broken.
     
  17. Jan 30, 2017 #16
    Yes.

    Because W boson is a gauge boson of the local gauge symmetry which is also responsible for *existence* of the electric charge in general (and this local gauge symmetry is not Abelian).

    Electromagnetism is the result of existence of unbroken local gauge symmetry subgroup U(1) in a bigger electroweak symmetry SU(2)*U(1). [Note that these two U(1) are different. Both SU(2) and U(1) in EW are broken, but there is a combination of them which still "functions"].

    Local gauge symmetries (even broken ones) give rise to force carrying bosonic fields.
    SU(2) has three, W1 W2 W3, and they are charged under it.
    U(1) has one, B, and it is uncharged (because U(1) is an Abelian group).
    Since they are both broken, their four boson fields are "mixed and redistributed" into four other boson fields: W+, W-, Z, and photon.
    (The "mixing" here refers to choosing a "coordinate basis" in which formulas look least complex. It is possible to not "mix" the original bosons, but then description of e.g. electron emitting a photon would be impossible - the electron would need to emit a superposition of W3 and B bosons. Quickly becomes way too messy).

    Since there is an unbroken electromagnetic local gauge symmetry subgroup U(1)em, one of those bosons is _its_ bosonic field: it's photon. It is uncharged because U(1) is an Abelian group. It is massless because U(1)em is unbroken.

    Since there is an unbroken electromagnetic local gauge symmetry subgroup U(1)em, no-longer-conserved SU(2) and U(1) charges nevertheless have a conserved linear combination - electric charge.

    Even without doing any calculations, it's clear the remaining W+ W- and Z bosons can't possibly all have that linear combination be 0 (they would fail to span the entire space of their group). Simple math shows that W+ and W- have EM charges +1 and -1. Z has 0.

    More math here (section "Bosons"):

    https://en.wikipedia.org/wiki/Standard_Model_(mathematical_formulation)
     
    Last edited: Jan 30, 2017
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