How can gravitons have anti particles?

[BadgerBadger92]

https://physics.stackexchange.com/q...ery-of-anti-graviton-i-e-the-graviton-antipar

How can a graviton have an anti particle even though they are essentially the same thing? Same thing with the photon. Any help would be great

[Mentor Note: The PSE link is okay, with a reasonable question and initial answer. The subsequent replies in that thread contain facetious woo, so please only consider the initial question and reply. Thanks.]

 

[PeterDonis]

How can a graviton have an anti particle even though they are essentially the same thing? Same thing with the photon.

Photons and gravitons are their own antiparticles. Why is that a problem?

 

[BadgerBadger92]

Photons and gravitons are their own antiparticles. Why is that a problem?

I thought their properties would be the the exact opposite

 

[topsquark]

I thought their properties would be the the exact opposite

What properties would you expect to be opposite? They have no charge, no magnetic moment, etc. Pretty much the only non-zero property they have is helicity, and the helicities of particles and anti-particles is the same anyway. So the properties of a photon and anti-photon are opposites... but as the properties are equal to 0 we just get the same particle back as we started from.

-Dan

 

[BadgerBadger92]

What properties would you expect to be opposite? They have no charge, no magnetic moment, etc. Pretty much the only non-zero property they have is helicity, and the helicities of particles and anti-particles is the same anyway. So the properties of a photon and anti-photon are opposites... but as the properties are equal to 0 we just get the same particle back as we started from.

-Dan

Thank you! This post helped

 

[BadgerBadger92]

What properties would you expect to be opposite? They have no charge, no magnetic moment, etc. Pretty much the only non-zero property they have is helicity, and the helicities of particles and anti-particles is the same anyway. So the properties of a photon and anti-photon are opposites... but as the properties are equal to 0 we just get the same particle back as we started from.

-Dan

Just one more question. If their properties are zero across the board, what makes the difference between photons and gravitons?

 

[vanhees71]

Gravitons have spin 2, photons spin 1. Since both are massless they have nevertheless both only two polarization-degrees of freedom, labelled by helicity, being $\pm 2 \hbar$ (gravitons) or $\pm 1 \hbar$ (photons) respectively. The helicity is the projection of total angular momentum to the direction of the momentum of the graviton or photon.

 

[dextercioby]

And one more thing: photons are known to exist for sure as discrete portions of electromagnetic field energy-momentum, while gravitons are the necessary hypothesized particles in the quantization of a free spin-2 field (Pauli-Fierz 1939), which coincides with the gravitational field (Hilbert-Einstein) in the weak-field limit. We have found experimental evidence of gravitational waves. Quantizing HE solutions in form of gravitational waves is done with gravitons.

 

[topsquark]

Just one more question. If their properties are zero across the board, what makes the difference between photons and gravitons?

It's also about what they are associated with. It's not just a listing of the properties of the particle that is important, it's also in what context they appear in theory. For example, the photon couples to electric charge; the only time you will see a photon exchanged between particles is if they both have electric charge. The graviton, however, couples with pretty much everything. Photons and gravitons act differently in theory as well which makes a big difference between them.

-Dan

 

[.Scott]

Photons and gravitons are their own antiparticles. Why is that a problem?

So I am thinking about particle annihilation - and I am probably not going to get this right - but here's my naïve whack at it:

As I understand it, both Photons and Gravitons are massless - so they travel at light speed and can carry a range of energy. We can have relatively low-energy microwave photons and relatively high-energy gamma photons. My guess is that there must also be corresponding low-energy and high-energy gravitons. Is that reasonable?

I always thought of particle-antiparticle annihilation as resulting in zero, two, or more non-photon particles with attributes that "add up" to zero and photon(s) that make up the "change", providing whatever energy and momentum is needed to balance out the input and output of the interaction.

But now I'm wondering about these gravitons. Couldn't they also be used to "make up the change"? How do they participate in particle-antiparticle annihilations?

A photon-photon interaction fits in with this pattern nicely. If the photons are sufficiently energetic, you might get two or more particles and new photons to make up the change. Otherwise, you would just get photons - perhaps the two you started with.

But what about a graviton-graviton interaction? Can gravitons annihilate each other yielding only photons?
Can gravitons carry enough energy to create other particles (eg, electron/positron pairs) when they collide? Would these interactions be reversible?

 

[PeterDonis]

now I'm wondering about these gravitons. Couldn't they also be used to "make up the change"? How do they participate in particle-antiparticle annihilations?

In principle, yes, you could have gravitons (if they exist--note that we do not have a theory of quantum gravity, nor do we have any evidence showing quantum aspects of gravity, so everything about gravitons is hypothetical at this point) involved in these interactions. In practice, since gravitons are equivalent to fluctuations in the spacetime geometry, the energy and momentum they would contain as a result of typical particle-antiparticle annihilations would be negligible by many orders of magnitude, since typical particle-antiparticle annihilations have negligible effects on the spacetime geometry by many orders of magnitude.

 

[BadgerBadger92]

Gravitons have spin 2, photons spin 1. Since both are massless they have nevertheless both only two polarization-degrees of freedom, labelled by helicity, being $\pm 2 \hbar$ (gravitons) or $\pm 1 \hbar$ (photons) respectively. The helicity is the projection of total angular momentum to the direction of the momentum of the graviton or photon.

If gravitons have spin 2, wouldn’t that mean anti gravitons have -2 spin?

 

[PeterDonis]

If gravitons have spin 2, wouldn’t that mean anti gravitons have -2 spin?

No. Spin-2 means the magnitude of the spin is 2 (or more precisely $2 \hbar$). A spin measurement on a graviton can therefore result in either 2 or -2. (Just as a spin measurement on a photon can result in 1 or -1.)

 

[BadgerBadger92]

No. Spin-2 means the magnitude of the spin is 2 (or more precisely $2 \hbar$). A spin measurement on a graviton can therefore result in either 2 or -2. (Just as a spin measurement on a photon can result in 1 or -1.)

This may seem to be like a weird question

Are there similar amounts of gravitons with spin 2 and spin -2?

 

[.Scott]

This may seem to be like a weird question

Are there similar amounts of gravitons with spin 2 and spin -2?

I don't believe that the spin in intrinsically positive or negative. It's relative to the measuring device - or to the particles it interacts with.

 

[topsquark]

This may seem to be like a weird question

Are there similar amounts of gravitons with spin 2 and spin -2?

What you seem to be talking about here is a measurement of the z component of the spin (actually, helicity.) This merely depends on the direction that the graviton is coming from.

-Dan

 

[PeterDonis]

Are there similar amounts of gravitons with spin 2 and spin -2?

The vast majority of gravitons would not be in an eigenstate of spin so they would not have a well-defined spin value at all.

 

[PeroK]

I wonder how the spin of a graviton could be measured?

 

[.Scott]

I wonder how the spin of a graviton could be measured?

As I understand it, that would be essential in recognizing a particle as a graviton - so lots of people wonder how to detect that spin.

 

[BadgerBadger92]

I wonder how the spin of a graviton could be measured?

I hope you weren’t laughing at me and my posts, based on your reactions. I hope you can forgive me, I’m not an expert with these things, I just enjoy it.

 

[.Scott]

I hope you weren’t laughing at me and my posts, based on your reactions. I hope you can forgive me, I’m not an expert with these things, I just enjoy it.

No one is laughing at you. Your curiosity is at the heart of Physics.

 

[PeroK]

I hope you weren’t laughing at me and my posts, based on your reactions. I hope you can forgive me, I’m not an expert with these things, I just enjoy it.

To be honest, I wasn't entirely sure whether you were pulling our leg on this thread!

You opened it at an I level, which should imply some knowledge of the subject.

 

[PeterDonis]

I wonder how the spin of a graviton could be measured?

Graviton spin, like photon spin, can be measured by measuring polarization. The only difference is that the orientations of the two orthogonal polarizations would be 45 degrees apart instead of 90.

 

[ohwilleke]

Just one more question. If their properties are zero across the board, what makes the difference between photons and gravitons?

Both have zero rest mass carrier bosons associated with long distance forces.

Photons have spin-1 and definitely exist, while gravitons have spin-2 and may or may not exist although their properties flow naturally from very generically defined quantum gravity theories.

Photons couple to electromagnetically charged particles with a strength that is a function of the fine structure constant "alpha". Photons are not themselves electromagnetically charged which means that they don't interact with other photons and that their behavior can be described by an Abelian gauge theory. The charges with which photons interact also come in discrete units equal to plus or minus 1/3rd of the electron charge. The interactions of photons can be described well with perturbative theories that are renormalizable and calculations with them converge in a comparatively small number of terms.

Gravitons couple to all particles that have mass and/or energy with a strength that is a function of Newton's constant "G". Since gravitons themselves carry energy, although not rest mass, gravitons have self-interactions with other gravitons which means that their behavior has to be described by a non-Abelian gauge theory (much like QCD in which gluons have self-interactions) which results in non-linear gravitational behavior that is absent from electromagnetism even in the quantum mechanical Quantum Electrodynamics (QED) version of electromagnetism. So far as we know, the mass-energy of any given particle is a continuous quantity rather than a discrete one, like electromagnetic charge that can differ from particle to particle by arbitrarily small amounts. The interactions of gravitons are not renormalizable which limits the usefulness of perturbative theories to describe their behavior.

 

[vanhees71]

If gravitons have spin 2, wouldn’t that mean anti gravitons have -2 spin?

No. The spin-quantum number refers to the eigenvalue of the square of the angular momentum. It's $s \in \{0,1/2,1,3/2,\ldots \}$ and the eigenvalue of the angular momentum squared is $s(s+1) \hbar^2$.

Massive particles have then $(2s+1)$ spin-$z$ eigenvalues (the angular momentum of states with momentum $0$), $\sigma \in \{-s,-s+1,\ldots,s-1,s \}$.

Massless particles have only two polarization-degrees of freedom. A "natural" eigenbasis are the helicity eigenstates, i.e., the projection of the total (!) angular momentum of the particle to the direction of its momentum. The helicities for massless particles with spin $s$ take the values $\lambda=\pm s$. For photons these are the circular-polarized states.

Antiparticles have the same angular momentum as the particles.