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In this video Dr.Lincoln says that the weak interaction is weak in low energies because the particles cannot emit the Z and W bosons easily. He mentions the decay of a neutron to a proton an electron and an electron antineutrino.He says the beta decay through the weak interaction happens slowly because for it to happen a W boson with mass 0.002 billion eV/c^2 must be emmited and the W boson has an average mass of 90 billion eV/c^2 -> it is very rare to find a W boson with such low mass which will cause the decay.

He made another video about quantum gravity which highlights some of the properties graviton must have:

averagely massless ( averagely moving at c , does not decay(infinite range) )

electrically neutral

quantum spin = 2 ( gravity is only attractive )

I was wondering according to his first video if gravitons were massless then it would be easy to be emmited from a particle ( fermion) and gravity should be really strong.But on the other hand gravitons must be massless because gravitational waves propagate at c ( special relativity).

So what's the point here?

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  • $\begingroup$ That's Nobel prize question! I suppose for example that gravitons are fermions. But who knows? $\endgroup$
    – kakaz
    Apr 13 '20 at 9:07
  • $\begingroup$ @kakaz Gravitons are spin-2 bosons. They aren’t fermions. $\endgroup$
    – G. Smith
    Apr 13 '20 at 16:19
  • $\begingroup$ It is just a surface. I suppose gravity is emergent phenomenon caused by fermions. Gravitons with spin 2 could be then collective quasiparticle $\endgroup$
    – kakaz
    Apr 15 '20 at 17:25
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Superficial answer:

Two factors determine the 'strength' of a force.

  1. The coupling constant and
  2. the mass of the gauge boson

Deeper answer:

The weak and electromagnetic force are unified, i.e. use the same gauge framework and the same coupling. The differentiation of electromagnetic and weak force only happens because of the W/Z masses. For massless W/Z bosons, these beta decays would happen instantly.

Gravity is a different kind of gauge theory that cannot easily be unified with the electroweak theory. It is an open question why the coupling constant is so small. We probably need a completely new unified field theory to explain this.

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  • $\begingroup$ Well weak interaction in the decay of top quarks is stronger than both electromagnetism and strong interaction. $\endgroup$
    – user256968
    Apr 13 '20 at 9:40
  • $\begingroup$ Thx for clearing that up $\endgroup$
    – user257090
    Apr 13 '20 at 9:45
  • $\begingroup$ Technically all forces are connected into one force which shows the theory of everything. If distances get really small all forces merge into one single force. $\endgroup$ May 8 '20 at 2:37
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The confusion comes because the video is talking of virtual particles as if they are real particles.

All particle interactions and decays are calculated using Feynman diagrams. The diagrams are an iconic representation of the interactions , the diagram can be rigorously translated to the integral of the interaction, which integral will give the crossection or the decay distributions of the real particles exiting an interaction

It is called a virtual photon because its invariant mass is off mass shell.

virtual

An internal line in an interaction also allows us to see how the quantum numbers are conserved in the interaction.

The strength of an interaction depends on the coupling constants, whether gravitationsl, electromagnetic,weak, strong, and a propagator representing the virtual particle .

Do not forget that in quantum mechanics everything is distribution of probability. The probability of exchanging an 80 gev particle is zero, because if the real mass was reached in the integral, there would be a real(on mass shell) W decay and energy conservation would be violated. So the video is hand waving on this part. It just means that the overall probability is depressed because the 80GeV W mass is in the denominator of the propagator, diminishing the integral further than the coupling constants diminish it.

I was wondering according to his first video if gravitons were massless then it would be easy to be emmited from a particle ( fermion) and gravity should be really strong.But on the other hand gravitons must be massless because gravitational waves propagate at c ( special relativity).

Spontaneous emission does not happen either with photons, an interaction has to happen for a photon to be emitted from a charged particle, see this as an example, otherwise energy would be violated.

The same is true for graviton radiation, and as the coupling of gravity is so very small, it is not detectable in the lab, the way bremsstrahlung is .

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