Can particles interact without a mass?

  • #1
Summary:
Is it possible for particles, any of them to have had some role in the beginning of the universe, despite not having mass?
I recall reading someone questioning if it were possible for the higgs boson to be the primordial atom which led to the start of the universe. I was just wondering if that could even be possible, since the higgs field was zero and couldn't have given these particles any mass. (please be kind, I'm not a physicist, I'm not in school for physics, I was just reading about it and was curious.)
 

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  • #2
Orodruin
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Mass is not a requirement for interactions.
 
  • #3
mathman
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All particles have "mass", although for photons it is called energy.
 
  • #4
Orodruin
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All particles have "mass", although for photons it is called energy.
This is not correct. Mass by definition in relativity is rest energy and a photon has none.
 
  • #5
mathman
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This is not correct. Mass by definition in relativity is rest energy and a photon has none.
Rest mass is not the same as (total) mass. It's simply a matter of definition. Think of the Lorentz transformation which defines "mass" in terms of rest mass and velocity.
 
  • #6
Orodruin
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Rest mass is not the same as (total) mass. It's simply a matter of definition.
In modern nomenclature, there is only one mass - what was previously referred to as rest mass or invariant mass. A photon has zero mass. Concepts such as relativistic mass have fallen completely out of fashion as they are not needed and lead to confusion.
 
  • #7
PeterDonis
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It's simply a matter of definition.
Even to the extent this is the case, the OP of this thread makes clear that he is using "mass" to mean "rest mass", since he talks about the Higgs boson giving particles mass, and the only meaning of "mass" that makes sense in that context is "rest mass".
 
  • #9
kimbyd
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Summary:: Is it possible for particles, any of them to have had some role in the beginning of the universe, despite not having mass?
So, you've already gotten your direct answer, but I thought I might extend it a little bit because I think it's fascinating.

As others noted, mass is simply not a requirement for any interactions. For an interaction, you essentially need to just have two particles which connect in their equations of motion. In practice what happens is that the interaction has a charge associated with it, and a force-carrying particle which connects to that charge. Any particle that has that type of charge connects to the relevant force-carrier. Like with the electromagnetic force, the charge is the electric charge, and the force carrier is the photon.

There are also cases where the force carrier itself has charge, such as with the strong nuclear force.

But the real reason why I find your question so interesting is that the equations which describe all of this stuff do not allow mass. Period. It's easy enough to modify the equations of motion to add mass to them, but it leads to a contradiction (I forget the details).

This is where the Higgs mechanism comes in: it's an interaction in the standard model which gives particles effective mass depending upon how they interact with the Higgs field. And it's why the Higgs boson was predicted decades before it was discovered.

So, far from lack of mass preventing interactions, interactions between massless particles are what creates mass in the first place!
 
  • #10
Vanadium 50
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But the real reason why I find your question so interesting is that the equations which describe all of this stuff do not allow mass. Period. It's easy enough to modify the equations of motion to add mass to them, but it leads to a contradiction (I forget the details).
Probably because this isn't quite right.

The force carrier cannot have mass.

The matter particles can, in general, have mass. There are cases where they don't, but it is not a genral prohibition.

If interested in discussing this further, it probably belongs in its own thread.
 
  • #11
PeterDonis
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The matter particles can, in general, have mass. There are cases where they don't, but it is not a genral prohibition.
Not a general prohibition, but a Dirac mass term for the fermions is prohibited in the Standard Model, since it would require left- and right-handed fermions to belong to the same representations of the gauge groups, and they don't.
 
  • #12
Vanadium 50
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So, we're going to hijack this thread, as well as take it beyond B level. OK, you're the Mentor.

In QED, a mass term is allowed.
In electroweak theory, it is not allowed because the couplings are chiral.
QCD is complicated. I don't know the answer there. The couplings certainly are not chiral, but flavor has many subgroups that I'd have to think about harder.
 
  • #13
PeterDonis
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In QED, a mass term is allowed.
In electroweak theory, it is not allowed because the couplings are chiral.
QCD is complicated. I don't know the answer there. The couplings certainly are not chiral, but flavor has many subgroups that I'd have to think about harder.
In the Standard Model, though, all fermions are in some representation of all of the gauge groups, so if a mass term is not allowed for any of them, it's not allowed, period.
 
  • #14
Vanadium 50
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Well sure. Nature has all the forces in it.
 
  • #15
kimbyd
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To sum up a bit of the above discussion, because it is a B thread:

The Higgs mechanism was originally proposed to solve some weirdness with the weak nuclear force. As Vanadium 50 mentioned, the force carriers very clearly cannot have mass. But the force carriers for the weak force do. The Higgs mechanism was proposed to solve that problem.

It was later discovered that matter particles can pick up mass from the Higgs field as well through a different mechanism. And the fact that the weak force is not left/right symmetric turns out to also exclude fundamental mass for the matter particles.

So it really does look like mass isn't a fundamental property of anything, at least in our universe.
 
  • #17
ohwilleke
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Photons undeniable have a "role in the beginning of the universe" and lack rest mass. So do the gravitons that are generically parts of any quantum gravity theory. So do gluons which give rise to baryon mass dynamically but do not themselves fundamentally have rest mass.
 
  • #18
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Summary:: Is it possible for particles, any of them to have had some role in the beginning of the universe, despite not having mass?

I recall reading someone questioning if it were possible for the higgs boson to be the primordial atom which led to the start of the universe. I was just wondering if that could even be possible, since the higgs field was zero and couldn't have given these particles any mass. (please be kind, I'm not a physicist, I'm not in school for physics, I was just reading about it and was curious.)
*If* you believe in the Higgs field in its present form (I dont) then its energy was high when the field was zero.
 
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  • #19
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*If* you believe in the Higgs field in its present form (I dont) then its energy was high when the field was zero.
What about gluons?
 
  • #21
PeterDonis
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*If* you believe in the Higgs field in its present form (I dont)
This comment is out of place in a "B" level thread, and even in a more advanced thread, it would need to be supported by some kind of reference to an alternative model. Otherwise it is personal speculation and off limits here at PF.
 
  • #22
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What about them? You realize you're responding to yourself, right?
Was I responding to myself? I cant see that. I have to get used a bit here. It's my day...☺ Dont gluons have zero mass while interacting with one another?
 
  • #23
PeterDonis
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Dont gluons have zero mass while interacting with one another?
Yes. As was already indirectly implied in post #9 and stated explicitly in post #17.
 
  • #25
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Yes: the quote in that post is from a previous post by you.
So I should have posted the comment on its own? I will figure out the bedt ways to do it. Would you know if I made this comment for you on its own?
 

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