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If they are zero-dimensional but have different masses, wouldn't that have to suggest they have different densities meaning they are composed of different things?

Qmechanic
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Tiger Rollen
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  • Density has 0 in the denominator, so it's infinite. But, indeed, if electrons and quarks were composite, of which there is no sign, they would be very different. – Cosmas Zachos Nov 14 '22 at 22:23
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    You're thinking "classically". All data to date suggests that particles of the Standard Model have no spatial extension. Terms like "size" and "volume" or density, are not well defined concepts in the realm of quantum physics. – joseph h Nov 14 '22 at 22:41
  • relevant https://physics.stackexchange.com/q/19424/ – anna v Nov 15 '22 at 04:58

1 Answers1

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Short, SciComm-like answer

One of the consequences of they being pointlike is that they are not composed of anything else. They are as small as it gets, and hence it doesn't really make sense to speak of them being made of different things. They aren't really made of anything else. Their masses are different because they are different, but there doesn't need to be a smaller particle of a fixed mass to make everything up.

Longer, More Technical Answer

The notion of particle is actually just a useful concept to grab intuition in some situations, but it really isn't fundamental. When we speak of an electron or a quark, we don't mean a little ball made of matter as we see everyday. Instead, they are very weird objects that are excitations of things known as quantum fields. I can't give much of an intuition on what a quantum field is because I don't know of such an intuition, which is why SciComm often doesn't mention it at all. They are quantum versions of fields such as the electromagnetic fields that one finds in classical physics, and, for example, in your microwave. Photons are quantum excitations of the electromagnetic field, electrons of the electron field, and so on.

The different masses of the particles are actually related to the properties of the fields. Different fields behave in different way, and have different "energy costs" to create an excitation (i.e., a particle). This energy cost is then interpreted as the particle's mass. It also related nicely to the notion of mass one finds in Relativity, for example, and hence it makes sense to interpret it in this way.

Hence, they don't really have different densities. They can't be thought of as classical balls, as we often picture them. The different masses do not suggest they are made of smaller stuff, they just behave in slightly different ways.

Níckolas Alves
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