What is matter?

[Demiurge]

From the point of view of quantum physics... Could physicists agree what matter is?

We see it, we describe it yet it resists intuitive understanding.

Is matter really just energy and measurement, decoherence, splitting, whatever process that brings out single outcomes aka "matter"?

 

[PeterDonis]

From the point of view of quantum physics... Could physicists agree what matter is?

"Matter" is a word. Physics is not done in words. It's done in math. So what the word "matter" means depends on what thing in the math you use that word to refer to.

 

[DaveC426913]

I think we've explored this before in another thread.

I offered the rough-n-ready description that - with a few notable exceptions* - matter encompasses the gamut of fermionic particles - the particles that obey the Pauli Exclusion Principle, and as a result "takes up room", in the way that, say, light does not. (You can fill a box with electrons until it's full, but you cannot "fill up" the same box with light.)

* Helium-3 is one of those notable exceptions that is matter, yet apparently does not seem to obey PEP.

That may not suit everyone, but I think it's a pretty good starting point.

 

[weirdoguy]

Is matter really just energy

Whatever "matter" means, for sure energy is a property of matter, not matter itself.

 

[Roberto Pavani]

Undergraduate answer to an undergraduate question: matter is anything that has inertial mass, i.e.,
anything that resists acceleration: $m = F/a$.
If you push it and it pushes back, it's matter.

 

[PhyEnthusiast]

I would have said: matter is simply condensed energy.

You might run into minor problems with this defintion because of energy-mass equivalence (most of an atom's mass comes from the binding energy of its constituents, after all). But the adjective 'condensed' captures quite well that it's something that now exists in reality, something you can somehow touch, and something that stays there without just disappearing again.

 

[PeterDonis]

Helium-3 is one of those notable exceptions that is matter, yet apparently does not seem to obey PEP.

He-3 does "take up room"; the fact that it is superfluid at low enough temperatures does not prevent it from occupying volume. "Superfluid" is not the same thing as "acts like light".