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Q & A: Matter and particle questions

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Most recent answer: 05/16/2013
What about matter warps the spacetime around it? How do you define matter or what space is? P.S. Are there any stable, neutral subatomic particles? If not, why not? Thanks
- Reuben (age 42)
Thatís a bunch of interesting questions.

Einsteinís famous equation in general relativity says that the stress-energy tensor is directly proportional to the curvature tensor (thereís also a cosmological constant term which may possibly not vanish, but fortunately you didnít ask about that one). The stress-energy tensor is a 4x4 matrix of numbers, each component of which is a function of position and time. One component is the energy density, which includes energy in rest mass, but also kinetic and potential energies. One column of components is the momentum density (imagine a fluid of particles -- each particle contributes momentum and thereís an average density which enters the Einstein equations). A row of components is the energy flux, such as heat transfer. And a 3x3 submatrix is the momentum flux. All of these components contribute to the curvature of space.

Space is just the set of coordinates we need to specify where an event takes place. Time is the coordinate we need to give to say when something happens. Some sets of coordinates are more convenient than others.

Some stable, neutral subatomic particles:


neutrino (actually, only one of the three mass eigenstates we suspect exist is truly stable -- the others can decay into it on timescales longer than the age of the universe).

Whatever dark matter may be composed of (my favorite candidate is a supersymmetric neutralino, but itís by far not the only possibility).


(published on 10/22/2007)

Follow-Up #1: gravity and particles

Does an electron and/or light follow the rules of momentum, power, gravity and so on...just on such a unmeasurable scale? So due to an electron(s) mass, isn't it impossible for it to go in a straight vector due to gravity caused by obviously larger pulling forces?
- Dwight (age 37)
Edmonton, AB Canada
Sure, electrons are affected by gravity. It's hard to measure the gravitational acceleration directly because the electrical forces are large. Light also is affected by gravity. The rules for light are different from what you'd guess from Newtonian rules. Because light travels at the speed of light, it turns out that General Relativity says (correctly) that the curvature of the path is twice as large as what you might guess. Other particles traveling near the speed of light (relative to the massive body) also show this enhancement.

Mike W.

(published on 05/20/2008)

Follow-Up #2: inside the particle dolls?

There is one troubling question where I was afraid to think about because it might make scientists very uneasy and lost but here I go?? What's the point of doing all these particle accelerating experiments when all we do is go deeper and deeper into matter but get more and more particles coming back at us to study? Think of it like like that Japanese doll. There is one big doll, open it up and there's a smaller one inside, open that up, and there's an even smaller doll. It never ends we "unlock" an atom and find theres more particles inside and open up the quarks and there's some more complicated crap to deal with and open up whatever quarks are made up and we get more thousands of particles inside that things get smaller and smaller but more "things" are made up of even more things. I know I put it in a really wierd way...but where is humanity going???
- Azeem Notta (age 19)
Toronto, Canada
That's a question that's often raised. Before answering it, I should try to clarify a bit what's happened so far.

The probing into deeper and deeper layers has not resulted in a cascade of ever more complicated ingredients. In some ways it's the opposite. The force laws governing these interactions have now been boiled down to two: the unified electroweak interaction and the quantum chromodynamic interaction. There's also gravity, important on different scales, and so far untouched by accelerator experiments. That's much simpler than the welter of different forces that show up in more superficial mechanics. That shower of new particles you describe turns out to require only a few new ingredients- six types of quarks. There is still some complication, in that a few dozen numbers are needed to describe the whole theory (various ratios of particle masses, etc.) but that's  already a lot simpler than, say, the properties of about 100 elements.

There's very good reason to expect that with a bit more probing a single unified law for the electroweak and chromodynamic interactions will emerge. Perhaps an overall unified description including gravity will be developed, maybe along the lines being explored in string theory.

So far we've seen a few doll layers. First, full atoms. Second nuclei plus electrons. Third, protons and neutrons and electrons. Fourth, quarks and leptons. The only level that even superficially looked simpler than the current one was the 'protons, neutrons, electrons' level. However, since there was no good simplification for the interactions at that level, it was really more complex than where we are now.  As for whether at some point there's an inmost doll, with nothing further inside, we don't know. If there is, we don't know if it will ever be reachable by our species. But why not give it a shot?

p.s. There's a beautiful discussion of exactly this issue in one chapter of The Character of Physical Law, by Richard Feynman.

Mike W.
thanks, Inga

(published on 05/16/2013)

Follow-up on this answer.