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Author Topic: Dark matter found  (Read 8772 times)
Halleck
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Re: Dark matter found
« Reply #30 on: October 22, 2006, 06:09:52 pm »

Yes, that's kind of the point...
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Re: Dark matter found
« Reply #31 on: October 24, 2006, 07:45:29 am »

I don't really like Modified Newtonian Dynamics. The dark matter explanation seems perfectly suitable, and this additional evidance lends yet more weight to it. There are also some galactic rotation curves that don't seem to make sense with TeVeS gravity.
We might learn more on this matter soon, but for now, I wouldn't count on TeVeS gravity being correct.
I wonder how TeVeS gravity would impact universal curvature and explansion models?
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Re: Dark matter found
« Reply #32 on: December 16, 2006, 05:43:00 am »

I may be a high school physics teacher, but I am no physicist. That said:

A common misconception is that anti-particles are the same as regular particles, but they have opposite charge. But we know there are anti-neutrons, so that doesn't work. A better description is anti-particles are the same as regular particles, but experience time in the other direction.  I cannot defend this statement except to say that I got it from a real physicist who makes a living looking for neutrinos and teaching particle physics at a good university.

My last year at that school left me seriously questioning ideas of truth and knowledge - Schroedinger's cat, and all that jazz leave me completely weirded out.
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meep-eep
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Re: Dark matter found
« Reply #33 on: December 16, 2006, 06:07:04 am »

Both the neutron and the antineutron have no charge, and the opposite of no charge is no charge.
But the neutron isn't an elementary particle. A neutron consists of two down quarks and one up quark. An antineutron consists of two anti-down quarks and one anti-up quark.
The quarks that form the antineutron do have opposite charge of those of the regular neutron.
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Re: Dark matter found
« Reply #34 on: January 08, 2007, 07:33:25 pm »

I may be a high school physics teacher, but I am no physicist. That said:

A common misconception is that anti-particles are the same as regular particles, but they have opposite charge.

They aren't exactly the same, but they're awfully similar. Another difference beside the one mentioned by meep-eep is that... well, let me back up a little. Neutrinos are peculiar particles in that they only come in one 'handedness', that is, they spin only one way in respect to their direction of motion. Anti-neutrinos have this backward as well.

Also, there are some more subtle asymmetries which are harder to describe, buried in the way that antiparticles mix quark flavors.

A better description is anti-particles are the same as regular particles, but experience time in the other direction.

This is a kind-of-okay description under certain circumstances, like if you're only interested in a momentary look at the mathematics of their interactions. However, once you get into a large enough system that entropy makes sense, it really screws around with the concept of time. Most physicists I know, including myself and also including my quantum field theory professor, think this description is pretty awful. He stressed that the field equations maintain causality, he showed how they do. We had to show how a different set of field equations maintained causality as part a homework assignment.

Now, the time-reversal symmetry, as an abstract tool, is an extremely useful description; but that does not mean that  the particles actually are time-reversed.

My last year at that school left me seriously questioning ideas of truth and knowledge - Schroedinger's cat, and all that jazz leave me completely weirded out.

It does that more than it needs to because of the weird interpretations that people have come up with. To a great extent, these interpretations are optional.

Schroedinger's cat is one of them, kind-of. The state of the universe contains some live cat and some dead cat, whether or not we look. But when we look, the parts of us that go with the live or dead cat are no longer similar.

The cat is not saved or killed by our looking at it.
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