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Messages - bobmath

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1
Physics questions / Re: Double Slit from a Photon's Perspective
« on: January 07, 2015, 06:21:44 PM »
The thing is, in QM, photons don't have to move at the speed of light. It's just much more likely for them to do so, if they're traveling more than a really tiny distance.

That wikipedia article on Wheeler's delayed choice is pretty bad. It keeps talking about how "the photon decides whether to be a wave or a particle", which is nonsense because photons don't "decide" anything. They always act like photons (not waves or particles).

2
Physics questions / Re: Double Slit from a Photon's Perspective
« on: January 05, 2015, 10:23:32 AM »
I don't think Ben has answered a question here in a while, so I'll just do my best Clippy impression and say, "it sounds like you're asking a question about relativistic quantum mechanics." Which I haven't studied. Time dilation is a special relativity thing, wave/particle duality is a quantum mechanics thing, and SR+QM=RQM.

On a related note, does anyone know where I can find a nonterrible description of Wheeler's thing? The ones I've found online are ridiculous.

3
Oh yeah / CP violation song
« on: December 11, 2014, 11:41:41 AM »
Heard Ben was doing a show on CPT symmetry violation, but have you done your research?

4
I don't eat bicycle tires. Why should I eat a little pink one?

So is it fair to say that the light and dark areas in a NMR image represent differences in the way the matter in that location responds to the "probe" magnetic field? (Exactly what differences are represented sounds like it could be very complicated.)

As far as actually constructing the image, one of your guests said that they're measuring a return voltage from the sample. It sounds like they're not using an array detector to pick up the entire image at once, but some kind of swept detector that picks up little bits of the image in rapid succession. (Maybe they're doing something Really Clever like exciting the sample with a series of Fourier basis functions and reading off the coefficients?)

Anyway, another great episode!

5
Physics questions / Re: What is energy?
« on: September 22, 2014, 11:04:46 PM »
Warnocked.

Please enjoy this bunny.


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Physics questions / What is energy?
« on: September 15, 2014, 11:11:37 AM »
Serious question. Don't say "it's the ability to do work," because we both know that work is "change in energy."

In freshman physics, you've got your forces and accelerations, but everything interesting is a mess of differential equations you can't solve. Then someone comes along and says, "here's a neat trick: just calculate the energy like this and forget about everything else," and that turns out to be a great way to look at things. But when I got to BabyQM, there's Schrode's equation with its energy term, and suddenly I realized that I have no idea what energy is. It was always just a trick for solving mechanics problems. So what's going on with this energy term? You could apply a voltage between a couple of plates and shoot your electron in between there. But that's just electron-electron repulsion, so it seems like E is just some kind of statistical average of how likely your electron is to interact with other nearby electrons. It's classical thermodynamics all over again. Or am I way off base here?

So anyway, does anyone have a snappy non-self-referential answer to the question "what is energy"?

7
Science... Sort Of / Re: Beer
« on: July 15, 2014, 07:11:58 AM »

8
Cool, looking forward to it.

9
There's another reason fusion is so slow: fusing two protons gives you Helium-2, which is very unstable and usually decays back into two protons. Once in a while, it decays to deuterium (a proton and a neutron) instead. Once you have some deuterium built up, you can start fusing it with regular hydrogen to make Helium-3. That's stable, but Helium-4 is more energetically favorable, so the Helium-3 tends to go through more reactions and end up in that form. This is called the proton-proton chain reaction.

10
<a href="http://www.youtube.com/v/lEF1F-VswsM" target="_blank" class="new_win">http://www.youtube.com/v/lEF1F-VswsM</a>

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<a href="http://www.youtube.com/v/Zpcwpv4haxU" target="_blank" class="new_win">http://www.youtube.com/v/Zpcwpv4haxU</a>

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Science... Sort Of / Re: Trailer Trash Discussion
« on: February 15, 2014, 03:07:38 PM »
I occasionally watch the trailers, if they sound interesting.

13
The Titanium Physicists Podcast / Episode 40: Snowlines and No Rhymes
« on: December 23, 2013, 07:01:26 PM »
Another great episode. I just wanted to chime in on the theory/law discussion at the end. Mathematicians also use the term "theory" (set theory, graph theory, field theory, etc), and they are definitely not implying any uncertainty in those areas. A theory is a body of knowledge, whereas a law is usually a fairly short statement. You might talk about the "law of commutativity," for example (though it seems to be more common to say "commutative property" nowadays, since it doesn't apply to all situations).

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Oh yeah / Re: stressed?
« on: November 14, 2013, 11:05:10 AM »
Huzzah, mavericks fixed my time machine!

15
The Titanium Physicists Podcast / Re: Episode 38: Particle Accelerators
« on: November 01, 2013, 09:14:27 AM »
Heh

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