Friday, January 2, 2015

Ask me about physics

I'm not sure if I have enough readers to do this sort of thing, but it's worth a try.

Does anyone have any physics questions?  Whenever I look at physics blogs, I see lots of commenters with questions in the comments, most of which never get answered.  And while there are physics FAQs out there, you usually can't just send in any question and have it be answered.

Since I'm only a physics PhD student, I'm willing to answer questions informally, and even offer my subjective opinion.  If I have a short answer, I'll put it in the comments.  If I have a longer answer I'll write a post.

I study superconductivity, but I also know a lot about cosmology, quantum theory, and quantum interpretations.  And when I read popular articles about physics, I tend to have a better understanding than the typical lay person.


miller said...

I still don't understand the difference between energy, temperature and pressure. If all three are (more or less) about the kinetic energy of the molecules, then why does a fire burn me but not knock me over, but a strong wind will knock me over but not burn me.

miller said...

I'm extremely confused about the nature of space itself. Is it a fundamental aspect of the universe, or is it emergent? Is it just a convenient way to conceptualize a number of forces that are usually linked? When I first read about general relativity, I came to think of space and time as being linked. But then, when I learned about the metric expansion of space, I misunderstood it at first, because I was thinking that if space itself was expanding, it was changing scale. And then I thought, for the speed of light to remain constant, then every time the scale of space doubled, the pace of time would have to slow by half. Which kind of made sense to me, since I was already thinking of space and time as being linked, so they would be expanding together. But then, of course, that would have some really wild consequences that obviously didn't fit into the standard cosmological model of the universe, like, if space were infinitesimal in scale at the Big Bang, then time.... Well, at any rate, once I realized that's not at all what cosmologists were talking about, that it was a metric expansion, not a scalar expansion, it made me wonder again, what exactly is this "space", that can expand independently of time in the expansion of the universe, yet gets warped together with time by mass?

Also, in what way is the idea of space important in quantum theory?

miller said...

I'm sure you've seen my posts about temperature vs energy.

I may write another post, specifically addressing the question of wind vs fire.

miller said...

It seems that one of the ways that space is defined is in terms of lines of potential inertia. And when I looked up inertia on Wikipedia, my understanding of what the article said is that it has been equated with the instantaneous effect of the gravitational fields of everything in the universe. If that's true, then I'm also wondering if the expansion of all the matter in the universe *within* space could therefore explain the metric expansion of space itself. But I don't pretend to understand any of this stuff in mathematical terms, so that's just idle speculation.

miller said...

I have indeed. Still opaque to me. That's probably why I'm an economist instead of a physicist.

miller said...

Yes, the geometry of space defines a set of "inertial reference frames" (which is what I think you mean by "lines of potential inertia". An inertial reference frame is basically a reference frame in where F=ma is true. So in absence of any force, objects will stay in motion on their own velocity.

The expansion of the universe is actually dissimilar to the outward motion of all matter in the universe. If all matter were simply moving outwards, then the relative speed of any two objects would be less than the speed of light. In big bang theory, objects that are sufficiently far apart recede faster than that.

For reference, I once wrote about big bang theory. For perhaps a better popularization, I recommend this article from Scientific American.

miller said...

Since it was so long ago, I'm likely to tackle the problem from a different angle. These days I have more distance from physics classes so I'd write something less technical.

miller said...

Thank you, yes, inertial reference frame is what I meant. And I had heard that the expansion of the universe was different from the motion of objects within the universe, although obviously that understanding isn't firmly grounded in my head. Thanks also for those links. Your article helped to address another question I had: the explanation for the bell shaped history of the expansion of the universe.

So, if space is linked to inertial reference frames, and inertia is linked to the instantaneous effect of the gravity fields of everything in the universe, does the expansion of the universe involve a kind of positive feedback loop, where the more it expands, the more it will expand?

I'm so very interested and confused about many so aspects of cosmology, and I have more questions than it seems polite to burden you with. I'd also be happy with any book recommendations for general readers. I have a higher tolerance for math than most general readers, but I could easily see a more scholarly book being way beyond my depth. Anyway, I'm happy to get links to more articles, book recommendations, anything. I really appreciate your blog on so many levels, and so I'd be eager to take any recommendations you could make.

miller said...

Sorry, I'm the wrong person to ask for book recommendations. I don't usually read books about physics.

Since the trajectories of matter and energy depend on the expansion of the universe, and since the space-time metric depends on matter and energy, there is in fact a feedback loop. The sign and magnitude of the feedback loop depends on how the matter/energy dilutes as the universe expands. Matter and radiation create a negative feedback loop. Dark energy, because it doesn't dilute, creates a positive feedback loop. I don't understand the technical details.

miller said...

That's okay (regarding no book recommendations). I just finished that article from Scientific American that you linked, and it was wonderful! It really cleared up a lot for me. Thanks for that.