ask a physicist

[Landifarne]

I'd have to go and find the article again [written by a UCLA emritus professor of economics about 15 years ago] to answer whether the figure included administration costs. I don't think it did, and I believe UCLA had the lowest overall percentage spent upon undergraduate instruction. Having cycled through it, my alma mater always left a bad taste in my mouth, and, from what my students inform me, it hasn't changed much in that regard (felt like a number, just churned through...sink or swim).

My friend was a principal researcher in biology, overlapping neuroscience. He did research for the DoD on echolocation in bats. Before all the grant money dried up, he stated that the sheer paperwork involved with grant writing and keeping in compliance with federal regulations/red tape took up 80% of his time. He had a good run doing cutting edge research for 20 years, but eventually gave it up to take a less stressful job (did maintain his pension, though, unlike all of the junior, non-tenured professors).

 

[gribble]

I've read through the thread and found it fascinating - thanks very much for giving your time to answer!

I know this thread has technically ended, but I have a couple of questions if anyone has the time to address them. Both are probably very naive, and reflect an undergrad understanding that has been blunted by nearly 20 years of work largely outside of Science/Physics (I work in software development), so please bear with me.

1) At one point freyar said "If your electrons became massless, the electromagnetic force would no longer be able to bind them to protons". Can someone please give an explanation of why? I thought the electromagnetic force concerned the interactions of charge and was unrelated to mass - i.e.: even if the electrons became massless, the electromagnetic force would still bind them to protons as long as they didn't lose their charge, wouldn't it?

2) There was discussion about our observable universe being only a tiny fraction of the known size of the universe (a figure of less than 1% of 1% was given), but we can see the CMB... which I thought was the remnant of the Big Bang and hence the edge of the universe? What am I misunderstanding?

As I said, likely very naive/ignorant questions, but they were the two things (apart from the bits that were just totally over my head) that didn't make sense to me.

 

[Umbran]

1) At one point freyar said "If your electrons became massless, the electromagnetic force would no longer be able to bind them to protons". Can someone please give an explanation of why? I thought the electromagnetic force concerned the interactions of charge and was unrelated to mass - i.e.: even if the electrons became massless, the electromagnetic force would still bind them to protons as long as they didn't lose their charge, wouldn't it?

I don't know for sure what freyar was thinking when he said this. But one answer goes like this

It turns out that massless particles must move at the speed of light. The electromagnetic force propagates at the speed of light (via photons). Therefore, the force coming from the proton will never catch up with the electrons, so they cannot be bound.

2) There was discussion about our observable universe being only a tiny fraction of the known size of the universe (a figure of less than 1% of 1% was given), but we can see the CMB... which I thought was the remnant of the Big Bang and hence the edge of the universe? What am I misunderstanding?

You are misunderstanding that it isn't at the edge of the universe - it *fills* the universe. When the universe was young, its entire volume was filled with hot, dense energy. As the universe expanded, the entire volume cooled, so the entire volume is filled with the resulting cool microwaves.

 

[gribble]

It turns out that massless particles must move at the speed of light. The electromagnetic force propagates at the speed of light (via photons). Therefore, the force coming from the proton will never catch up with the electrons, so they cannot be bound.

Ah, cool. That makes sense, thanks... although wouldn't the electron be moving perpendicular to the electromagnetic force (in a particle model), not directly away from it? So the photons should still be able to "catch" the electrons?

You are misunderstanding that it isn't at the edge of the universe - it *fills* the universe. When the universe was young, its entire volume was filled with hot, dense energy. As the universe expanded, the entire volume cooled, so the entire volume is filled with the resulting cool microwaves.

Right, it is an issue with my mental model then. I have a mental picture of the universe being like an inflated balloon, and the CMB being the inside of the balloon as seen by a point inside it. So the CMB (the stretch the analogy) is actually the air inside the balloon, and we can only see it as far as we can detect?

 

[Umbran]

Ah, cool. That makes sense, thanks... although wouldn't the electron be moving perpendicular to the electromagnetic force (in a particle model), not directly away from it? So the photons should still be able to "catch" the electrons?

No. Consider an image: there's a little guy with a photon gun standing on the proton, ready to fire the force-carrying particle at the electron. The electron is moving quickly across his field of view, right? So, even if his shot is moving as fast as the electron, he must aim at where the electron *will be*, not where it is.

But, in reality, the proton has now way of knowing where the thing will be. It has no prescience.

Right, it is an issue with my mental model then. I have a mental picture of the universe being like an inflated balloon, and the CMB being the inside of the balloon as seen by a point inside it. So the CMB (the stretch the analogy) is actually the air inside the balloon, and we can only see it as far as we can detect?

This will probably confuse you, but... In the traditional balloon analogy, everything you know in the universe is actually on the surface of the balloon. It is an example of what would seem to be a two-dimensional surface (the balloon material if it were laid out flat) being curved in a third dimension, and expanding. The air in the balloon is not in the universe.

The better way to think of it is probably like this - when you, with your eyes, look up at the sky, you see stars. They all to your naked eye look like they are painted on a surface, the vault of the sky. However, in reality, they are spread through a volume. It is like looking into a fog - it all looks like it is at a distance, but really, it's everywhere.

 

[freyar]

It turns out that massless particles must move at the speed of light. The electromagnetic force propagates at the speed of light (via photons). Therefore, the force coming from the proton will never catch up with the electrons, so they cannot be bound.

Ah, cool. That makes sense, thanks... although wouldn't the electron be moving perpendicular to the electromagnetic force (in a particle model), not directly away from it? So the photons should still be able to "catch" the electrons?

Keep in mind that Umbran's explanation is very heuristic and just illustrates what happens. You of course need the mathematical results. What I was thinking about is that if you look at the mathematics describing atoms and take the electron mass to zero, you find that (1) the atomic orbitals become infinitely large and (2) the binding energy goes to zero. So that means the atoms aren't held together.

Right, it is an issue with my mental model then. I have a mental picture of the universe being like an inflated balloon, and the CMB being the inside of the balloon as seen by a point inside it. So the CMB (the stretch the analogy) is actually the air inside the balloon, and we can only see it as far as we can detect?

The idea that the CMB fills the universe is right. What we see is the CMB at our location that is entering our telescope right now. These CMB photons have been travelling through the universe since the universe became transparent, so the CMB we see and measure reflects the conditions in our universe at that moment long ago and also very far away (since the photons are travelling at the speed of light that whole time). That's also the farthest distance we can see right now because looking a farther distance away would require looking back in time to a point when light couldn't travel because the universe was too hot and dense.

 

[AbdulAlhazred]

No. Consider an image: there's a little guy with a photon gun standing on the proton, ready to fire the force-carrying particle at the electron. The electron is moving quickly across his field of view, right? So, even if his shot is moving as fast as the electron, he must aim at where the electron *will be*, not where it is.

But, in reality, the proton has now way of knowing where the thing will be. It has no prescience.

I don't think this has anything to do with speed of light though. Its true of ALL force-carrying particles, one must ask how do they 'know' to appear where they are going to interact with another particle? An EM field for instance fills all of space, so does this imply that a magnet fills all of space with photons continuously? Where does the energy for this come from? Its one of those things that makes us question the nature of our views of both time and locality.

This will probably confuse you, but... In the traditional balloon analogy, everything you know in the universe is actually on the surface of the balloon. It is an example of what would seem to be a two-dimensional surface (the balloon material if it were laid out flat) being curved in a third dimension, and expanding. The air in the balloon is not in the universe.

The better way to think of it is probably like this - when you, with your eyes, look up at the sky, you see stars. They all to your naked eye look like they are painted on a surface, the vault of the sky. However, in reality, they are spread through a volume. It is like looking into a fog - it all looks like it is at a distance, but really, it's everywhere.

Right, there IS no 'surface of the fireball' at the big bang, we're INSIDE IT still, and always forever. Its just getting bigger and cooling off. No outside, no inside, just everywhere.

 

[gribble]

Ok, thanks guys. I think I understand the electron thing, though more from the math than Umbran's explanation... as AbdulAlhazred points out, the "where will it be" question isn't intrinsic to moving at the speed of light. However, if the math says that without mass the orbital becomes infinitely large (which I can kind of understand from a gravitational analogy, even if that perhaps isn't strictly accurate in this case), then it also makes sense that the binding energy would go to zero (if something is infinitely far away, then it intuitively makes sense to me that the force it would exert due to charge would be zero).

Still really confused about the size of the universe / CMB thing. The best analogy I can come up with that seems consistent with what Umbran is saying is to imagine an LED on the inside surface (or even the outside surface I guess) of a balloon. The surface of the balloon represents the bounds of the universe, but the amount of the surface illuminated by the LED (much smaller than the size of the balloon) is what we see as the CMB... is that at least somewhat accurate?

 

[AbdulAlhazred]

Still really confused about the size of the universe / CMB thing. The best analogy I can come up with that seems consistent with what Umbran is saying is to imagine an LED on the inside surface (or even the outside surface I guess) of a balloon. The surface of the balloon represents the bounds of the universe, but the amount of the surface illuminated by the LED (much smaller than the size of the balloon) is what we see as the CMB... is that at least somewhat accurate?

We certainly do only see a part of the CMB, as we see a part of the universe generally. you could think of that as like the 'horizon' in your balloon analogy, space goes on, perhaps forever or perhaps not, and its all filled with these CMB photons. Anyone anywhere will see basically the same CMB, since it was nearly at thermal equilibrium when the photons decoupled from matter.

 

[gribble]

We certainly do only see a part of the CMB, as we see a part of the universe generally. you could think of that as like the 'horizon' in your balloon analogy, space goes on, perhaps forever or perhaps not, and its all filled with these CMB photons. Anyone anywhere will see basically the same CMB, since it was nearly at thermal equilibrium when the photons decoupled from matter.

Yeah, I guess this was what I was meaning by the LED. If we're on the outside of the balloon rather than the inside, and there is a source of light outside the balloon, then the horizon of the balloon will suffice. However if we're inside, or there isn't some external source of light, the LED serves this purpose.