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Wednesday, January 17, 2018

A few questions about basic physics

Here are a few questions I have about physics after reading (or attempting to read) several books.I can't seem to find consistent answers to these. I don't have the base of understanding necessary to ask good questions yet, but I hope most of these questions are at least interesting!

(Apologies for typos! My voice recognition seems to insist that molecules are "made up of Adams"...)

Section 1: About general relativity:

One of the starting example situations that often seem to be used in general relativity is this idea of someone who is traveling on a train or other moving base, who shines one light forward in the direction they're traveling and another light backward. 

There are two targets that are equally distant from them on the train, in the forward direction and backward direction. If a rider on the train turns both lights on at the same time, naturally the rider perceives the lights hitting those targets at the same time.

But if there is an observer who is stationary on the ground outside the train, watching the train pass them by, then because the speed of light will be absolute relative to them, the light can't be traveling forward at the speed of the train *plus* the speed of light. Here's where standard explanations say something like "To our outside observer the beams will be going at a different speed relative to the train than what the rider sees." The outside observer will observe the light hitting the backward target first, which in these examples is supposed to demonstrate the weird implications of the absolute speed of light relative to any observer.

There are a lot of different assumptions here that aren't being addressed, which I don't understand. First of all, the example above seems to mix subjective experience and objective descriptions of reality. What does it mean to say that "to" the outside observer, the light hits the backward target first? The outside observer doesn't have magic, objective-time goggles that can instantly communicate when the light hits a target. 

But they can see when the light that hit the target bounces to them and strikes their eye or camera or whatever. All they can do is receive reflected light. 

But suppose it *was* possible for light to travel faster than the official speed of light. That would mean it would be possible for there to be one objective reality between the two observers: the light is moving forward at the speed of light that the observer on the train observes, plus the forward speed of the train. The light moving backward is moving at slightly slower than the speed the observer on the train perceives. And the two targets really do got struck at the same time. But because the forward Target is farther away from the outside observer than the backward Target, the light that reflects towards the outside observer would not reach them at the same time. They would perceive the backward target to have been struck first. That's the order in which they perceived the targets to be struck by light actually does not demonstrate the absolute speed of light limit.

In this scenario, it really only depends on the relative position of the train to the outside observer when the targets are struck. If the train is moving towards the outside observer, and is relatively far away, they will see the light from the forward target first. If the train is past the outside observer, they will see the light from the backward target first.

But, dropping my counterfactual, that's the same outcome that's predicted by the general relativity example. Only in a very tiny range of starting locations for the train, almost but not quite to the position of the outside of server, do my counterfactual and general relativity differ on which target the outside observer will appear to observe first.

I ultimately Einstein is right, and I think I've argued myself back to a position of agreeing with him, but I'm just doing a ton of extra scenario building and going way past the point where I have any idea if what I'm saying is right or totally wrong. The simplified scenarios that are illustrated in example after example seem to me not to actually demonstrate the difference that general relativity predicts. Sometimes I wonder with some scenarios if, for information to return to all of the original observers, it's like it has to undo the effects of general relativity.

I read a quick explanation of the Michelson Morley mirror experiment, in which (so I've read) it was observed that light traveling in the direction that the Earth is spinning or moving around the Sun or something didn't move faster than light traveling some other direction. I'm sure I am wildly misstating this. But the way it was explained to me seemed to have nothing surprising at all. Because if the light was indeed moving at the suppose it speed of light plus the speed of the Earth, but we are ourselves moving at the speed of the Earth, how would we perceive that extra speed?

Again, I'm sure that from the perspective of someone who really understands this stuff my questions are totally misinformed, I just can't fill in that context myself with the books I have.

So my overall question is, what is a scenario in which our conventional expectations are subverted by what we observe, and we can only conclude that general relativity is correct?

Question 1: is my description of the train scenario correct?

Question 2: what is a scenario in which are conventional expectations are supported by what we observe, and we can only conclude that general relativity is correct?

Section 2:

Regarding atomic resistance to contact:

I read in several places that as electrons in different atoms (which are each magnetically balanced) approach each other, they exchange photons. That is, photons are the mechanism by which electrons communicate their proximity to each other. 

Or maybe photons are the mechanism by which atoms communicate their proximity to each other. I don't really begin to understand. 

I've read that you could think of electrons using a metaphor of two people passing a dodgeball back and forth, where they're able to do it much faster as they get closer together, which means in effect their resistance to each other is higher. And as atoms get too close, their electrons push each other away and that pushes the whole atom away. 

But another thing I've read is that atomic nuclei just really don't like to be too close together, that there's a sort of proton exclusion principle or something, and that's why two atoms don't like to be pushed together. 

The source of my question is why is it, fundamentally, that two objects don't just merge as they collide? What is changing within them as they get close together? When I stand on a floor, I understand that the floor is pushing back at me with 200 lb of force. What I'm surmising is that that force comes from the compression of the floor, whose atoms are getting pushed together more than they like, and something is happening within them that repels those atoms from each other, and repels my shoe atoms from the floor, and those atoms repel my foot atoms, Etc. But I can't figure out which law of physics is the main one that's at work here.

A related question: I've been told that water is essentially incompressible, that it doesn't shrink and heat up as you compress it the way gases do. 

But say two people are standing on top of two big plungers, where the space under the plunger for one person is filled up with water and for the other person is filled up with air. I assume that what's happening within the chamber in each case is that the material is being compressed and creating resistance that pushes back against the plunger with an equal force to the weight of the person, at least as soon as the plunger initially depresses to adjust to the person's weight. What is the source of the force in each case? 

I've been told in a gas it is the rate of Brownian motion collisions from the gas atom bumping upwards against the plunger. But I gather that's not the case with the water. If not, what is it? Are there two totally different laws operating in the two cases? Or is it more a matter of degree, where the sort of atomic explosion I've been talking about is indeed still at work in the gas and that's why gas atoms bump each other and communicate motion, and similarly there is a bit of Brownian motion in the water as well but it's just not a significant way of describing the water's resistance to compression? Is there anything relating to that electrons playing dodgeball with photons idea that makes sense here?

Question 3: why is it, fundamentally, that two objects that are pressed together don't just merge or pass through each other?

Question 4: is my description of the Brownian motion of the gas producing force correct?

Question 5: by what atomic-level mechanism is the water producing upward force against the plunger?

Question 6: what happens within a single atom as another atom (which it doesn't need in the sense of forming a molecule with it) approaches it?

Section 3:

My next question is about reflection of light. I know there are some aspects of quantum mechanics that it's foolish to expect to understand intuitively, and maybe this is one of them. From what I (shakily) understand, incoming light is being reflected by a mirror at all sorts of angles, and the only reason we perceive an angle of light shining on a mirror at 10 degrees as also reflecting off the mirror at 10 degrees (or, rather, 170 degrees) is that the average of the possible angles of the reflected light is 10 degrees. I'm sure I have said that wrong, I'm not sure if this is an issue of quantum mechanics or of the wave/particle nature of light, or both.

I'd love a good explanation of that aspect of reflection, but I'm trying to keep my questions a bit more focused. So instead, I want to ask:

Question 7: when a photon hits an object, say a speck of green paint, what happens within that within that atom that causes a photon to be sent back out? 

Question 8: in a very light absorbent substance like vantablack, what is happening when one of its atoms is struck by a photon?

Question 9: is it correct to assume that a surface a single atom thick would still, in theory, be capable of reflecting light at a complementary angle to the one the photons came in on?

Question 10: on the atomic scale, why should a photon that makes contact with the atom at one angle be sent out at the same angle?

Question 11: say you had many such single atom think surfaces, reach oriented at a different angle. You shoot a stream of photons at each one, striking each one at a single atom. How do the photon and atom know what angle to send the photon out at? Isn't the scenario identical, for all intents and purposes, for each of these atoms being struck by a photon?

About the two slit experiment, I don't understand enough to ask a good question. I guess I fundamentally don't really understand what we're talking about when we say light acts as a wave. Sometimes it seems as though the concept of it being a wave is something far beyond our ability to have an intuitive understanding of. Other times, it seems practically Newtonian and straightforward. For instance, I get that some substances are transparent to some wavelengths of light and opaque to others; and that's like a measurable thing, you can imagine AM radio waves hitting all kinds of obstacles in all there going back and forth that higher bandwidth FM waves sail past. Or with polarized filters, you can say only light that is moving back and forth along some particular rotation angle will get through. All this has given me the sense that light waves involve light photons essentially moving up and down as they travel forward, varying their position relative to the average center of their direction of travel by inches or even feet.

But this understanding doesn't seem like it can be right. Because you can't like catch a photon several inches away from the center of its vector of travel, right? Because it seems to be that would mean our eyes would have no idea where any of the photons it receives are coming from.

And then you go to the two slit experiment and suddenly we're talking about light waves not being a phenomenon centered along one direction of travel, but something where the waves are a more elusive concept than the somewhat concrete sense I get from the ease of polarization.

Anyway, I'm far from being able to articulate good questions about the two slit experiment, or even just polarization. I just know there's a lot of background that I don't understand!