epenguin said:
I come back after an absence to this forum and this thread. I think Xezlec has missed the point of my question.
Not only that, I didn't even realize you intended to ask a question. I interpreted your post to be saying something like: "These scientists are dumb. This obviously doesn't make sense because I don't understand it." I apologize if I misunderstood. Maybe I was a little testy that day.
In any case, you appeared to claim that quantum mechanics was "clearly" different from other physics in the sense of not being based on evidence, and the main thing I wanted to say was that that is very, very, very false.
To put it another way is there a physically intuitive explanation at fairly popular level? Say Scientific American level. There are books or articles that make a fair stab at explaining the not exactly intuitive quantum mechanics to laymen.
Have you read them? Did they explain it? I think the problem is you are reaching the limits of what these kinds of coffee-table simplifications can do for you. Perhaps you want more understanding, but haven't yet resigned yourself to the fact that the stuff the simplifications don't explain is exactly the reason that they are simplifications. If you really want to understand why spin only occurs in certain values and what those numbers mean, I think you are going to continue to feel unsatisfied with the fuzzy answers you get until you finally make up your mind to just take the plunge and actually learn the painful complexity of details that together form the true answer to your question. I could be wrong but that's my guess.
I am reminded of a story I read in a Philosophy of Religion class. There was an argument about whether everything had to have a single cause, and some philosopher proposed the following situation: you walk up to a bus stop and find 5 Eskimos standing there. You ask yourself what could be the cause of such a strange event. But what if it turned out there was no single satisfying answer? It's possible that one of them immigrated due to global warming and was going to school nearby and decided to catch the bus home, another is from Canada and works at the Canadian embassy in hopes of beefing up a resume to go into politics one day, and each Eskimo in turn has a different -- and completely unrelated -- reason for being there.
I can relate to this story as follows. If you learned all of that by talking to all of them, let's say some stranger walks up and asks why all those Eskimos are standing there. You start explaining each one's story but he stops you, saying he doesn't want all that detail. He just wants the bottom line. What the hell can you possibly tell him? That's how I feel in this situation. All I can say is that each of these 10,000 Eskimos of physics has a story and probably a Nobel prize behind it for the leap of reasoning that it took to show how some experiment proved some amazing new fact. Or, I can explain each one and its reasoning in detail. But I don't know what else to do.
Sure. I can try to help explain what little of this I understand, though I warn you that I'm still trying to learn it myself. However, the thing about simplified explanations is that they are simplified. That means they won't be completely correct and they probably won't completely make sense. If I said something that was both perfectly correct and didn't have any logical "holes", then it would be a literal physical law. And then you wouldn't be a layman.
The other problem is that quantum physics is notoriously unintuitive. It is probably safe to say that every physicist who has lived since the dawn of quantum mechanics has tried and failed to find a more intuitive way of thinking about the subject. Experienced physicists today just tend to say "oh, well, our hunter-gatherer brains just aren't designed to understand things at the quantum level, so our intuition will never be up to the task. Tough luck."
Have you at least read "The Elegant Universe"? It tries pretty hard to simplify these kinds of subjects. I personally didn't feel completely satisfied with its explanations, but I think that book is about the best anyone can ever possibly do.
And things like quarks too... forces explained as exchange of virtual particles…
I believe, after studying a lot of math very very hard, that I may be coming close to seeing what it means to say that forces can be comprised of an exchange of virtual particles. Let me see what I can do. This is going to be horribly inaccurate, so I'll probably get a flood of corrections from people.
To say things in a very oversimplified and foggy way: practically any kind of wave can be expressed as a moving distribution of "probabilities" of a particle being in different places. That probability is not really a probability in the typical sense of the word, since the particle actually sort of occupies all of those positions simultaneously up until its location becomes important for some reason, at which point the universe finally sort of uses the wave as a true probability and picks a random location for it to be. Oh, and this probability also has a constantly-rotating phase angle associated with it, and the probabilities add together in a funny way I can't explain without using details. The reverse is also true: any particle's position can be expressed as a wave.
Then, I must explain that a virtual particle is like a particle that has some "probability" (in the above sense of the word) of existing somewhere at some time, but never is capable of actually coming into existence there and then because it would violate some conservation law, but still affects the world in some indirect way nonetheless, by virtue of how that probability wave produces other waves, smooshes together with other waves, and stuff. Kind of.
Next, I could point out that forces must always be transmitted by waves. This one is a lot easier to explain, as it doesn't require any quantum. For example, if you push on an object, that force nudges the atoms touching you, and then they nudge the atoms touching them, and so on down to the end of the object. The same is true for electromagnetic and gravitation forces, but the wave is made up of something other than atoms: a field (instead of a force) "pushes on" empty space, and that sudden change then exerts a field in the empty space in front of it, and so on until finally the wave of fields reaches a charged object, at which point the field actually exerts a force on that object (in addition to exerting a field on the space it occupies, kind of).
Finally, we can sort of tie it all together (loosely!) by saying that if a gravitational force exists between two stationary things, then there must be waves of fields propagating that force between them, and since all waves are in some (extremely complicated) way a representation of the probability of the existence of some particle at different points, there must be particles traveling between them, but all this particle motion would take energy coming from nowhere, but that energy is also going to nowhere since the objects aren't giving way to the forces at all, so the force between them that we observe must be the effect of particles that sort of exist in the sense that they have probability waves that make something true, but can't possibly really exist in the measurement sense and don't even need to. *passes out from lack of oxygen*
I think that the above paragraph is particularly bad and wrong, due to my understanding falling off right about at that point. Maybe someone else can swoop in and clear that up a bit.
Particles can be observed to have lots of different properties. The Standard Model is the final result of decades of individual observations and theories to simplify all the interactions that have been seen. I mean, I don't know what you expect here. There are millions and millions of events that have been recorded by particle accelerators and the standard model is the smallest set of rules anyone has come up with that completely describes them all. Don't ask what all those rules are, because if you knew those you certainly wouldn't be a layman anymore. And I don't know much about them anyway.
Start with special relativity. Then add that an accelerated point of view feels exactly like a stationary point of view with a gravitational source nearby. Do a grotesque amount of math to flesh out that idea and you have a theory of propagating curvature of spacetime that is much too complex for my tiny brain to have ever accomplished a grasp of.
Spin of electrons or photons does have some intuitively understandable obeservable physical manifestations I know of that entitle it to be called spin.
Are you referring to the simple statement that it looks like "intrinsic angular momentum"? Let me show an example of how incomplete that description is.
In answer to your "2 what?" question, the angular momentum represented by a particle with spin s is given by: \hbar \sqrt{s(s+1)}. (I learned that from Wikipedia, along with most of the rest of the stuff I know). Now, I ask you, is spin a vector? Angular momentum is certainly a vector: it has some magnitude and some axis of rotation, right? So if spin really is angular momentum, it should be a vector too. But no, spin is just an integer multiple of 1/2. This is because no property of a particle is really definite; in quantum mechanics every particle is in a superposition of all possible states with different "probabilities". And because the quantum world is so complicated, it just turns out that the relative "probabilities" of different spin values for a given particle combine together to also tell you something about axis direction. However, what exactly they tell you, and why, is not easy to state without math.
But what gives similar confidence to think we know something of graviton spin?
As far as I understand, the type of wave that propagates the 4-dimensional warping of spacetime that corresponds to gravity can only sensibly be connected to the probabilities of a particle with spin 2. This is apparently due to the details of how waves in quantum mechanics describe the properties of particles they correspond to. I think it has something vaguely to do with the number of degrees of freedom that are available to a spin-2 particle and the number of degrees of freedom involved in the gravitational force. I'm not going to be able to do any better than that, being fairly ignorant of that subject.
To reflect back Xelec’s Just because he can’t think of an explanation of that kind doesn’t mean that no one can.
Indeed not. I wish you the best of luck. If you ever do come across a simple, intuitive way of explaining advanced physics concepts, please post it here! We will all appreciate it very much.
) though they never have been observed nor is there AFAIK any programme or hope of an observation. Nothing has ever been observed for which GR is not adequate.
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