Hacker Newsnew | past | comments | ask | show | jobs | submitlogin
Ask HN: What are physicists doing?
6 points by cool-RR on Dec 11, 2008 | hide | past | favorite | 11 comments
In this thread I would like to consult with you, HN readers, about a problem I have with physics. It is a sort of gap between the way I research physics, and the way mainstream physicists do it. This thread will probably appeal only to those of you who are interested/knowledgeable in physics.

A little introduction about myself is necessary. I'm 22 years old. I live in Israel. My main occupation right now is studying physics. However, I am not a student in a university, nor am I affiliated with any other kind of academic institution. It's been about a year and a half since I've started studying physics in this independent way. I used to be an official student, a few years ago. I was an undergrad in Electric Engineering. I quit there after one year. Then I decided to start studying math independently. I did that for about a year and a half. Then I stopped with math and decided to start studying physics, with the goal of figuring out how the universe works.

So I'm about a year and a half into my quest. I can say that up to now I studied Special Relativity and Classic Electromagnetism.

The thing is, I feel that I have diverted from the ways of academic physicists. Actually, after these 1.5 years it has come to a point where I feel like I'm speaking a different language than the one they're speaking. Perhaps I took a wrong turn somewhere and I'm heading into a dead end? Perhaps the mainstream physicists took a wrong turn and I'm taking the right one? Or maybe some other possibility? I am hoping that you, HN readers, will be able to shed some light on this gap.

What are these differences between me and them? It will take some exposition before I could explain. Some of the things I say about physics may seem wrong or provocative to you. If you feel the urge to tell me I'm wrong, please do it with a thorough, logical argument. If anything seems unclear, please ask. Here we go:

In Newtonian mechanics, life was simple. There was a collection of bodies in different places. Each pair of them exerted forces on each other. There were rules that said exactly how much force they exerted. With these rules it was possible to calculate exactly what the force on each body was. After you knew the force for a body, you could know the acceleration that that body would have, according to the revered formula, F=m a, or in its more useful form, a=F/m. After you knew the acceleration, you could advance the simulation by a small time-step. The bodies would then move a bit, and you would calculate the forces again, and so on.

If you took a small enough time-step, you could calculate the outcome of any physical system to any desired accuracy. All the subjects taught in Newtonian mechanics, such as angular momentum, centrifugal forces, conservative fields, and kinetic and potential energies would appear as emergent phenomena from these rules. They were just epiphenomena to the true axioms of physics: The force equations and F=m a.

That was Newtonian mechanics. In approaching Special Relativity, I expected the same style, maybe a bit more complicated. Eventually that's what I got, but it was hard work, and I had to build big parts of the system myself, with only hints from physics textbooks (more about that later.) It turns out that Special Relativity is just a tad more complicated than the above description of Newtonian mechanics. Instead of the formula a = F/m, there is a more complicated formula:

a = (F- v (F v)/c^2)/(m gamma)

(Where v is the velocity, gamma is some function of the velocity, c is the speed of light and that (F v) is a dot product.)

And the formulas for calculating forces become more complicated as well. I will not list them here, since the mathtext will become too cumbersome, but if anyone will insist I'll post them. These equations are eventually what is called Classic Electromagnetism. All the revered Maxwell equations turn out to be just special cases of these equations.

(Also, in Special Relativity there is the issue of Lorentz transformations: but that is important only if you want to change viewpoints, and even then it can be deduced from the rules above.)

Another note: I know that the system I described is not an end-all model of the world. It does not include Quantum Mechanics, and thus it will be valid only for macroscopic bodies. (It also does not include General Relativity, and thus it could not deal with gravitation, but that is less important in my opinion.)

I mentioned that I had to build most of that system myself, and that physics textbooks don't give this system explicitly. That is the biggest gap between the physics community and myself. Physicists do not seem to accept this system. The equation for the acceleration that I supplied above cannot be found in any textbook, or at least I didn't find it. That is even though it can be easily derived from known equations of Special Relativity. The equation for the Electromagnetic force is almost as hard-to-find, although it can be derived from the well-known Liénard-Wiechert potentials. Why are these things not mentioned in textbooks? Am I blind to something? What are physicists doing, how can they research anything without knowing this system?

About Quantum Mechanics: Even though the system I described will break down at the quantum level, I think it's indispensable for trying to figure out how the quantum world works. It is true that as you go smaller and smaller, the physical reality will deviate from this macroscopic model; But if you want to study and understand these deviations, you should understand the macroscopic model first, so you will know exactly what to compare the physical reality against!

That's my opinion. I may be wrong, and if I am, I would love to hear a well-reasoned rebuttal. I really hope you guys can shed some light on this.

Ram.



You say "physics textbooks don't give this system explicitly". They don't need to - they simply present the system that's most instructive to learn or is most interesting to their authors. As long as your formula or "system" is consistent with the accepted formulations of special relativity, you have no reason to worry.

PS - Most physics communities are getting by with their own methods, and if you really think yours adds something you should write a textbook (or a paper).


I understand that their systems may be simpler for their uses, and it's possible that for this reason they use Maxwell's equations and things like that: But how can they not mention it, even once? I would personally expect these equations to be revered in every Classic Electromagnetism book. Okay, so they are not revered, maybe my taste is a little peculiar, but they are not even mentioned! I just can't understand it. Think how much Maxwell's laws are revered, and this is one formula that contains them all and more, in one line. Don't you think that they would mention it?

That's what's baffling me. Do they not know this? Do they not care? How can they not care?

About your PS: Yes, I'm working on writing this.


So... Special relativity is called "special" because it specifically assumes no acceleration and therefore, no force. That formula you have is an approximation for "small" forces. General relativity is what actually explains space with acceleration (using Riemann Geometry, another fascination and extremely complex topic in of itself relying on tensor calculus, which is another fascinating and extremely complex topic in of itself). Secondly, the Lorentz transformation is critical to special relativity, because that is what gave rise to the entire theory and also because the Lorentz transformation is exact (up to QM effects and 0 acceleration). Also, the Lorentz transform is sufficient to derive the result e = mc^2 among many other useful results on momentum and particle collisions (or at least the aftereffects thereof, since we can have no forces) which is why special relativity is even taught in isolation.

As to the textbooks, all I can say is that you haven't come by the right textbooks yet. I would advise reading the Feynman's lectures on physics (all 3 volumes). Expensive but worth it and if you want, you could probably find free pdfs online. FLs does indeed go into the gory details of special relativity (v1 I think) and even shows how to derive most of the useful results including (I think) information on how to derive the Lorentz transformation from the Special Relativity's base assumption: the speed of light is constant in all time frames. ENM along with several applications is also discussed in FLs (v2 I think). FLs V3 covers QM is great detail (well... the parts that we humans can solve mostly). Another very good book on ENM would be Electricity and Magnetism by Purcell, Berkley Physics Course Vol. 2.

Oh btw. the central assumption of General Relativity is that force and gravity are just 2 different manifestations of the same phenomenon. Thus, for very weak gravitational fields, you could actually use the formula you have. Just that it would not be very useful because a) you would have to interpret it as constantly changing time frames over small time intervals, and b) because I don't think the Earth's gravitational field (or the moon's) is weak enough to justify the approximations inherent in your equation.

A note on finding good textbooks: Pick a good university. Any good university (I would advise Caltech but thats because thats where I study) and chances are very good that said universities will publish the course textbooks for their classes. Some will even publish the homework assignments and solution sets along with lecture notes. Eg. http://www.pma.caltech.edu/GSR/physicscourses.html is the place where CIT has currently placed a convenient listing of all the physics courses offered with links to their web pages which contain among other things: the textbooks, syllabus, and in most cases, the homeworks for the class. Finding textbooks from there shouldn't be very difficult.


There are many objections I have to raise to your post. But let's just look at the first thing you say.

You say that SR can't handle acceleration. I disagree. Let's examine this issue.

My first question about it is: Assume you have a "twin paradox" situation. One twin stays on earth for a year, while the other twin goes somewhere far away in space and returns, in relatvistic speed. The travelling twin accelerates smoothly in his journey. Given the twin's trajectory, the mission is to calculate in how many years he has grown old. Do you think that SR can't handle this case?

Second question. Observe the Lienard-Wiechert potentials. They say the exact value of the EM field at any given point. What is the significance of the value of the field E, if not the chrage of the body times the force that acts on it? If it does mean that, then "force" in which context, if not SR? Newtonian? GR?


The twin paradox cannot be handled by SR. The difference in aging comes about due to time dialation caused by the differential acceleration that the twins experience (the twin in the spaceship is accelerated and the other is not). SR can be used to judge the middl period after the acceleration but it will not give any useful information because you can always switch viewpoints. If you have two bodies moving at some relative velocities, both can claim that the other is moving slowly in time. The mathematics of communication and speed of light work out so that this system is consistent. The difference in measured time comes from the acceleration that one of the bodies must undergo to enter the same time frame as the other.

Secondly, electric fields have intrinsic effects as in EM radiation (colloquially known as light) and need not apply any force to have effects. The movement of emitters of electrical fields through space and acceleration thereof actually give rise to magnetic fields. Read Berkley Phys. Vol. 2 ch. 5 which shows how this happens in the framework of SR. It later hints how tensors can be used to generalize this result to GR. So... in response to your question, if force is applied, either it must be approximated in a Newtonian or SR context, or GR must be used in its full generality. Usually, the forces and particles involved are sufficiently small that approximations work (see success of Newtonian mechanics), but that is not the general case. Also, very common is the tendency to derive general results and then use them is special cases. Maxwell's equations are an excellent example. They refer only to EM fields and are exact (uptil QM of course).


Okay Newt, now we have something we can compare! Because I claim that SR can handle such a case. I propose that I will give the answer as I calculate it from my version of SR, and you can give it from GR or whatever you think is necessary. Does that seem like a worthwhile comparison to you? Let's say that our travelling twin takes a trip of one second (not one year, to make the calculation easier). Let's say he goes in the following trajectory: x(t)=0.9 c sin (k t) /k Where k equals (pi/second). How older will the twin be after the journey? I got the result of 0.7459255 seconds. What is yours?


Are you working on it?


There is a problem with your analysis. Switch viewpoints to the reference frame of the traveling twin. In that case, that twin stays in a constant position and the other twin travels with a trajectory of x(t) = -0.9 c sin(kt) / k. The negative sign is irrelevant as we are only interested in absolute velocity and the negative sign implies that the movement is in the opposite directions. Then by your calculations, the other twin should now experience 0.74... seconds. Ie. Without accounting for the acceleration that one of the twins experiences, you can always switch frames and therefore cannot draw any conclusive results for both twins simultaneously unless you fix the reference frame (but in that case, differential aging really makes very little sense).

As to calculation the actual difference in age, I don't know GR sufficiently well to be able to calculate the aging of the twins but I know some people who do. I have contacted them and will get back to you once they send me an answer.


Hello Newt,

Thanks for answering and for forwarding this to people who know GR, this might really help me.

Regarding your first paragraph: I didn't give you my analysis, I only gave you a final answer, so how can you say there's a problem with my analysis? It's more like, there's a problem with your guess of what my analysis was. I phrased the original question accurately and gave a final answer.

The contradiction you're talking about does happen if you try to switch reference frames without taking note of the acceleration. I didn't try to switch reference frames.


That is the problem. One of the central tenants of relativity is that what reference frames we choose is irrelevant as long as we account for all the relativistic effects. In you analysis, whatever that may be, you have two twins which separate, move around, and then return to the same reference frame and your result is apparently dependent on which twin's reference frame you are using. That is a contradiction. Thus the need to account for the acceleration that one of the twins experiences w.r.t. the rest of the universe.


We are having quite a funny argument, because it seems we both see the problem.

"One of the central tenants of relativity is that what reference frames we choose is irrelevant as long as we account for all the relativistic effects."

I think that applies only to inertial frames. At least in my version of SR that is so.

"your result is apparently dependent on which twin's reference frame you are using. That is a contradiction."

I know it is dependent. I don't see what's the contradiction. The earth twin has a special status - he is non-accelerating. His reference frame is therefore inertial. The other twin is accelerating. His reference frame is non-inertial.

Where is the problem here?




Guidelines | FAQ | Lists | API | Security | Legal | Apply to YC | Contact

Search: