# pmb

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379

1. ## Active vs Passive Gravitational mass

Yep. I'm quite aware of that, thanks. I.e. I've spent a great deal of time working on Lagrangian dynamics, as an under graduate student, in graduate school, and in personal independant studies, e.g. http://home.comcast....stic_energy.htm as well as Noether's theorem. Yep. I'm also aware of that as well as Noether's theorem which is what you're discussing. That does't tell us anything more than what swansont was trying to say .... unless you thought I didn't understand Lagrangian dynamics and Noether's theorem. In either case, I understand them both. Yay! Yep. I'm quite aware of that, thanks. I.e. I've sent a great deal of time working on Lagrangian dynamics, as an under graduate student, in graduate school, and in personal independant studies, as in e.g. http://home.comcast....stic_energy.htm Yep. I'm also aware of that as well as Noether's theorem which is what you're discussing. That does't tell us anything more than what swansont was trying to say .... unless you thought I didn't understand Lagrangian dynamics and Noether's theorem. In either case, I understand them both. Yay! Does anybody want to take a crack at figuring out the Lagrangian of a system of three particles, and expressed in terms of each particle's passive gravitational mass and active gravitational mass? Thanks. Pete
2. ## Elements

Relying on memory in science can be tricky. What the magazine meant by it said and your memory combines to be a problematic article retrieval in your memory. For example: Does the molecule have a spectrum which is the same as an atom etc. More later

4. ## Active vs Passive Gravitational mass

Establishing a relation by using the Lagrangian, Lagranges equations, and a conservation relationhip is a matter of derivation. It's not true because it was defined that way. There is something else which bothers me about your arguement which I can't put my finger on. If I figure it out I'll get back to this. I have a feeling that there is trouble forming a Lagrangian. In any case I'm having trouble trying to write down a Lagrangian for a simple ayarwn for, say, a system of three particles moving in the xy-plain. E.g. how does on express the potential terms in tems of acitve and gravitational mass?
5. ## Space curved in on itself but with no Boundary & finite.

Above you said if the space is really empty. The space can have a density so small that its possible to have a straight line and not run into one of the particles in the space or you can take that one particle out of the way a bit. What that means is that its possible to have a straight line (a geodesic in 3-d space) from the front of your head to the back of your head but not gonig through your head.
6. ## Elements

Not to a great extent. What I mean is this - It's possible to have two oxygen oxygen atoms with different mass. This is due to different number of neutrons in the oxygen nucleus.
7. ## Active vs Passive Gravitational mass

Something occured to me this morning. I've read/heard people argue about the distinction between active and passive gravitational mass. If these quantities were different then there'd be a violation in conservation of momentum. The conservtion of momentum is based on observation. The equality is therefore a law of physics, and not something which is true by definition. Let us compare this with the equality of a particle's inertial mass and passive gravitational mass. If this equality were not true then the Equivalence Principle would be wrong. If the equivalence principle was wrong then the rate of a particles rate of fall would depend on the particle's rest mass. Therefore this is also something which is true based on observed. The equality is therefore a law of physics and not something which is true based on definition. To take not take note of passive g-mass and active g-mass is therefore inconsistent with the taking note of the equality of passive g-mass and inertial g-mass.
8. ## Is gravity a pseudo force?

this post was a mistake)
9. ## Is something really there?

I'd hazard to guess that if the components of the stress-energy-momentum tensor of an object is not zero then that object exists. This was Einstein's view in that Eintein that if any of the components of that object do not vanish then there is matter in there is region of space where it is not zero. Are you familiar with tensors etc?
10. ## Is gravity a pseudo force?

On the funny part of this, there are now 5 posts discussing whether the threads was hijacked.
11. ## Is gravity a pseudo force?

Yes. That is not working.
12. ## Active vs Passive Gravitational mass

Thanks for pointing out the oversites on my part. So many equaions can make me go blind sometimes. Please don't hesitate to point them out in the future. And I thank you vedy vedy much! The definition of active and passive gravitational mass are as follows Yes. That is correct. I don't know where you got that from. Sometime people confuse proper mass with m(0) where m = m(v) = inertial mass = relativistic mass. If a particleis at rest in a gravitational field then then, as always, |P|/c is the invariant mass. However when v = 0 and the particle is in a g-field (or non-inertial frame of reference) then |P|/c != m(0) != proper mass I recommend that you try it out for the most simplest case you can think of. E.g. let the particle be at rest in a uniform g-field or something and see how it works out.
13. ## Gravity a force or natural motion?

Another way to address this problem is to look at it from another point of view. It's one I don't like but you might enjoy but you might. First - It seems to me that asking Is gravity really a force or is it something else? starts off similarly to asking Is electricity a force or is it something else? How did we come get to ask these questions? Electricity is a science so how can it come to be that a broad field of study has a field of force to it?
14. ## Is gravity a pseudo force?

I believe what you just did i not thread hijacking. I believe that continuing the discussion here is thread hijacking though.
15. ## Active vs Passive Gravitational mass

I stated Eq. (3) whereupon I decide to prove/derive Eq. (9) which I did. I just didn't say "this equals 3" since it was 3 which I stated I was proving. I assume that the reader doesn't forget what I stated 6 equations ago. Thanks for pointing it out though. I think that just might make it clearer.
16. ## Active vs Passive Gravitational mass

I was wondering if any of you folks have heard the terms Active Gravitational Mass and Passive Gravitational Mass? Theoretically they are proportional to each other. You can choose constants of proportionality such that they're differerent, However, regardlesso how you chose those constants the densities of these quantities have the same value. A longtime ago I wrote a web page addressing this matter. I placed it here http://home.comcast....e_grav_mass.htm The density is seen in Eq. (3). Let rho = density of active gravitational density. As you can see from Eq. (3) rhoactive g-mass = u0 + 3p. I don't derive it on that page (but will create a new page soon) but the passive gravitational mass density equals rhopassive g-mass = u0 + p. It is the case tht passive g-mass = active g-mass. I think that's equal by definition. Not sure. I forgot. The point I'm making is that even though you can choose constants so that they're equal, there is a very good reason to define two equantites since they have different densities in general. Consider now a particle moving in a static gravitational field. The 4-momentum of the particle is P = (mc, p) where m is the passive gravitational mass as well as the inertial mass. Then for a particls at rest in the field m2 = P2. The energy if the particle is P0 where the mass of the particle is The energy if the particle is P0. Note: m(0) is the rest mass butit is not equal to the particles proper mass. I.e. m(0) != |P|/c. In this thread I'm hoping to make it clear that one has to be careful with what they call mass in SR/GR. It's not as easy as it appears. Pete ps - When I create a new page on the subject I'll post a URL to it if there is a desire for somonme to read it.
17. ## Gravity a force or natural motion?

I'm not sure what you mean when you said Which is, by definition, non-Newtonian, and is what I meant. Can you explain it for me? What is it that is non-Newtonian?
18. ## Gravity a force or natural motion?

I disagree. Gravity is an inertial force and since Einstein built GR around the idea that the gravitational force was an inertial force and that inertial forces are real and GR was Einstein's theory then in GR gravity is a force. Well known experts in GR hold this same thing even today. For example. See Was Einstein Right? Putting General Relativity to the Test. Clifford M. Will - Updated and Newly Revised. See page 94 Here, Will is speaking about the velocity dependance of all forces in SR and how it has its counterpart in GR.
19. ## Is gravity a pseudo force?

To answer your question one needs to know what an inertial force is. The centrifugal force and the Coriolis force are psuedo-force and fictitious force. These terms are quite misleading so it is suggested that they not be used. For details please see http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm According to Einstein, the gravitational force is as real as the electric force and as such it is seen to be real. The resistance of a gravitational field depends on the frame of reference that one is using. Just as frames of reference can change, so too is the existence of the gravitons. This strange notion is found in the relative existence of photons in a falling charged particle. At this point I'd like to caution you regarding the presence of a gravitational field and curved spacetime. The presence of a gravitational field is determined by the existence of a non-inertial frame of reference, not tidal gradients. Therefore the presence of as a gravitational field is determined by the the presence is the affine components in a spatial Cartiansian coordinate system.
20. ## Your personal definition of science?

That is simply incorrect. Just because the name of the organization which published the article has the word physics in it can in no way be used as a valid excuse of it being the definition of science. The example you gave merely shows that synthetic chemistry, as you've used it, means that synthetic chemistry is not a valid branch of science.

22. ## Why does light travel at the speed of light?

Please post such a derivation. Typically such derivations are really identities.

24. ## Warped spacetime and gravity

That's not quite correct. The embedding diagram for a Schwarzschild geometry can be found in the book Exploring Black Holes by Taylor and Wheeler which is at http://www.eftaylor.com/pub/chapter2.pdf (Moderator - Don't worry. These sections are available to be placed on line by the authors). Download and turn to page 2-26. Look at Figures 6 and 7. Read what is stated in the diagrams for for r, dr, d(sigma). If a collection of rulers was layed down around the Black Hole then you would see a different set of distances than would be there had not the black hole been present. Pete
25. ## Your personal definition of science?

The following is the definition of science from the America Associatio of Physics Teachers. This is not copywritten material. http://home.comcast.net/~peter.m.brown/ref/what_is_science.pdf I prefer this definition to any other I hae been able to find. I hope that I don't get in trouble for pasting this here. Pete
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