Gravity and relativity

Hi Votorx,
Yes, work is being done. Which is why I specified that "nonremovable kinetic energy" is the minimum average KE in an inertial (non-accelerating) reference frame.

I'm not sure if we're talking on the same wavelength at the moment... what is was the point of our discussion again?
 
I don't know....Im not even thinking about what Im saying anymore, I'm done for today.


Both Person A and Person B have mass
Person B is circling around Person A at .9C
Relative to person B, Person A is spinning at .9C. Something that has a great acceleration over a small period of time produces a stronger gravitational field. Wouldn't it be true then that Person A, spinning at a speed of .9C relative to Person B,
would create a greater gravitational field due to the fact that he's accelerating greatly over such a small period of time?


That's what we're talking about or should be anyways...
 
Gotcha.
OK, so no matter how fast B goes, no work is being done on A in any inertial reference frame, so A's "nonremovable kinetic energy" stays at zero and A's gravitational field does not increase.
 
But what I don't understnad is why it doesn't. I can understand that in some reference frame his kinetic energy is 0, but in Person B's reference frame Person A has kinetic energy due to the fact that he's spinning at .9c.
 
Bear in mind that I really don't know what I'm talking about here... I'm way out of my depth, but...

Person A is not accelerating, so they are not producing gravity waves. So, the only gravity they have is their rest mass gravity. It doesn't matter that they have kinetic energy in other reference frames. I don't know why.
 
Person A is not accelerating, so they are not producing gravity waves. So, the only gravity they have is their rest mass gravity. It doesn't matter that they have kinetic energy in other reference frames. I don't know why.
Click to expand...

But...Person A would be accelerating relative to Person B. He's spinnig around relative to person B, changing direction continously.

Yes I realize that you aren't completely sure about what your talking about, but no one else is contributing and I'm trying to make sence out of it.
 
Votorx said:
But...Person A would be accelerating relative to Person B. He's spinnig around relative to person B, changing direction continously.
Click to expand...
Not relevant, because B's reference frame (the frame in which A is accelerating) is not inertial.

I don't know exactly why, because I'm incapable of doing the proper analysis, but I am sure that A does not produce gravity waves unless they are accelerating in an inertial reference frame.
 
Hi Votorx,

Did you see my post back on the first page? Pete drew my attention to this thread, but I think you may have missed my attempt at answering your question.

I think some of the confusion arises from the notion that acceleration is relative. It's important to remember that Einstein gave us the general theory of relativity, not the theory of general relativity. In other words, not everything is relative. There are still special nice frames that exist in little regions of spacetime which are essentially non-rotating frames in free fall. What the general theory does is abolish the notion of absolute space by defining these frames relative to a local dynamical entity, the gravitational field. It also gives us complete coordinate freedom to describe physics, so while you can find coordinates where a rotating system appears to not be moving, it doesn't mean that rotation is somehow arbitrary. In other words, it is possible to say that something is rotating. Rotating with respect to what, you rightly ask? Rotating with respect to the local gravitational field.
 
Thanks again, Physics Monkey.

Am I on the right track with the concept of "nonremovable kinetic energy"?

1) If a massive object is vibrating (orbiting, bouncing, whatever) in a confined space in some inertial reference frame, will the object's gravitational field at a moderate distance be larger than if it was not vibrating?

2) Is the vibrating object's distant gravitational field equivalent to the field of a non-vibrating with mass equal to the mass plus average kinetic energy of the vibrating object? (Average kinetic energy in the vibrating object's average rest frame)

(I hope that makes sense?)
 
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Hi Pete,

Yes, the notion "nonremovable kinetic energy" is a reasonable way to understand things, at least in the weak field limit. The relevant quantity is essentially the energy of the system in its rest frame i.e. the frame where the total 3 momentum of the system is zero. For any macroscopic material system, the total center of mass energy always has contributions from atomic and nuclear binding energies, thermal motion, and so forth. All these types of internal energy can therefore contribute to the field, but the typical situation is that these extra contributions are totally negligible since they are often several orders of magnitude smaller than the simple rest mass result.

While the essential idea remains the same, you can take my word for it that things become more complicated when the fields are no longer weak. For example, there is not even a universal definition of energy in the general theory owing to the difficulties associated with curved spacetime.
 
Newton and Mach and Einstein are on record as believing that rotation is absolute.

All three failed to rigorously define this.

All three believed that rotation was specifically relative to the majority of mass of the universe.

None of their public statements, that I am aware of, relate rotation to only the local environment as a local effect.

Which of their statements, as one more of many learning exercises for me, state a belief that rotation is relative to the local gravitational field?
 
Person A is the observer
Person B is the traveler

Person B is circling around Person A at .9c. Does Person A produce a stronger gravitational field because of this?
Click to expand...

According to GRT it depends on the reference frame.
 
Well I asked my teacher both questions and he gave me very different answers

For the first scenerio, about gravity, he said that this would not be so, since gravity is not relative. But, if gravity was strong enough, and lets say a person going .9C was to pass by it and accelerate, then it could cause relativity.

Then for the second scenerio, about the 2 people accelerating and deaccelerating, he said that, since they are in reference of each other, they never see each other deaccelerate and accelerate if they are both doing it at the same time. Therefore they never see work being done relative to each other.

It seems though that accelerating can still be applied to SRT, according to the other thread I made...
 
Is it safe to assume that 2 people going near the speed of light will never be aligned (side by side) relative to each other?
 
Hi Votorx,

I don't understand what your teacher means by "cause relativity", that statement makes no sense to me. It may be useful to clarify some concepts. First, there is only one gravitational field. Second, if person B has no appreciable mass, then this person qualifies as a "test particle" and doesn't affect the field. On the other hand, if person B has significant mass then person A is going to move along a free fall path as viewed by some distant observer unless person A uses rockets. If you want to avoid these complications then the simplest thing you might imagine is then the situation of a binary star system where person A and person B rotate about each other in free fall. In such a situation the rotation of the system definitely does matter. It contributes in principle to the field and more importantly it leads to gravitational waves.
 
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