General > General Technical Chat
Acceleration of gravity from earth, on objects, traveling at near the SOL.
Non-Abelian:
--- Quote from: Nominal Animal on December 19, 2020, 08:29:35 pm ---Unfortunately, Dr. Baird is wrong. It is obvious general relativity is way outside his area of expertise. I'm sure that a PhD specializing in optics finds the fact that even light bends space frightening.
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No, he is not wrong. His point is that the useful concept in relativity is the invariant mass, which is not a frame dependent quantity (which seems to be what you are trying to say, but don't realize it). He isn't trying to explain general relativity to anyone. Second, the curvature is in spacetime, not space. Third, you can always pick a frame in which spacetime is flat at a point.
--- Quote ---Like I said, in Einstein field equations energy and momentum cause the curvature of spacetime. Mass affects curvature only through energy and momentum. Dr. Bairds fundamental error is assuming that mass itself is what curves spacetime. That is simply not true.
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In the rest frame of a mass, E=m. (The c^2 is irrelevant since c is just a way to convert meters to seconds. It doesn't mean anything physically.)
--- Quote ---Relativistic mass is actually just a notional measure.
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So, what's the problem? That is what the web page you are railing against was pointing out.
--- Quote ---It is equal to the energy that causes the same spacetime curvature as the momentum of the point-like mass. It is only needed when you need something that has an analog in Newtonian physics. To correctly model spacetime, you need to use Einstein field equations.
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As noted above, the energy in the rest frame of a mass is just its mass. T_00 is its mass density.
Nominal Animal:
--- Quote from: Non-Abelian on December 19, 2020, 09:40:51 pm ---Second, the curvature is in spacetime, not space.
--- End quote ---
You managed to pick the one point where I miswrote spacetime, and made that a point.
Perhaps you should read my first post (fourth in this thread), and point out why you believe Dr. Bairds post invalidates my point.
MIS42N:
I have always been suspicious of so called time dilation of fast moving objects. There isn't a way to measure time, we rely on observing repetitive phenomena (like atomic vibration), and counting how many repetitions of the phenomenon occur between some other events (like the sun appearing at the same angle above the horizon). It is determined that repetitive phenomena in an object being accelerated occur slower (or faster, I don't know which) than one not being accelerated. This explains (to me) why clocks on satellites run at a different repetition rate than those on earth's surface. We live in a higher acceleration field (a different spacetime?) than the satellite. The satellite has to move around the earth at speed to stay in space, but is the different rate of its clock affected by the speed, or just by being in a different spacetime?
Consider an object moving away from earth near SOL. If we are in continuous contact with the object, it's clock will slow down while it is being accelerated, then it will appear to be running slower because of redshift in the received signal. But is it actually running slower when the object reaches its desired speed and is not being accelerated? From the point of view of the object, it will see the earth recede at the same speed, and signals from earth will be redshifted.
If a spaceship were able to go to Alpha xxx our nearest stars (say 4 light years away) at 1/2 the speed of light, then return at 1/2 the speed of light, I think what we would see is the spaceship taking 12 years to get there and 4 years to get back. Any people in the spaceship would see it take 8 years to get there and 8 years to get back. Apart from the slowdown due to acceleration, the people on earth and on the spaceship will age about the same amount. It seems to me any other outcome results in a paradox. Using the spaceship as a reference or using the earth as a reference, shouldn't the result be the same? The spaceship and earth start in the same spacetime and end in the same spacetime. But during the journey observations are distorted due to the speed of light. Does relativity deal well with correlating observers in different reference frames?
Non-Abelian:
--- Quote from: Nominal Animal on December 19, 2020, 10:06:09 pm ---
--- Quote from: Non-Abelian on December 19, 2020, 09:40:51 pm ---Second, the curvature is in spacetime, not space.
--- End quote ---
You managed to pick the one point where I miswrote spacetime, and made that a point.
Perhaps you should read my first post (fourth in this thread), and point out why you believe Dr. Bairds post invalidates my point.
--- End quote ---
His point was the same as the one you seem to be trying to make - that the invariant mass is the physically relevant quantity, which is correct. You can only disagree with that by trying to say physical quantities depend on what coordinates you choose, which would be ridiculous.
nctnico:
--- Quote from: MIS42N on December 20, 2020, 12:33:51 am ---Consider an object moving away from earth near SOL. If we are in continuous contact with the object, it's clock will slow down while it is being accelerated, then it will appear to be running slower because of redshift in the received signal. But is it actually running slower when the object reaches its desired speed and is not being accelerated? From the point of view of the object, it will see the earth recede at the same speed, and signals from earth will be redshifted.
If a spaceship were able to go to Alpha xxx our nearest stars (say 4 light years away) at 1/2 the speed of light, then return at 1/2 the speed of light, I think what we would see is the spaceship taking 12 years to get there and 4 years to get back. Any people in the spaceship would see it take 8 years to get there and 8 years to get back. Apart from the slowdown due to acceleration, the people on earth and on the spaceship will age about the same amount. It seems to me any other outcome results in a paradox. Using the spaceship as a reference or using the earth as a reference, shouldn't the result be the same? The spaceship and earth start in the same spacetime and end in the same spacetime. But during the journey observations are distorted due to the speed of light. Does relativity deal well with correlating observers in different reference frames?
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Unfortunately it doesn't work that way. You can only apply special relativity theory on objects which have a constant velocity compared to the reference object. As soon as there is accelleration / de accelleration involved you need general relativity theory. Examples of objects to which the special relativity applies to are satellites orbiting the earth.
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