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If gravity is the curvature of space and objects "fall" in to the gravity well because they are following a curved path, then, well, that can only happen if the object is moving, isn't it so?

I understand there is still molecular and sub-atomic motion in matter and this would be the major contributing factor if this is true.

If gravity is the curvature of space and objects "fall" in to the gravity well because they are following a curved path, then, well, that can only happen if the object is moving, isn't it so? I understand there is still molecular and sub-atomic motion in matter and this would be the major contributing factor if this is true. But then escape velocities exist and I get really confused.

Light doesn't curve because spacetime is curved. It curves because the speed of light is lower near the floor.

Caution: There are different uses of the term curve in general relativity. It this instance John is not using it to refer to curved spacetime. He's using it to refer to the fact that in the non-inertial frame that the observer is at rest in, e.g. a uniform gravitational field or a uniformly accelerating frame of reference, the worldline of a photon is not straight, but curved.

As Feynman said, since light has energy and anything that has energy has mass then light has mass, it follows that there is a gravitational force acting on the photon and that's accelerating it.

Just because the speed of light changes with potential that doesn't explain why the light moves from places having different potentials.

I'm a Feynman fan, but I have to say he didn't understand gravity

See for example ...sorry, you cannot view external links. To see them, please REGISTER or LOGINIf the light is moving parallel to the beam in one direction then its deflected. If its moving in the opposite of that direction than its not deflected but keeps moving in a straight line. Otherwise its deflected.Quote from: JohnDuffield... and it makes perfect sense. Sure it makes sense. But just because something makes sense it doesn't mean that there's more to it and the paper above is a perfect example.Quote from: JohnDuffieldAs you know light doesn't curve "because spacetime is curved", and the interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with. Of course I know. I wrote a paper on it 15 years ago. If I'm correct then that's how you learned of it, isn't it?

... and it makes perfect sense.

As you know light doesn't curve "because spacetime is curved", and the interpretation of gravity as a curvature in space-time is an interpretation Einstein did not agree with.

I'm a Feynman fan, but I have to say he didn't understand gravity. See for example ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN.

The same result can be obtained in still another way. A photon of frequency ω _{0} has the energy E_{0} =ℏω_{0}. Since the energy E_{0} has the relativistic mass E_{0} /c^{2} the photon has a mass (not rest mass) ℏω_{0} /c_{2}, and is “attracted” by the earth.

I've heard you say a lot of incorrect things and this one is near the top of the list.

What about it? What in that chapter do you claim is wrong?

Please clarify. If space is curved then spacetime is curved. What's wrong with that?

So what? First of all that section is very difficult to read. It's expressed in a language that is now almost 100 years old and as such unclear. In any case there are very few physicists who've read all of Einstein's papers. Why should Einstein be any different?

You have no proof that I'm wrong, do you?

All you know is that Einstein said something similar to this but not exactly what I was saying. He never demonstrated this for all possible gravitational fields. He only did it for a uniform gravitational field and the gravitational field of the sun. In those instances its true.

What is what Einstein said?

I'm saying that the speed of light varying with position is insufficient to cause light to be deflected. And that is a fact. It's clearly another one of the many facts in physics that you're unaware of.

An example of this was published back almost 100 years ago in the following paper: On The Gravitational Field Produced by Light, Tolman, Ehrenfest and Podolsky, Physical Review, Vol. (37), March 1, 1931, pg 602-615. You can also find it in the following textbook on relativity and cosmology: Relativity, Thermodynamics and Cosmology by Richard C. Tolman, Dover Pub.

I worked this out and uploaded it onto my website for just such an occasion. If you can follow the math then do so. See: ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN

Sure it makes sense. But just because something makes sense it doesn't mean that there's more to it and the paper above is a perfect example.

Of course I know. I wrote a paper on it 15 years ago. If I'm correct then that's how you learned of it, isn't it?

Actually in that chapter he talks about the relativistic mass of the photon.

QuoteThe same result can be obtained in still another way. A photon of frequency ω _{0} has the energy E_{0}=ℏω_{0}. Since the energy E_{0} has the relativistic mass E_{0} /c^{2} the photon has a mass (not rest mass) ℏω_{0} /c_{2}, and is “attracted” by the earth. This is another example of Feynman saying that light is attracted to the earth because it has mass.

The same result can be obtained in still another way. A photon of frequency ω _{0} has the energy E_{0}=ℏω_{0}. Since the energy E_{0} has the relativistic mass E_{0} /c^{2} the photon has a mass (not rest mass) ℏω_{0} /c_{2}, and is “attracted” by the earth.

And you and I both know that that's a measure of energy, and that a concentration of energy causes gravity, and that "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".

Well it's wrong, because light is "attracted" twice as much as matter...."Before Einstein developed the full theory of General Relativity he also predicted a deflection of 0.875 arcseconds in 1913, and asked astronomers to look for it. But World War I intervened, and during the war Einstein changed his prediction to 1.75 arcseconds, which is twice the Newtonian deflection."

Quote from: JohnDuffieldAnd you and I both know that that's a measure of energy, and that a concentration of energy causes gravity, and that "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". You have it backwards. There is only one reason that energy is a source of gravity and that's because there is an equivalence between mass and energy. There is no other reason. This is exactly what Einstein and other gravitational physicists argue. E.g. Misner, Thorne and Wheeler state the same thing in their text Gravitation.Quote from: JohnDuffieldWell it's wrong, because light is "attracted" twice as much as matter...."Before Einstein developed the full theory of General Relativity he also predicted a deflection of 0.875 arcseconds in 1913, and asked astronomers to look for it. But World War I intervened, and during the war Einstein changed his prediction to 1.75 arcseconds, which is twice the Newtonian deflection."Once again you're misinterpreting the facts. The reason that Einstein was off by a factor of two was that at the time of his calculation in 1911, i.e. the last one before he got GR right, was due to the fact that he didn't take into account the curvature of space. When he accounted for the curvature of space he got the right value. It turns out that the curvature of space accounts for an exact value of a factor of two in the amount of deflection.Let's drop this nonsense. It has nothing to do with the topic of the thread.

Does gravity only work on moving objects?

You have it backwards. There is only one reason that energy is a source of gravity and that's because there is an equivalence between mass and energy. There is no other reason. This is exactly what Einstein and other gravitational physicists argue. E.g. Misner, Thorne and Wheeler state the same thing in their text Gravitation.

Once again you're misinterpreting the facts. The reason that Einstein was off by a factor of two was that at the time of his calculation in 1911, i.e. the last one before he got GR right, was due to the fact that he didn't take into account the curvature of space. When he accounted for the curvature of space he got the right value. It turns out that the curvature of space accounts for an exact value of a factor of two in the amount of deflection.

Let's drop this nonsense. It has nothing to do with the topic of the thread.

The first good estimate of the Earth's mass was produced by the Cavendish experiment [nofollow], which measured the attraction between heavy balls in a torsion balance. This attraction still exists when the balls are stationary, relative to each other.

Pete the factor of 2 pops up a lot in the equations to do with gravitation so I'm in your camp.

The special theory of relativity has led to the conclusion that inert mass is nothing more or less than energy, which finds its complete mathematical expression in a symmetrical tensor of second rank, the energy-tensor.

Abstract - I describe how relativistic field theory generalizes the defining property of material systems to possess mass to the requirement of them having a mass-energy-momentum density tensor T_{uv} (energy tensor for short) associated with them. I argue that according to general relativity T_{uv} is not an intrinsic property of matter, looking at how the energy tensor for a relativistic material system can be derived in a Lagrangian framework. It will become evident that the matter fields alone are not sufficient for such a derivation. The metricfield g_{uv} plays a prominent role in obtaining the energy tensor of a material system, and occurs explicitly in a generic T_{uv} . Accordingly, since g_{uv} represents the geometry of spacetime itself, the properties of mass, stress, energy and momentum should not be seen as intrinsic properties of matter, but as relational properties that material systems have only in virtue of their relation to spacetime structure.

Does gravity only affect moving objects? No. A "relative gravimeter" is essentially a sensitive force balance, which measures the extension of a spring or the current required by an electromagnet to suspend a superconductor. In either case the test mass is stationary.Classical textbook measurements of G admittedly detect the asymmetry of a swinging torsion pendulum in the presence of a large mass, but the underlying physics is the theoretical deflection of a stationary bob weight.

Quote from: alancalverd on 20/12/2014 07:54:50Does gravity only affect moving objects? No. A "relative gravimeter" is essentially a sensitive force balance, which measures the extension of a spring or the current required by an electromagnet to suspend a superconductor. In either case the test mass is stationary.Classical textbook measurements of G admittedly detect the asymmetry of a swinging torsion pendulum in the presence of a large mass, but the underlying physics is the theoretical deflection of a stationary bob weight.Sub-atomically the are moving. I mentioned this.

Piffle. The constituent atoms and subatomic bits of a body may well be moving but (a) they are moving at random and therefore the net motion within the body is zero (b) G does not depend on temperature and (c) as far as astonomers can tell, G has the same value for rocks and gases.

Sub-atomically the are moving. I mentioned this.

Pete said:"What is what Einstein said? I'm saying that the speed of light varying with position is insufficient to cause light to be deflected. And that is a fact."This is a very important point. Light being slowed IS insufficient to cause the deflection of light as can be demonstrated by a photon traveling directly away from the centre of gravity. In these circumstances there will be no change of trajectory. It is only by following an angular path to the direction of a gravitational field where we find deflection. The question is do we need tidal forces for this to occur or can it occur in a flat gravitational field?