# The speed of propagation of gravity

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17 hours ago, studiot said:

That's interesting thanks. +1

I know almost nothing about the any theories of gravity waves.

If you can somehow get hold of a copy of Misner/Thorne/Wheeler, I cannot recommend it highly enough! He talks about all these things in detail, it’s a truly excellent text, and considered the gold standard for a good reason. Sadly, it’s difficult to obtain and very expensive. I was lucky to find an old and damaged copy (still legible though) on eBay some years ago.

18 hours ago, SergUpstart said:

The speed of propagation of electromagnetic waves is slowed down in the material environment. There is no absolute emptiness Natura abhorret vacuum.

As others have pointed out, for all intents and purposes we are in a vacuum here. The deviation from “true” vacuum is negligibly small.

18 hours ago, SergUpstart said:

I consider the gravitational field as a material medium

This isn’t the right way to look at it, spacetime is not a medium.

19 hours ago, SergUpstart said:

the velocity of electromagnetic propagation varies depending on the strength of the gravitational field

c does not change, the only thing that changes if you vary gravity is the path that an electromagnetic wave travels through spacetime (which is always a null geodesic).

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5 hours ago, Markus Hanke said:

c does not change, the only thing that changes if you vary gravity is the path that an electromagnetic wave travels through spacetime (which is always a null geodesic).

The same phenomenon can be described in different ways

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7 hours ago, Markus Hanke said:

This isn’t the right way to look at it, spacetime is not a medium.

So after all, the force field is not space-time.

Edited by SergUpstart

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18 hours ago, SergUpstart said:

The same phenomenon can be described in different ways

A variation in c, and a variation in a particle’s world line in spacetime, are not equivalent.

16 hours ago, SergUpstart said:

So after all, the force field is not space-time.

There is no force field. An accelerometer attached to a test particle in free fall will read exactly zero at all times, so gravity is not a force.

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25 minutes ago, Markus Hanke said:

There is no force field.... so gravity is not a force.

But if you were going to do a field treatment of gravity, space-time geometry would be the field.

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3 hours ago, Markus Hanke said:

gravity is not a force.

If gravity is not a force, but a curvature of space-time, then in order for gravitational waves to exist, space-time must have elasticity and density, that is, space-time must have physical properties

Edited by Strange

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3 hours ago, SergUpstart said:

If gravity is not a force, but a curvature of space-time, then in order for gravitational waves to exist, space-time must have elasticity and density, that is, space-time must have physical properties

So length and time are physical entities? Do you have evidence for this?

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1 hour ago, swansont said:

So length and time are physical entities? Do you have evidence for this?

Gravitational waves are experimentally detected. Waves are an oscillatory process, oscilations are impossible without inertia, and mass is a measure of inertia. For wave propagation, an elastic medium with inertia is necessary. Therefore, if you believe that gravity is not a force field, but a curvature of space-time, you must give this space-time physical properties, elasticity and inertia, or density. This is an unsolved problem in General relativity.

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5 hours ago, SergUpstart said:

If gravity is not a force, but a curvature of space-time, then in order for gravitational waves to exist, space-time must have elasticity and density, that is, space-time must have physical properties

Can I borrow a liter of it? How much mass do dimensions have?

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1 hour ago, SergUpstart said:

For wave propagation, an elastic medium with inertia is necessary.

Did somebody say 'aether' ?
( gravitational, this time )

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2 hours ago, SergUpstart said:

Gravitational waves are experimentally detected. Waves are an oscillatory process, oscilations are impossible without inertia, and mass is a measure of inertia. For wave propagation, an elastic medium with inertia is necessary. Therefore, if you believe that gravity is not a force field, but a curvature of space-time, you must give this space-time physical properties, elasticity and inertia, or density. This is an unsolved problem in General relativity.

So what has inertia in light propagation?

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3 hours ago, swansont said:

So what has inertia in light propagation?

You will not deny that electric and magnetic fields have mass.(Mass-characteristic of inertial properties)

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4 hours ago, SergUpstart said:

You will not deny that electric and magnetic fields have mass.(Mass-characteristic of inertial properties)

Photons, which contain oscillating electric and magnetic fields, have zero mass.

Any static field you might find can’t be disassociated from the mass that creates the field. The whole system has mass. But you’re talking about a vacuum, so this is moot.

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21 hours ago, SergUpstart said:

If gravity is not a force, but a curvature of space-time, then in order for gravitational waves to exist, space-time must have elasticity and density, that is, space-time must have physical properties

Gravity - and hence also gravitational waves - are a geometric properties of spacetime. It has neither elasticity nor density, nor any other material property. It is not a material of any kind. Nonetheless it does have degrees of freedom, but these are of a geometric nature.

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7 hours ago, swansont said:

Photons, which contain oscillating electric and magnetic fields, have zero mass.

The mass of the photon m=hf/c^2.

If the mass of the photon is zero, then the pulse  of the photon is zero, do you think that the solar sail is impossible?

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19 hours ago, SergUpstart said:

Gravitational waves are experimentally detected. Waves are an oscillatory process, oscilations are impossible without inertia, and mass is a measure of inertia.

This is an example of the fallacy of begging the question.

And it is wrong. Why not learn some physics, instead of making stuff up?

2 hours ago, SergUpstart said:

The mass of the photon m=hf/c^2.

If the mass of the photon is zero, then the pulse  of the photon is zero, do you think that the solar sail is impossible?

Wrong again.

The momentum of a photon has nothing to do with mass. It is given by $p = \frac{h}{\lambda}$

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2 hours ago, Strange said:

The momentum of a photon  p=h/lambdap=hλ

Who is arguing with this

p=mc = (hf/c^2)*c = hf/c = h/cT = h/lambda

2 hours ago, Strange said:

The momentum of a photon has nothing to do with mass.p=hλ

And the energy of a photon, which is equa E=hf, is also not related to its mass in your opinion??

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p=mc seems incorrect for photons since photons do not have an invariant mass m.

My opinion, based on (limited knowledge) about this is that it seems to mix classical mechanics that do not apply to the model of photons.

But I may be wrong, so I would be happy for reference where photon's momentum is calculated from it's invariant mass "m" and the speed of light, "c".

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5 hours ago, SergUpstart said:

The mass of the photon m=hf/c^2.

no, it’s zero, as is anything that travels at c

5 hours ago, SergUpstart said:

If the mass of the photon is zero, then the pulse  of the photon is zero, do you think that the solar sail is impossible?

Photons have momentum. Reflection, absorption and emission all involve a change in momentum of the target.

42 minutes ago, SergUpstart said:

Who is arguing with this

p=mc = (hf/c^2)*c = hf/c = h/cT = h/lambda

And the energy of a photon, which is equa E=hf, is also not related to its mass in your opinion??

All of physics is arguing with this. p=E/c

m=0

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2 hours ago, SergUpstart said:

And the energy of a photon, which is equa E=hf, is also not related to its mass in your opinion??

No. Mass means rest (or invariant) mass. Photons are never at rest and their rest mass is zero.

You can calculate an equivalent "relativistic mass" if you wish. But relativistic mass is just a measure of energy, so that seems a pretty pointless exercise.

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54 minutes ago, Strange said:

No. Mass means rest (or invariant) mass. Photons are never at rest and their rest mass is zero.

That's right. But relativistic mass is also a characteristic of inertial properties

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52 minutes ago, SergUpstart said:

That's right. But relativistic mass is also a characteristic of inertial properties

We’re not referring to relativistic mass, so this is moot.

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1 hour ago, SergUpstart said:

That's right. But relativistic mass is also a characteristic of inertial properties

"Relativistic mass" is just a way of describing energy. A lot of people think it shouldn't be used because it confuses people (like you).

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That confusion is one of the reasons why the current terminology is the invariant mass for rest mass and variant mass for the relativistic mass.

I wouldn't describe the relativistic mass as an energy term without describing the energy type. It's better to use the kinetic energy term for its association to the momentum term.

Photons having no invariant mass tells us that it does not couple with its fields of interaction. It is the coupling strength that gives rise to the invariant/rest mass. This is the potential energy terms.

If the photon did have invariant mass then the range of the EM force would not be infinite.

That confusion is one of the reasons why the current terminology is the invariant mass for rest mass and variant mass for the relativistic mass.

Edited by Mordred

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44 minutes ago, Mordred said:

That confusion is one of the reasons why the current terminology is the invariant mass for rest mass and variant mass for the relativistic mass.

I wouldn't describe the relativistic mass as an energy term without describing the energy type. It's better to use the kinetic energy term for its association to the momentum term.

Photons having no invariant mass tells us that it does not couple with its fields of interaction. It is the coupling strength that gives rise to the invariant/rest mass. This is the potential energy terms.

If the photon did have invariant mass then the range of the EM force would not be infinite.

Then the main question is, does the relativistic mass satisfy the classical definition of mass as a measure of resistance to changes in inertia?

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