Gravity (Split from The speed of gravity = the speed of light. Does that mean gravity and light are the same thing?)

[swansont]

5 minutes ago, NTuft said:

If it has no mass, must we say it is pure kinesis instead? Humor my simplicity, please: if it is energetic, and has no mass, by what equations or ideas did it acquire momentum?

No. It’s not pure anything.

E^2 = m^2c^4 + p^2c^2 (for a particle at rest, this becomes the familiar E = mc^2)

If m = 0, then E = pc

You can also find this classically for electromagnetic waves

[NTuft]

  

13 minutes ago, swansont said:

We frown on such piggybacking. If your question is not directed at/replying to the OP, it should be asked in a new thread.

My good moderator,
OP: "The speed of gravity = the speed of light" -- I say we must account for units, and how these mesaurements are being made. 

 

3 minutes ago, swansont said:

No.

E^2 = m^2c^4 + p^2c^2 (for a particle at rest, this becomes the familiar E = mc^2)

If m = 0, then E = pc

You can also find this classically for electromagnetic waves

Wonderful, thank you for the equation; Paul Dirac extended it. So it's energy is defined then by its momentum and the speed of light. Its energy is only a function of it's momentum then if we take c as constant.

  

4 minutes ago, swansont said:

Speed is length/time.

Speed of gravity ≠ gravity itself

gravity itself = ?

speed of light is in units length/time, which he tried to equate with speed of gravity. helpful to parse it apart.

Edited June 19, 2022 by NTuft

[swansont]

7 minutes ago, J.Merrill said:

Massless particle are purely energy. "It's sufficient for a particle to have energy to have meaningful sense of existence" Says Flip Tanedo, is an assistant professor of physics at the University of California, Riverside.

Word for word, I wish I had a way to Definitively credit this with proof. 

No they aren’t. Taneda is wrong, and you’ve been provided rebuttals of this.

3 minutes ago, NTuft said:

OP: "The speed of gravity = the speed of light" -- I say we must account for units, and how these mesaurements are being made. 

Speed is length/time.

Speed of gravity ≠ gravity itself

Start a new thread to discuss how the speed of gravity has been determined

[MigL]

This is almost ridiculous ...

Gravity affects light, forcing it to travel along geodesics, and reducing/increasing its wavelength when moving radially, but it does NOT affect photons in a way that is predictable.

Gravity and its model, GR, along with EM waves, are classical models.
Photons, on the other hand, are quantum mechanically modelled.

The two are currently incompatible, and often confused.
 

[Markus Hanke]

20 hours ago, J.Merrill said:

This is relative only to the observer though?

No, all observers agree that the frequency shift happens in this scenario.

20 hours ago, J.Merrill said:

Person A shoots a few thousand partials of light 90° In to space at Person B. who is just outside of earths “gravitational  gradient”. Would person B experience a red shift in light waves or blue?

Both observers agree that light emitted at A will be redshifted once it arrives at B.

20 hours ago, J.Merrill said:

So is gravity still having an effect on light as a constant or is it affecting the observer’s perception only?

I don’t know what you mean by this, but it’s a real, measurable effect that isn’t just a visual ‘artefact’ of some kind. 

15 hours ago, J.Merrill said:

Then that is to say light leaving a stars gravity is red shifted too

Yes, but the effect is pretty small.

15 hours ago, J.Merrill said:

even if it’s moving closer to us by nature of the universe.

If there is relative motion of the star with respect to us, then the different effects will combine - there is redshift due to the star’s own gravity, blueshift due to Earth’s gravity, and either blue or redshift due to relative motion. What the overall net frequency shift will be then depends on the relative magnitude of each effect.

15 hours ago, J.Merrill said:

On another note, we have been able to observer and confirm things moving away from the earth in our universe shift more towards red light

Yes, that’s cosmological redshift. For large distances, this effect will be much larger than any local effects, so light from far-away sources is overwhelmingly redshifted.

9 hours ago, NTuft said:

Humor my simplicity, please: if it is energetic, and has no mass, by what equations or ideas did it acquire momentum?

By the relativistic energy momentum relation:

\[E=\sqrt{p^2 c^2 + m^2 c^4}\]

which is just the magnitude of the energy-momentum vector, being part of the energy-momentum tensor, which follows as a conserved quantity from Noether’s theorem for time-translation invariance. So the idea behind it is symmetries of spacetime.

For m=0 this gives p=E/c.

[exchemist]

9 hours ago, J.Merrill said:

Massless particle are purely energy. "It's sufficient for a particle to have energy to have meaningful sense of existence" Says Flip Tanedo, is an assistant professor of physics at the University of California, Riverside.

Word for word, I wish I had a way to Definitively credit this with proof. 

 

Aha but that’s different. It’s one thing to say HAVING energy, as a property, is enough for an entity to be said to exist in some sense. It’s quite another to say - as apparently he didn’t - that it IS “pure energy”. It is that which is ballocks, because it implies that energy is something that can have independent existence. It can’t. 

And you don’t provide the context in which his remark was made, which would shed more light on what he meant by what he said. Taking individual remarks out of context is always liable to create misunderstandings as to what was meant.