"Speed of gravity" (big news)

[wolfriverjoe 1,523]

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One of the reasons I went into solid state physics. Mostly it makes sense.

Yeah, right! Quantum tunneling makes perfect sense.

Someone (Neils Bohr?) once said that if you think you understand Quantum Mechanics, then you really don't have any clue.

"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

[JackC1 0]

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.....
One of the reasons I went into solid state physics. Mostly it makes sense.

Yeah, right! Quantum tunneling makes perfect sense.

Someone (Neils Bohr?) once said that if you think you understand Quantum Mechanics, then you really don't have any clue.

QM seems more intuitive to me than GR, but then I've studied a lot more QM than GR. The trick is to forget all the "interpretations" and realize that in the same way that Pythagoras Theorem doesn't necessarily predict the existence of an anti-hypotenuse, QM doesn't necessarily predict many worlds or wave function collapse, it's just a useful theory so shut up and calculate.

[billvon 3,116]

> Is light (still traveling at c by definition) confined within the event horizon because
>the "intensity" of the gravitational field has slowed time to the point that there is not
>enough time (to an outside observer) for the photons to make it out?

You could think of it that way. Another way to think of it is that the light is redshifted past zero frequency.

> Or does gravity not act on itself?

Good question; I don't think we understand the propagation of gravity well enough to be able to answer that.

>Or is gravity really ("the fabric of") space itself and gravitational waves
>are propagated on the "surface" of gravity (space) at the speed of light?

That's a pretty common visualization abstraction for gravity; the "rubber sheet" analogy. It represents the idea that gravity is warping of space-time rather than a physical something that emanates from massive objects.

[muff528 3]

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..... Would there even BE an L2 in a binary black hole system?....

Imagine an orbiting system with a "heavy" black hole and a "lighter" black hole in which their event horizons are merged. To an outside observer there is an "elongated" black hole ...like a bowling pin shape. The L2 position, as long as it is outside the event horizon, might still exist (The L2 position may or may not be outside the "combined" black hole EH). Question would then be would it have relevance ...i.e., could a satellite reside at that orbit and stay put, so to speak? Like throwing the bowling pin so that it spins around its CG, there will be same classical stable Lagrangians as for 2 separate bodies. Or maybe I'm trying to apply a classical example to a non-classical system.

[lawrocket 3]

I’m kinda confused. I’m not necessarily thinking of event horizons except for concern about the size of the black hole necessary. i.e., if the Langrangian point falls within the event horizon then it’s not too useful.

You are describing the bowling pin sort of plotlines when a larger and smaller Hill spheres intersect. Intersecting event horizons, it seems to me, would mean that there could be no sphere of influence between and wherever an object ends up ultimately is chaotic. But outside, I’d think that you’d run into standard hill spheres.

Plus – there’d be no L1 that one could confirm.

And thinking of your example, you may want to therefore put a satellite at L3. You’ll be closer to the event horizon but not as affected by tidal forces if you are looking at a supermassive black hole.

But then the weird part for me - two black holes. We know that time slows down for the observer looking at the satellite. Does that mean that gravity also propagates more slowly between the satellite and the observer? Could a person be moving away from the black hole at high speed but the observer not realize it? Or perhaps the object moving AWAY from the black hole could APPEAR to the observer to be moving at near light speed because of the rate of time differential due to the extreme gravity close to event horizon? (“Whoa! Look at that dude! We saw him one moment and the next thing we saw was a blue streak!”)

Is the speed of gravity constant? If time itself isn't constant then...

My wife is hotter than your wife.

[PLFKING 4]

Yoiu guys are too smart to be jumping out of perfectly good airplanes.


Don

"When in doubt I whip it out,
I got me a rock-and-roll band.
It's a free-for-all."

[BigMikeH77 0]

You know whenever I hear people say that, I want to let them know that it feels as though I'm fortunate that the little plane made it to altitude.. packed with as many skydivers as it is... and that they would want to jump from it too.

[muff528 3]

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I’m kinda confused. I’m not necessarily thinking of event horizons except for concern about the size of the black hole necessary. i.e., if the Langrangian point falls within the event horizon then it’s not too useful.

You are describing the bowling pin sort of plotlines when a larger and smaller Hill spheres intersect. Intersecting event horizons, it seems to me, would mean that there could be no sphere of influence between and wherever an object ends up ultimately is chaotic. But outside, I’d think that you’d run into standard hill spheres.

Plus – there’d be no L1 that one could confirm.

And thinking of your example, you may want to therefore put a satellite at L3. You’ll be closer to the event horizon but not as affected by tidal forces if you are looking at a supermassive black hole.

But then the weird part for me - two black holes. We know that time slows down for the observer looking at the satellite. Does that mean that gravity also propagates more slowly between the satellite and the observer? Could a person be moving away from the black hole at high speed but the observer not realize it? Or perhaps the object moving AWAY from the black hole could APPEAR to the observer to be moving at near light speed because of the rate of time differential due to the extreme gravity close to event horizon? (“Whoa! Look at that dude! We saw him one moment and the next thing we saw was a blue streak!”)

Is the speed of gravity constant? If time itself isn't constant then...

Yeah, "DUH" on your point about L1! . My bad.

I was only wondering how a satellite at L2 (if it can exist) would see the BH pair. Without considering the math or seeing equations I don't even know if L2 can be far enough away to be outside of the (combined) event horizon or if it would be much farther away. Again, trying to apply classical thinking to a relativistic system?

I'm only considering the event horizons as a way to define the binary black hole as an "object" that might be observed from a distance.

example for clarity
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[muff528 3]

Regarding the Hill Spheres. I think that the extreme close proximity of the two black holes and the great gravitational field strengths of each object would cause the Hill Spheres of both objects to contain each other and there might be no L2 location that exists outside the less massive object's sphere but still within the more massive object's sphere (depending on their difference in mass). So a satellite would probably have to be so far away that it orbits the binary system but only sees it as a single object emitting all kinds of weird, pulsating stuff.

[lawrocket 3]

So that being the case it would not be a binary black hole - at least not a binary black hole that we know about.

My wife is hotter than your wife.

[JohnMitchell 16]

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Fascinating stuff. Light cannot escape a black hole event horizon. What about gravity?

One of the reasons I went into solid state physics. Mostly it makes sense.

Damn, I didn't even come close to wondering that. But amazingly good question now that it's out there.

Hmmm, light is an electromagnetic phenomenon of photons, which have a slight mass, correct? So a gravitational field with an escape velocity > C will trap them forever.

But gravity, as I understand it, is a curvature of spacetime we perceive as a force. Although it appears this curvature, which has some wave-like fluctuations, travels at the speed of light, its mechanism of propagation must be massless.

Is there a graviton particle? How DOES gravity tell space how to shape itself? I sure don't know.

[turtlespeed 226]

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Fascinating stuff. Light cannot escape a black hole event horizon. What about gravity?

One of the reasons I went into solid state physics. Mostly it makes sense.

Damn, I didn't even come close to wondering that. But amazingly good question now that it's out there.

Hmmm, light is an electromagnetic phenomenon of photons, which have a slight mass, correct? So a gravitational field with an escape velocity > C will trap them forever.

But gravity, as I understand it, is a curvature of spacetime we perceive as a force. Although it appears this curvature, which has some wave-like fluctuations, travels at the speed of light, its mechanism of propagation must be massless.

Is there a graviton particle? How DOES gravity tell space how to shape itself? I sure don't know.

It is all interdependant on itself.

I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

[lawrocket 3]

That's what I'm confused about. Photons have mass. But something with mass cannot travel at the speed of light (it's how we know neutrinos have mass - the have the ability to change form, meaning that time is present for neutrinos, which means they must go slower than light and therefore have mass).

DO they or don't they? Does light have momentum?

My wife is hotter than your wife.

[turtlespeed 226]

Found this:

The use of words can make a lot of confusion. Unfortunately, the word "mass" has been used in two different ways in physics. One was the way Einstein used it in E=mc2, where mass is really just the same thing as energy (E) but measured in different units. This is the same "m" that you multiply velocity by to find momentum (p), and thus is sometimes called the inertial mass. It's also the mass that provides the source of gravitational effects. Light has this "m" because it has energy. So it is indeed affected by gravity- not just in black holes but in all sorts of less extreme situations too. In fact, the first important confirmation of General Relativity came in 1919, when it was found that light from stars bends as it goes by the Sun.

The other way "mass" is often used, especially in recent years, is to mean "rest mass" or "invariant mass", which is sqrt(E2-p2*c2)/c2. This is invariant because it doesn't change when you describe an object at rest or from the point of view of someone who says it's moving. Obviously that's a good type of "mass" to give when you want to make a list of masses of particles. For a light beam traveling in a single direction, E=pc, so this "m" is zero. There is no point of view from which the light is standing still!

However, once you consider light traveling in a variety of directions, the E's from the different parts just add up to give the total E but the vector p's don't. In fact the total p can be zero if there are beams traveling opposite ways. So for many purposes the older definition of m (the inertial mass) is more convenient than the invariant particle mass, since it's the inertial mass that's just the sum of the inertial masses of the parts. For light moving equally in all directions, like the light bouncing around inside a star, total p is zero, so both definitions just give m=E/c2.

I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

[kallend 2,148]

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That's what I'm confused about. Photons have mass. But something with mass cannot travel at the speed of light (it's how we know neutrinos have mass - the have the ability to change form, meaning that time is present for neutrinos, which means they must go slower than light and therefore have mass).

DO they or don't they? Does light have momentum?

p = h/lambda where lambda = wavelength.

Equation for the energy of a body of rest mass m in the relativistic case is

E²=(mc²)²+(pc)²

Where p is the momentum of the body. For a photon m=0, so we get E=pc. According to Planck, the energy E of a photon is hf, so hf=pc. Rearranging we get p=h/lambda. Notice also that when p=0 (i.e. in the rest frame) this becomes E=mc².

NOTE: I took Relativity from Otto Frisch and Quantum Mechanics from Neville Mott, both giants of 20th Century physics. As a consequence I am often confused about this stuff since they were both dreadful instructors.

...

The only sure way to survive a canopy collision is not to have one.

[lawrocket 3]

Thanks! And thanks to Turtle.

My problem is I study this stuff in no particular order so I miss out. Time to do some of the math and figure it out there, too.

My wife is hotter than your wife.

[JackC1 0]

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That's what I'm confused about. Photons have mass. But something with mass cannot travel at the speed of light (it's how we know neutrinos have mass - the have the ability to change form, meaning that time is present for neutrinos, which means they must go slower than light and therefore have mass).

DO they or don't they? Does light have momentum?

Photons do not have mass, but they do have momentum. Any talk of photon mass is misleading, as is it not a real mass, it's not even a useful concept to think of a photons mass. If you absolutely must think of photon mass, think of it as an effective mass in the sense that a photon has momentum which would be equivalent to a particle of mass m traveling at speed c, but that's as far as it goes. No real mass.

This comes from
E=Pc from relativity.
E=hf from Quantum mechanics
P=mv from Newtonian physics.

If you put the first two together you have hf=Pc, rearrange to get P=hf/c, then equate this to P=mv where v=c for a photon, and rearrange to give m=hf/c^2. But this isn't real mass, it's mathematical trick mass.

Physics is full of stuff like this and it is confusing unless you really grasp the basics of where it all came from. You'll often see a particles mass quoted in units of MeV (mega electron volts) which is a unit of energy, not mass. What they often neglect to say (because physicists already know) is that you need to convert to mass by dividing by c^2 (and a factor of electron charge but that just confuses things even more). But no one ever bothers because quantum mechanics deals with energies easier then it does with mass, so all those c^2 terms just get in the way.

Another example is noise temperature. Take the power P (in Watts), the bandwidth B (in Hz) and divide P by B and you have units of energy. Equate this to Boltzmanns constant k multiplied by a temperature T and again we have units of energy. You can then rearrange for T. Noise however, is not really hot.

[turtlespeed 226]

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That's what I'm confused about. Photons have mass. But something with mass cannot travel at the speed of light (it's how we know neutrinos have mass - the have the ability to change form, meaning that time is present for neutrinos, which means they must go slower than light and therefore have mass).

DO they or don't they? Does light have momentum?

p = h/lambda where lambda = wavelength.

Equation for the energy of a body of rest mass m in the relativistic case is

E²=(mc²)²+(pc)²

Where p is the momentum of the body. For a photon m=0, so we get E=pc. According to Planck, the energy E of a photon is hf, so hf=pc. Rearranging we get p=h/lambda. Notice also that when p=0 (i.e. in the rest frame) this becomes E=mc².

NOTE: I took Relativity from Otto Frisch and Quantum Mechanics from Neville Mott, both giants of 20th Century physics. As a consequence I am often confused about this stuff since they were both dreadful instructors.

Is there any way to make a photon truly be at rest? If that is possible would't it have mass at that point?

I guess what I'm asking here is if a black hole or singularity has enough gravity to keep photons from escaping, its hold, wouldn't some of the photons be stacked up together forming solid light?

What kind of gravitational forces are we talking about that can affect energy

I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

[JackC1 0]

From the photons point of view, you could say it is at rest. The photon sees time slow to a complete standstill so it can be everywhere in the universe all at the same time.

But the gravitational field from a black hole doesn't really stop a photon in the sense that it slows it down, it just red shifts it away to zero frequency. That's not stopping the photon at all, it's killing it stone dead.

If you want to slow a photon down, you need to pass it through a material. Super cold sodium for example will slow a photon down from 186000 miles a second to a mere 38mph (the current record)