Photons, mass, and black holes

[Philbert1893]

If photons are light particles, and they lack mass, how is it possible that they are affected by gravitational pull from a black hole?

Super sorry if this has been asked / answered before, I couldn't find it on this forum if it has...
Caveat No.2: My physics knowledge was limited to high school (many years ago at that)!

Cheers

 

[phinds]

If photons are light particles, and they lack mass, how is it possible that they are affected by gravitational pull from a black hole?

Super sorry if this has been asked / answered before, I couldn't find it on this forum if it has...

It has been asked many times. Mass is only part of the total energy of a photon and it is the totality that responds to the space-time geometry that we call "gravity"

 

[Philbert1893]

So photons are not massless?

 

[phinds]

So photons are not massless?

Where on Earth did you get that idea? Please read what I said.

See if this helps:
https://en.wikipedia.org/wiki/Stress–energy_tensor

 

[Arman777]

If photons are light particles, and they lack mass, how is it possible that they are affected by gravitational pull from a black hole?

Super sorry if this has been asked / answered before, I couldn't find it on this forum if it has...
Caveat No.2: My physics knowledge was limited to high school (many years ago at that)!

Cheers

Photons are not directly affected by the gravity, but affected by the space-time curvature that the heavy objects made.

Theres no force or any other direct interaction between light and the star. In the picture as you can see star bends the Space-Time, creates a curvature, and light follows that curved path.

 

[Vanadium 50]

Photons are not directly affected by the gravity, but affected by the space-time curvature that the heavy objects made.

This is incorrect and unhelpful. There is no distinction to be made between "gravity" and "spacetime curvature".

 

[Arman777]

This is incorrect and unhelpful. There is no distinction to be made between "gravity" and "spacetime curvature".

I specifically said "not directly" which I tried to describe that photons are not affected by any kind of "force" due to heavy object. The force is not cause of this affect, but the space-time curvature.

Simply its pure GR effect and we can't think this situation in the sense of Newtonian gravity.

 

[Vanadium 50]

There is no distinction to be made between "gravity" and "spacetime curvature" It is both wrong and unhelpful.

 

[Arman777]

There is no distinction to be made between "gravity" and "spacetime curvature" It is both wrong and unhelpful.

I didnt understand why you said the same thing. I am talking about Newtonian Gravity. If you are claiming Newtonian Gravity is the space-time curvature then go ahead.

 

[jbriggs444]

I didnt understand why you said the same thing. I am talking about Newtonian Gravity. If you are claiming Newtonian Gravity is the space-time curvature then go ahead.

Perhaps what you meant to say is that photons are not affected directly by the massive body near which they are passing. They are directly affected by gravity. That gravity is the space-time curvature that goes with the presence of the massive body.

Explaining why photons curve in terms of Newtonian gravity in a relativity forum would be of questionable value.

 

[Arman777]

By saying "Newtonian gravity", I am trying to say that we can't use the Newtonian gravitational force to explain this effect, In other words, we can't use
$F=\frac {MmG} {r^2}$ for light. That's all. The question of OP was;

If photons are light particles, and they lack mass, how is it possible that they are affected by the gravitational pull from a black hole?

I think OP thought that "photons don't have mass then how can they be affected by gravity since we can't put it in the gravitational force equation
And the answer is that it's purely GR, space-time curvature effect, which I tried to explain in my pre-posts.

Explaining why photons curve in terms of Newtonian gravity in a relativity forum would be of questionable value.

I agree but I think that's what OP thought.

 

[PeterDonis]

By saying "Newtonian gravity", I am trying to say that we can't use the Newtonian gravitational force to explain this effect

As @jbriggs444 has already pointed out, this is the relativity forum and we don't do Newtonian gravity in this forum; questions and answers involving Newtonian gravity belong in the Classical Physics forum. Particularly since in Newtonian gravity there is no clear answer to the OP question anyway, and even more particularly since the OP question asked about a black hole. A proper answer in this forum must use relativity. See below.

If photons are light particles, and they lack mass, how is it possible that they are affected by gravitational pull from a black hole?

As others have pointed out, in relativity, "gravitational pull" is not a good way to view gravity. Gravity is not a force in GR. It's spacetime curvature, which affects the motion of everything.

It is also true that, in GR, light can act as a source of gravity. This is because light has stress-energy, and stress-energy (not rest mass, which light does not have) is the source of gravity in GR.

 

[PeterDonis]

the answer is that it's purely GR, space-time curvature effect, which I tried to explain in my pre-posts.

But I agree with @Vanadium 50 that you didn't do that very well, since you tried to draw a distinction between "gravitational pull" and "spacetime curvature", as though objects that have rest mass are affected by "gravitational pull" while light is only affected by "spacetime curvature". That is not correct.

 

[Arman777]

But I agree with @Vanadium 50 that you didn't do that very well, since you tried to draw a distinction between "gravitational pull" and "spacetime curvature", as though objects that have rest mass are affected by "gravitational pull" while light is only affected by "spacetime curvature". That is not correct.

Whats the correct way then ?

 

[Ibix]

Whats the correct way then ?

I'd put it this way.

In Newtonian physics, gravity is a force between masses. The status of massless things like photons is, indeed, rather dubious.

In relativistic physics, gravity is the curvature of spacetime, it is not a force. Everything is affected by it because it's the "shape" of spacetime. Loosely, paths are curved because spacetime is curved (that's horribly over-simplified, but this is a B-level thread). Furthermore, the source of gravity is not just mass, but includes energy and momentum, although not in a straight forward way. So under some circumstances photons can attract each other gravitationally despite being massless because they have energy and momentum.

 

[Arman777]

I'd put it this way.

In Newtonian physics, gravity is a force between masses. The status of massless things like photons is, indeed, rather dubious.

In relativistic physics, gravity is the curvature of spacetime, it is not a force. Everything is affected by it because it's the "shape" of spacetime. Loosely, paths are curved because spacetime is curved (that's horribly over-simplified, but this is a B-level thread). Furthermore, the source of gravity is not just mass, but includes energy and momentum, although not in a straight forward way. So under some circumstances photons can attract each other gravitationally despite being massless because they have energy and momentum.

I understand, thanks for your reply. What kind of circumtances exactly ?

 

[Ibix]

I understand, thanks for your reply. What kind of circumtances exactly ?

Photons traveling parallel to each other don't attract. Pairs of photons traveling in any other direction do attract (including anti-parallel - in fact, if the photons aren't traveling parallel then you can always transform to a frame where they are moving anti-parallel to each other). The effect is beyond tiny, though, so this is purely theoretical. No laser we've got can produce the kind of energies needed to detect any gravitational effect between beams of light, and I don't think I'd like to be on the same planet as one that could.

 

[Arman777]

Still, I didn't understand why I am the "wrong one" here.

If the gravitational effect of the photons are purely negligible,

The effect is beyond tiny, though, so this is purely theoretical.

and it happens only the case of

under some circumstances photons can attract each other gravitationally despite being massless because they have energy and momentum.

where,

Pairs of photons traveling in any other direction do attract

Hence, It's clear to say that even light can act as a source of gravity (which I never claimed it cannot, and even it does it will be purely negligible).
We can say that light is affected by the space-time curvature in most of the general cases. Including black holes and stars etc.

only affected by "spacetime curvature". That is not correct.

I never said "only", I just claimed that it's affected by space-time.

The Question is about the relationship between light and the black hole (a heavy object), In that case, the only thing that matters is the space-time curvature of the heavy object.

 

[PeterDonis]

The Question is about the relationship between light and the black hole (a heavy object), In that case, the only thing that matters is the space-time curvature of the heavy object.

Yes. But you claimed that this is not true of an object with nonzero rest mass; you said that for that kind of object, there is a "gravitational pull" in addition to the space-time curvature of the heavy object. That is not correct. The space-time curvature of the heavy object is sufficient to explain the motion of all test objects in its vicinity, whether they are massless or not.

 

[Arman777]

But you claimed that this is not true of an object with nonzero rest mass; you said that for that kind of object, there is a "gravitational pull" in addition to the space-time curvature of the heavy object.

Where did I say it?

In the whole time, I was talking about the light, I never claimed such idea or mention it.

That is not correct. The space-time curvature of the heavy object is sufficient to explain the motion of all test objects in its vicinity, whether they are massless or not.

I know that.

Yes.

If yes to the part where I said,

The Question is about the relationship between light and the black hole (a heavy object), In that case, the only thing that matters is the space-time curvature of the heavy object.

Then why we are even discussing it for a long time. I was claimed "wrong" or "unhelpful" for my post so many times?? I said the same thing in my previous posts.
For example,

And the answer is that it's purely GR, space-time curvature effect, which I tried to explain in my pre-posts.

or

Theres no force or any other direct interaction between light and the star. In the picture, as you can see star bends the Space-Time, creates a curvature, and light follows that curved path.

I am not sure what's going on here. We are on the same page, but I am being misunderstood repeatedly, claimed wrong, and that really annoys me.

 

[PeterDonis]

Where did I say it?

Here:

Photons are not directly affected by the gravity, but affected by the space-time curvature that the heavy objects made.

As both @Vanadium 50 and I commented, the distinction you were drawing in this quote between "gravity" and "space-time curvature" is not valid. "Gravity" is "space-time curvature".

 

[PeterDonis]

In the whole time, I was talking about the light

No, you weren't. You did say this about the effect on light:

Simply its pure GR effect and we can't think this situation in the sense of Newtonian gravity.

But then you said this:

I am talking about Newtonian Gravity. If you are claiming Newtonian Gravity is the space-time curvature then go ahead.

Which makes no sense unless you were trying to talk about something other than light.

We are on the same page, but I am being misunderstood repeatedly, claimed wrong, and that really annoys me.

I know you think you are on the same page, but IMO you were not conveying things in a way that makes that clear. Particularly in a "B" level thread where the OP might not have the background to interpret what you are saying correctly.

 

[PeterDonis]

why we are even discussing it for a long time

Here is the quote from you that I responded to:

The Question is about the relationship between light and the black hole (a heavy object), In that case, the only thing that matters is the space-time curvature of the heavy object.

And here is my response:

Yes. But you claimed that this is not true of an object with nonzero rest mass

It's true that you did not explicitly say this; but what you said can be interpreted that way, and that's how I interpreted it, and how the OP might well have interpreted it. Again, this is a "B" level thread; you can't expect the OP to be able to draw the kind of fine distinctions that people more knowledgeable in the subject can draw. The clearest and simplest way to state what GR says is what I said here:

The space-time curvature of the heavy object is sufficient to explain the motion of all test objects in its vicinity, whether they are massless or not.

Once you have said that, you have answered the OP's question, and done so in a way that, IMO, minimizes the chance of misinterpretation. If the OP follows up with questions about how all this relates to Newtonian gravity, that's fine; but so far he hasn't posted at all in response to you, or me, or anyone in this thread but @phinds right at the start (his last post is post #3).

 

[Arman777]

Here:
As both @Vanadium 50 and I commented, the distinction you were drawing in this quote between "gravity" and "space-time curvature" is not valid. "Gravity" is "space-time curvature".

I tried to explain what I was trying to say many times. There's no point in bringing old arguments or what we have discussed before.

No, you weren't. You did say this about the effect on light:

But then you said this:

Which makes no sense unless you were trying to talk about something other than light.

Those quotes have a history which I replied and I am sorry but that's just really silly to deduce that.
I was trying to make a point in there when I said

Simply its pure GR effect and we can't think this situation in the sense of Newtonian gravity.

I was saying that this situation cannot be treated as Newtonian Gravity problem.

If you are claiming Newtonian Gravity is the space-time curvature then go ahead.

In here I was just trying to say that Newtonian Gravity cannot be treated as GR.

It's true that you did not explicitly say this; but what you said can be interpreted that way, and that's how I interpreted it,

This is how you are responding me from the beginning. Just interpreting things that I didn't even say and try to win the argument or Idk what your purpose in here. Since whenever I say you will look from a different angle, which I have no intention to say such things, there's no point in continuing this.

Again, this is a "B" level thread; you can't expect the OP to be able to draw the kind of fine distinctions that people more knowledgeable in the subject can draw. The clearest and simplest way to state what GR says is what I said here:

We are talking about light, my post is sufficiently enough. Also this is not our point of discussion. You are just diverting the topic with all of these.

 

[Arman777]

It's true that you did not explicitly say this, but what you said can be interpreted that way, and that's how I interpreted it, and how the OP might well have interpreted it.

We are not doing literature here, you understand that way cause you want to take it that. If I say

The Question is about the relationship between light and the black hole (a heavy object), In that case, the only thing that matters is the space-time curvature of the heavy object.

And you conclude that I am saying that,

But you claimed that this is not true of an object with non-zero rest mass;

Then it's clear that its about your understanding problem. There's NOTHING wrong with my claim. Which has been told repeatedly WRONG!

Again, I didn't understand how you concluded that also, your counterclaims (in post #22 about this non-zero rest mass discussion) has no makes sense at all. I was talking about totally different things.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

The thread will remain closed.