# If photon is mass-less why can it be pulled into blackhole?

Discussion in 'Physics & Math' started by Saint, Aug 7, 2017.

1. ### SaintValued Senior Member

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If photon is mass-less why can it be pulled into blackhole?

3. ### Michael 345New year. PRESENT is 69 years oldValued Senior Member

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Not totally sure on this one but from another thread in the forum

the speed of the photon gives it mass property

I'm sure I'll be corrected if wrong and/or somebody will give a far better explanation than mine

5. ### DaveC426913Valued Senior Member

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Things are not pulled into black holes because of their mass. That's a Newtonian expectation.

Black holes are an Einsteinian artifact.

Black holes warp space. All things - matter as well as photons - follow this warped space.

Another way of putting it:

as far as the photon is concerned, it is following a straight line - it's just that the straight line is bent. (This is called a geodesic.)

7. ### The GodValued Senior Member

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Any photon directed towards a black hole is pulled into (you meant inside Event Horizon?). A photon inside an EH never comes out. So where is the problem?

8. ### The GodValued Senior Member

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Can you elaborate this?

9. ### ajantaRegistered Senior Member

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Photon can generate gravitational force, so there will be attraction between photon and BLACK HOLE( that can produce strong gravitational force).

10. ### DaveC426913Valued Senior Member

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At the risk of opening the can of worms I suspect you're wielding...
In a Newtonian universe, gravity is a force; one that acts on masses to pull them toward the source.
So, massless photons being affected by this gravitational force doesn't make sense. You can calculate it, but you'll get the wrong answer.

In an Einsteinian universe, gravity is a field; it does not affect mass directly; it affects space-time by bending it**. Mass and EMR (photons) both follow the curvature of space-time.

**Sorry, this is lazy terminology. Gravity does not affect space-time by bending it; gravity is the bending of space-time.

11. ### James RJust this guy, you know?Staff Member

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DaveC has this right. Also...

That kind of idea comes from the idea of "relativistic mass". But most physicists don't use that concept, because the "relativistic mass" of something varies depending on the observer's frame of reference. This is particularly problematic for photons, because we can't work in the frame of reference of anything moving at the speed of light.

To avoid confusion, most physicists talk only about rest mass when they refer to mass. Rest mass is an invariant quantity, and photons have zero rest mass.

To bring this conversation right up to date: since the discovery of the Higgs boson, maybe the best thing to say about photons is that they have no (rest) mass because they don't interact with the Higgs field. And because they don't interact with that, they have no way of ever getting mass from anywhere.

The apparent bending of photon paths by gravity is due to the curvature of spacetime, which is caused by mass and energy. Stars and black holes curve the spacetime around them because they have a lot of mass.

If you try to mix and match the Newtonian idea of gravitational force with the idea of mass-energy equivalence from special relativity, the result is something like what you describe. But the problem here is that the observed effect (how much photons curve due to gravity) turns out to be larger than what you calculate if you try to calculate it this way.

Really, the best way to think about things is to draw on general relativity. In that theory, everything, with or without mass, follows the closest thing to a straight line through spacetime (i.e. a geodesic), unless some non-gravitational force acts on it. In general relativity, gravity is not a force at all. It is a description of how mass and energy bends spacetime, which is what makes those "straight lines" in spacetime turn into curved paths in space.

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12. ### SaintValued Senior Member

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the theory says that if you fell into blackhole, you will be pulled apart.
how about photon, will this massless particle be pulled apart?

13. ### James RJust this guy, you know?Staff Member

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Regarding photons: inside the event horizon, we know that that photons will make their way towards the centre of the black hole. But individual photons aren't divisible, as far as we can tell. You can't pull a photon into two separate pieces. We don't know what happens at the centre of a black hole, so we can't say what, exactly, happens to photons there.

14. ### SaintValued Senior Member

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will photon be annihilated inside blackhole?

15. ### DaveC426913Valued Senior Member

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As JamesR says, we don't know.
We describe the centre of a black hole as a singularity. 'Singularity' is mathspeak for 'we have no rules that apply here'.

16. ### river

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Because the photon is energy . E=mc^2.

To be clear though , BH , is nonsense , to me .

Last edited: Aug 9, 2017
17. ### The GodValued Senior Member

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Einstein general relativity is a metric theory, what is the physical reality of metric, none knows. It is the mathematical representation. The space-time, singularities and many associated terms are the artifacts of this mathematical representation. There is absolutely nothing real about singularity, it is just that our knowledge fails us. I am inclined to say that something is blocking the scientific community, may be the huge inertia of so called paper success, to come forward with new ideas.

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18. ### Confused2Registered Senior Member

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Newtonian physics does not (and cannot) address why the speed of light should be constant or why there should be a constant which we call 'the speed of light'. Could be time to leave High School behind.

19. ### QuarkHeadRemedial Math StudentValued Senior Member

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It is. And far as I am aware, all theories in physics are metric theories. Physics is, at the end of the day, about what can be measured and very little else.
A bold statement indeed! Is it not the "reality" of the distance between objects, and the time elapsed between events?

20. ### The GodValued Senior Member

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you are ok with the literal meaning of the word metric, but it is loosely used for "metric of spacetime".

Try this...

https://en.m.wikipedia.org/wiki/Alternatives_to_general_relativity

21. ### The GodValued Senior Member

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Which theory can address that ?

22. ### QuarkHeadRemedial Math StudentValued Senior Member

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Well I thank you for your expert approval

When I said in an earlier post that physics (in fact all science) is about measurement, it was intended as a "global" assertion - it was intended to embrace the General Theory of Relativity.

No thanks - I am content with the General Theory.

If you want to know exactly what the metric is in GR (it's actually a field), I can give 2 arguments, one of which requires some familiarity with the differential calculus. But both involve mathematics, which you appear to despise as applied to physics.

So I do not expect you to take me up on this offer.

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