# Black Holes

So black holes are incredibly dense points of matter that affect everything in their area with very powerful gravity field, including light. \

So, asuming that gravity affects only matter, that by default says that light is a physical particle.

So why don't the regular laws apply to it? When light moves through the air in the atmosphere, why doesn't it slow due to air resistance like all other objects with mass?

Related Astronomy and Astrophysics News on Phys.org
mathman
So why don't the regular laws apply to it? When light moves through the air in the atmosphere, why doesn't it slow due to air resistance like all other objects with mass?
When light travels through the atmosphere its speed is slightly lower than in vacuum.

So, asuming that gravity affects only matter, that by default says that light is a physical particle.
You assumtion is wrong. Gravitation affects mass and energy. Under gravitation mass and energy is equivalent.

As has been stated, your assumption is incorrect. If you look at general relativity, gravity is a manifestation of the curvature of spacetime. Light, or photons, travel along null geodesics, which are essentially a generalization of straight lines in curved space. So if a photon is traveling along spacetime it is going to follow that generalized straight line path, which in curved spacetime means it will appear to be bent (giving rise to gravitational lensing).

And at any rate, light does behave as it should. In mediums denser than vacuum, the speed of light is less than it is in the vacuum (this gives rise to index of refraction for example).

thanks for the help. I have two more question:

Does light continue slowing down as it moves through more of an atmosphere?

If light is in the electromagnetic spectrum, then all other wavelengths of the spectrum travele at the same rate. Correct?

- No, the velocity only changes at the interface.
- In vacuum, yes. In media, no, it depends. Otherwise there would be no rainbows..

If light is bent around a black hole, does it bend around all points of gravity, however small?

And even if it doesn't, do the other waves of the spectrum bend around black holes?

If light is bent around a black hole, does it bend around all points of gravity, however small?

And even if it doesn't, do the other waves of the spectrum bend around black holes?
yes! light bend around all points of gravity!

yes! the other waves of the spectrum also bend around black holes!

thanks for the help. I have two more question:

Does light continue slowing down as it moves through more of an atmosphere?

If light is in the electromagnetic spectrum, then all other wavelengths of the spectrum travele at the same rate. Correct?
As the medium becomes denser then in general yes, the effective speed of light decreases. For example light travels fastest in a vacuum, slower in air and slower still in water.

And correct, light is just an electromagnetic wave. What makes it special to us is we can see a sliver of the electromagnetic spectrum and have called it light. Light is the same thing as x-rays or radio waves. All that differs is the frequency (or wavelength).

If light is bent around a black hole, does it bend around all points of gravity, however small?

And even if it doesn't, do the other waves of the spectrum bend around black holes?
Yes, as stated it will move around all sources of gravity. But the effects will be very very very small for small sources of gravity.

you have to remeber that black holes have one of the strongest gravitational pulls. they pull in a lot of thing, including light.

If light is bent around a black hole, does it bend around all points of gravity, however small?

And even if it doesn't, do the other waves of the spectrum bend around black holes?
The bending of light around the periphery of Sun was one of the experimental confirmations of General Relativity.

physically speaking light really slows down a bit

When light travels through the atmosphere its speed is slightly lower than in vacuum.
Nothing [:yuck:] anyone said in this thread so far is really correct. If anyone really cares, ask me why not.
So melodramatic.. won't you enlighten us all then to the error of our ways?

Last edited:
In the presence of a medium of refractive index n, the speed of light is reduced to c/n. Since the atmosphere has an refractive index of around 1.000295 at sea level for visible light, it is not slowed down much.

You may interested to know that light beams have been slowed down to a walking pace and even stopped completely (http://www.afrlhorizons.com/Briefs/Jun02/OSR0201.html [Broken] just from google).

Last edited by a moderator:
ranger
Gold Member
Nothing anyone said in this thread so far is really correct. If anyone really cares, ask me why not.
Wont it just be easier to tell us? I'm eager to know why what mathman said is wrong.

Chris Hillman

Somewhat to my surprise, people care. Hmm... it wasn't nearly as bad as I said... sorry!.

Wont it just be easier to tell us? I'm eager to know why what mathman said is wrong.
Oops, nothing. I was "kicking" about a bunch of posts which came later on. Here are my pedantic corrections/comments:

If light is bent around a black hole, does it bend around all points of gravity, however small?
I think madphysics is trying to ask whether light bends around any compact object, or indeed any concentration of mass-energy in some region, or just around black holes. The answer is of course "yes".

And even if it doesn't, do the other waves of the spectrum bend around black holes?
Light bending does not depend upon the frequency of the light--- this is very important, if it weren't true, we'd see colored fringes in those Hubble images of lensed galaxies. But in any case I suspect madphysics meant to ask if "massless" radiation which is not electromagnetic radiation--- in particular, gravitational radiation, also experiences bending. The answer is "yes", because wave packets of such radiation have (roughly speaking) world lines which are null geodesics (in vacuum).

yes! light bend around all points of gravity!

yes! the other waves of the spectrum also bend around black holes!
Well, from what I just said you can see that I wouldn't put it this way.

As the medium becomes denser then in general yes, the effective speed of light decreases.

[snip]

Yes, as stated it will move around all sources of gravity. But the effects will be very very very small for small sources of gravity.
Sorry, Brad, that looks OK too.

you have to remeber that black holes have one of the strongest gravitational pulls. they pull in a lot of thing, including light.
All objects with mass M have pretty much the same "gravitational pull". The interesting thing about neutron stars and black holes is that they are so much more compact than ordinary objects, which means that you can get a lot closer without striking "the surface" (or encountering the horizon). Since the tidal forces scale like M/r^3, smaller r for given M means much larger tidal forces.

physically speaking light really slows down a bit
sanjeeb is thinking of the "Shapiro light delay effect", but this name is potentially misleading since, at the level of tangent spaces, light always travels at c (in vacuum). This effect is really a global effect; in a sense "the effective speed of light" over a large course can be different from c. One way to understand that this is no contradiction is to realize that even in flat spacetime there are multiple operationally significant notions of distance in the large for accelerating observers (which at very small scales all reduce to the notion given by the metric tensor). In particular, radar distance, the notion relevant to the Shapiro effect, can exhibit odd behavior when a light path travels near a massive object such as the Sun.

These corrections are probably incomplete since at least two of the other posters are in my ignore list, in part because I tired of correcting their frequent misstatements concerning gtr.

OK, shame on me for not reading more than the last half of the thread. I'll try not to make such sweeping statements in future

Last edited:
I think people were a little offended by your comments Chris because it just came across as very arrogant that you accused *everyone* of being wrong and you wouldn't even justify your reasoning, instead we would have to ask you for the privlidge of divulging your greater widsom upon us.

In scientific circles, these kind of sweeping statements are highly offensive. But your apology is welcome and I won't hold it against you in future!

Last edited:
Chris Hillman
your apology is welcome and I won't hold it against you in future!
Thanks, natski, I appreciate that!

OK, we now return you to the original thread

Sorry i hope you dont mind me posting this here rather than starting a new topic

I believe i'm right in saying that light cant escape a black hole due to it following a pathway through bent space which starts and ends in the blackhole.

so if somehow we could survive the trip through the EV of a blackhole and then tryed to look out through the EV back into normal space what would we view the EV from inside as being ,bright due to all light trying to escape following pathways back in or black.

Last edited:
Chris Hillman
I believe i'm right in saying that light cant escape a black hole due to it following a pathway through bent space which starts and ends in the blackhole.
If you are picturing null geodesics inside the event horizon, any outgoing ones must fall back without ever getting outside the horizon.

So if somehow we could survive the trip through the EV of a blackhole and then tryed to look out through the EV back into normal space what would we view the EV from inside as being ,bright due to all light trying to escape following pathways back in or black.
See Andrew Hamilton's website (see my sig) for some pointers, then ask again.

Will do thank you.

Light bending does not depend upon the frequency of the light--- this is very important, if it weren't true, we'd see colored fringes in those Hubble images of lensed galaxies.
That statement seems a bit too strong.

In general relativity if we have two objects with a given mass-energy the amount of "bending" depends on both objects since each object produces some Weyl curvature. Of course the mass-energy of a photon is almost nothing in comparison with an an object like the Sun, so we conveniently ignore it.

According to GR, each particle in the universe that has either energy or mass must be surrounded by some Weyl curvature. The more energy or mass the more curvature. Thus a higher frequency photon must be surrounded by more Weyl curvature than a lower frequency photon.

Last edited:
oh by the way, is it true that a more massive object will exert a greater gravitational pull?

Chris Hillman