Science Questions

If light has no mass, how can a black hole capture it?

Sun, 14th Sep 2008

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Sri Adibhatla Soma Sekhar asked:

If light has no mass, how can a black hole capture it?


Dave -  Light has no mass but it is affected by the shape of space.  What Einstein worked out in general relativity was if you have a very large mass it distorts the shape of space.  If you shine light across a curved piece of space it will get bent by it.  A black hole is basically so heavy that it bends space so much that light will get bent a bit like a lens and come straight back into the black hole again.

Chris -  That’s why they look black, because there’s no light coming back out.


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sri adibhatla soma sekhar asked the Naked Scientists:

sir, as we know that the gravitational force could exist between two bodies having masses then how could blackholes (massed) capture (massless i.e gravity-less) photons (light rays)?

What do you think? sri adibhatla soma sekhar, Fri, 12th Sep 2008

You have to be careful when saying that light has no mass. It is true that a photon has no rest mass, but photons are never at rest.

A thread appeared some time ago which elicited a comprehensive reply to this question. I'll try to find the thread and post a link to it here.

In the meantime, maybe someone else will interject with an answer. DoctorBeaver, Fri, 12th Sep 2008

Although not well written I'm told in the thread " Is a Black Hole a point? " I attempt to help some of you understand a black hole. There is no actual proof that light, is in fact captured by black holes ! Gabe2k2, Fri, 12th Sep 2008

But the significant concept here is not the gravitational force but the gravitational field. If an object's mass where important, then why Galilei found that all masses have the same acceleration when they fall from Pisa's tower?

Take an object of 1 kg mass and release it in the void and it will move towards the Earth in a certain way.
Now take an object of 0.001 kg mass, then of 0.0000001 kg, ecc. Will you expect some difference in the trajectory and in velocity?
At the limit for m-->0 there will be any difference? lightarrow, Fri, 12th Sep 2008

There is no experimental proof, that's true. But every theory says that it is so. DoctorBeaver, Sun, 14th Sep 2008

There is no experimental proof, that's true. But every theory says that it is so.
If we admit the existence of black holes, then we have to admit there is one in our galaxy's centre and there is nothing emitting light there so... lightarrow, Sun, 14th Sep 2008

Alberto - it is certainy true that what evidence we have points strongly to there being a black hole at the centre of the galaxy; but there is no definitive proof per se. DoctorBeaver, Sun, 14th Sep 2008

The speed that stars can be seen orbiting the centre of the galaxy points to a something of extreme mass that could only be a SMBH, either that or it would have to be something new to science.

Because the mass of a black hole bends space .Gravity has no direct effect on light as the photon contains no mass, Gravity does however bend space.
Imagine space being a bunch of fibre optics, if all the fibre optics  started and ended inside the black hole their would be no escape for the photons traveling along them.
ukmicky, Sun, 14th Sep 2008

Precisely - As I said, "There is no experimental proof, that's true. But every theory says that it is so."

We cannot rule out all the impossibles it we don't know what all the impossibles are. DoctorBeaver, Sun, 14th Sep 2008

Ok Sir!  lightarrow, Sun, 14th Sep 2008

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