Light and Gravity

[Tuko246]

Was just watching a khan academy video on youtube about black holes, and khan was going over how once light (or anything) reaches the event horizon, it is pulled back into the black hole. The question then popped into my mind, " If light is slowed down and pulled back into a black hole, isn't light emitted by anything pulled back slightly?" So right now I'm wondering if light ever reaches the speed of light, because it must be emitted by something, and that something has to have mass and therefore gravity, which will pull on the light it emits. All answers are appreciated.

This is the link to the video in case you want to watch it.

[ollypenrice]

I think that the black hole's gravity does not slow light down but bends the paths on which it travels. Also I think that light can lose energy without losing velocity. This happnens when it is redhifted. Looking forward to the professionals' response on this though.

Olly

[JulianO]

Yes - light always travels at the speed of light Light trying to leave a black hole will find that space is so warped, no matter which way it goes, it always ends up back where it started. It would be like trying to run off the Earth into moon - there isn't a path you can take that gets there!

As to energy, that's the gravitational red shift. As photons leave the Earth, for example, they lose a little energy as the fight their way out of the gravity well. This causes them to lose energy by getting a longer wavelength. This effect is used directly in astronomy, in measurements like the integrated Sachs-Wolfe effect.

[Vlad]

Lose energy, yes. Lose velocity, no. How so? the wavelength changes from, say a blue, to a red. so, light that was blue but is now red, still travels at "C", but with less energy.

I have wondered about this for years and years. Also about the big bang??? Can't get my head round it .

[Dave S]

A bit overpowering this article but it has most of the answers

http://www.speed-light.info/speed_of_light_variable.htm

Dave...

[Alastair Rae]

The trouble is trying to apply billiard ball concepts to the quantum world. I'm currently reading The Quantum Universe: Everything that can happen does happen (Cox & Forshaw) and they bang on about how particles and waves etc are just analogies. Things in that world do not really behave like anything in the macro world. So a photon does not accelerate like a real particle going from 0 to c.

[ollypenrice]

Lose energy, yes. Lose velocity, no. How so? the wavelength changes from, say a blue, to a red. so, light that was blue but is now red, still travels at "C", but with less energy.

I have wondered about this for years and years. Also about the big bang??? Can't get my head round it .

This needn't be a difficult idea, the drop in energy as the wavelength lengthens. In fact colour vision would be in a pickle if longer wavelengths travelled more slowly because the red would arrive after the blue!

The whole spectrum travels at the same velocity but different parts of it are more energetic than others. In the high energy end there are lots of waves waved every second and at the lower end there are fewer waves waved and so less energy. Or so it seems to an amateur like me.

Olly

[SionR25]

Lose energy, yes. Lose velocity, no. How so? the wavelength changes from, say a blue, to a red. so, light that was blue but is now red, still travels at "C", but with less energy.

I have wondered about this for years and years. Also about the big bang??? Can't get my head round it .

I think it is because photons are mass-less, the energy they contain is not needed to move it as a photon/wave is a way of transferring energy. The definition of a wave is a transfer of energy. A photon is one little part of a wave. Imagine it as water, a water wave when pushed will keep going until the energy you gave it in the push is dissipated by the water hitting a physical barrier. With light that energy is dissipated in the same way, transferring the energy to an electron.

Don't know how clear that explanation is but hope it helps.

Sion