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why are photons affected by gravity

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I would go along with kpax. Some of the explanations I've seen effectively say that gravity warps space - light follows the curvature of space etc. See gravity at wikipedia.org

Gravity can also slow down light and I thought that physicists now accepted that light does have a mass/exerts a pressure.

There is an interesting victorian toy called a radiometer. It has 4 vanes suspended in vacuum in a bulb. The vanes are painted black on one side white on the other. It was suspended in a glass bottle in vacuum. Light makes it turn. See for yourself

If that link doesn't work or you want to buy one use



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Beamish take a look at Olber's paradox on wikipedia.org. Don't think that the introduction is very good so. If the universe if infinite and contains an infinite number of stars and is mostly empty why is the night sky dark? That's Olber's paradox. The maths is a bit like black hole maths. Stars give out light in a sphere so the intensity of the light drops of rapidly as the sphere gets bigger and bigger. As there are an infinite number of stars the night sky should still be light. That's Fred Hoyle's way of explaining it. It's infinity mathematics. These don't make much sense in some ways as one can always add another number to any other number. In this case though a rigorous calculation would say that the night sky shouldn't be dark. Hoyle basically says that there isn't an easy way of explaining it. There is another version based on the heat output of an infinite number of stars too. We should all be burnt to a crisp. Some web sources sometimes state that as Olber's paradox. I also wonder what a light ray would do if it passed an infinite number of stars.

As the age of the universe has been revised several times I've always liked the thought that photons/light waves decay and loose energy over huge astronomical distances - hence red shift and the apparent expanding universe etc. Not that I'm qualified to have an opinion. I just like it. Every now I come across something that makes me like it even more - like jets that are travelling faster than light, needing dark matter etc. One of the problems with astro physics is that we can't actually measure the distances past a certain point. 1st it was trigonometry, then cephid variables, now it's super nova and red shift.


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Do/will photons travelling in a straight line , not colliding with anything in it's path, ever slow down/stop/fizzle out/run out of "energy" ???? :scratch: :hello2:

In an expanding universe (like ours) the answer is yes. They don't slow down but they lose energy to gravity, manifested as redshift. The microwave background photons were very high energy when they were created - all other freely propagating photons in the universe lose energy over time in the same way. Freely moving particles are slowed down by the same effect - ultimately they would all come to a halt, after an infinite length of time. I think I'm right in saying that this result still goes through in an expanding unvierse with no matter (except for the test particle). So Newton's First Law isn't true.

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