# Light bending?

_Muddy_
Hello,

not too sure if this is the right section but i do remeber reading it in a book about astrophysics and cosmology. The reason I am asking is that a can't find where i read it.

It said something like light bends slightly due to gravity or something like that. There was a man in a spacecraft and the height from which he shone a torch the light hitting a screen on the other side of the spacecraft was slightly lower and so the light had bent for some reason.

If anyone has absolutly any idea of what I'm talking about please could you tell me and explain fairly simply what is going on

Thanks _Muddy_, sorry for the rather poor and vague explanantion

## Answers and Replies

Staff Emeritus
Gold Member
You seem to be referring to the thought experiment dealing with the path of light in an accelerating spacecraft .

The gist of it is this: If you have an accelerating spacecraft and you shine a light from one side to the other(perpendicular to the acceleration), the beam will follow a curved path as seen by someone in the space craft, due to the fact that the velocity of the ship changes as the beam crosses the width of the ship. Now, according to General Relativity, there is no difference beween the effects of acceleration and gravity(as far as anyone in the ship knows they could be sitting motionless in a gravity field rather than accelerating.
Given this, it follows that if accleration causes the beam to curve, so will gravity.
Astronomically, this has been confirmed by noting how the gravity of galaxies can cause the light from further galaxies along the same line of sight, to curve as it passes the nearer galaxy.

Gold Member
Now, according to General Relativity, there is no difference beween the effects of acceleration and gravity(as far as anyone in the ship knows they could be sitting motionless in a gravity field rather than accelerating.
More specifically known as the Principle of Equivalence.

blimkie.k
It said something like light bends slightly due to gravity or something like that.

Yes light bends in high gravity fields, for example beams of light traveling in a line near the sun (rectilinear propagation) will bend towards the sun due to the high gravity. If you type "light bending around sun" or something like that in google images you will get some nice diagrams.

_Muddy_
I thought that light consisted of photons and that photons had no mass? I didn't know that gravity would have an effect on something without a mass?

Staff Emeritus
Gold Member
In general relativity a mass warps spacetime and light follows these curves in space. I'm sure you've seen the rubber sheet analogy for general relativity. Confusion comes about because we are originally taught Newton's view of gravity which involves the concept of forces between two objects with mass so it seems absurd that light can be affected by gravity, but as others have mentioned it has been directly observed.

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blimkie.k
It gets kind of complicated if you haven't studied the theory of relatively. A better way to word it would be that the light itself does not bend it follows the curvature of space time. Imagine space as a web of fibres, as it nears a large gravitational field such as the sun or a black hole it becomes warped. So basically the light just follows the space time curvature. Check it out on google and if you go on youtube you can probably find some animated videos which will explain relativety and space time. Or maybe someone else on PF can explain it better as I myself have limited knowledge on the subject.

blimkie.k
there we go Kurdt beat me to the post

_Muddy_
thanks everyoen and thanks again kurdt

dslowik
The fact that everything travels in straight lines in inertial reference frames, and that these frames accelerate in a gravitational field, shows that everything has mass.