Is Light Really Bound by Gravity?

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Discussion Overview

The discussion centers around the nature of light and its interaction with gravity, exploring concepts such as the bending of light, the curvature of spacetime, and the implications of these phenomena in both theoretical and practical contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants recall learning that light cannot bend, while others reference literature suggesting that light can be bent by gravity.
  • One participant explains that gravity warps spacetime, causing light to follow curved paths rather than straight lines.
  • Another participant questions the concept of a "curved universe," expressing frustration over the topic.
  • It is noted that light appears to travel straight from the perspective of inertial observers, but may appear bent to those in different frames of reference.
  • Participants discuss the possibility of light beams returning to their source due to the curvature of space, particularly around massive objects.
  • There is mention of the photon sphere, where light can orbit a massive body, with some participants providing differing opinions on the specifics of its location relative to the Schwarzschild radius.
  • One participant argues that while light follows geodesics, not all geodesics represent the shortest path globally, suggesting a nuanced understanding of light's trajectory.
  • Another participant asserts that gravity can bind light, while others point out that various factors, including changes in refractive index, can also affect light's path.

Areas of Agreement / Disagreement

Participants express a range of views on the bending of light and the implications of gravity on its path. There is no consensus on the nature of light's trajectory or the implications of geodesics in this context, indicating ongoing debate and exploration of the topic.

Contextual Notes

Some statements rely on specific definitions of geodesics and the conditions under which light travels, which may not be universally agreed upon. The discussion includes various assumptions about the nature of spacetime and light that remain unresolved.

orgthingy
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i remember in middle school that the light can NEVER be bend! I've just read a book " a briefer history of time" by : s.hawking... (im in high school now) but i really didn't understand y! :P
 
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i read in " a briefer history of time" that light can be bend! i rly didn't understand how :P
 
Gravity warps spacetime. Light follows the shortest path from point to point, which, in curved space is not a straight line.
 
and y the hell "the universe is curved" ?
 
Why does that make you so angry?
 
HallsofIvy said:
Why does that make you so angry?
Wait until he gets to college--then he'll really be pissed. :-p
 
Doc Al said:
Wait until he gets to college--then he'll really be pissed. :-p

(you are funny, Doc-----)


and then he'll still come back to the PF and will still get these same types of answers
 
Actually, gravity is caused by warped spacetime, which is caused by mass.
 
Light does go straight, but only from the point of view of what are called inertial observers (IOs). Standing still on the surface of the Earth doesn't make you an IO, but being in free fall does (approximately and, of course, temporarily).

So if someone was falling towards the earth, they would see nearby beams of light traveling along straight lines. But if you watched this from the ground, these beams look bent, in the same way the path of a stone through the air, which the falling person sees as moving in a straight line, appears to you to be bent (although the light bends to a much smaller degree due to its much higher speed). So that's how light can "look" bent, even though, in an important sense, you're right to say it never really bends.

Actually, because of the curvature of space, even IOs can see distant light bending, because the curvature means roughly that an IO at one point won't look like an IO to an IO in another location.
 
  • #10
Is it possible to send out a light beam, that returns to the emitter due to space curvature?
 
  • #11
A.T. said:
Is it possible to send out a light beam, that returns to the emitter due to space curvature?

Yes. If you were on the surface of a massive object (a star, say) you could emit a light beam with an angle such that it circles the star and returns to you.
 
  • #12
Yes -- around any strong gravitational field, such as around a black hole. However, the orbit is unstable -- if you were not exactly on the correct orbit, you'd either decay into the hole or escape. I can't actually remember, but it's something like twice the Schwartzchild radius or something -- can an astrophysicist correct me?
 
  • #13
I'm pretty sure that it's 1.5 times that Schwartzchild radius.
 
  • #14
daniel_i_l said:
I'm pretty sure that it's 1.5 times that Schwartzchild radius.

Yes, the photon sphere is at 1.5 times the Schwarzschild radius, and at the photon sphere light can orbit a massive body. In theory, by looking in the right direction, you could see the back of your head at the photon sphere (the image might be dim and distorted, howeer). The orbit is unstable against small pertubations.
 
  • #15
Doc Al said:
Wait until he gets to college--then he'll really be pissed. :-p

Surely gravity can bend the light. this is already proved by einstein in his theory on bending effect of light
 
  • #16
surely light will have bending effect
 
  • #17
pervect said:
Yes, the photon sphere is at 1.5 times the Schwarzschild radius, and at the photon sphere light can orbit a massive body.

Thanks for your answers. The reason that I asked this, is the statement that "light follows the shortest path from point to point".

This is maybe true locally, but if a photon send out from point A orbits a massive star and arrives back close to point A, then this is certainly not the shortest path between two points globally. The shortest path between two points, is always a geodesic. But not every geodesic is the shortest path.

One could say: "Light follows the straightest path from a point in a given direction"
 
  • #18
A.T. said:
Thanks for your answers. The reason that I asked this, is the statement that "light follows the shortest path from point to point".

This is maybe true locally, but if a photon send out from point A orbits a massive star and arrives back close to point A, then this is certainly not the shortest path between two points globally. The shortest path between two points, is always a geodesic. But not every geodesic is the shortest path.

One could say: "Light follows the straightest path from a point in a given direction"

The path that a photon (or anything else) travels between two events is the path with the smallest distance through spacetime - not the shortest distance through space.
Lets look at an example - throwing a ball in the air. The two events are:
A: the ball is thrown in the air, h=0, t=0.
B: the ball is on the ground again, h=0, t=10.
Now let's see what path gives us the shortest ST interval between those two events. If the ball stays in the same place for the whole time then the space interval is 0 and the time interval is 10. But according to the SC metric the time interval (dt) contributes more to the interval the closer it is to the Earth - so if during the 10 seconds the ball is a little higher in the air (h>0) the total interval will be smaller. But if it's too high the interval will get bigger (because it goes a long way in the 10 seconds). It turns out that the best path to take is one where the ball goes up and comes down in a ballistic trajectory.
 
  • #19
so, gravity can BIND light!
 
Last edited:
  • #20
To be honest, lots of things bend light. Changes of refractive index being the obvious one. It's certainly not true that light travels in straight lines... The full story behind going from working out the principle behind the path that light takes (Fermat's Principle) to modern day mechanics is jam-packed with the most fun kind of maths.

Now that I look at that sentence, it's not as much of an endorsement as I wanted... :(
 

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