• Visit 'Mars' in Space.com's new 'Space Traveler' mini-documentary - Watch here! 🚀

Bending light

Status
Not open for further replies.
T

technoid

Guest
Hi, i was just wondering, how much magnetic (or other) force there will be needed to bend light? And I would really like to know wether you can do it with a electro magnet, and if so, how much power for how much bend?
 
K

Kessy

Guest
Since photons don't have an electric charge, I don't *think* an EM field, no matter how strong, should bend light. Unless someone knows more about it?
 
S

SteveCNC

Guest
As far as I know only gravity or optics can bend light , and I suppose any amount of gravity will bend light but your ability to measure the bend will be doubtful unless you have a whole lot of gravity like that of the sun . I'm sure your already aware of the experiments done during full solar eclipse plus there's the whole gravitational lensing thing also .

Many things can cause optical effects for instance heat currents in the atmosphere can cause a mirage .
 
C

captdude

Guest
Hi, i was just wondering, how much magnetic (or other) force there will be needed to bend light? And I would really like to know wether you can do it with a electro magnet, and if so, how much power for how much bend?
To be technical; there is no actual bending of the lights trajectory. What is "bent" is the space that the light travels through due to the gravitational effects of a local body of mass. So unless I am mistaken, nothing other than gravity can effect the trajectory of a photon.
 
M

Mee_n_Mac

Guest
captdude":3csithd4 said:
Hi, i was just wondering, how much magnetic (or other) force there will be needed to bend light? And I would really like to know wether you can do it with a electro magnet, and if so, how much power for how much bend?
To be technical; there is no actual bending of the lights trajectory. What is "bent" is the space that the light travels through due to the gravitational effects of a local body of mass. So unless I am mistaken, nothing other than gravity can effect the trajectory of a photon.
I would disagree. Let me propose the following thought experiment. We have an universe, empty except for a neutron and a laser. The laser is incredibly intense and has a beam which doesn't diverge at all (impossible I know but that's not germane ATM). We turn on the laser and let the beam propagate some far distance such that the laser itself introduces negligible "gravity" at some point in space just 1 nm from the beams edge. At that spot we place the chargeless neutron, giving it zero velocity wrt the beam.

What happens ? Does the neutron ever move ?

I say yes it does. Again neglecting any gravity from the laser's source, the beam itself is energy and that energy warps space just as mass does. You just need a whole lot of it to ever be noticable, that whole E=Mc[super]2[/super] thing. Energy, not just mass, is a component of the stress energy tensor in GR that describes the warpage of space. The neutron eventually moves towards the beam.



Now hopefully somebody like SpeedFreek or Ramparts will come along and give you the truth ! :mrgreen:
 
R

ramparts

Guest
Truth is on its way! ;)

First of all, to pick nits: you can't really put anything at rest with respect to a beam of light. That's one of the fundamentals of special relativity!

Anyway, you're right in that the neutron wouldn't stand still in any reference frame (except its own), since the radiation will gravitate and attract it, even if only slightly. Your laser would probably be pretty massive and gravitate a lot more, but I'm happy to ignore that :) You're also correct that E=mc^2 is at the heart of that. All energy gravitates just as mass does - the object doing the gravitation in Einstein's theory (taking the place of mass in Newtonian gravity) is an object called the energy-momentum tensor. As the name suggests, it takes into account not only mass but energy and velocity as well. As far as spacetime is concerned, energy is just another facet of momentum, exactly the way that time is another facet of space. Since the two are interchangeable, depending on your frame of reference, you can't have one gravitating without the other.

In fact, while the gravity from light is usually negligible (and is today), for the first 40,000 years or so of the Universe's history, the radiation filling the Universe was the main source of gravity, outweighing the contributions from things like matter and dark energy, and that caused the Universe to expand in a different way than it did later when matter dominating the Universe, and much differently than now when dark energy dominates.
 
D

dryson

Guest
As far as I know only gravity or optics can bend light , and I suppose any amount of gravity will bend light but your ability to measure the bend will be doubtful unless you have a whole lot of gravity like that of the sun . I'm sure your already aware of the experiments done during full solar eclipse plus there's the whole gravitational lensing thing also .Many things can cause optical effects for instance heat currents in the atmosphere can cause a
Do optics actually bend light or is the effect an optical illusion? I think that because of the curvature of the optic being used the light gathered would appear to be bent across the surface of the optic when in fact the light photon is actually travelling in a straight vector from it's point of origin.

How can gravity effectively bend a light photon if a light photon does not have a positive or negative charge for gravity to effect the light photon with?
 
R

ramparts

Guest
dryson":2fv0ocr5 said:
How can gravity effectively bend a light photon if a light photon does not have a positive or negative charge for gravity to effect the light photon with?
Because gravity has nothing to do with charge. Charge is relevant to the electromagnetic force. All objects follow paths in curved spacetime, so all objects are subject to gravity.

Dryson, surely you're not going to say you don't believe that gravity affects light? If you do, I'd be very curious to hear your explanation for the observations of light bending around the Sun that first gave evidence for GR, or for the (many) observations of light from quasars being split around less distant galaxies. These observations agree precisely with the expectations from Einstein's theory.
 
J

Jerromy

Guest
captdude":3cmf2jyx said:
To be technical; there is no actual bending of the lights trajectory. What is "bent" is the space that the light travels through due to the gravitational effects of a local body of mass.
ramparts":3cmf2jyx said:
All objects follow paths in curved spacetime, so all objects are subject to gravity.
I don't mean to haggle frivilous details but I'm curious if I am mistaken... I feel that spacetime isn't really "bent" in the sense that an object would continue in an undisturbed straight line from any perspective. I think a gravity "well" is just a description of how matter is affected at various distances from the disturbance. The closest thing to a straight "curved" path I can imagine would be all matter in one spot except for a photon circling at a stable orbit.
 
R

ramparts

Guest
Think about the Earth's surface. Curved. If you look at the Earth's surface from outside, paths which connect two points by the shortest distance appear to be curved. This is why when you plot out an airplane's trajectory on a map, it appears curved, even though it's usually close to the shortest possible route from the departure to the destination.

The curvature of spacetime is similar, although obviously it's much harder to visualize a three-dimensional space (or four-dimensional spacetime!) that's curved. Remember, the Earth's surface is technically a two-dimensional space, which we can easily imagine being curved.

The mathematics describing curved spaces - which was developed quite a while before Einstein ever applied it to gravity - happens to describe gravity brilliantly, matching all observations done to high precision and in an elegant package. That said, one can never prove anything in physics, only provide more evidence, so it remains possible this picture is wrong. But if that's what you think, you should probably support it with more than "I feel that...". Your feelings and scientific evidence are (thankfully) completely different things.
 
M

Mordred

Guest
I'd like to add a though experiment to this though I don't believe this has ever been tested. Or it may have been.

Take light passing through something like a bohr's-Einstein condensate. We have managed to slow light down to a few meters per second. ( I can't recall the name of the other extremely cold substance more commonly used for this) However regardless its still an extremely cold fluid. At this speed could the fluidic flow cause light to bend.??? or would any change in direction merely be a refraction. As far as optic tubes are concerned you are not bending light just confining it to continuosly refract off the inner surface of the optic tube similar to electrons bouncing off the outer edges of a wire.

However in the latter case a superconductor will cause those electrons to flow a lot straighter I would think the Bohr's Einstein condensate could have similar effect however you would not be able to maintain its temperature once light enters it due to too much energy. So this is merely a thought experiment thats not really practical
 
J

Jerromy

Guest
Thanks for the replies... I may have an unorthadox method of understanding physics but it has worked so well for me since childhood. I blame relationship counselling for the "I feel" approach to debating!

My debate is simply this: the math supports the path that photons take through space where gravitational influences are concerned. The part I have trouble conceptualizing is that spacetime itself is bent rather than the path the photon follows being changed through flat space.

My thoughts on the speed of light: all EMR propagates at exactly the same speed in a vacuum, when confronted by any obstruction in its path there is an interaction. I am still uncertain whether the impactor transfers energy to another particle which is ejected or the original particle is delayed (perhaps orbiting the obstructing atom). My only point is that there is no reason to believe "slowing down" light would make it more or less susceptable to diversion.

A final thought on this subject relates to gravitational acceleration, this is the only logical reason that I need to dwell on to conclude whether space IS curved or not. If an object is approaching a large mass devoid of atmosphere it is accelerated towards the mass as it approaches and decelerated if it clears the mass as it departs the vicinity. Since light should not experience these effects in velocity I would think there would be an equivelent shift in wavelength. Not sure if that would prove anything, I'll have to sleep on that...
 
R

ramparts

Guest
Mordred - light always travels at c. In a Bose-Einstein condensate (not Bohr's!) or another thick substance, the slowing of light is just an apparent effect as photons are constantly absorbed and re-emitted. Bending of light in such a medium would be the same thing - an illusion.

Jerromy - You can interpret the curved paths objects take in gravitational fields however you'd like. If you want to see them as objects following curved paths in flat spacetime, feel free, but when you write down the mathematical expression for those paths, you'll be shocked by their uncanny resemblance to geodesic paths in curved spacetime!

As it is, there's no a priori reason for spacetime to be flat, except that we happen to like it. We know of no physical process which should restrict the curvature to be flat, so it seems obvious we should consider general spacetime curvatures and see what happens.

A final thought to consider on this: classically, we expect light to follow least time paths. This is how, for example, Snell's law is derived; light which refracts according to that rule will reach its destination faster than light following any other path. Maxwell's laws, which give rise to electromagnetic radiation like light, predict the same. In the absence of a good reason for light to suddenly behave differently when gravity is involved, doesn't positing that run against the spirit of Occam's Razor?

Light does change wavelength as it moves towards/away from a gravitational source, due to conservation of energy and the constancy of the speed of light.
 
J

Jerromy

Guest
Doesn't quantum foam sharpen Occam's razor? I care nothing for position when vectorial velocity is my focus. I yearn to conclude what Einstein assumed. Period.

Either spacetime is an integer grid and gravity attracts mass on its trip or spacetime is curved by mass and gravity attracts nothing but apples to Sir Issac's head.

Mathematically the difference is irrelevant. Idealogically the fundamentals of true particle physics relies on the distinction.

Inquiring minds want to know!?!?!?! Perhaps the shift in wavelength holds the logical key... does it shorten the wavelength on approach which would increase the energy towards gamma rays? That might make sense for the lack of gravitational attraction, because I would think gravity would stretch the wavelength longer (towards infrared) if it were pulling photons away from their normal wavelength. I'm out on a limb here shaking some apples loose but I'm guessing the approach would red-shift light. I think I just dumbfounded myself because I am not even sure if the wavelength describes the length of the path between waves or the width of the waves... just when you think you know too much you realize that you don't KNOW nearly enough!
 
L

Last89er

Guest
Light doesn't "bend". In optics atoms act as delay lines bringing photons into and out of phase with adjacent portions of the group. Modified spacetime also acts as delay lines.
 
R

ramparts

Guest
Jerromy":3pcd0i6z said:
Doesn't quantum foam sharpen Occam's razor? I care nothing for position when vectorial velocity is my focus. I yearn to conclude what Einstein assumed. Period.
Erm..... what?

Also, Occam's Razor is a guideline, not a law (I made sure to word that sentence somewhat carefully in my last post, in fact). It's tough to say that any physical discovery does much to affect it.

Either spacetime is an integer grid and gravity attracts mass on its trip or spacetime is curved by mass and gravity attracts nothing but apples to Sir Issac's head.

Mathematically the difference is irrelevant. Idealogically the fundamentals of true particle physics relies on the distinction.
I remain perplexed.

Inquiring minds want to know!?!?!?! Perhaps the shift in wavelength holds the logical key... does it shorten the wavelength on approach which would increase the energy towards gamma rays? That might make sense for the lack of gravitational attraction, because I would think gravity would stretch the wavelength longer (towards infrared) if it were pulling photons away from their normal wavelength. I'm out on a limb here shaking some apples loose but I'm guessing the approach would red-shift light. I think I just dumbfounded myself because I am not even sure if the wavelength describes the length of the path between waves or the width of the waves... just when you think you know too much you realize that you don't KNOW nearly enough!
Gravitational redshift - the shift in wavelength in a gravitational field - is an effect of general relativity. It would be a stretch to think it holds the "logical key" (to what, by the way?). The effect of gravity is actually pretty intuitive. If I shine a light outward from the surface of a gravitating body, the light will be redshifted (to longer wavelength), which you can think of as the gravity "pulling" on the light, stretching the wave out. Approaching a gravitational field, the light is blueshifted to shorter wavelengths.

More specifically, you can think of the effect as conservation of energy. As an object enters a gravitational field, it loses gravitational energy; since that energy can't disappear, it goes into the photon's energy. A photon's energy is inversely proportional to its wavelength, so the photon's gain in energy is the same as a shortening of its redshift. The reverse argument applies for redshift when leaving a gravitational field.

Wavelength is the length of a single complete wave "cycle" - from crest to crest, or trough to trough.
 
J

Jerromy

Guest
So the wave length of light varies as in the distance between waves in the ocean, not as in the height of the waves?
 
M

MeteorWayne

Guest
That correct. Wave-Length (1/frequency), not Wave-Amplitude.
 
A

Agelesslink

Guest
ok i have a question on this matter. ive recently watched an episode that touched on einstein's theory with spacetime. the word spacetime is almost impossible for my mind to perceive, but it was shown as a bowling ball put on a trampoline. later they explained how scientists tested this theory with the star that was shown behind the sun during the eclipse. in my mind im imagining a 2d grid as space and the bowling ball stretching the grid. the coordinates by the sun would be stretched. the beam of light from the star behind the sun would still have passed through the sun being blocked from earth. this almost seems like a bottleneck at the sun to me. it seems that it would be better described by splitting the y' coordinates in the middle and plopping a bowling ball in the empty space. does this not make sense. im trying to grasp this concept in laymans terms so don tbe harsh. ive seen what you guys do to dryson.lol
 
M

MeteorWayne

Guest
The beam of light from the stars would have been behind the sun (and moon) and been invisible the earth, but the path the light took made it appear at a shifted position.
 
M

Mee_n_Mac

Guest
Agelesslink":376xsom5 said:
ok i have a question on this matter. ive recently watched an episode that touched on einstein's theory with spacetime. the word spacetime is almost impossible for my mind to perceive, but it was shown as a bowling ball put on a trampoline. later they explained how scientists tested this theory with the star that was shown behind the sun during the eclipse. in my mind im imagining a 2d grid as space and the bowling ball stretching the grid. the coordinates by the sun would be stretched. the beam of light from the star behind the sun would still have passed through the sun being blocked from earth. this almost seems like a bottleneck at the sun to me. it seems that it would be better described by splitting the y' coordinates in the middle and plopping a bowling ball in the empty space. does this not make sense. im trying to grasp this concept in laymans terms so don tbe harsh. ive seen what you guys do to dryson.lol
No doubt you saw something like the depiction below. Alas, if you look at it very carefully, it's not a great visual as to what happens. The idea is that mass distorts spacetime and the 3D planet is distorting the 2D example of space(time). The grid pattern shows (not exactly) this distortion happening. The stars light rays/path do represent what happens but the grid pattern they've laid out to show the aforementioned distortion is inaccurate when compared with the light's path. Ideally the light's path should have the same geometric relationship to the grid pattern at any local grid location. The grid should show the same distortion that the light path takes. It doesn't and I've yet to see one that does (properly). But it's just supposed to be a 2D analogy to what happens in the real 3D world and in this case (trying to match the light's path to the grid) it's an analogy that gets stretched too far.

 
A

Agelesslink

Guest
thank you for a "REAL" answer to my question. i'm glad to see that someone understands what ive bee trying to say and according to the way grids work, it doesnt give a common sense view of how this works. fortunaely from actually explaining what happens to my wife my mind was able to slightly comprehend what happens, but i still can't understand how it works.lol of course i don't have a degree in physics or anything so thats to be expected.
 
Status
Not open for further replies.

ASK THE COMMUNITY

Latest posts