weight of light.

[dstebbins]

I was just watching the Science Channel. The show was called "Cosmos," starring Carl Sagan, and he made a comment that at a gravity level of 1 million g, light continues to travel in a straight line, but a 1 billion g, light begins to bend towards the source of the gravity, as happens with a black hole.

 

Correct me if I'm wrong, but I believe this would suggest that light actually has mass. I know that light is reffered to in the scientific community as a "wave-particle duality," but the particle part referres to a ball of energy, doesn't it? If light didn't have mass, then it shouldn't be affected by gravity.

 

Am I thinking right?

[GutZ]

From what I know about GR it's not that gravity affects light, but that gravity is due to space-time curvature by massive objects, and that the light follows that curvature (path).

 

http://en.wikipedia.org/wiki/General_relativity#Bending_of_light

 

But don't take my word for it. I know nothing.

[insane_alien]

well, technically the light is travelling in a straight line but the space its traveling through is curved to such an extent that we can actually see it deflect light beam. light in itself does not have mass only momentum. you could say it has relativistic mass as it has energy but has no rest mass which is the important bit for weight.

[Klaynos]

An interesting effect of this is gravity lensing:

 

http://en.wikipedia.org/wiki/Gravitational_lens

[GutZ]

High Five! I wasn't completely wrong.

[swansont]

If light didn't have mass, then it shouldn't be affected by gravity.

 

Am I thinking right?

 

The notion that gravity affects only mass and vice-versa was discarded (at the higher levels of physics) when Einstein came up with General Relativity. Mass and energy change the shape of space, and light follows a curved path. Most people aren't exposed to this concept, so your thinking is wrong because your knowledge is incomplete. From a Newtonian perspective, your thinking is valid, but from a relativistic one, it turns out that light is massless.

[dstebbins]

That can't be possible. If space and time is getting distorted by super-strong gravitational forces as you suggest, then as light curves to follow that distortion, it should appear perfectly straight to us. Keep in mind that the only thing that allows us to see with our eyes is light, so if light is bending to follow the dented path, so should our vision, and they should cancel out.

[blike]

That can't be possible. If space and time is getting distorted by super-strong gravitational forces as you suggest, then as light curves to follow that distortion, it should appear perfectly straight to us. Keep in mind that the only thing that allows us to see with our eyes is light, so if light is bending to follow the dented path, so should our vision, and they should cancel out.

Take a look at this picture to see how light is bending around the object in the center. That's the concept they're trying to convey.

[swansont]

That can't be possible. If space and time is getting distorted by super-strong gravitational forces as you suggest, then as light curves to follow that distortion, it should appear perfectly straight to us. Keep in mind that the only thing that allows us to see with our eyes is light, so if light is bending to follow the dented path, so should our vision, and they should cancel out.

 

It is possible, because it has been observed. Consider that the curvature is generally not uniform, i.e. it differs from place to place.

[GutZ]

I thought Gravity is created by the distortion of spacetime? Like the more massive the object the more warped spacetime becomes, having a stronger gravitational affect?

[ParanoiA]

I thought Gravity is created by the distortion of spacetime? Like the more massive the object the more warped spacetime becomes, having a stronger gravitational affect?

 

This is how I understand it too Gutz. And I believe that fits with what's been said so far. It's probably not quite right, but I think of it like mass squeezing itself into spacetime, creating the pressure or curvature of the localized space. Like taking a full bottle of water and forcing an ice cube in it without increasing the volume (I don't even know if that's possible, but it works in my noggin). In my mind, this creates pressure on the ice cube - the ice cube being mass, the water being space. Of course, the problem with that example is the pressure is distributed evenly in a bottle of water, whereas in space it is localized - at least I think that's true.

[swansont]

I thought Gravity is created by the distortion of spacetime? Like the more massive the object the more warped spacetime becomes, having a stronger gravitational affect?

 

Yes, we perceive the distortion as gravity.

[GutZ]

If space and time is getting distorted by super-strong gravitational forces as you suggest

 

Oh ok, I guess this confused me then.

[dstebbins]

It is possible, because it has been observed. Consider that the curvature is generally not uniform, i.e. it differs from place to place.

 

that's not the point. The point is that we use light to see, so if light is following a distorted path, then our vision should also follow a distorted path, and the two should cancel each other out.

 

Besides, if gravity is just the result of an object attempting to squeeze itself into "spacetime" as you call it, then where does the "force that pulls you towards the source of gravity" come into play? That's what gravity has always meant in laimen's terms, so are we just redifining gravity?

[RyanJ]

Besides, if gravity is just the result of an object attempting to squeeze itself into "spacetime" as you call it, then where does the "force that pulls you towards the source of gravity" come into play? That's what gravity has always meant in laimen's terms, so are we just redifining gravity?

 

What do you mean by " if gravity is just the result of an object attempting to squeeze itself into "spacetime""? If you are using an analogy like that the closest you could have would be it pushes spacetime away (even though I don't think it happens like that) creating a curve.

 

If you believe relativity its the distortion in space time its self. Anything within the influence of the distortion can feel its gravitational effects. If we are above and object and fall towards it we can be described as "sliding down the gravitational distortion in spacetime". Sorry, I'mnot very good at explaining things.

[dstebbins]

What do you mean by " if gravity is just the result of an object attempting to squeeze itself into "spacetime""? If you are using an analogy like that the closest you could have would be it pushes spacetime away (even though I don't think it happens like that) creating a curve.

 

If you believe relativity its the distortion in space time its self. Anything within the influence of the distortion can feel its gravitational effects. If we are above and object and fall towards it we can be described as "sliding down the gravitational distortion in spacetime". Sorry, I'mnot very good at explaining things.

 

And I can see why. Sorry for any offense, but those English skills need work.

 

But if the object isn't squeezing itself into spacetime, then what is causing this distortion?

[RyanJ]

Well technically an object is inside space time anyway - the 2D sheet representation sucks for explaining stuff like this.

 

If your referring too how an object actually distorts spacetime there are ideas that involve the wave-particle duality. Light can be a photon or an EM wave, in the same way gravity can be a gravity "wave" or a graviton which is the theoretical transmission particle for gravitational attraction. For a better explanation one of the physics guys will probably be able to give more details. Although the graviton has not been observed yet (or has it?) they believe it may exist

as a particle with 0 mass that travels at light speed like a photon (its also in the same category, Gauge bosons).

 

My English is very poor at the moment because I'm really tired

[timo]

that's not the point. The point is that we use light to see, so if light is following a distorted path, then our vision should also follow a distorted path, and the two should cancel each other out.

Why would the effect cancel out instead of doubling ? Ok, that was just a joke question but I hope my message that it´s very speculative to draw conclusions about the behaviour of an unknown/unfamiliar mechanism came through. In this case, I think your problem might stem from a misconception of how human vision works:

 

When I program a raytracing program (a type of program that visualizes how an observer would view a virtual 3D world), then for each pixel on the screen I shoot an appropriate ray from the observer's position through the world. At some point, the ray will collide with some object in the 3D world. I figure out the color at this point of collision and paint the pixel accordingly. That is an easy way to determine what an observer within a world sees but it's not how it physically works. Your eyes don´t emit light rays that scan through the area. Light from the surrounding falls into your eyes. Construction of the image is done by your brain which usually -in lack of better information and usually also correctly-so- assumes that the light ray reached your eye in a straight line. That can lead you to see object at the wrong position.

You get a simple example of seeing things in incorrect position when you take a look into a mirror: The light emitted by you is reflected in the mirror and arrives at your eye - that is the part of the process where light is bent. But in the second step, the reconstruction of the image by your brain, there is no bending effect that undoes the previous bending. Your brain assumes the light ray having travelled in a straight line and it therefore reconstructs a picture in which you are standing inside/behind the mirror.

In short: Your vision does not correct for effects of light bending.

 

Let´s get the previously-said back onto the topic of light being bent by gravity and extend the scope to the question how you acutally figure out that light is bent. I´ll do so using the image below; it´s the result of a numerical calculation enhaced by my extraordinary skills with windows paint:

 

 

The bending of light in a gravitational field can be observed when a ray of light passes close by a large mass. Let´s stay to the lower guy first. The light from the object you want to observe (the red ball) is emitted, bent on a massive object (a black hole here, but anything does in theory) and hits your eye. You brain, not knowing about the black hole (it´s black, you can´t see it ) thinks the ray came in straight and reconstructs the object at a different position (green dot).

Ok, so far you know that you´ll see an object in an incorrect position when the light emitted by it is bent on its way to your eye. That doesn´t yet tell you how you can ever figure out if light bending really exists (for most objects being observed actually travelling there to check out the real position is not an opportunity). You can check the bending if the position of the bending object changes. The amount of bending is directly dependent on the distance of the light ray from the bending object. Look at the upper guy. The bending object (still the black hole from the lower guy) is farther away from the light ray than before. As you can see, for the upper guy the misconception about the red balls position is significantly smaller than for the lower guy. That´s the key to one method to verify light bending: Observe a star for which you know a massive object will soon cross the connection line between it and you. If you´re lucky, you can see that the appearent position of the star will change when the massive object passes by and restore to the original one after the massive object is gone again.

 

Enough for now, I´m already scared at the thought of proof-reading what I just wrote

 

Besides, if gravity is just the result of an object attempting to squeeze itself into "spacetime" as you call it, then where does the "force that pulls you towards the source of gravity" come into play? That's what gravity has always meant in laimen's terms, so are we just redifining gravity?

I find it hard to give a definite answer here because every easy thing I could say here without going deep into detail would be wrong to some extend. Let´s say it this way: In modern physics forces play a much lesser role than in Newtonian physics (including Newtonian gravity). Conceptually, the important feature in GR is the geometry of spacetime. Forces felt by the objects as a result of this do in some sense exist, but they usually play a minor role. In short: Yes, we´re redifining gravity (whatever that is ...).

[swansont]

that's not the point. The point is that we use light to see, so if light is following a distorted path, then our vision should also follow a distorted path, and the two should cancel each other out.

 

Atheist has already shown the diagram; we can see the effect because (as I stated earlier) the effect varies in space. Similar to why one sees a stick bend when it is put in water — the refraction occurs because of a change in the index of refraction. No change in index, no effect.

 

Besides, if gravity is just the result of an object attempting to squeeze itself into "spacetime" as you call it, then where does the "force that pulls you towards the source of gravity" come into play? That's what gravity has always meant in laimen's terms, so are we just redifining gravity?

 

Laymen's terms are known to not jibe with scientific terms on many occasions. But which do you think are right, and which are the watered-down approximation? As I explained earlier, most people don't learn about general relativity. Gravity has been redefined for ~90 years now, but that affects a really small fraction of people. If you want to learn about it, great, but as with quantum mechanics, you will need to discard the notion that your Newtonian intuition will apply.