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banquo's_bumble_puppy
2015-Aug-11, 11:51 PM
Does anything other than gravity bend light? Actually...mirrors do? I think that's wrong...they deflect light. Am I wrong? I'm sure there is something else.

danscope
2015-Aug-12, 12:07 AM
Perhaps a prism ??? :)

Ken G
2015-Aug-12, 12:09 AM

Buttercup
2015-Aug-12, 12:54 AM
Does water?

cjameshuff
2015-Aug-12, 01:11 AM
Transparent objects and gases will refract light, as will an opaque object with the right pattern of holes, slits, etc:
https://en.wikipedia.org/wiki/Zone_plate
http://www.dtic.mil/dtic/tr/fulltext/u2/a531869.pdf

This refraction is dependent on wavelength (https://en.wikipedia.org/wiki/Dispersion_(optics)), though, unlike gravitational deflection.

schlaugh
2015-Aug-12, 01:51 AM
Strictly speaking it's not gravity that bends the light but the distortion of space-time caused by gravity. The light continues to follow a "straight" line, although the gravitational well has bent the space around itself. Hence the light simply follows space.

Is this not the case?

Noclevername
2015-Aug-12, 01:53 AM
Strictly speaking it's not gravity that bends the light but the distortion of space-time caused by gravity. The light continues to follow a "straight" line, although the gravitational well has bent the space around itself. Hence the light simply follows space.

Is this not the case?

Assuming that gravity is something separate from the curvature of spacetime caused by mass. Which is still an open question IIRC.

Squink
2015-Aug-12, 02:22 AM
Refractive index (https://en.wikipedia.org/wiki/Refractive_index)
Reflection (https://en.wikipedia.org/wiki/Reflection_%28physics%29)

schlaugh
2015-Aug-12, 02:45 AM
Assuming that gravity is something separate from the curvature of spacetime caused by mass. Which is still an open question IIRC.

Arrgh...yes, should have said mass is distorting ST, not gravity.

Jens
2015-Aug-12, 04:02 AM
Does anything other than gravity bend light? Actually...mirrors do? I think that's wrong...they deflect light. Am I wrong? I'm sure there is something else.

By perhaps naive answer would be "no." Gravity bends light, as schlaugh wrote, by distorting space, not by actually doing anything to the photons. But with the other processes, like refraction, what's actually happening is that the photon is being absorbed by an atom and then being re-emitted in a different direction. So it's not really being "bent" in the same sense.

Ken G
2015-Aug-12, 05:31 AM
It's hard to say, it's a pretty similar process. You have interference between a bunch of different ways the photon can get from point A to point B, and the path of constructive interference is the path the photon takes. If you could ever say "which atom" absorbed the photon in a lens, the refraction wouldn't work, so it's problematical to claim refraction happens by absorption and re-emission. You could probably imagine there is some kind of virtual absorption and re-emission going on, but a sum over interfering virtual processes is always one way to picture any type of wave/particle duality, including the wave/particle duality of gravitational lensing. It's true that there's a null geodesic with GR that is not there with an ordinary lens, but the reasons a photon follows a null geodesic can be viewed as pretty similar to why it follows a curved path in your eye-- in both cases that's the path of constructive interference.

Trebuchet
2015-Aug-12, 04:52 PM
Does water?

Certainly, as does glass. And they do it differently for different wavelengths. Hence, rainbows!

Amber Robot
2015-Aug-12, 05:08 PM
But with the other processes, like refraction, what's actually happening is that the photon is being absorbed by an atom and then being re-emitted in a different direction.

This depends on what you mean by "absorb", and also, since the "bending" in refraction happens only at the interface between two materials, it's not really an atom 'absorbing' and re-emitting in a different direction unless you can specify what is peculiar about the atoms at the surface of a material versus atoms in the bulk material. I think Ken G has it right below in that refraction is more of an interference effect than a corpuscular one.

Darrell
2015-Aug-12, 06:03 PM
I thought that refraction was due to the speed of light being different in different media. That the change in the speed of the wave at the boundary between two different media causes the effect of a change in direction? This effect can be demonstrated mechanically at macro scales, i.e. a change in speed at an interface causing a change in direction. And that both the speed and the wavelength change. Also, the one case where a change in speed at a boundary doesn't result in a change in direction is when the direction of the wave is perpendicular to the boundary.

And of course there are other effects that can occur at boundaries, like reflection and diffraction.

Jeff Root
2015-Aug-12, 06:54 PM
Strictly speaking it's not gravity that bends the light but
the distortion of space-time caused by gravity.
I say that mass-energy distorts spacetime, and the
distortion is gravity.

I also say that gravity bends the light's path, rather
than bending the light. :)

-- Jeff, in Minneapolis

Ken G
2015-Aug-12, 07:10 PM
I thought that refraction was due to the speed of light being different in different media. That the change in the speed of the wave at the boundary between two different media causes the effect of a change in direction?Yes, in the approximation that the boundary is sharp. Or one can imagine a gradually changing index of refraction, yielding a gradually changing speed of light, and that will bend the light gradually.

One can view gravitational lensing the same way, where the coordinate speed of light can be viewed as changing, causing light to bend. Or you can say what Jeff Root said, that the "straight" path is itself being bent, and all the light is doing is having the usual interference effects cause the photon to follow the new straight path.

Solfe
2015-Aug-12, 07:11 PM
I don't know if this is light bending, but it is cool.

Mineral oil and pyrex have close to the same visual qualities. One mind bending trick is to pour the oil into a pyrex container, as soon as the oil passes the rim you can't see it. Then take a pyrex rod and stir. Of course, the rod vanishes as soon as it enters the oil. Adding an oil based paint causes the oil and rod to reappear in a crazy swirl of color.

malaidas
2015-Aug-13, 08:11 AM
Yes the disappearing glass trick is cool. The light bending is all typical refraction IIRC though, so there is nothing special going on, except that the two have the same refractive index causing the glass to be invisible.

banquo's_bumble_puppy
2015-Aug-13, 10:47 AM
What I meant to ask or should have asked: are there any forces in nature that can bend light in the sense that strong gravitational force can "bend" light? I put quotation marks around bend. With the understanding that gravity doesn't actually bend light, but distorts space. So...is there anything that distorts space other than gravity? Are there forces that have been conjectured that can distort space?

banquo's_bumble_puppy
2015-Aug-13, 11:44 AM
Would dark gravity be one possibility?

cjameshuff
2015-Aug-13, 12:22 PM
What I meant to ask or should have asked: are there any forces in nature that can bend light in the sense that strong gravitational force can "bend" light? I put quotation marks around bend. With the understanding that gravity doesn't actually bend light, but distorts space. So...is there anything that distorts space other than gravity? Are there forces that have been conjectured that can distort space?

As we currently see it, gravity is a distortion of space-time, so that question doesn't make sense.

Would dark gravity be one possibility?

"Dark gravity"?

banquo's_bumble_puppy
2015-Aug-13, 01:06 PM
Anti-gravity? The question is: might there be other forces that could distort space and change the path of light? Maybe my question is nonsense...

Grey
2015-Aug-13, 03:11 PM
Anti-gravity? The question is: might there be other forces that could distort space and change the path of light? Maybe my question is nonsense...In principle, probably, though I'm not aware of any evidence that there actually is such a thing. But there's a semantics issue at work here, too: at this point, anything that distorts spacetime like gravity does would probably just be considered gravity. That is, if we discovered something like that, we might have to modify Einstein's field equations to take it into account, but we'd probably just call that a change in our understanding of how gravity works, rather than calling it a separate force.

It would probably be reasonable to call dark energy just such a modification to gravity, but since dark energy in particular seems to be uniform throughout space, I think it would be specifically not able to cause light to bend.

schlaugh
2015-Aug-13, 03:47 PM
Would dark gravity be one possibility?
Or did you mean dark matter which exhibits gravitational lensing?

JohnD
2015-Aug-13, 05:41 PM
Does a mirror 'bend' light?
Is the reflected beam made up of the same photons? Or retransmitted ones, with the same properties?
John

Jeff Root
2015-Aug-13, 05:51 PM
What Grey said.

I expected Grey or Ken to explain general relativity. :)

General relativity is a theory which describes the relationships
between space, time, and matter. It describes the effect of
matter on spacetime as a distortion, or warping, and the
distortion or warping is detected as gravity. Physicist John
Wheeler expressed the relationship this way: "Spacetime tells
matter how to move; matter tells spacetime how to curve."

The equation which defines the relation quantitatively can be
seen in the box on the right-hand side near the top of this

https://en.wikipedia.org/wiki/Introduction_to_general_relativity

That equation replaces Newton's law of gravity. The two give
essentially identical results in situations where the energies,
forces, and relative speeds are low. Newton's law contains
only mass (m), distance (r), and the constant G as factors in
gravitational field strength. Einstein's equation takes into
account mass, energy, momentum, and stress, the last of
which may be further decomposed into pressure and tension.
These factors comprise the "stress-energy–momentum tensor".
In Einstein's equation, they define the amount of curvature of
spacetime, which is effectively equivalent to the Newtonian
gravitational field strength.

"Geometry and gravitation", which includes the subsections
"Sources of gravity" and "Einstein's equations".

-- Jeff, in Minneapolis
.

Darrell
2015-Aug-13, 06:22 PM
Does a mirror 'bend' light?
Is the reflected beam made up of the same photons? Or retransmitted ones, with the same properties?
John

I am not sure there is a good answer to that. The photons interact with electrons in the surface of the reflector. I know "absorbed" and "retransmitted" are often used to describe that, but I think "interact" is a better word to use. The properties of the photons are changed by the interaction, namely their momentum.

Also, an analogy, if some molecules of water are absorbed by a sponge and then released from it, are they the same water molecules?

malaidas
2015-Aug-13, 06:47 PM
Nice analogy. My question about it would go further ;) do the photons even exist in between the two events?

The answer to which cuts to the very heart of what we mean by photon

Eta: if you think this is a silly question, then consider the following, a particle adsorbs a photon gaining energy, it then undergoes a particle interaction with a different transferon, yielding that energy. Was it the same energy being transferred? How can we possibly distinguish such.? So it's not even just a matter of not being able to tell photons apart.

banquo's_bumble_puppy
2015-Aug-13, 07:27 PM
Or did you mean dark matter which exhibits gravitational lensing?

It does? Then you answered my question...I could ask what dark matter is...tbd...to be determined I think.

Jeff Root
2015-Aug-13, 08:05 PM
Practically the only thing we think we know about dark matter is
that it has the same gravitational effects as any other matter.

-- Jeff, in Minneapolis

Ken G
2015-Aug-13, 10:31 PM
Nice analogy. My question about it would go further ;) do the photons even exist in between the two events?

The answer to which cuts to the very heart of what we mean by photonI'd say that a useful principle in the quantum realm is "the question that is not posed in the apparatus is also not answered in that apparatus." So if there is nothing in the apparatus to establish that the photon existed in the "in between" realm, then it cannot be said to have existed in there.

malaidas
2015-Aug-13, 11:37 PM
I'd say that a useful principle in the quantum realm is "the question that is not posed in the apparatus is also not answered in that apparatus." So if there is nothing in the apparatus to establish that the photon existed in the "in between" realm, then it cannot be said to have existed in there.

I knew you would catch my meaning Ken.

Darrell
2015-Aug-14, 12:27 PM
Actually, I think that works in any realm. Well, except in the realm of imagination.

Ken G
2015-Aug-14, 01:39 PM
Yes that's true, it's just the quantum realm where you run into the possibility of not posing various questions so you have to think about what you want to know-- in most other situations, there's something going on that poses the questions for you!

Grey
2015-Aug-14, 04:20 PM
Well, and there's an important difference. You can model (say) the trajectory of a thrown baseball, and you can assume that it exists and follows a definite path, even if part of that path is through a region where it's not directly observed. If you do that, you get all the right answers about how the baseball is moving, so it doesn't cause any problems to assume the baseball is still really there if it's not being observed. However, if you try to make those same kinds of assumptions for a photon (that it's a real physical object following a single definite path, and that it has a definite location even when not observed), you get the wrong answers for how the photon behaves.

When your apparatus isn't set up to observe a baseball's position for part of its trajectory, the baseball behaves like you don't know where it is, which doesn't actually affect the way the baseball behaves at all. When your apparatus isn't set up to observe a photon's position for part of its trajectory, the photon behaves like it doesn't even have a definite position or trajectory in that region, which can have a significant effect on the behavior of the photon.

Ken G
2015-Aug-14, 05:46 PM
But the difference is, for the baseball, there are other things going on in the "apparatus", writ large, than just the things that the physicist has decided to measure. There could be air molecules bouncing off the baseball, but even if the baseball is moving through a vacuum, there are internal modes of behavior that "decohere" the things you don't know about the baseball, ruining their ability to interfere and change observable outcomes. One has to go back to the preparation of the baseball, to find all the things going on "in the apparatus" that we are not explicitly tracking, but which we could have tracked without changing anything-- so that is what we might call "classical" absence of information (by which I mean, information that is there but we have simply chosen not to take into account). The quantum realm is where you have more "intrinsic" absence of information, which is what I mean by "the question not posed." So my statement above takes a very inclusive meaning of "apparatus"-- more akin to a notion of "reality." (In fact, I originally had the word "reality" in place of "apparatus", but I decided to go with the more scientific sounding word!)

(Incidentally, there is an interpretation of quantum mechanics that would dispute my dichotomy of classical and intrinsic absence of information-- the Bohmian interpretation. That approach claims that all absence of information is always classical, so even in a quantum system it is still classical absence of information about the preparation that is creating the illusion of uncertainty in the outcome. In that interpretation, the photon always does exist, even when there is nothing in the apparatus that poses the question, because there was already something in the preparation of the photon that posed that question, we just didn't track the preparation well enough to establish the photon existence in those "in betweens." But the more standard interpretation is that quantum uncertainty is intrinsic, so the state is prepared with certain indeterminacies that are simply never resolved, unless some later interaction in the apparatus resolves them.)

JohnD
2015-Aug-14, 08:49 PM
As an analogy to reflected photons, the water absorbed by a sponge and then squeezed out again is not good!
First of all, the water is absorbed by capillary action, unless the sponge is squeezed first and then allowed to expand in the water, so the process uses energy.
I don't think reflection does.

Then, the water molecules that come out when the wet sponge is squeezed are clearly the same as those that were absorbed.
Unless it's a Magic Sponge, that can transmute atoms and molecules into identical atoms and molecules!

John

Darrell
2015-Aug-14, 10:19 PM
I agree, that is not a good analogy. I'll attempt to relate what I was trying to convey more directly. The question "are they the same photons" can not be answered definitively at present. The only reasonable answer is "don't know," or possibly "what does that mean(?)." The question may not even make sense in the context of quantum level phenomena. It may be one of those questions that afford much philosophical exercise, but never a definitive, concrete answer.

malaidas
2015-Aug-15, 09:27 AM
But the difference is, for the baseball, there are other things going on in the "apparatus", writ large, than just the things that the physicist has decided to measure. There could be air molecules bouncing off the baseball, but even if the baseball is moving through a vacuum, there are internal modes of behavior that "decohere" the things you don't know about the baseball, ruining their ability to interfere and change observable outcomes. One has to go back to the preparation of the baseball, to find all the things going on "in the apparatus" that we are not explicitly tracking, but which we could have tracked without changing anything-- so that is what we might call "classical" absence of information (by which I mean, information that is there but we have simply chosen not to take into account). The quantum realm is where you have more "intrinsic" absence of information, which is what I mean by "the question not posed." So my statement above takes a very inclusive meaning of "apparatus"-- more akin to a notion of "reality." (In fact, I originally had the word "reality" in place of "apparatus", but I decided to go with the more scientific sounding word!)

(Incidentally, there is an interpretation of quantum mechanics that would dispute my dichotomy of classical and intrinsic absence of information-- the Bohmian interpretation. That approach claims that all absence of information is always classical, so even in a quantum system it is still classical absence of information about the preparation that is creating the illusion of uncertainty in the outcome. In that interpretation, the photon always does exist, even when there is nothing in the apparatus that poses the question, because there was already something in the preparation of the photon that posed that question, we just didn't track the preparation well enough to establish the photon existence in those "in betweens." But the more standard interpretation is that quantum uncertainty is intrinsic, so the state is prepared with certain indeterminacies that are simply never resolved, unless some later interaction in the apparatus resolves them.)

And yet even if the bohmian interpretation were true, we would still be stuck not just because we cannot detect such a photon, but also because we don't know what such are outside of the observable properties we used to theorize their existence. we can only talk about photons in the context of electro magnetic radiation. They simply dont occur within our models except in this way. It simply doesn't make coherent sense to describe the number of photons another particle contains for example. Rather we consider how much energy that particle has, which is increased or decreased through different interactions which are described using transfer particles, such as a photon. So even with a classical approach, we are still left with the existence of the photon in between times as reasonably uncertain, not because the substance, )let's call it energy for sake of a placeholder) ceases to exist, but because we dont know what form it is. It is reasonable to assert as with the baseball that so long as it is unbound that it exists even when we are not observing it, in this interpretation but I would say its not reasonable within the in between state. Of course as you say the more typical interpretations don't even allow this.

Eta: as a quick thought experiment let's consider another major issue with the idea of the photons continued existence. A particle adsorbs a photon with energy n later on it emits 2 photons with energy n/2. Returning it to its original state. Where did the second photon come from? And yet we see this basic set of events happening all around us, every day.

Ken G
2015-Aug-16, 12:05 AM
And yet even if the bohmian interpretation were true, we would still be stuck not just because we cannot detect such a photon, but also because we don't know what such are outside of the observable properties we used to theorize their existence. If we take the Bohmian interpretation as true, then we can assert the photon does exist "in between", simply because that's what that interpretation says. But we cannot know that interpretation is true-- the only way to check if something is true is if we can set up an experiment capable of saying if it is true, and if we do that, we cannot falsify the claim that "the question not posed is also not answered" because we are having to make sure the question is indeed posed in order to test its answer. So we are still "stuck," as you say-- we are left with having to make a choice about what to believe, that we cannot demonstrate, or else we are forced to accept that we must adopt a stance of agnosticism on any question whose answer makes no predictions one way or the other. The way I would put this is, one of the most important things science does is figure out how to make questions answerable, but any question that we cannot figure out how to make answerable must not be regarded as a scientific question until such a time as we can make it answerable.

Eta: as a quick thought experiment let's consider another major issue with the idea of the photons continued existence. A particle adsorbs a photon with energy n later on it emits 2 photons with energy n/2. Returning it to its original state. Where did the second photon come from? And yet we see this basic set of events happening all around us, every day.That's not really the "in between" I was talking about, I was thinking more in terms of a two-slit experiment, where you fire a photon toward two slits and detect it at the other end, and it's not clear that it can be said to exist in between because it cannot be said that it goes through one slit or the other.

malaidas
2015-Aug-18, 09:18 AM
Yes, I guess we are coming at the question from slightly different angles, but I also think it comes down in both cases to just what we mean by photon and of course just what we mean by exist.

Ken G
2015-Aug-18, 03:24 PM
Yes, that is very true. We always want to use regular language when talking about what is going on, because part of understanding is being able to make contact with shared experiences. This is the fundamental problem in quantum mechanics, because our shared experiences are all at the classical level, and quantum mechanics is fundamentally non-classical. So this brings us to the various interpretations of quantum mechanics, which seek to solve this problem from very different approaches. This is how some of the main interpretations would address the question of existence in the slits:

Copenhagen: this is a highly empiricist approach, meaning that we can only assert as true that which we can experimentally demonstrate as true. Indeterminism is thus treated agnostically in this approach. This is often viewed as the default interpretation.

Many worlds: this is a highly rationalistic approach, which asserts that the mathematics is the reality. So we do not say the particle goes through the slits, we say the particle is in a superposition of classical states, and only those classical states go through the slits-- the particle state is a superposition. Modifications to the experiment could establish the classical states as real, but then the experiment in some sense branches into many different worlds-- each different world houses one of the classical outcomes, but the "true state" of the universe is still a superposition of all these "worlds." This means the "true state" of the particle is to be not only in a superposition of existing in the slits, but also a superposition of never having existed at all!

Bohmian: this approach is the most classical. It maintains that our goal as physicists is to put everything that happens into a language we understand, and that means a classical language. So we have no choice but to interpret the events as being classical. Hence particles always follow perfectly classical trajectories, and the particle goes through one slit or the other, never both at once. Indeterminacy is handled by a combination of burying it into unknown aspects of the preparation of the particle, and the dynamical evolution of the indeterminacy is handled by a kind of ephemeral causative field called the "pilot wave" that is never itself observed, and travels both forward and backward in time. To some, that seems like a lot of trouble to go to in order to allow classical pictures to prevail, but others feel that classical pictures are required in order to have language we actually understand.

So we see that the three main interpretations have totally different ways of answering "what does the particle do in a two-slit experiment." All three are consistent with the outcomes of the two-slit experiment. I think this puts in great jeopardy Jeff Root's as yet unsupported claim that "the evidence is clearly and unambiguously overwhelming" that the particle must exist in one slit or the other or both, in order for us to correctly describe the outcome of a two-slit experiment. Indeed, we now see that this statement is not in the spirit of any of those three main interpretations! It's not Copenhagen, because it's not agnostic about the particle existence. It's not Many-Wolds, because it rules out that the true state of the particle includes a superposition of states in which the particle does not exist anywhere. And it's not Bohmian, because with Bohm, you can also rule out "both" slits.

This really brings up how badly massacred gets the key concept of "evidence" in this forum-- all too often, the term seems to be taken to mean nothing more than "whatever I believe to be true, and no one can prove me wrong, so that's my evidence."

Jeff Root
2015-Aug-18, 06:17 PM
I agree that my description of what happens to photons in
the two-slit experiment does not match any of your three
alternative descriptions. I would contend that the main
reason it doesn't match any of them is that mine is more
general, not attempting to specify where the photon is or
which slit it passes through, or whether it behaves more
like a particle or more like a wave at any particular time,
but merely that it clearly does pass through the slits and
therefore exists as it does so.

-- Jeff, in Minneapolis

Ken G
2015-Aug-18, 06:47 PM
I agree that my description of what happens to photons in
the two-slit experiment does not match any of your three
alternative descriptions. I would contend that the main
reason it doesn't match any of them is that mine is more
general, not attempting to specify where the photon is or
which slit it passes through, or whether it behaves more
like a particle or more like a wave at any particular time,
but merely that it clearly does pass through the slits and
therefore exists as it does so.

-- Jeff, in Minneapolis
[By the way, I see that I have crossed this thread with the "two-slit" thread, because now we are onto similar topics in both threads. It's off topic for this thread, so perhaps the mods can pull all the stuff about quantum interpretations out of this thread. My apologies for accidentally crossing the threads.]

Going back to the discussion about the two-slit experiment, it is simply untrue that your statement is more general, because your statement is flatly contradicted by both the Copenhagen interpretation and the many-worlds interpretation. It is only more general than the Bohmian interpretation, and if one takes that interpretation, the more specific statement made there is of central importance. What this means is, no proponent of any of those interpretations would find value in the way you framed the situation, which certainly contradicts your claim that your framing is "unambiguously true."

malaidas
2015-Aug-19, 06:55 AM
Yes I wasn't meaning for a massive discussion on the nature of quantum physics when I extended darrels analogy. But I brought it up, so I'll go back to what I was originally agreeing with, which is that reflection is not really an example of bending light if it involves the adsorbtion and subsequent emission of a photon because we cannot say that the photon emitted is the same as the one adsorbed.