Light completely stopped for a record-breaking minute(newscientist.com)
newscientist.com
Light completely stopped for a record-breaking minute
http://www.newscientist.com/article/dn23925-light-completely-stopped-for-a-recordbreaking-minute.html?cmpid=RSS%7CNSNS%7C2012-GLOBAL%7Conline-news#.UfEVLFM6dEl
17 comments
"Thus continuing my inability to understand [...]"
You will enjoy this beautiful lecture given by Dr. Feynman entitled, "Probability and Uncertainty - The Quantum Mechanical View of Nature." [1] In this lecture, he describes the double-slit experiment, which is tied to the topic of quantum superposition.
This particular section blows me away every time:
"Here are the circumstances: source, strong light source; tell me, behind which hole will I see the electron? You say, 'Well, the reason you can't tell through which hole you're going to see the electron is, it's determined by some very complicated things back here: if I knew enough about that electron - it has internal wheels, internal gears, and so forth - and that this is what determines through which hole it goes. It's 50/50 probability because, like a die, it's set sort of at random - and if I were to have studied it carefully enough, your physics is incomplete: if you get a complete enough physics, then you'll be able to predict through which hole it goes.'
That's the 'hidden variable' theory, so called.
Well, that's not possible.
It is not due to a lack of detailed knowledge that we cannot make a prediction, because I said that if I didn't turn on the light, I should get this interference pattern.
If I have a circumstance in which I get that interference pattern, then it is impossible to analyze it in terms of saying, it goes through here or here, because that curve is so simple, mathematically - a different thing than the contribution of this and this as probabilities.
So if it were possible for you to have determined through which hole it was going to go if I had the light on, the fact that I had the light on hasn't got anything to do with it! Whatever gears there are back here that you observe, which permitted you to tell me whether is was going to go through 1 or 2, you could have observed if I had the light off.
And therefore you could have told me with the light off which hole - each time an electron goes - which hole it's going to go through.
But if you can do this, then that curve would have to be represented as the sum of those that go through there and those that go through there - and it ain't.
Therefore, it's impossible to have information ahead of time as to which hole it's going to go through when the light is out - or when the light is on, or out - in a circumstance where the experiment is set up that can produce this interference pattern.
It is not a lack of unknown gears - a lack of internal complications - that makes nature have probability in it; it seems to be in some sense intrinsic.
Someone has said it this way: 'nature herself doesn't know which way the electron is going to go.' A philosopher once said (a pompous one): 'it is necessary for the very existence of science that the same conditions always produce the same result.' Well, they don't: if you set up electrons in any way - I mean, you set up the circumstance here, in the same conditions every time, and you cannot predict behind which hole you'll see the electron.
They don't - and yet the science goes on in spite of him."
[1] https://www.youtube.com/watch?v=OCFX_NHBefI
You will enjoy this beautiful lecture given by Dr. Feynman entitled, "Probability and Uncertainty - The Quantum Mechanical View of Nature." [1] In this lecture, he describes the double-slit experiment, which is tied to the topic of quantum superposition.
This particular section blows me away every time:
"Here are the circumstances: source, strong light source; tell me, behind which hole will I see the electron? You say, 'Well, the reason you can't tell through which hole you're going to see the electron is, it's determined by some very complicated things back here: if I knew enough about that electron - it has internal wheels, internal gears, and so forth - and that this is what determines through which hole it goes. It's 50/50 probability because, like a die, it's set sort of at random - and if I were to have studied it carefully enough, your physics is incomplete: if you get a complete enough physics, then you'll be able to predict through which hole it goes.'
That's the 'hidden variable' theory, so called.
Well, that's not possible.
It is not due to a lack of detailed knowledge that we cannot make a prediction, because I said that if I didn't turn on the light, I should get this interference pattern.
If I have a circumstance in which I get that interference pattern, then it is impossible to analyze it in terms of saying, it goes through here or here, because that curve is so simple, mathematically - a different thing than the contribution of this and this as probabilities.
So if it were possible for you to have determined through which hole it was going to go if I had the light on, the fact that I had the light on hasn't got anything to do with it! Whatever gears there are back here that you observe, which permitted you to tell me whether is was going to go through 1 or 2, you could have observed if I had the light off.
And therefore you could have told me with the light off which hole - each time an electron goes - which hole it's going to go through.
But if you can do this, then that curve would have to be represented as the sum of those that go through there and those that go through there - and it ain't.
Therefore, it's impossible to have information ahead of time as to which hole it's going to go through when the light is out - or when the light is on, or out - in a circumstance where the experiment is set up that can produce this interference pattern.
It is not a lack of unknown gears - a lack of internal complications - that makes nature have probability in it; it seems to be in some sense intrinsic.
Someone has said it this way: 'nature herself doesn't know which way the electron is going to go.' A philosopher once said (a pompous one): 'it is necessary for the very existence of science that the same conditions always produce the same result.' Well, they don't: if you set up electrons in any way - I mean, you set up the circumstance here, in the same conditions every time, and you cannot predict behind which hole you'll see the electron.
They don't - and yet the science goes on in spite of him."
[1] https://www.youtube.com/watch?v=OCFX_NHBefI
This is only true for local hidden variables. It is possible to explain this with hidden variables in broader sense. Which you could also call hidden connections.
"This is only true for local hidden variables. It is possible to explain this with hidden variables in broader sense. Which you could also call hidden connections."
Can you explain further?
The reason I ask is that, it would appear quite sound and logical (based on our knowledge of discrete objects in the real world) to say that the electron goes through either slit 1 or slit 2 in all cases, but this is not accurate.
The reason is this:
When an observation is made, then it can be determined which slit the electron went through; in this case, it always goes through one slit or the other.
However, this observation is impossible to make without simultaneously disturbing the electron and destroying the interference pattern; that is to say, the resulting arrival distribution curve is not equal to the arrival distribution curve that is generated when no observations are made.
Therefore, how can it be determined which slit the electron will go through, while at the same time, not destroying the interference pattern?
To me, this is one of the "mystery of mysteries" of our universe.
Can you explain further?
The reason I ask is that, it would appear quite sound and logical (based on our knowledge of discrete objects in the real world) to say that the electron goes through either slit 1 or slit 2 in all cases, but this is not accurate.
The reason is this:
When an observation is made, then it can be determined which slit the electron went through; in this case, it always goes through one slit or the other.
However, this observation is impossible to make without simultaneously disturbing the electron and destroying the interference pattern; that is to say, the resulting arrival distribution curve is not equal to the arrival distribution curve that is generated when no observations are made.
Therefore, how can it be determined which slit the electron will go through, while at the same time, not destroying the interference pattern?
To me, this is one of the "mystery of mysteries" of our universe.
There is a perfectly good theory which allows one to have electrons just going through one slit at a time. It has a few names: de Broglie-Bohm theory/pilot-wave theory/Bohmian mechanics. The idea is that we have a wave AND a particle. The wave goes through both slits and interferes. The particle travels along the wave, like some wood on the ocean.
Unlike an ocean wave, the quantum wave exists in 3N dimensional space where N is the number of particles in the universe (fundamentally) or the number of relevant particles in an experiment (practically). When an observation is made of which slit it went through, the environmental variables get entangled with the experimental system and separate the two branches of the wave functions (kind of like 2d waves that become separated vertically) and thus they can no longer interfere. The particle, always there and traveling along the wave, no longer is on a wave with an interference pattern. The system is no longer isolated from the environment and the behavior changes as a result.
The weirdness is not the hidden variable (position of the particle!), but rather the quantum wave function. It lives on configuration space, meaning that, in theory, it uses the configuration of all the particles of the universe at a single time. This somewhat contradicts relativity. Bell, once he saw Bohm's theory, realized that nonlocality is the central issue. He tried to do better. He failed and then proved that nature itself is simply nonlocal. No one likes it, but that is simply the way nature is. Relativity lost to quantum nonlocality.
As for randomness, one can prove in Bohm's theory that it is impossible to know the precise location of a particle. The best we can do, theoretically regardless of technology, is the quantum mechanical rule of psi-squared probability.
The theory has global existence and uniqueness of solutions which classical mechanics does not even have. It agrees completely with standard quantum predictions. It is not very magical. It can even be derived more easily than the Schrodinger equation itself. In fact, all one has to do is assume that one has particles with positions and the guiding equation can be derived from at least five entirely different points of view.
Unlike an ocean wave, the quantum wave exists in 3N dimensional space where N is the number of particles in the universe (fundamentally) or the number of relevant particles in an experiment (practically). When an observation is made of which slit it went through, the environmental variables get entangled with the experimental system and separate the two branches of the wave functions (kind of like 2d waves that become separated vertically) and thus they can no longer interfere. The particle, always there and traveling along the wave, no longer is on a wave with an interference pattern. The system is no longer isolated from the environment and the behavior changes as a result.
The weirdness is not the hidden variable (position of the particle!), but rather the quantum wave function. It lives on configuration space, meaning that, in theory, it uses the configuration of all the particles of the universe at a single time. This somewhat contradicts relativity. Bell, once he saw Bohm's theory, realized that nonlocality is the central issue. He tried to do better. He failed and then proved that nature itself is simply nonlocal. No one likes it, but that is simply the way nature is. Relativity lost to quantum nonlocality.
As for randomness, one can prove in Bohm's theory that it is impossible to know the precise location of a particle. The best we can do, theoretically regardless of technology, is the quantum mechanical rule of psi-squared probability.
The theory has global existence and uniqueness of solutions which classical mechanics does not even have. It agrees completely with standard quantum predictions. It is not very magical. It can even be derived more easily than the Schrodinger equation itself. In fact, all one has to do is assume that one has particles with positions and the guiding equation can be derived from at least five entirely different points of view.
And could also call non-local (faster-than-light) magical thinking.
I'm glad you linked to Part 6. This is the greatest QM lecture ever given.
My layman's reading of this.
We used a crystal that is normally opaque (not see-through) and fired laser beams at it because, well... it seemed like fun! Apparently the laser beam affected the crystal and made it see-through but only for a narrow range of light frequencies (e.g. think of a filter that only allows one colour of light through). Then we thought Hey, we know how to make this here crystal see-through for a particular wavelength of light (think 'colour' if wavelength doesn't work for you). What if we (1) made it see-through with the laser, (2) shone some of that colour light in and then (3) turned off the laser so that it's no longer see-through. I wonder what happens? <time passes> zOMG! We trapped light!
In other words, I do think it's possible to get the message across without needing to refer to 'quantum' anything. However, this article is from NewScientist and the intended audience is likely to be vaguely familiar with concepts of superposition via stories of Schrödinger's cat [1]
[1] http://en.wikipedia.org/wiki/Schrödinger's_cat
We used a crystal that is normally opaque (not see-through) and fired laser beams at it because, well... it seemed like fun! Apparently the laser beam affected the crystal and made it see-through but only for a narrow range of light frequencies (e.g. think of a filter that only allows one colour of light through). Then we thought Hey, we know how to make this here crystal see-through for a particular wavelength of light (think 'colour' if wavelength doesn't work for you). What if we (1) made it see-through with the laser, (2) shone some of that colour light in and then (3) turned off the laser so that it's no longer see-through. I wonder what happens? <time passes> zOMG! We trapped light!
In other words, I do think it's possible to get the message across without needing to refer to 'quantum' anything. However, this article is from NewScientist and the intended audience is likely to be vaguely familiar with concepts of superposition via stories of Schrödinger's cat [1]
[1] http://en.wikipedia.org/wiki/Schrödinger's_cat
Your understanding is actually pretty good (lines up with mine, at least), and I liked your explanation except for the needlessly conversational tone. Basically by only cutting things out:
> We used a crystal that is normally opaque (not see-through) and fired laser beams at it [which] made it see-through but only for a narrow range of light frequencies (e.g. a filter that only allows one colour of light through). [Then] we made [the crystal] see-through with the laser, shone some of that colour light in and turned off the laser so that it's no longer see-through. We trapped light!
> We used a crystal that is normally opaque (not see-through) and fired laser beams at it [which] made it see-through but only for a narrow range of light frequencies (e.g. a filter that only allows one colour of light through). [Then] we made [the crystal] see-through with the laser, shone some of that colour light in and turned off the laser so that it's no longer see-through. We trapped light!
This may be a dumb question...
but how are they able to validate that the light was stopped for one minute?
They switched the crystal to opaque... and time passed... until the light exited the crystal??? How much time? How did they validate the light left the crystal? Did they have to do anything to induce the light to leave? If they did, why couldn't this storage crystal hold the light indefinitely... until we trigger the release state?
Sorry if these are dumb questions, just can't really figure that part of it out. I understand the crystal going from opaque, to color filter, to opaque... that is essentially the "writer". How does the "Reader" work?
but how are they able to validate that the light was stopped for one minute?
They switched the crystal to opaque... and time passed... until the light exited the crystal??? How much time? How did they validate the light left the crystal? Did they have to do anything to induce the light to leave? If they did, why couldn't this storage crystal hold the light indefinitely... until we trigger the release state?
Sorry if these are dumb questions, just can't really figure that part of it out. I understand the crystal going from opaque, to color filter, to opaque... that is essentially the "writer". How does the "Reader" work?
This is based on my fuzzy knowledge of quantum physics:
- electrons exist at varying levels of excitement "around" a molecule. These states are discrete; an electron can only have certain values of energy
- photons can "collide" with electrons and impart more energy, provided the photon's energy (correlated to the wavelength of light) plus the electron's existing energy puts it into an allowable state
- when an electron falls back from a high energy state to a low energy state it will emit a photon
- crystals have low electron mobility; it's hard for an electron with a lot of energy to escape from a molecule. The molecules also don't move within the lattice.
- in this experiment, the first laser raises the electrons to a higher energy state, where this new state + the wavelength of the light is also an allowable state
- the second laser excites the electrons into an even higher state. These electrons can't escape from their position (they're fixed in space), but they encode the energy of the second laser. When the first laser goes away, the distance the electrons now have to fall back to their natural (ground) state is very large, so it takes some time. This is what limits how long you can keep the light "stopped" for.
-the 'Reader' is just a photodiode (or a hemisphere of them) which emits electricity when hit by a photon. So you pulse laser #2, then wait for the diodes to 'light up', which means they've been hit by light. The 1 minute figure is the time between turning laser #2 on and seeing the diodes turn on.
- electrons exist at varying levels of excitement "around" a molecule. These states are discrete; an electron can only have certain values of energy
- photons can "collide" with electrons and impart more energy, provided the photon's energy (correlated to the wavelength of light) plus the electron's existing energy puts it into an allowable state
- when an electron falls back from a high energy state to a low energy state it will emit a photon
- crystals have low electron mobility; it's hard for an electron with a lot of energy to escape from a molecule. The molecules also don't move within the lattice.
- in this experiment, the first laser raises the electrons to a higher energy state, where this new state + the wavelength of the light is also an allowable state
- the second laser excites the electrons into an even higher state. These electrons can't escape from their position (they're fixed in space), but they encode the energy of the second laser. When the first laser goes away, the distance the electrons now have to fall back to their natural (ground) state is very large, so it takes some time. This is what limits how long you can keep the light "stopped" for.
-the 'Reader' is just a photodiode (or a hemisphere of them) which emits electricity when hit by a photon. So you pulse laser #2, then wait for the diodes to 'light up', which means they've been hit by light. The 1 minute figure is the time between turning laser #2 on and seeing the diodes turn on.
I don't have enough background knowledge to ascertain if this is the correct understanding of the problem, but this explanation reads very well, and is the best 'mechanical' explanation I've seen in this thread so far.
Thank you.
Thank you.
That makes it sound to me like the light was not stopped, merely the light's energy was stored. Which is quite a different thing?
The important thing is that whatever information was encoded in the light was retained instantly. The only other way to do this is expensive photonics kit to convert light to electricity, then back to light. We're basically developing tools to achieve with light what we now do with electricity.
Also, light gets absorbed and retransmitted all the time in real life. The particular photons involved aren't really important in the typical notion of light.
Edit: to clarify, light in a medium is absorbed and reemitted, which is why light in atmosphere is slower than light in a vacuum. Between atoms it's just as fast, but there's latency in colliding, collapsing and reemitting.
Also, light gets absorbed and retransmitted all the time in real life. The particular photons involved aren't really important in the typical notion of light.
Edit: to clarify, light in a medium is absorbed and reemitted, which is why light in atmosphere is slower than light in a vacuum. Between atoms it's just as fast, but there's latency in colliding, collapsing and reemitting.
There is no other way to do this, in this case they're maintaining information about the quantum properties of the beam (entanglement, etc.), which get destroyed if you convert it to electricity. When talking about quantum cryptography or, in general, quantum computing (as in "computations that are performed using the fundamental quantum interactions between particles"), the particular photons are indeed very important :)
Just to clarify (I'm pretty sure, but you seem knowledgeable), the quantum state is maintained when photons are absorbed/re-emitted by matter, right? Otherwise it seems like there would be a lot more vacuums involved. That was what I meant by 'the particular photons'; to my mind if you transmit your energy to an electron and are re-emitted you become a 'different' photon. It's a pretty silly point to argue about, though.
They're pretty sure it's the same light because (in other experiments with stopped light, not sure about this paper in particular) it maintained the same quantum information as the input light. Stopped light maintains superpositions etc.
I didn't read it all yet, but I imagine they made it transparent again and saw the beam come out. It likely couldn't ever hold it indefinitely as thermal entropy would likely scramble it to undetectable levels again. But this could be a really cool way to start building some digital optical delay lines depending on how it works.
This part implies some kind of answer, right? "They also used the trap to store and then retrieve an image consisting of three stripes".
I'm not sure that answers exactly how the "Reader" work, though.
I'm not sure that answers exactly how the "Reader" work, though.
> "... except for the needlessly conversational tone."
Yes, I agree it's not necessary to get the basic information across but it was definitely a lot more fun to write that way!
If I were really writing piece for an article or blog post I'd take a lot more care and probably consider using better analogies.
Yes, I agree it's not necessary to get the basic information across but it was definitely a lot more fun to write that way!
If I were really writing piece for an article or blog post I'd take a lot more care and probably consider using better analogies.
> Yes, I agree it's not necessary to get the basic information across but it was definitely a lot more fun to write that way!
I agree; for me, "conversational tone" is usually much more fun to write, and much more fun to read. I find it a sign of a good understanding of the topic if one can phrase his explenation in a conversational tone. You have to know your stuff well to be able to correctly express complicated ideas in simple terms.
I agree; for me, "conversational tone" is usually much more fun to write, and much more fun to read. I find it a sign of a good understanding of the topic if one can phrase his explenation in a conversational tone. You have to know your stuff well to be able to correctly express complicated ideas in simple terms.
'Tone' may have been the wrong word. Obviously he understands, he did explain it well. I was just trying to point out that he added a bunch of noise under the guise of being conversational. Cutting out the noise makes it concise will remaining accessible - I don't think my edits actually changed the tone.
I liked and agreed with your description and particularly the transparent reading of your `conversational tone'. There's a career for you editing mathematics journals my friend.
So, the light trapped in the crystal is stationary, and not just "bouncing around" inside, where we can observe it?
I guess the question could be phrased - is the light really stopped, or just confined to a REALLY small area (but the light still vibrates, wobbles, spins, or whatever else light does when trapped in a dark place)?
I guess the question could be phrased - is the light really stopped, or just confined to a REALLY small area (but the light still vibrates, wobbles, spins, or whatever else light does when trapped in a dark place)?
The light is transformed into a "spin wave", an excitation of a certain atomic angular momentum describable by your good old e^i*theta.
http://en.wikipedia.org/wiki/Spin_wave
http://en.wikipedia.org/wiki/Spin_wave
That's where the above analogy doesn't really work. Without reading the paper, I'd guess that the crystal enters a different energy state. A bit like a jet of water that's been 'absorbed' by a sponge, rather than sloshing around in a hollow ball [1].
[1] I know that's a bad analogy but I can't think of anything better off the top of my head right now.
[1] I know that's a bad analogy but I can't think of anything better off the top of my head right now.
For intended purposes like the "Quantum repeater" I believe it does not matter if the photons really stopped or wobbling in some very small space. For the repeater they need to time shift photons. (IMHO, I have no idea what I'm talking about :) )
[deleted]
I'm certainly misunderstanding something (considering they did it), but what I don't understand here is how the change of state of the crystal could have spread faster than light could escape the crystal. Is the information spreading faster than the speed of light in that case?
I'm not a physicist and the paper is paywalled so I'm speculating, but the speed of light in this crystal is probably much lower than the speed of light in a vacuum, and it's the latter that puts an uppper limit on the rate at which information can propagate (excluding quantum entanglement).
> excluding quantum entanglement
IANAP either, but AFAIK (ah, acronyms :)) quantum entanglement can not transfer information FTL.
IANAP either, but AFAIK (ah, acronyms :)) quantum entanglement can not transfer information FTL.
I'm also not a physicist, but as I understand it entangled particles can affect each other instantaneously regardless of distance, but cannot transfer information due to random instabilities.
IIRC it's not due to random instabilities, but rather due to the no cloning theorem: http://en.wikipedia.org/wiki/No-cloning_theorem
I'm a biochemist, not a physicist, but I have had some quantum chemistry. Here's my (probably incorrect) thinking:
Light passing through the "open state" crystal may travel significantly slower due to interaction/absorbance with the crystal atoms along the beam path. Energy injection from the lasers is probably necessary for maintenance of the "open state", and as soon as the laser is shut off there may remain photons that are being absorbed/emitted by the atoms in the crystal lattice. When the crystal is in the "closed state", the photons will be "trapped" (continuous absorption or non-emission or something?).
It's all about quantized energy levels. At certain energies, the atoms will interact differently.
Someone please correct my thinking here.
Light passing through the "open state" crystal may travel significantly slower due to interaction/absorbance with the crystal atoms along the beam path. Energy injection from the lasers is probably necessary for maintenance of the "open state", and as soon as the laser is shut off there may remain photons that are being absorbed/emitted by the atoms in the crystal lattice. When the crystal is in the "closed state", the photons will be "trapped" (continuous absorption or non-emission or something?).
It's all about quantized energy levels. At certain energies, the atoms will interact differently.
Someone please correct my thinking here.
A question for you since you seem to know about light in media: my understanding is that the speed of light is also the speed events propagate through spacetime. So, considering a galaxy made entirely out of pure crystalline diamond, would events propagate slower in that galaxy? In other words, does time itself slow down under those circumstances, does it not change?
It could also be that the speed of light is constrained by the speed events propagate through spacetime and not the other way around, but I am most probably thoroughly confused.
It could also be that the speed of light is constrained by the speed events propagate through spacetime and not the other way around, but I am most probably thoroughly confused.
> So, considering a galaxy made entirely out of pure crystalline diamond, would events propagate slower in that galaxy? In other words, does time itself slow down under those circumstances, does it not change?
It doesn't change. See e.g. https://en.wikipedia.org/wiki/Cherenkov_radiation where electrons travel faster than the speed of light in a specific medium.
It doesn't change. See e.g. https://en.wikipedia.org/wiki/Cherenkov_radiation where electrons travel faster than the speed of light in a specific medium.
Maybe because speed of light in the crystal is slower than speed of light in the vacuum (the latter being the theoretical limit for speed of information transfer)?
AFAIR light traveling through material is not just flying through; it's constantly absorbed and reemited - thus "slowing down".
AFAIR light traveling through material is not just flying through; it's constantly absorbed and reemited - thus "slowing down".
Maybe they had multiple lasers arrayed around the outside to cause the condition of the crystal, and stopped them all at the same time while the light was in or approaching the center of the crystal (allowing for time for change to begin). Wouldn't the state change start at the outsides and move in?
I really have no idea whether you can actually reason about this stuff like that...
I really have no idea whether you can actually reason about this stuff like that...
There is a Wikipedia article about "Quantum superposition". https://en.wikipedia.org/wiki/Quantum_superposition
In quantum states, you cannot know the actual state something is in unless you measure it. Beforehand, you just have different probabilities of where it might be.
A quantum superposition of two states means that there are two states the atoms can be in, with specific probabilities assigned to each state.
In quantum states, you cannot know the actual state something is in unless you measure it. Beforehand, you just have different probabilities of where it might be.
A quantum superposition of two states means that there are two states the atoms can be in, with specific probabilities assigned to each state.
Right, and issue three of this article's "multiple issues" is "This article may be too technical for most readers to understand. (April 2011)"
APS typically features important results with articles that are not only understandably by the quantitative laymen, but also do a good job of communicated content. (They're supposed to be the least common denominator for interdisciplinary physicists.)
Here's the article for this paper: http://physics.aps.org/articles/v6/80
Here's the article for this paper: http://physics.aps.org/articles/v6/80
I think a good way to think about sentences like that is that the word quantum is short for "belonging to quantum mechanics", like how "classical gravitation" doesn't mean that there's anything classic about gravitation. The quantum quantifier is there to differentiate the superposition they mention from "classical" wave superposition.
It's very complicated stuff but not "very, very complicated" stuff, you can read Feynman's QED book (http://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/069..., <$9 used with shipping) and obtain a pretty good working knowledge of things.
It's very complicated stuff but not "very, very complicated" stuff, you can read Feynman's QED book (http://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/069..., <$9 used with shipping) and obtain a pretty good working knowledge of things.
I think the article struck the right balance... Stories like these first appear in the more serious media circles first, and then filter out into the mainstream. A long time ago I remember reading scientific americans that tended to be more technical then whats out in the mainstream nowadays. There's room for news sources that fit between the raw whitepapers and the mainstream.
http://davidjarvis.ca/entanglement/
I wrote a few pages explaining quantum entanglement in rudimentary terms... and without relying on math, which you'll find too much of on the Wikipedia pages.
I wrote a few pages explaining quantum entanglement in rudimentary terms... and without relying on math, which you'll find too much of on the Wikipedia pages.
I almost feel like this is the one field where if you know enough to explain it, you are working in the field, not in the business of explaining it. If that makes sense...
I work in a photonics lab and for all of the new fancy techniques you see show up in popular science publications for slowing and stopping light... they're all impractical. In the real world when we need to slow down light we still just use delay coils. Wrap tens to hundreds of kilometers of fiber into a coil (using a pump laser amplifier at some point if it gets too long) and just letting the light spin around in circles for the desired amount of time.
> when we need to slow down light
Why do you need to do that? I'd never heard of "photonics" before now.
Why do you need to do that? I'd never heard of "photonics" before now.
Delaying light is sometimes used for a buffer in optical networks and signal processing, it is also used in the internal workings of certain opto-electronic devices such as optical oscillators; sometimes also used for testing and calibration of optical systems, making sure things are coming out as clean and at the same time as they do through the 'test' delay coil.
Could you give an example of a real world application for stopping/delaying light?
Optical switching?
The trouble is that the power levels required for a nonlinear optical material are often much higher than the light beam being switched. So you might need to use a 1000W beam to switch a 1W beam (for example).
The trouble is that the power levels required for a nonlinear optical material are often much higher than the light beam being switched. So you might need to use a 1000W beam to switch a 1W beam (for example).
Most of the switches we use are still opto-mechanical, just mechanically change the connection.
Wouldn't that impart an improper spin on the light?
-cough-
-cough-
How big do 100km bundles of fiber get?
The dimensions would vary depending on the form factor needed for the particular application, but a spool with a 9 inch by 9 inch spool with a 2 inch hub (which works for 1310nm applications) would hold about 120km
Wow. That's pretty impressive. I was imagining massive cylinders the size of refrigerators.
The fiber we're talking about has an outer diameter of 250µm (standard single mode) so it packs pretty densely; we can actually do a bit better using some more exotic (expensive) fibers and some of our proprietary tricks to packing a bit more in there, but the size I gave is pretty typical. The largest ones we have made fit in a 5U rack enclosure, but I can't remember the length we had on those offhand and some of the internal space was used for amplifiers.
I have a dream of a mirror that reflects the light of yesteryears.
You would look into it and see the reflections of the people and scene from the exact place and time as you were now - but 10, 20, 50, 100 years ago.
It would be a type of city attraction, where people would come to visit daily, so there would always be new and interesting people (and fashions) to see.
We actually have the technology to implement this right now, with video cameras, monitors and hard drives.
But the simplicity and beauty of a device that actually slows down light to accomplish this - now that would be something.
You would look into it and see the reflections of the people and scene from the exact place and time as you were now - but 10, 20, 50, 100 years ago.
It would be a type of city attraction, where people would come to visit daily, so there would always be new and interesting people (and fashions) to see.
We actually have the technology to implement this right now, with video cameras, monitors and hard drives.
But the simplicity and beauty of a device that actually slows down light to accomplish this - now that would be something.
You're thinking of 'slow glass', from Bob Shaw's short story 'Light of Other Days': http://www.tor.com/blogs/2010/10/slow-glass-seen-from-all-ar...
Analog, 1966 - Well, I guess he didn't get the idea from me, then.
Melancholy synopsis from Wiki:
While on vacation, they visit a slow glass merchant and notice the man's wife and child through the window, playing inside the house. When they enter, however, the house is deserted; only then do they realize that the window is made of slow glass, giving the lonely man his final glimpses of his long-dead wife and child.
It's funny how stories from your youth can permeate into your memories.
Melancholy synopsis from Wiki:
While on vacation, they visit a slow glass merchant and notice the man's wife and child through the window, playing inside the house. When they enter, however, the house is deserted; only then do they realize that the window is made of slow glass, giving the lonely man his final glimpses of his long-dead wife and child.
It's funny how stories from your youth can permeate into your memories.
>We actually have the technology to implement this right now, with video cameras, monitors and hard drives.
Jeez, this sounds really fun to build and exhibit...
Jeez, this sounds really fun to build and exhibit...
It's nice for once to see evolutionary algorithms being used for something completely impractical.*
*mostly tongue-in-cheek
*mostly tongue-in-cheek
What I want to know is: Does the crystal gain mass with the light stopped inside of it?
I don't think there's an energy -> mass -> energy transformation going on. Rather the crystal enters a different energy state while it 'has' the photon.
well its energy has increased, so yes, but probably not measurably.
I though light was massless which was the reason it was able to travel at c and that anything with mass would travel at less than c?
If so then how does a massless object add weight to an object by being trapped in it.
If so then how does a massless object add weight to an object by being trapped in it.
Photons have zero rest mass, but light has an energy of $h\nu$, where $\nu$ is the frequency, so this gets added on to the mass of our thing.
Oversimplified explanation: Photons are massless so they can go as fast as they want without needing kinetic energy, but even massless things can store energy (think of the massless springs you studied in high school physics). Energy is the same thing as mass, but we still refer to photons as massless, in the same way that we think of an (imaginary) massless spring as such; we don't like to think of potential energy as _being the object_, just somehow _there_.
This makes more sense if you study physics, in no small part due to the fact that we make it all rigorous (though not to the satisfaction of the mathematicians, I guess...).
Oversimplified explanation: Photons are massless so they can go as fast as they want without needing kinetic energy, but even massless things can store energy (think of the massless springs you studied in high school physics). Energy is the same thing as mass, but we still refer to photons as massless, in the same way that we think of an (imaginary) massless spring as such; we don't like to think of potential energy as _being the object_, just somehow _there_.
This makes more sense if you study physics, in no small part due to the fact that we make it all rigorous (though not to the satisfaction of the mathematicians, I guess...).
Does this mean that if you move an object away from Earth's gravitational influence it will increase in mass because it has more potential energy?
What about if you heated something up, does it gain mass then?
How about sound? Does the sound of my voice cause imperceptible ripples of increased mass as it vibrates everything that it touches?
Is a charged battery heavier than an empty one?
Is glass heavier with light travelling through it?
What about if you heated something up, does it gain mass then?
How about sound? Does the sound of my voice cause imperceptible ripples of increased mass as it vibrates everything that it touches?
Is a charged battery heavier than an empty one?
Is glass heavier with light travelling through it?
> Does this mean that if you move an object away from Earth's gravitational influence it will increase in mass because it has more potential energy?
Gravitational potential energy is a little different. It's the energy that the object would have if it fell to earth, it doesn't actually exist until it starts falling. (correct this, i'm sure i'm wrong)
> What about if you heated something up, does it gain mass then?
Yes, but it's incredibly miniscule to the point that it's not worth looking at in most situations
> How about sound? Does the sound of my voice cause imperceptible ripples of increased mass as it vibrates everything that it touches?
This one I think does but it's going to be so small that you also won't notice it. What you would be able to notice though are the subtle changes in density at the peaks and troughs of the wave.
> Is a charged battery heavier than an empty one?
Maybe. This depends on the chemistry of the battery more than anything. Some of them will oxidize as it gives out energy and give off the oxygen when charging. While the energy will contribute slightly to the mass because of the chemcical bonds, the battery will change mass more because of the lost/gained atoms than any other cause.
> Is glass heavier with light travelling through it?
I really don't know how to answer this one. The photon never really becomes part of the glass as far as I'm aware, but some will be absorbed and raise the temperature so maybe?
Gravitational potential energy is a little different. It's the energy that the object would have if it fell to earth, it doesn't actually exist until it starts falling. (correct this, i'm sure i'm wrong)
> What about if you heated something up, does it gain mass then?
Yes, but it's incredibly miniscule to the point that it's not worth looking at in most situations
> How about sound? Does the sound of my voice cause imperceptible ripples of increased mass as it vibrates everything that it touches?
This one I think does but it's going to be so small that you also won't notice it. What you would be able to notice though are the subtle changes in density at the peaks and troughs of the wave.
> Is a charged battery heavier than an empty one?
Maybe. This depends on the chemistry of the battery more than anything. Some of them will oxidize as it gives out energy and give off the oxygen when charging. While the energy will contribute slightly to the mass because of the chemcical bonds, the battery will change mass more because of the lost/gained atoms than any other cause.
> Is glass heavier with light travelling through it?
I really don't know how to answer this one. The photon never really becomes part of the glass as far as I'm aware, but some will be absorbed and raise the temperature so maybe?
The wikipedia page on the subject seems to say that potential energy does increase mass [1].
[1] https://en.wikipedia.org/wiki/Mass-energy_equivalence#Meanin...
[1] https://en.wikipedia.org/wiki/Mass-energy_equivalence#Meanin...
Interesting. I wonder how that reconciles with having multiple gravitational bodies then.
Yes, yes, yes, yes. Not sure about the last one as I'm not sure about the way photons travel through glass.
Think about the opposite - nuclear fission. The energy released when nuke goes off comes from the difference of mass between original atoms and products of the split. This difference is the energy stored in strong nuclear force, binding protons and neutrons together.
I belive I kind of understand what you are saying.
I am confused as to whether or not the photons actually stop moving. Since they halt them by shutting off the transparency it seems that the photons would get trapped and end up bouncing around being absorbed slowly.
Is that why the effect is temporary? The light eventually gets absorbed and disperced though the crystal?
I am confused as to whether or not the photons actually stop moving. Since they halt them by shutting off the transparency it seems that the photons would get trapped and end up bouncing around being absorbed slowly.
Is that why the effect is temporary? The light eventually gets absorbed and disperced though the crystal?
(Irrelevant aside) Unfortunately HN doesn't have MathML, but you can use unicode: ℎν.
Actually it could be a good idea to have MathML or TeX on HN; we tend to use math-like expressions here quite a lot.
Yes. Since light is effect by gravity, it must have a mass. The increase should however be quite small on the crystal.
correction: See below.
correction: See below.
> Since light is effect by gravity, it must have a mass
Sorry, this is completely false. Light is affected by gravity because gravity is really a curvature of spacetime.
* I guess "curvature of spacetime" is perhaps not the most satisfying explanation of why light is redshifted as it moves out of a gravitational potential well (i.e. if you shoot a laser from earth into space). Nonetheless the answer is still "because of relativity".
Sorry, this is completely false. Light is affected by gravity because gravity is really a curvature of spacetime.
* I guess "curvature of spacetime" is perhaps not the most satisfying explanation of why light is redshifted as it moves out of a gravitational potential well (i.e. if you shoot a laser from earth into space). Nonetheless the answer is still "because of relativity".
Light does have mass (it has zero rest mass, but it's always moving at c, and as we know m = E/c^2...)
Uh, E = mc^2 is not the full equation. That is, in fact, a special case for an object with no momentum in a given frame.
The actual equation is E^2 = (mc^2)^2 + (pc)^2, where the p is momentum. And, unsurprisingly, the old "p=mv" isn't the whole story, either. The (magnitude of the) momentum of a photon is given by h/λ where h is Planck's constant and λ, of course, is the wavelength.
As such, at no point is it necessary for light to have mass and it is believed that, in fact, photons are entirely and truly massless. Relativistic mass--which some take to mean photons actually gain mass--is a confusing and misleading concept that doesn't really have a whole lot to do with actual mass and doesn't really help explain anything.
"Effective mass"--which is more related to stopping photons, but not really related to the Einstein equation you gave--also doesn't really mean the photon gains mass but it does mean that interactions within the crystal give effects similar to if the photon had mass.
The actual equation is E^2 = (mc^2)^2 + (pc)^2, where the p is momentum. And, unsurprisingly, the old "p=mv" isn't the whole story, either. The (magnitude of the) momentum of a photon is given by h/λ where h is Planck's constant and λ, of course, is the wavelength.
As such, at no point is it necessary for light to have mass and it is believed that, in fact, photons are entirely and truly massless. Relativistic mass--which some take to mean photons actually gain mass--is a confusing and misleading concept that doesn't really have a whole lot to do with actual mass and doesn't really help explain anything.
"Effective mass"--which is more related to stopping photons, but not really related to the Einstein equation you gave--also doesn't really mean the photon gains mass but it does mean that interactions within the crystal give effects similar to if the photon had mass.
I'm not sure this is a useful thing to say.
I thought light was not effected by gravity but by the increased distance it had to travel due to warping effect on time/space caused by gravity?
Keep in mind these descriptions are just interpretations of observations. Another intelligent civilization could explain the observations in a wholly different way. The observations show that light is affected by gravity (more precisely, by centers of gravitational attraction) the same as material objects are. We have alternate generally accepted ways of looking at this, such as the river model of space [1], where space is moving inward toward a center of gravitational attraction, carrying along in its "current" everything including light rays.
[1] http://arxiv.org/abs/1204.0419
[1] http://arxiv.org/abs/1204.0419
Correct. Photons are responding to the curvature in space-time, not directly to the gravitational field. Doing a second check, light has inertia-mass, but not rest-mass.
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Yes, conservation of energy.
here is the original paper: http://prl.aps.org/abstract/PRL/v111/i3/e033601
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This article was pretty poorly written, leading to a couple of questions:
1) It seems to me that the scientists could have simply stored the energy from the light for a period of time? Which would not actually be "stopping" the light.
2) I understand a super position and quantum physics, but I still have vague idea of what happened in the experiment. If I actually understand the terminology and have done various experiments, research, etc. but still don't fully understand, how would the standard reader (without further explanation) understand the article?
1) It seems to me that the scientists could have simply stored the energy from the light for a period of time? Which would not actually be "stopping" the light.
2) I understand a super position and quantum physics, but I still have vague idea of what happened in the experiment. If I actually understand the terminology and have done various experiments, research, etc. but still don't fully understand, how would the standard reader (without further explanation) understand the article?
> It seems to me that the scientists could have simply stored the energy from the light for a period of time
Strained analogy: It's more like recording a hologram for a minute and then playing it back. It's a one-minute buffer for light, the same way we have video buffers, but for real life.
Strained analogy: It's more like recording a hologram for a minute and then playing it back. It's a one-minute buffer for light, the same way we have video buffers, but for real life.
I strongly dislike headlines like this (which all the crappy science news digests, like New Scientist, frequently indulge in).
If light could actually be "stopped", it wouldn't be light any more. It also can't be "slowed down"; it always travels at exactly the speed of light in whatever medium it's in. OTOH, if the light's energy is absorbed by electrons, it's not "light" any more. All matter does this trick all day every day - it's not news.
This sort of sentence only sows confusion in a lot of minds.
If light could actually be "stopped", it wouldn't be light any more. It also can't be "slowed down"; it always travels at exactly the speed of light in whatever medium it's in. OTOH, if the light's energy is absorbed by electrons, it's not "light" any more. All matter does this trick all day every day - it's not news.
This sort of sentence only sows confusion in a lot of minds.
"Tens of seconds of light storage are needed for a device called a quantum repeater, which would stop and then re-emit photons used in secure communications, to preserve their quantum state over long distances."
That's quite a lot of latency. Assuming many repeaters, does that mean a quantum-encrypted ping from one side of the world to the other might be measured in hours?
That's quite a lot of latency. Assuming many repeaters, does that mean a quantum-encrypted ping from one side of the world to the other might be measured in hours?
Not necessarily. It's not so much that a 10-second delay will be introduced at every repeater, but that a practical system/implementation requires that a delay of up to ten seconds may be introduced when required by traffic, etc. A straight-pipe, latency-free, synchronous-only system would require a lot more infrastructure to support (analogous to the earliest direct-connection telephone systems).
Wait so is the story basically along the lines of, "Light hits black paint. Absorbed, stops. Comes out later." ?
I asked a physicist. The headline and article are both stupid but the science is good. It's like the paint thing except that when light hits paint, the paint affects the light, they interact, the information in the light gets messed up. With the fancy crystal thing they are preserving the information.
Radiolab had a segment about stopping light back in February. It's a great listen: http://www.radiolab.org/2013/feb/05/master-universe/
This is interesting... every time I've read a "light slowed down" article over the last 10 years it's been Professor Lene Hau. This time it isn't.
I look forward to our Middle Earth future.
http://en.wikipedia.org/wiki/Silmarils
http://en.wikipedia.org/wiki/Silmarils
this seems like the coolest thing in the world to see, but yet no pictures? What does stopped light look like?
To the naked eye, nothing. If it's stopped, it can't hit your retina!
my head just exploded trying to grasp this... objects don't move but i can see them, but i can only see them because of light. therefore, if light doesn't move, if the object does, can i see it on the object?
If the light doesnt hit you in the retina you don't see it. So no.
My guess: The object that does not reflect light to you ceases to exist in your vision. Nothing comes from that direction. Your brain would then attempt to compensate -- as it does for the blind spot in both your eyes -- and fill in the void, perhaps with a fuzzy version of continuity of whatever colors and textures surround the object.
We don't need to make wild hypotheses about what an object that doesn't reflect light looks like. It's so mundane we have a word for it: "black".
It would look black to a light sensor. As he says, your brain does play tricks and fill-in-the-black more than you might think. If you have a blind-spot you don't always perceive the absence of light - you may perceive what your brain is expecting. There are also disorders where it does this too much. I knew a guy who used to do things like overfill his coffee mug all the time, because the mug kept looking empty and then suddenly it was overflowing - his brain just expected the mug to stay empty.
A thing that absorbs light is not a "blind spot". It's black.
Are you on the good stuff or something? It's black. Just because someone muttered "quantum" does not make this magic. It's black. What is so complicated about this concept? It literally can not get much more normal than this.
Are you on the good stuff or something? It's black. Just because someone muttered "quantum" does not make this magic. It's black. What is so complicated about this concept? It literally can not get much more normal than this.
I never implied anything to do with quantum physics or "magic". I also never said it wasn't black or an absence of light. Both I and the GP comment are referring to how your brain fills in the gaps and causes you to think you see things that are not there. So no light reaches you eye, yes, and that is referred to as being "black". We're talking about what your brain would then cause you to perceive or "see". Now what is so complicated about that?
"The object that does not reflect light to you ceases to exist in your vision. Nothing comes from that direction. Your brain would then attempt to compensate -- as it does for the blind spot in both your eyes -- and fill in the void, perhaps with a fuzzy version of continuity of whatever colors and textures surround the object."
This is gibberish. Unless you're high on acid, when you see a black thing, your visual system does not go nuts trying to fill in the missing data... it sees black.
You're trying to explain a phenomenon that doesn't exist. Your brain does not see a black object as a "gap"... it sees it as black. There's nothing complicated to explain here. There's no "illusion". There's no weird perceptual thing to explain, beyond the usual "what is seeing red, really?" (red for some reason being the usual color chosen for this question), which is clearly not what is being referenced. There's no gap here. No optical illusion. Just black.
I mean, find something in your visual field that is black, and look at it for a bit. Do you see your brain "filling in the void, perhaps with a fuzzy version of continuity of whatever colors and textures surround the object"?
The only hard thing about understanding this is that there's no hard thing.
This is gibberish. Unless you're high on acid, when you see a black thing, your visual system does not go nuts trying to fill in the missing data... it sees black.
You're trying to explain a phenomenon that doesn't exist. Your brain does not see a black object as a "gap"... it sees it as black. There's nothing complicated to explain here. There's no "illusion". There's no weird perceptual thing to explain, beyond the usual "what is seeing red, really?" (red for some reason being the usual color chosen for this question), which is clearly not what is being referenced. There's no gap here. No optical illusion. Just black.
I mean, find something in your visual field that is black, and look at it for a bit. Do you see your brain "filling in the void, perhaps with a fuzzy version of continuity of whatever colors and textures surround the object"?
The only hard thing about understanding this is that there's no hard thing.
It would be interesting to see some ultrablack material in person http://www.wired.com/wiredscience/2009/03/ultrablack/
Wouldn't it just be dark? I think the blind spot is a different phenomenon. This would be no different than being in a pitch black room.
Maybe the critical question is: If no light at all comes from the object, is that the same as it being dark, or is it a complete lack of stimulus?
If the brain receives noise, it fills in the blanks (http://www.sciencedaily.com/releases/2000/06/000601164617.ht...). In sensory deprivation, the brain also fills in the blanks (would cite but ended up finding a ton of supporting material in a web search, including an interesting study about the effects of anxiety on amount of hallucinations).
If the brain receives noise, it fills in the blanks (http://www.sciencedaily.com/releases/2000/06/000601164617.ht...). In sensory deprivation, the brain also fills in the blanks (would cite but ended up finding a ton of supporting material in a web search, including an interesting study about the effects of anxiety on amount of hallucinations).
When you turn off the lights before you go to bed... what happens?
Do the things move in the dark, while you can't see them, because there's no light?
What if you didn't have eyes? What then?
If tree falls in the forest, and no one is there to see it... does it make a sound?
Do the things move in the dark, while you can't see them, because there's no light?
What if you didn't have eyes? What then?
If tree falls in the forest, and no one is there to see it... does it make a sound?
I think it's for light that entered the object, unsure if any light reflects off the surface.
It could be a glass ball which turns to look like a shiny ball.
It could be a glass ball which turns to look like a shiny ball.
That is the most mind-blowing thing I have read all day.
This is so basic but incredible to think about. How do they know that it is there when it's stopped, provided that it neither has mass nor emits "something"? My brain just stopped.
Probably by deduction and careful experimental design. If it's not anywhere else, there's only one place left. Also, it is emitted at some point so it's not trapped forever.
They know it's there because when they 'unfreeze' it, the light continues on its path and they can detect it then.
They sent an image thru the crystal. Three stripes.
When they made the crystal transparent again, the same image came out.
When they made the crystal transparent again, the same image came out.
If I understood the experiment correctly, it doesn't look anything exceptional. The crystal would just turn opaque after the first laser is shut off, and when they shot the laser at it to turn it transparent again, the photons from the second laser would be re-emitted in the same trajectory. It's not really "light frozen mid-air", but rather a crystal emitting light after being hit by a laser.
But why? Why not stop time instead?
Someone could already be constantly stopping and resuming time. There'd be no way of knowing.
Indeed, maybe the guy with the rocks takes a break right now. (https://xkcd.com/505/)
The ultimate solution for information storage?
Thus continuing my inability to understand what the fuck anyone is talking about whenever the word "quantum" is used in a sentence.
I mean, seriously. This has to be the one area of general reader science journalism in which the journalists -- having about as much as idea as I do about what the physicists are talking about -- just copy down whatever the guys in the lab coats are saying and print it.
I work in research computing. When I start talking to someone from our communications department and use a word like "interoperability," they say, "Whoa, slow down. You have to spell that out for our readers." But "quantum superposition?" Oh, what the hell.
I understand that is this very, very complicated stuff, and that they can't explain the whole thing from top to bottom every time they mention quantum mechanics. But pithy little paragraphs like this explain absolutely nothing.