Light completely stopped for a record-breaking minute
newscientist.com
newscientist.com
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.
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."
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.
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.
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]
> 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!
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?
- 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.
Thank you.
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.
I'm not sure that answers exactly how the "Reader" work, though.
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.
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 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)?
[1] I know that's a bad analogy but I can't think of anything better off the top of my head right now.
Also the imprinted image may be lost or fuzzier.
IANAP either, but AFAIK (ah, acronyms :)) quantum entanglement can not transfer information FTL.
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.
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 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.
AFAIR light traveling through material is not just flying through; it's constantly absorbed and reemited - thus "slowing down".
I really have no idea whether you can actually reason about this stuff like that...
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.
Here's the article for this paper: http://physics.aps.org/articles/v6/80
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 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.
Why do you need to do that? I'd never heard of "photonics" before now.
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).
-cough-
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.
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.
Jeez, this sounds really fun to build and exhibit...
*mostly tongue-in-cheek
If so then how does a massless object add weight to an object by being trapped in it.
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...).
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?
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?
[1] https://en.wikipedia.org/wiki/Mass-energy_equivalence#Meanin...
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?
correction: See below.
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".
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.
In general relativity, space-time curvature is the gravitational field, so I'm not quite sure what distinction you're trying to draw?
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?
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.
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.
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?
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.
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.
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).
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?
It could be a glass ball which turns to look like a shiny ball.
When they made the crystal transparent again, the same image came out.