Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: kenhikage on 14/09/2010 15:14:04
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Or to rephrase, do we know if light can be slowed down (without interacting with matter)? And if so, is it still visible? Does it become matter? How about dark matter?
Obviously my understanding is limited to the "Huh?" [???] side of math.
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It's a nice question.
Can we prove photons, definitely.
But can we prove them traveling?
I'm not sure of it. But when you cool photons, you first use a magnetic trap holding and slowing down the molecules of a gas, you then cool it even more by bouncing photons, using a laser, on the atoms in that gas. And so cooling something becomes the absence of motion which to me is rather cool :) Maybe it's possible to use only lasers for it, very difficult of course but still?
If it was, would that be 'cooling' without interacting with matter?
And no, cooling a gas and then sending in a beam of light into that 'gas' will slow it down, even stop it, but what seems to happen is that the beam becomes some sort of imprint on the Bose Einstein condensate that the gas have became, storing the information. What we need to remember is that the condensate also get boson-like properties when it's cooled. When you then let the condensate gets warm the beam will continue out of the condensate, acting as normal as I understands it.
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Hi. I read some time back where a lady had not only slowed down light but had actually stopped it. After a short while she restarted it with a laser beam and it quickly regained its' original speed. Thanks for comments. Joe L. Ogan
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No. Light always travels at the speed of light in a vacuum, which is constant. That's one of the postulates of special relativity.
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Could Joe be referring to something to do with Bose-Einstein condensates?
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Light cannot slow down but it can be absorbed and dissipated in particle vibrations (heating, laser, photovoltaic effect). But then, it interacts with matter. That is what come to my mind for an answer to your question. When matter and antimatter collide, it produces light (and probably neutrinos). But can we combine light to produce matter and antimatter? I don't think so. Photons seems to be the ultimate chaos (or entropic particle).
http://en.wikipedia.org/wiki/Entropy
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But can we combine light to produce matter and antimatter?
Yes: http://en.wikipedia.org/wiki/Pair_production
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thx JP, I knew antimatter could be produced, but i did not expect it was this way...
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Light does slow down slightly when travelling through transparent materials such as air or glass.
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Nice to see you back Joe.
Yep Geezer, I too think that this was what Joe was thinking of, and, she's a very cool lady that one :)
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Another thing :)
When light becomes a imprint on a Bose Einstein condensate (Hau stopping light. (http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html)), have it interacted with matter or with bosons?
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Using sodium atoms and two laser beams, they made a new kind of medium that entangles light and slows it down.
The CfA researchers used an easier method. They shot laser beams through a dense cloud of rubidium and helium gas. (Rubidium, in its solid or natural form, is a soft, silver-white metal.) The light bounced from atom to atom, gradually slowing down until it stopped.
So, I guess the consensus is that we haven't done it.
The reason I ask is because I'm wondering if mass isn't just extremely bent and slowed light (electromagnetism). That is, everything is an expression of light. Matter being slowed light that can interact with light, dark matter being slowed light that can't interact with it. More info on what exactly I mean in this thread: http://www.thenakedscientists.com/forum/index.php?topic=33912.25;topicseen
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Joe, I made a link about Hau's experiments before. -Stopping light- And make it jump? (http://www.thenakedscientists.com/forum/index.php?topic=20457.0) As I wasn't sure if I understood the idea the article I had read presented. It's quite understandable, I hope :)
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Hi. I read some time back where a lady had not only slowed down light but had actually stopped it. After a short while she restarted it with a laser beam and it quickly regained its' original speed. Thanks for comments. Joe L. Ogan
After reading all of the learned discussion, (Some of which I understood) I am not so sure about my statement being correct. I hope that the part I remember about the lady getting a $500,000.00 tax free grant to do with as she sees fit is correct. I gather that it was a very significant discovery. Thanks for comments. Joe L. Ogan
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Photons don't bounce on atoms, they are absorbed and another photon is emitted... This takes some times so you could think it slows down the photon but it is not the same one...
Lasers work on this principle. For gas or liquid lasers, you use 2 concave mirrors in front of each other, if you have the proper atoms or molecules (lasing material)in the middle (which can emit, absorb and re-emit light in an organised way), you can create a resonant cavity by adjusting the distance between the 2 mirrors. Some photons will bounce from the molecules or atoms to the mirrors and and finally between both mirrors. Everytime it bounces back and forth, some photons will be absorbed by the lasing material and other photons of the same energy (frequency) will be emit, creating a laser beam. One mirror is less reflective and let a part of the beam going out.
Light Amplification by Stimulated Emission of Radiation
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well, if I remember right I actually used the articles 'terminology' there, and here too :) Sloppy wording I admit :)
"We adjust the frequency of the laser radiation so that the atoms absorb it and then reradiate photons. An atom can absorb and reradiate many millions of photons each second, and with each one, the atom receives a minuscule kick in the direction the absorbed photon is moving. These kicks are called radiation pressure." Or as the article , and I did, 'bouncing'.
This page (http://www.fortunecity.com/emachines/e11/86/bose.html) is very informative for how they do it..
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Photons don't bounce on atoms, they are absorbed and another photon is emitted... This takes some times so you could think it slows down the photon but it is not the same one...
Interesting. What causes it to emit, just having too much energy to store it all? If so, it seems strange that it would accept more energy and kick out old, rather than simply letting the original photon pass by.
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It can be explained in form of its momentum I think. That radiation pressure is also its momentum, which is treated as an 'immaterial' force alike a kinetic energy. If we accept that an atom is matter then you might look at it as a very small ball hitting another much larger one. The 'kinetic energy' transfered through that :) 'Bounce' will deliver extra energy to the atom and its electrons orbitals while destroying the original one (photon originale:), as I think of it, which in its turn will create a new photon as the atom stabilizes again. You can look at photons and 'black body radiation' for an even finer image of how it's thought to work. But for me it's the momentum that gives it the 'kick' aside. I'm not sure of how to define the 'energy' transfered otherwise. It's the same with CO2 in the atmosphere, when the molecules delivers their heat they do it through 'bouncing' against each other as I understands it.
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As for the 'forces' transmitting this kinetic energy, the accepted idea, for now, seems to be 'virtual photons' outside Planck-time that carries it (the momentum?) around between the original photon and the atoms 'parts'. Well, never said it was simple, even though I would like it to be :) 'Energy' is a very weird thing and 'virtual photons'? Well, we know they exist mathematically and in the Casimir effect (http://en.wikipedia.org/wiki/Casimir_effect) we can see a reflection, as I see it, of those virtual photons, even though it here is treated as a 'field' instead of 'singular virtual photons'.
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I don't get that bit about virtual photons, but let me if I've got the rest. Basically, the atoms don't have any choice in whether or not they absorb the light, because light has so much energy/moment that it shoves its way in. The atom is then too cramped, so it shoots out some energy in the form of a photon.
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Virtual photons are weird creatures :)
And the rest of what you wrote seems okay to me.
The small ball is of course a 'ball' only in its momentum, as it has no 'rest mass', like matter has. "The invariant mass, intrinsic mass, proper mass or just mass is a characteristic of the total energy and momentum of an object or a system of objects that is the same in all frames of reference" and that is what defines matter, that it always will have an invariant mass, even though the 'weight' might differ. But it works for me when thinking of it, and using the analogy I can connect the way molecules release heat, to the way photons interacts with atoms. And, that's probably why I let 'bounce' slip through, two times :) and now three. ::))
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In some ways the definition is a little vague though, thinking of it. The same way a photon can be seen to be red or blue shifted depending on your 'frame of observation', traveling towards it or from it, so can a piece of matter be seen to have more, or less, 'total energy' depending on its direction relative yours. And that's also what's called 'relative mass' or 'potential energy'. The 'potential energy' here is a description of your 'frame of reference' relative the 'frame of reference' of the photon, towards you, or, away from you.
'Away' will show itself as a redshift with a photon/wave, but with matter you won't see any difference, except in its 'relativistic geometry', it shrinking with speed/velocity, I think? Coming towards you the photon/wave will show itself as a blue shift, but with matter you won't see a difference, except in the way its geometry transforms.
But the same way as matter consists of this 'invariant mass', so can a photon be defined to be of a so called 'light quanta'. That light quanta, again as I understands it, will also be 'invariant' no matter what frame you observe it from. Meaning that looked on as a 'light quanta' you can say that this photon also have a defined 'invariant energy', well, as I understands it :) Analogous to matters 'invariant' mass.
There have to be something more to that definition. They say that it is invariant in all 'frames of reference' right? And treated so it can only be the matter itself, and its own transformation into energy, that defines invariant matter, not the 'total energy' (aka + velocity/speed) as it seemed to say to me when I reread it.
But the question is still, if a 'light quanta' also is of a invariant energy/momentum, doesn't that definition of invariant mass lose some of its uniqueness? The only way around it seems to be to assume that a light quanta only is defined versus your frame of reference, meaning that it have no defined energy/momentum, only the combination of yours, as well as its own, 'frame of reference'?
(Light quanta: In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force.)
Now why did I have to wonder about that one?
Sh*
Short history of light quanta (http://en.wikipedia.org/wiki/Photoelectric_effect#Einstein:_light_quanta)
And if you read that one you will wonder what the he* they meant by 'intensity'.
"According to Einstein’s photoelectric effect in where he proposed that light is made up of packets of energy called photons. Photons have no mass, but they have momentum and they have an energy given by: Energy of a photon : E = hf (where E= energy in J and F = frequency unit in 1/s or hertz and h= 6.693*10^-4J*s, or Planck's constant) So, E= Planck's constant (h) multiplied by its frequency (f)
The photoelectric effect works like this. If you shine light of high enough energy on to a metal, electrons will be emitted from the metal. Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted. Light above the threshold frequency, even if it's not very intense, will always cause electrons to be emitted. It takes a certain energy to eject an electron from a metal surface. This energy is known as the work function (W), which depends on the metal. Electrons can gain energy by interacting with photons. If a photon has an energy at least as big as the work function, the photon energy can be transferred to the electron and the electron will have enough energy to escape from the metal. And a photon with an energy less than the work function will never be able to eject electrons. Knowing that light is made up of photons, it's easy to explain now. It's not the total amount of energy (i.e., the intensity) that's important, but the energy per photon."
The point being here that it was the 'discrete light quanta/photons' that dislodged the electrons.
Anyway.
I must be missing something simple here.
It's probably much clearer mathematically :)
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Photons don't bounce on atoms, they are absorbed and another photon is emitted... This takes some times so you could think it slows down the photon but it is not the same one...
Interesting. What causes it to emit, just having too much energy to store it all? If so, it seems strange that it would accept more energy and kick out old, rather than simply letting the original photon pass by.
Atoms exist only in specific quantum energy level states. Each state is associated with electrons energy levels. An atom can be excited to another higher energy quantum state if it is hit by a photon of the same energy as the difference of the 2 quantum states. It results in a higher quantum vibration state of an electron revolving around the atom nucleus. The atom will then emit another photon in order to return to a more stable quantum state (probably of the same energy as the prior photon).
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Now, there is another way to look at bouncing :)
Think of a mirror, and the photons 'bouncing' there. Somewhere I read that bouncing the photons gives them twice the impulse/pressure on whatever they 'hit' than if it just absorbed them. And while thinking of sending photons into a ultra-cold condensate. Do photons have a 'vibration'? If you think it has, what happens with that in deep space, does it stop 'vibrating'?
As I assume this vibration to be an equivalence to its energy that is :)
If I'm wrong there, and vibrations/motion have no relevance to a photons energy, then what have?
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I mean, we know that a photon can contain different amounts of energy, as proved by the way 'black body radiation' works, right? And as I assume temperature to be an expression of energy too, I take the liberty of assuming this relation to go all the way here ( for the question :)
You might say that the vibration here is their 'motion' in SpaceTime, but how can photons then be of different energies, as they all seem 'identical' in all other ways, except in their polarization/spin possibly?
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Normally a definition of the photons energy is made from its 'frequency' or inversely its wavelength.- Look here -. (http://en.wikipedia.org/wiki/Photon#Physical_properties). But when looking at one single photon, how do you define a frequency to that? I don't expect it possible, other than as a 'vibration' or like a defect in the fabric of SpaceTime. Also, 'virtual photons' have no such restrictions. So, how do they do it?
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photons are defined by their energy...
Normally a definition of the photons energy is made from its 'frequency' or inversely its wavelength.- Look here -. (http://en.wikipedia.org/wiki/Photon#Physical_properties). But when looking at one single photon, how do you define a frequency to that? I don't expect it possible, other than as a 'vibration' or like a defect in the fabric of SpaceTime. Also, 'virtual photons' have no such restrictions. So, how do they do it?
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it is defined by its energy...
Er, what is defined by what exactly?
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Photons don't bounce on atoms, they are absorbed and another photon is emitted... This takes some times so you could think it slows down the photon but it is not the same one...
Interesting. What causes it to emit, just having too much energy to store it all? If so, it seems strange that it would accept more energy and kick out old, rather than simply letting the original photon pass by.
Atoms exist only in specific quantum energy level states. Each state is associated with electrons energy levels. An atom can be excited to another higher energy quantum state if it is hit by a photon of the same energy as the difference of the 2 quantum states. It results in a higher quantum vibration state of an electron revolving around the atom nucleus. The atom will then emit another photon in order to return to a more stable quantum state (probably of the same energy as the prior photon).
Late late reply, sorry. That makes sense. What determines the energy requirements of a quantum state?
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How Do You Catch An Atom And Pin It Down? (http://www.npr.org/templates/story/story.php?storyId=130268498) [:o]
Could one do this to light using, say, leptons? Something else?
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yes, light slow down in denser transparent medium. such as water, glass, and prism..
http://en.wikipedia.org/wiki/Speed_of_light
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yes, light slow down in denser transparent medium. such as water, glass, and prism..
http://en.wikipedia.org/wiki/Speed_of_light
Please see the first post on this topic.
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I understand that the subliminal speed of light in a material of refractive index of more than 1 is commonly attributed to its absorbsion and re emission by atoms.
Is this not now considered rather over simplistic what other explanations are offered.