This professor, Dr. Bowley I believe is his name, is so gifted in teaching science. He may never see this comment, but I just started teaching labs as a graduate student and I have been constantly studying Dr. Bowley's teaching through Sixty Symbols. It is amazing.
1 Baff is the amount of work needed to bring a particle reacting to light of 671.005 nm into a state where it instead reacts to light of 671.000 nm. It is all explained in the video :D And it ffints nicely into the SI system where every unit is calibrated to basic laws of physics.
I have always wondered how that works! Also, you folks are getting better and better at editing together videos that do a good job of leading the viewer into ideas. You are excellent.
Just subscribed today and already learned something I've wanted to know. How they cool with lasers. Of course I could have googled it but for some reason never did. Keep up the videos!
This Sixty-Symbols Series is brilliant! And to a Fellow Aussie, (I think Meghan gave that away a couple of vids ago) Well done. You've made me sound smart again.
The professor is so good at explaining science because he has a humility and understanding of the lack of understanding of his audience - I wish I had had him as a lecturer at university!
On a complex science subject there exists union of a subject expert and a joyful demonstrator expert to make us understand at least partially and also realize the complexity of such systems and situations. Thanks.
Always enjoy learning something brand new, off to read-up a little on Doppler-cooling, which I guess is the whole point, so cheers. ... and Professor Roger is of course correct, nothing beats a really good sneeze.
Very good job editing. I was able to easily understand what they were talking about. If you remove 1 key scene from this video, it doesn't make any sense.Well done Brady!
I never thought it would be possible to make laser cooling sound any more complex but that guy just did it.... Bravo! not so much complex as it was long but still hahaha. Great video.
i have a question if anyone knows the answer, now we hit the atom with photons in the opposite direction of the atom's motion to slow it down. but in this direction due to doppler shift the atom sees a frequency closer to its resonance frequency o it will absorb it. so what about its emission? i mean that it still absorb energy and re-emit it dost these collision due to compton effect that we consider this atom like a free particle that absorbs part of the photons energy and changes in its momentum ?
@ByakuyaZERO No it is significant: this is where the kinetic energy of the atom is lost bit by bit so that the atom loses its kinetic energy. It recoils when it absorbs the photon and goes into an excited state; then it re-emits a photon which can go in any direction so on the average there is no recoil, and some of the kinetic energy is lost. Also the entropy of the gas goes goes down as well as it cools, but the entropy (disorder) of the photons increases so all is well.
Oh WOW!! I've wondered for so long how they get stuff into such tiny temperatures. and YES i did think laser alway heated up or burned up stuff :D Thanks for clearing the misconception.
@mr0myster Yes! Also it is improper to state "degrees kelvin" Both rules are often broken. It is sufficient to state "zero kelvin" without the absolute or the degrees.
does this mean, if you accidentally start with a laser frequency that's too *low*, you'd heat them up instead? since they might catch the light as they're moving away from it instead of towards it?
I have one question. Because the atoms are absorbing I assume that the electrons of the atoms are going in to a higher energy state. I know that this increase in energy doesn't imply a temperature change (electron energy != kinetic energy). But, why don't the electrons fall back in to a lower energy state and eject a photon which would counteract the momentum change? Is it because this kind of cooling is only feasible for gasses which are receptive to photon absorption but less susceptible to ejection or is it because the photons aren't being absorbed by electrons but by some other particle (something in the nucleus maybe?)?
Is it so that because of the Doppler effect, the atom emits a photon with a larger wavelength and energy than the one it initially absorbed; also does this cause the reduction in the kinetic energy of the atom (and cooling due to repetition of this process)?
@IngeniousSheep the atom does absorb the photons, and later the spontaneous emission of these photons will contribute to cooling atoms, while induced emission of such photons does not help. Wiki page about "laser cooling" gives same explanation as seen in the "Doppler cooling" part.
Doesn't photo electric emission take place when you but the Na atoms with photons of the correct frequency? Also, why are sodium or rubidium chosen for the experiment?
@RupertsCrystals I think he said the energy of the photons when absorbed by the atoms turn into the "momentum" of their electrons, making the atoms change into an excited state. The professor said there's a recoil or "nudge" or "push" whenever this happens. What I want to know is: how do photons -massless- hitting an atom have an effect on its kinetic energy? Why do the lithium atoms slow down when they become excited?
I understand mostly everything going on here with the doppler effect and the shifts which occur, but why do the photons add on once the particle matches the frequency of the laser?
If absolute zero is the absence of molecular motion, is there a corresponding opposite temperature? A point beyond which you can no longer add heat to a system? Would that be the temperature of gas molecules moving at the speed of light?
I'm confused by something. So you need the right frequency for the atom to be affected, you need to change the laser light, like he said. But unless all of the atoms get hit and stay at the speed they need to be, won't some "fall off the bus," so to speak? In that if I need frequency X to slow the atoms down, and one of them doesn't get hit by any photons, and then the frequency is changed to Y which is no longer what it needs to be for those particular atoms, are they just left as they are?
so you shoot a photon that has a certain amount of energy into another moving particle that has energy and and the resulting energy is less because the energy difference is stored in the particle itself by exciting an electron? is that correct? if not where does the energy go? and isnt the particle eventually going to go back into its ground state and emit a photon and thus start moving again? i hope i can get some answers! thanks for the great videos!! keep it up!!
So when the photon is absorbed, it's quickly re-emitted, right? Is it re-emitted back in the direction it came in from, or is it randomized? I realize that in either case the interaction will effectively steal momentum from the subject on average, but I'm curious.
Nifty thing about this: because the nature of laser cooling is that the mass of the target atoms are directly related to their resonance, this technique can be used for (and has been adapted to) isotopic enrichment.
Mass and energy are interchangeable so you just look for the energy required for a particle to have an effective mass so great the particles individual gravity causes the escape velocity from the particle is greater than the speed of light.
if movement of atoms mean temperature, then there have to be a maximum temperature because atoms can't move at the speed of light right? what's that temperature limit?
Question: So the atom gets exited and gets slown down due to a "recoil".but does the atom emit an EM-wave with a higher frequentie than the incoming laser light frequentie ? because you'd otherwise be losing energy because the kinetic energy of the atom gets smaller. Hope my question is clear :p
Where does the energy go, though? Don't the electrons on the atoms have to re-emit the photons to return from their energised state, regaining the momentum they lost (albeit in a random direction)?
wait a minute. when the electron of a given atom absorbs a photon and the atom goes to an exited state it gains energy. how can you be cooling down the gas if you continuously ADD energy to it's atoms? Are these atoms all ending up at higher and higher states as they are cooled?
I have some questions: 1. If its in space or somewhere with 0 gravity, does its time to cooling off increase or no? 2. If you made a sphere of lasers would it go faster, or the time is the same even if you use just 2 mirror pointing at each other back and forth? 3. So it's affected by the Doppler effect? And what would happen if you...put your hand of an object on the middle where its cooled off? Very interesting video btw. ^.^
Yes, that is correct. When the photon hits the atom it does so with some momentum, this will impact the movement of the atom slightly,thus slowing it down. In doing so the photon causes an electron jump into a higher energy state, and because electrons don't like being in this state it will return to its original energy level, though the emission of energy, taking the form of a photon...
Does anyone know of any diagrams for a laser cooling setup? I have access to multiple laser diodes of the right wavelength and power, as well as stuff for the optics.
I'm curious where you obtained such materials? It's difficult to find precision lasers for this purpose, so I'm wondering what company or lab you got laboratory quality materials from.
MegaSqueakymouse just rewatched the video, I realized that the diodes sold by DTR are not the right wavelength. Using the diodes from a cd burner and a beam combiner would probably be the cheapest way for someone to get the diodes for a project like this.
So how do they fine-tune the laser frequency as they hit the atoms?....if i understood the basic idea correctly, the laser frequency constantly has to undergo a change to match the atom's frequency to cool it...
@bmbirdsong v=0 is just the vibrational quantum number. This isn't equal to T=0 or absolute zero. The reasons behind this are pretty complex, but it's due to anharmonic properties of molecular vibrations and fun things like that. Wikipedia is your friend on this. :)
@anonymousbl00dlust I also am not an expert. I think, that when the photon hits it slows the atom down. Then the photon is re-emit in a random direction and will gain momentum again, but since it does this a lot of times and the direction is random it will equal out at some time and only the slowing down effect of the photon hitting will matter, because it always hits from the same direction. Someone pleas correct me if I'm wrong.
@elflordbob1 Why would that be? I would imagine converting energy into matter would only occur at very high energies, even if it's an unknown form of matter. Though I suppose dark matter particles could be of very low mass...
Einstein once wondered what it would be like to travel alongside a beam of light. As I recall he pondered what the world around him would look like as he cruised along at 186,000 miles per second AND he pondered what the beam of light itself would look like as he traveled alongside it. My question is this, we have managed to slow a beam of light down to a crawl inside a Bose-Einstein Condensate. Aside from its speed, is the properties of light still the same regardless of its speed and can we study and learn things about light inside the BEC that we could only speculate about before the advent of the BEC?
What's the point of all the mirrors and lenses etc. if all that ends up happening is the lasers get routed through a fiber optic cable to some other spot? The lasers are already, presumably, coherent and everything, so what more needs to be done?
+Ryan Lanzetta There is just one laser, but they need many beams from all directions... The apparatus is meant to split the beam into many beams that are then routed to the cooling chamber...
+Jake K. I don't think coherence is the major issue here. The reason one laser is used is because the laser used here is expensive piece of equipment - so a mirror assembly is just more economical than having three lasers!
Also, as I understand from the video, if a different frequency is needed, there's some arrangement that can shift the frequency of the laser a tiny bit.
why does not an atom get momentum in random direction when it absorbs photon? I mean electrons absorb the photon and gets excited so how this process give momentum in specific direction to the atom?
+Mad Ad They get the initial momentum from the incoming photon in the direction of that photon. Through spontaneous emission they re-emit a photon in random directions and get a commensurate momentum kick in the opposite direction of the re-emitted photon. However, since the direction of the spontaneous emissions are random over the entire solid angle, the average effect is zero over many emissions, so the net effect is to be kicked in the direction of the incoming photons from the laser.
@bmbirdsong There's no such thing as an 'absence of molecular motion'. Molecules will still possess a zero-point energy, and can never reach absolute zero. On the other hand vibrational energy levels go from v=0 to v=∞, so there is no maximum temperature. Or put another way, to get something to the speed of light would require infinite energy. Thus unless you can get to ∞°C you won't get an atom/electron or any particle with mass to the speed of light.
So if you have a laser cooling something very cold, and a laser heating something very hot, you could create a heat-exchanger (peltier arrangement) that allows you to re-capture the energy?
@gamesbok The photon is absorbed by the atom and an electron goes to an excited state. The electron goes back to the ground state and a photon is emitted isotropically, that is all directions of emission are equally probable. On the average (the photon can be emitted in any direction) the atom loses momentum, and also a bit of its kinetic energy is taken away by the photon. Repeat the process ten thousand times and the atom slows down and nearly stops. A Nobel prize results for this idea.
I believe he's retired now. I think there's a video on it called "The Retired Professor." I liked any time he made an appearance on this channel too ;)
@xXmatthdXx That seems unlikely. You need a gas or at least a liquid for this to work. On a CPU, which is opaque, you could at best shoot lasers on it from the top etc., but not from all directions, and the laser wouldn't reach into the CPU very far (or at all).
so really with laser cooling you are only slowing down the gas particles by impacting them with high speed photons. now they slow down because they are being hit by an in coming particle that is moving in the opposite direction. so why doesnt it speed back up in the other direction? if you cancel motion in one direction wouldnt it pick up in the other?
So interesting! I have 2 questions! 1- Where does the energy go? 2- Is that theory (about the mechanism of how it works) confirmed or is it just a hypothesis?
I think I found the answer to question one! The cooled atom will emmit a photon immediately. But now I have a new question! 3- How can you tweak / fine-tune the frequency of light with that precision?!
I don't know if this question has been asked already, but I thought Rubidium and Sodium were solid at room temperature. So when you are lowering the temperature of this gas doesn't it just turn into a solid? How do they keep the atoms seperate while lowering the temperature to such extreme levels?
john hall melting/boiling points are different at different pressures, remember this is being done in a vacuum for example at water freezes at a lower temperature at high altitude (lower pressure) than at sea level (higher pressure) so because these particularly unstable metals are in a vacuum, they can be kept as gases at much lower temperature
No expert but I found somewhere that they almost immediately release a photon afterwards in a random direction, with a tiny bit more momentum than the original photon, thus everything is conserved.
Alex is right, in fact this results in a cap to the amount of cooling you can achieve with lasers alone. This cap is called the "Doppler Limit". We can, however, cool atoms past the doppler limit by adding things like an external magnetic field as in a MOT (magneto optical trap), and polarization gradient cooling which uses polarized laser light to further cool atoms.
...This emission is in a random direction, and carries with it its own momentum, meaning that it also affects the movement of the atom. This may be either slowing it down or speeding it up, but because of the large numbers of photons that are being shot at the atoms by the 3 lasers, and the random nature of the direction of the photon's emission, the result is a net cooling of the substance. Hope I've explained that well... If not let me know :)
@MainsOnTheOhmsRange The atoms absorb one frequency of light but then emit photons. I'm guessing they probably emit photons at a different frequency than they absorb leading to a net reduction in the energy of the atom.
@mathiaspaul1987 Same question here. I guess if the atoms give energy of, they emit light at the same wavelength they had absorbed, so if the frequency had already been change by the time they return to their ground state, the light is kind of "tainted" in the container. Or does is the energy just spent on the momentum change? I'm just guessing really :P
Laser cooling is awesome. You can call it laser compression. You're using an electric field to counter the motion of another electric field. It works, and it still has stuff to tell us.
Haha this professor is awesome, and I never knew molasses was an American word! Treecool? Is that what y'all call it? Haha, awesome video. And it's not sad he retired! Working your whole life and getting to retire is a glorious thing!
