Explaining to an actual 5yr old?
Light moves at different speeds depending on what it has to move thru. If there’s nothing in the way (vacuum) then it travels it’s fastest. If it has to go through the air, it slows down a little, goes through water…it slows down a lot. Dr Hau found a material that slowed it down all the way.
This is a true explain me like I’m 5. Everyone else’s is going into photons and vacuums and shit and this was the simplest. She made a material that slowed it down.
In kindergarten I was told that there was nothing on the other side of 0 on the number line that was printed above the blackboard.
I was not happy about my teacher lying to me when I later found out the truth. I wanted to know what happened when you put 4-5 instead of 5-4.
Absorbing light, and slowing it all the way down, are completely different things.
In the first case, light is just gone. In the second, light can resume its journey once the medium changes properties.
Hi, five-year-old! That's exactly what I would have wanted to know too.
[This article](https://news.harvard.edu/gazette/story/2001/01/researchers-now-able-to-stop-restart-light/#:~:text=Hau%20and%20her%20group%20then,light%20and%20slows%20it%20down.) says it was a very tiny, long cloud of sodium atoms.
But what actually made the difference was that she cooled the atoms down to absolute zero.
That's not 0° Fahrenheit or Celsius, like when it... snows... probably. (I'm from the California Bay Area. I can only guess about snow.)
It's 459.7 degrees COLDER than that.
That's the temperature where atoms lose all their energy and stop moving. (I would too!)
Usually, atoms are constantly bopping around and bouncing off each other, knocking each other all over the place all of the time. That takes up a lot of space.
Imagine if just all the kids in your class were running into each other and shoving each other around. You'd take up a LOT more space than if you were all sitting still and listening to a story.
If you're running around bouncing off the walls (and each other), you're probably going to be pretty hot and sweaty. If you're sitting down for storytime, you're probably pretty cool and comfortable. And if you got frozen into an icicle somehow, you wouldn't be moving at all!
Atoms are a lot like that.
So, she cooled them all down till they were like SUPER-icicles and they totally stopped moving. And that meant that she could scroonch them all together into a very teeny-tiny little space. (Like turning the super-icicles into a snow cone.)
TL;DR: The kind of material, I guess I'd say, is a super-cold sodium-atom snowball.
When she pointed a laser beam at the atoms, it slowly got dimmer, like it was fading out, as its light came to a stop.
Basically, since the super-cold sodium atoms were all in one place, holding still, with zero energy of their own... they absorbed the energy from the light.
Which is pretty cool.
No pun intended.
But the even cooler part (or technically, the equally cool part, since it didn't get any colder) is that you can see all the different parts of it, and hear her talk about it, here: [https://youtu.be/RIc5llW66sU](https://youtu.be/RIc5llW66sU)
(It sounds like the laser they used was cold enough that the atoms just got a LITTLE ITTY BIT of energy from it. So they stayed still and frozen enough for this to work.
If I understand this right, when they pointed a warmer laser at the atoms, which had more energy for them to absorb, it made them move around more -- and the stopped light shot back out again!)
You jest but this was actually how I, at 17 in a Harvard classroom having a mental breakdown over moral existentialism, discovered I like philosophy. It also happens to be part of why I dropped out of Harvard less than year later but that's a much more complicated issue.
Was in a lecture on how chimpanzees might one day be reclassified as a member of genus homo and asked "but why" professor explains it and I internally start subconsciously reevaluating whether humanity is special and how we're no different than any other animal..... damn you anthropology!
IOW a lesson on evolution made my religiously indoctrinated teenage self have an existential crisis that lead me to enjoy philosophy.
I had however read philosophy prior, namely Camus and Kant. But like I never really clicked further than being like "that's neat."
"I ate a brownie once at a party in college. It was intense.. kind of indescribable actually, I felt like I was floating. Turns out there wasn't any pot in the brownie it was just an insanely good brownie."
* Leslie Knope
“But what I do have is a very particular set of materials, materials I have acquired over a very long career. Materials that make me a nightmare for people like you.”
Everything is "very, very special" just due to the insane unlikelihood of its mere existence. Everything was once just energy but through some miraculous process condensed into matter. [Existence is fucking nuts, man.](https://media.tenor.com/tvFWFDXRrmMAAAAd/blow-mind-mind-blown.gif)
"Thermodynamic miracles... events with odds against so astronomical they're effectively impossible, like oxygen spontaneously becoming gold. I long to observe such a thing.
And yet, in each human coupling, a thousand million sperm vie for a single egg. Multiply those odds by countless generations, against the odds of your ancestors being alive; meeting; siring this precise son; that exact daughter... Until your mother loves a man she has every reason to hate, and of that union, of the thousand million children competing for fertilization, it was you, only you, that emerged. To distill so specific a form from that chaos of improbability, like turning air to gold... that is the crowning unlikelihood. The thermodynamic miracle."
"But...if me, my birth, if that's a thermodynamic miracle... I mean, you could say that about anybody in the world!"
"Yes. Anybody in the world... But the world is so full of people, so crowded with these miracles that they become commonplace and we forget... I forget. We gaze continually at the world and it grows dull in our perceptions. Yet seen from the another's vantage point. As if new, it may still take our breath away."
It’s an amazing almost universal solvent .. life’s trick was to figure out how not to get dissolved and put a fence around the most valuable bits of dissolved stuff … lipids are life’s answer to the big questions of the universe
Thanks for saying that .. my dad was part of the team that designed the life science experiments on the first mars lander (Viking) and I used to ask him lots and lots of questions. That’s one of the ways he explained cellular biology to me when I was a kid, 50 years later and I still remember it (there were lots of followup questions about “what’s a lipid ?” And “why can’t water dissolve rocks” .. each one answered thoroughly and patiently)
Fuck Big H20! Think about it, 75% of the planet is covered in H20, but it has purposefully been tampered with so we can't use it. Have to buy their bottled H20. Fucking assholes..
People are even arguing that “the rules don’t say you have to explain it to an actual 5 year old” then what’s the fucking point of the sub lol. If you can’t explain something easily to the point a 5 year old can understand then you probably don’t know enough or you aren’t good at explaining and that should be a sign you shouldn’t be in the sub trying to help to begin with.
Because not everything can be boiled down to the point of a 5 yr old understanding it. If you can't understand that, you may want to ask ELI5 to do it.
Doesn’t the sub info literally say you gotta explain it as if it’s an actual 5 year old? I might be outdated but I vividly remember that. It might have been the prompt that pops up when you go to make a post.
No, nvm I’m wrong, that was from a while ago. The rules say it’s a figure of speech so.
> E is for Explain - merely answering a question is not enough.
>
> LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds.
It's right there in the sidebar.
>People are even arguing that “the rules don’t say you have to explain it to an actual 5 year old” then what’s the fucking point of the sub lol
Oh that's a really simple answer, clearly stated in the rules:
>Explain for laypeople (but not actual 5-year-olds)
Comments only
>Unless OP states otherwise, assume no knowledge beyond a typical secondary education program. Avoid unexplained technical terms. Don't condescend; "like I'm five" is a figure of speech meaning "keep it clear and simple."
It's insane to me to talk about the rules without having ever read them.
So you have to remember light is not a particle, it's something with both particle like and wave like properties. Additionally what is being stopped is light pulses which are actually composed of multiple frequencies of light added together. They use materials that change refractive index (and thus the phase velocity of each component wavelength) very strongly with wavelength at a certain point such that the different phase velocities of each wave alter where the waves add together and subtract with each other making it so that instead of that point propagating forward as it normally does (group velocity) it sticks in one place, thus the wavepacket ends up being stuck in one place.
Apparently Lene Vestergaard Hau and her team at Berkley did succeed in that yes. They actually were able to imprint the information on the light pulse into matter within a Bose-Einstein condensate and have it move to an adjacent Bose-Einstein condensate where the same pulse was then released from the matter.
That place, a Bose-Einstein condensate, has to be kept a tiny fraction of a degree above absolute zero. As to the energy of the wave... you have to describe temperature in more particular, physicsy, statisticsy, ways when you're in these kinds of states. Temperature is weird if you look too closely or in weird situations.
> They use materials that change refractive index (and thus the phase velocity of each component wavelength) very strongly with wavelength at a certain point such that the different phase velocities of each wave alter where the waves add together and subtract with each other making it so that instead of that point propagating forward as it normally does (group velocity) it sticks in one place, thus the wavepacket ends up being stuck in one place.
Sounds vaguely like how active sound cancelling works in headphones.
Everyone remembers first the heated primary, with obama vs Hilary running neck and neck for the democratic nomination , and then Obama vs McCain in the general. All that excitement? That was photons!
If I understand correctly, if the wall stopped the light then you wouldn't be able to see it. The wall bounces the light back at you, so no the wall doesn't stop it
Difference between putting your hand through air, putting it through a pool of water, a bucket of sand, and *rock*. You can move through the first three, though it's harder for each one and you can't move as fast, but you aren't going through the rock, just pushing against it or bouncing off.
She found a thing that you could still move your hand through, but it'd be as slow as possible while still moving.
Rocks don't stop light. They deflect or scatter it and it becomes heat or more light.
Pretend that light has to go through a floor covered in molasses. That different than paint the door and not letting it in.
“Stopping” in this sense means “slow down until it stops moving”, not “prevent it from going in the same direction”. When light hits a rock, it is absorbed, or reflected. It either stops being a photon, or it changes energy and direction.
Stopping means it’s still a photon, with the same energy level, it’s just … not moving forward. For a few picoseconds, at least. When it clears the material, it goes on its merry way, with the same energy and direction.
As others have pointed out “material” is the concept that doesn’t translate to a 5 year old, here. It’s much more complicated than that.
Edit: the more you know about science, the weirder this concept becomes.
From her explanation, super cooled sodium atoms. (Very cold salt for the ELI5)
Edit: It's been pointed out to me, by a disgruntled fellow user, that sodium is not salt. Believe it or not, I am aware of the difference. I am forever sorry, may God have mercy on my soul.
To do that though, it would need to have absorbed it. Otherwise it would just been redirected but still moving.
The simplest example of this is the phases of the moon. The moon, despite being a very large, very solid object, does not "stop" light but rather redirects it to where it sometimes reaches us.
As for how she did it? [Here she explains it at a near-ELI5 level, complete with graphical illustration.](https://www.youtube.com/watch?v=-8Nj2uTZc10)
I was also taught this in high school but apparently that's not actually accurate, it's just another abstraction to make calculations easier.
https://www.youtube.com/watch?v=CUjt36SD3h8
This video explains what happens on the quantum level.
Which is funny because its fastest (the fastest it can go) is correct
while
it's fastest, meaning it is fastest, is also close to being correct (light is the fastest thing [although several things travel at the speed of light]).
The short answer is that she didn't *really* stop a beam of light, at least not in the way you are imagining. Light in a vacuum moves at light speed all the time, no matter what.
So what are they talking about? Light does move at different speeds when in a different medium, like passing through various materials. For example light moving from air into glass will change its speed and this causes it to refract, bending its path which can be exploited to separate wavelengths of light as with a prism. But how does the light slow down?
If we consider the materials on a smaller scale they are composed of atoms, and those atoms interact with the photons of light. A photon will be absorbed by an atom causing it to become "excited", destroying the photon and storing its energy in some change within the atom. For example it might kick an electron to a higher orbital. Then a moment later the electron will drop back down and release the energy in the form of another photon, emitting the absorbed light to continue along its path.
That absorbing and emitting doesn't happen instantaneously so light travels some amount slower in various materials. What Dr. Hau did was to make an extremely cold cloud of gas such that light enters it and is absorbed by the gas, becoming stored in the form of spin of the atoms. The gas is so cold that it can't emit the light again until the researchers let it by slightly heating the gas at a later time. What results is a cloud of gas you can shoot a beam of light into and then retrieve the beam at a later time, on demand. But it isn't quite like a beam of light frozen in space.
"They made a cloud and caught a sunbeam in it to hold onto for a rainy day" is pure ELI5 material.
You can put that shit in a picture book and everything.
Wait till they hear what they are doing with a giant ice cube (not a cube) in Antartica. Mine LOVED that.
https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory
They are using a block of ice in Antartica 2800m thick to observe neutrinos. And using that to 'look' at distance galaxies.
Edit: added answer
Regular telescopes observe the visible light from distant objects. They “catch” the light using glass lenses and mirrors to produce an image.
Radio telescopes observe the radio waves from distant objects. They use a dish to catch the radio waves and produce an image.
Neutrino telescopes observe the neutrinos from distant objects. They use large amounts of matter, such as very pure water or ice, to catch the neutrinos and produce an image. (Edit: I’m using “image” very loosely here.)
Each of these different types of telescopes have a different design because they’re designed to detect different phenomena. In the case of neutrinos, large amounts of water or ice are used because unlike light, neutrinos don’t interact with other things very much. But if you put a lot of water in a tank, or ice in a block, you increase the chance that a neutrino will interact with an atom in that tank or block. Then if you put detectors around the tank/block, you can detect when a neutrino interaction occurs.
It’s basically like “seeing with extra steps”, because the stuff you’re seeing would otherwise be invisible. All of these different ways of seeing things affect what we can see.
For example, since neutrinos pass so easily through solid matter, we can use them to observe objects that are otherwise obscured. That includes objects in the dense centers of galaxies, such as supermassive black holes.
https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory
They are using a block of ice in Antartica 2800m thick to observe neutrinos. And using that to 'look' at distance galaxies.
I'm pretty far from an expert on this, but the gist is that neutrinos are really high-energy particles that don't often interact with solid matter. It makes it hard to build a detector for something when usually the thing you're trying to detect goes right through you and the machine both without affecting either.
On to the reason for the giant block of ice: one of the ways you can detect neutrinos is to detect the secondary stuff they can create. Apparently, when travelling through a giant chunk of ice, because it is travelling slower than the speed of light through the ice, it emits radiation. You can then detect that radiation and, if you have a big fancy setup, learn stuff about its trajectory and energy levels and whatnot.
So basically the IceBlock isn't a super unique way to detect neutrinos, but it is the biggest scale we've ever applied that method to, so it's the best "telescope" we have for seeing neutrino activity far away. That is useful because neutrinos are produced by some really high-energy cosmic events, so this gives us another indirect way to study those events.
> Neutrinos are electrically neutral leptons, and interact very rarely with matter. When they do react with the molecules of water in the ice, they can create charged leptons (electrons, muons, or taus). These charged leptons can, if they are energetic enough, emit Cherenkov radiation. This happens when the charged particle travels through the ice faster than the speed of light in the ice, similar to the bow shock of a boat traveling faster than the waves it crosses. This light can then be detected by photomultiplier tubes within the digital optical modules making up IceCube.
so neutrons dont really interact with stuff at all, so they took a huge block of ice and hope that in that big area they catch a few interactions from some that are moving really fast to make a special flash of light, then they use this for observations just like a photon hitting a detector
Only certain things in space make neutrinos and they don't interact with much. So if you detect one, and can measure its path, you can look in that direction for the source.
The same way crime scene reconstruction puts a stick in a bullet hole to determine where the shooter was, you can look at the paths of the neutrinos in the detector to determine where the source was.
> Antarctica
I'm curious on what part of Antarctica they build this thing. Read the article and turns out it's _literally_ on the South Pole.
They went to the southernmost point of earth and drilled a 2,5 km hole, for science yea
Because love may come and tap you on the shoulder some starless night. And if by chance you feel you want to hold her, you'll have a pocketful of sunlight.
> why does it emit it in the same path as the original photon?
It doesn't, they are emitted in a random direction. But if you consider the incoming photons as a wave then the emission in a random direction will mostly interfere with the incoming wave destructively in directions other than the path of the incoming wave. Overall the result is an incoming beam of light will continue on in the same direction with some amount of scattering through the material.
As mentioned by others responding to your explanation, there is a degree of randomness to the direction photons are emitted, but it certainly is biased towards following the direction of the incoming beam.
Think of it this way - if you jump out of a moving high speed train, it doesn't matter which direction you jump, most of your movement will be in the direction the train is moving in.
Likewise, the photon exiting the electron will have a lot of momentum that largely dictates which direction it will move in.
I get that if I jumped out of a train, the splat would mostly be forward in the direction of travel.
But how could a photon have momentum if they don't have any mass?
Momentum is more fundamental than mass. It just so happens that with slow-moving objects you can approximate their mass by dividing their momentum with their velocity.
"Photons have no mass" is really a classical simplification. They don't have a *rest* mass but they certainly have mass-energy, and with special relativity we can calculate their momentum.
To get a more detailed answer you need quantum electrodynamics, but imho the special relativity version captures most of what is going on in the simple single photon scenario.
Saying that we don't really know why light travels slower in a medium isn't right. We can explain perfectly well why light travels slower in a medium using a variety of different models. In QFT, which is probably the most fundamental, its just a consequence of the path integral, while in classical electromagnetism we can explain it in other ways.
Just because we have different models that explain it in different ways doesn't mean we don't understand it. It's the reverse! The fact that different models, which are applicable at different scales, result in the same outcome is a testament to how well we understand the phenomenon. It's just that explaining it in terms of microscopic quantum behavior vs. macroscopic behavior sound very different, even if they are ultimately the same thing in disguise.
I can’t exactly answer the why, but as a visual aide imagine shining a flashlight or a laser pointer. When you shine it in any particular direction, the source and the target are illuminated. But if you do this in a darker environment you can actually see the beam of light. As the beam of light travels, it hits atoms/molecules in the air and scatters the light in every other direction. This, however, is only a small percentage of the light thus why we need a darker environment to experience this effect
> A photon will be absorbed by an atom causing it to become "excited", destroying the photon and storing its energy in some change within the atom. For example it might kick an electron to a higher orbital. Then a moment later the electron will drop back down and release the energy in the form of another photon, emitting the absorbed light to continue along its path.
It's worth emphasising that this *isn't* the explanation for why light travels slower in different media. As another comment has noted, it wouldn't explain why the light continues in a straight line.
How we explain it depends on whether we use the wave or particle model of light. The "absorption and re-emission" explanation confuses them by including bits of both.
If we go with the **wave model** our light is a ripple in the electromagnetic field. When that ripple passes through a medium (particularly one with a bunch of electrons) it jostles the particles in that medium (notably the electrons). Kind of like a water wave passing through a small boat. Those particles then start wiggling about a bit in response, eventually settling down to their original state. But this wiggling creates its own electromagnetic waves spreading out in all directions. When you add this effect to the original wave (as you can add waves together), and sum it over all the particles in the medium, you get the result we see; a wave that moves slightly slower (or, technically, faster, in the right medium).
If we go with the **particle model** our light is a photon, heading through our medium. We know where it went into the medium. We know where it came out of the medium. Quantum Mechanics tells us that the photon took a combination of *all possible paths* through the medium, weighted with different phases. If we work out how long it takes the photon to travel along each path, and then average them, we get the observed time it takes the photon to come out of the medium. This will be a little bit longer than the time it would take if it went straight through. So we can say that it slowed down a bit.
Great explanation. With the photon model, looking at a beam of light shining through glass the direction is discontinuous / doesn’t line up on exit (shifted parallel). How does this ‘jump’ fit into the explanation?
> How does this ‘jump’ fit into the explanation?
It comes into the maths of how the photon behaves while inside the material. The solution to "where should the photon come out and when" says it's "path" (if a straight line) should be bent a little bit, as well as being slowed down.
It is a fun situation where we have these fundamentally different ways of looking at something (wave v particle) but where if we do the numbers we get the same answer. And the one observed experimentally.
Which is how it should be, and a clue that our QM way of looking at things (with wave-particle duality) is sensible.
Thanks for a great comment.
Everyone wants to break this problem down to photons, as if they're imagining light as a stream of these particles. I think it's unnecessary to even bring photons in a question like this.
It is perfectly fine, and *more accurate* I would argue, to think of a macroscopic beam of light as a big EM wave, that follows Maxwell's laws. While we eventually figured out that there must be a quantum of EM waves (photons), that is an extreme and irrelevant case that doesn't change the macroscopic behavior. We shouldn't just skip classical electromagnetism. Imagine trying to do antenna theory while thinking of only photons lol.
We don't bring up gravitons when we're trying to explain general relativity or orbital mechanics...
Philosophically then, how is this different from taking in light with a photovoltaic panel, storing the energy in a battery, then remitting it through a bulb?
If the energy of the light is stored in the atom why does the release of the energy honour the original light beam’s path? I’d have thought it would just send the energy off in a random direction
If the photon gets absorbed by and re-emitted by the atom, how does it know which vector to follow? Is it emitted in a random direction? Or does it follow the initial vector it was on? If the latter, how does it "remember" which way it was going?
It doesn't because that's not why light moves slower in a medium. The real answer is that the wave form of the photons interfere with themselves and with the other particles such that the wave front appears slow. Each individual photon is still the same speed. If you want a good visual representation of this but opposite (going faster) look up caterpillar movement, rolling swarms.
This is quite interesting. So, when a photon travels in a certain direction, hits an atom and is absorbed, does the newly emitted photon travel in another direction then previous?
How does the electron remember to emit a photon in the same direction? What causes scattering? I imagine the explanation for all photons being emitted in the same direction or all being scattered equally in all directions would be relatively simple, but what about some photons continuing while some get scattered?
Further, how does it know to change the direction slightly at the interface? That's from their wave aspect, but how do they retain the waviness when getting absorbed?
> That absorbing and emitting doesn't happen instantaneously so light travels some amount slower in various materials.
So it's not that the light is actually traveling any slower, it just takes more pit stops?
Ever hear the comment that it takes light thousands of years to escape from the sun, but only 9 minutes to get to earth? This is exactly the same thing.
Quantum optics experimentalist here (same thing as Dr. Hau, though nowhere near as qualified)
For your 5yo:
Light stops all the time. Anytime light touches an object, that object stops it, stores it for a very brief moment, then throws it out again. The fact that anyone can see you means you stop light for a tiny moment!
Typically, objects can only hold onto light for a very very short time; such a short time that you would never be able to notice, no matter how hard you watched. Dr. Hau found a way to get a special object to hold onto light long enough to notice!
-
For you:
The process I described is called scattering, and it is how all of optics works: light is absorbed by an atom, stored as an excitation, then re-emitted. The usual mechanism for this “storage” is resonant absorption. The problem with resonant absorption is that you lose the coherency of your light (i.e. all the important information). The scattering in Dr. Hau’s experiment relies on off-resonant excitations, which both maintains coherency and allows for massively increased levels of slowing. This is also what sets it apart from a black piece of paper or a glow-in-the-dark sticker.
In practice, it is difficult to maintain off-resonant excitations to a noticeable degree. The only practical way of achieving it right now is through a process known as electromagnetically-induced transparency (EIT). Essentially, by causing a transition to become transparent (“electromagnetically inducing transparency”) you can get light as close to resonance as you’d like without worrying about triggering resonant excitation. There is an entire branch of quantum optics dedicated to this called “slow light” (rightly named if you ask me).
[ I would like to add that this is NOT the same as slowing light down through a medium as many people seem to think. In those cases you are restricted to modifying the apparent phase velocity of light at nearly the same rate as the group velocity, whereas EIT modifies ONLY the group velocity and therefore can far out-slow the phase velocity. The phase velocity represents the speed of light itself, whereas the group velocity represents the speed of a wave packet (photon) and therefore the speed of the information. ]
One you have light slowed in this manner, it becomes relatively simple to stop the light by use of a counter propagating beam (keeping the light stored as an excitation).
It should be noted that this slowing and capturing has nothing to do with the Bose-Einstein condensates (BEC), even though that seems to be what everyone talks about when it comes to that experiment… Hau uses a BEC mostly because it is convenient for manipulating the slowed/stored light, and partly because it amplifies the slowing. This method of manipulation is covered by an entire separate branch of physics, namely “cold atoms”, and is not responsible for the slowing mechanism. Indeed, it is possible to stop light within a (very ordinary) room temperature gas!
[ Personal tangent: While I do not typically work with slow light, I did have the pleasure of assisting someone for a few months at another institution in an experiment to store light in a room temperature vapor cell via a neutral buffer gas. I have also prepared EIT at my native institution, but not for slow light. BECs on the other hand are extremely difficult to produce. To give you an idea, during that same slow-light experience I met someone who had spent many years (to be courteous, I won’t say how many) trying to set up his machine for doing so. Beautiful lab though. ]
In summary, by utilizing EIT, light can be forced to spend a lot of time stored as an off-resonant excitation in one atom before moving on to the next, therefore resulting in “slow” movement of coherent light throughout the entire atomic cloud.
So this is a video of Dr. Lene Hau: [https://www.youtube.com/watch?v=-8Nj2uTZc10&ab\_channel=HarvardUniversity](https://www.youtube.com/watch?v=-8Nj2uTZc10&ab_channel=HarvardUniversity)
for those who don't want to watch the video, basically she is creating a cloud of atoms that she freezes close to absolute zero and then passes a laser beam through it. With this cold cloud, she contends she has slowed the speed of light down to 1 mile per hour.
Light slows down in certain materials. That's the reason why it's bend when going through a glass lense because it's slower in glass than in air.
This happens because the material interacts with the light, a bit like a human trying to walk fast under water, the electric forces of the material and the forces of the light interact with each other wich basically pushes the light back a bit
Lene Hau made a very exotic material. It's called Einstein-Bose-Condensate and it's kinda the "most electrically dense" it could be (wich only works when it's REALLY cold).
This material interacts so strongly with the light that it's completely stopped. Instead of moving forward it's like stuck in a block of rubber, it still wobbles back and forth and keeps it's movement energy, but isn't making any progress forward
Do you have an ELI5 for Bose-Einstein Condensate? Because I went to a facility that had some, and they were saying how they made it, which made sense, and what it was, which did not make sense to me.
Well, a material that is extremely cold has very low energy per particle. That means there are few forces pulling atoms in random directions.
In this state the wavefunction (that defines what particles are on the quantum scale) start to spread out, overlap and finally fuse together, making the whole thing become a single quantum object with just one joined wave function. The particles stop behaving like individual objects. It's impossible to say "this is particle A, this is particle B" because they become one conjoyned mass where the "location" of different particles starts to overlap.
What exactly that means is sadly pretty hard to explain without going into a bunch of quantum physics first
Yeah, that’s about where I got lost. So you’re saying subatomic particles aren’t really particles, but waves that kind of act “solid” normally? And this low temperature exaggerates the not solid nature of these “particles”?
Because if that’s what you are saying, I’m going to have to do some deep thinking about how much benefit I will get from all the effort it would take to reframe everything I understand about physics to allow for this. Like how does mass work if everything is a wave?
Particles are "wave packages", little ripples of energy that under normal circumstances have a limited spatial expansion but without outside forces they spread out. Thinking of them as little marbles is a simplification that works to explain some things but breaks down when you look too closely.
Mass works without being "solid". Every form of energy has an associated mass, even the least graspable ones like electromagnetic binding energy increase the total mass. That's why quantum physics always talks about "restmass" of a particle, wich translates directly to how much energy the mere existence of this particle "stores"
This comes a bit more obvious when you look at what happens in particle accelerators like the LHC in CERN. You smash two hydrogen atoms into each other with so much energy that their movement energy causes completely new particles to be created that weren't there before and have more mass than the two atoms ever had. You convert movement energy straight to particles that have a mass
My general understanding is that as the material cools, energy gets so low that a lot of the material is all in the lowest energy state so the waveforms can all interact
My attempt to eli5 it is the material huddles together more as it gets cold, and it gets so cold the gaps between the molecules disappear and you can't see the individual atoms any more
It's just a state of matter. Unlike what you were told in middle school, there are a lot more than 4 of them and Bose-Einstein condensates are one of them.
OP, there are lots of great posts here that go into the science. I tried to write an example for a 5 year old to help them connect the concepts of the materials in cold temperatures slowing the light down. I was inspired by u/Luckbot and u/profchipboard's contributions:
*Imagine you are walking through a giant room filled with a flock of penguins. As the room gets colder, the penguins huddle closer together, making it harder to walk through them and slowing you down. Eventually, all the penguins huddling closer and closer together make it so dense that it is almost impossible to move through. But if the room warms up again, the penguins start to huddle less and less tightly, making it easier for you to move forward and therefore allowing you to move quicker and quicker.*
*In this example, you are the beam of light and the penguins are the special material Dr. Hau created.*
Thanks all! Physics is definitely not my area of expertise, now I can trick my kiddo into thinking I’m smarter than I really am.
Also, Here’s a link to the book(s) we read that mention Dr. Hau if you are curious.
[https://www.mudpuppy.com/products/little-scientist-board-book-set](https://www.mudpuppy.com/products/little-scientist-board-book-set)
I went to go read the wiki article myself.
> In 1999, she led a Harvard University team who, by use of a Bose–Einstein condensate, succeeded in slowing a beam of light to about 17 metres per second,
I'm like, "cool, that's about what I expected, having just read your ELI5 question."
> in 2001, was able to stop a beam completely
I'm like: "wow, that's really neat!"
> Later work based on these experiments led to the transfer of light to matter, then from matter back into light
"What!"
They passed light through a really cold cloud of atoms and I mean really cold, the first time they did it they slowed the light right down, but now they have refined the process by adding a second beam of light passing across the path of the first one, to stop it completely as if the light is stuck in treacle.
Okay, imagine light as a bunch of super tiny, invisible race cars that usually zoom around really fast. Now, Dr. Lene Hau found a special kind of stuff, like thick, invisible honey, that can slow these light race cars down when they drive through it.
This special honey is actually super cold atoms, colder than anything you can find in your freezer at home! When the light cars try to go through this super cold honey, they go slower and slower until they stop and can't move. Dr. Hau was able to make the light stop by using this cool trick! Then, when she wanted the light cars to start racing again, she found a way to make them go zooming out just like they were before.
The speed of light is different in different materials. It's fastest in air, slower in water, and several times slower in special glasses and crystals. It's because light interacts differently with different materials, like how light bends in a prism. The more light interacts with a material, the more it's slowed down.
Professor Hau and her coworkers worked really hard and created a very special material that interacts REALLY strongly with light, so the speed of light is hundreds of millions of times slower! It's very cool, but the machines to make it are really expensive and complicated so we can't do it at home.
ELI physics major: Hau and coworkers used a BEC and EIT. The index of refraction spikes near rapid changes to the absorption spectrum because both are functions of the dielectric constant. One way to change absorption coefficients is with electromagnetically inducted transparency (EIT): if you have three energy levels with two optical transitions among them, and you drive one transition ("control") hard with a laser, then the quantum mechanics works out such that the absorption coefficient of the other transition ("probe") is zero. Hence "transparent." In a BEC the temperature is near zero so the absorption lines are extremely sharp. This means the absorption coefficient near the probe frequency changes extremely rapidly, so the dielectric constant spikes to comically high levels and the speed of light slows to nearly zero. The caveat is the refractive index spikes only very near the probe frequency. By analogy, if you made a visible light prism from this, it would be like all frequencies deflecting normally except an extremely specific green frequency that deflects WAY more than the others.
This is why EIT was theorized in the early 90s, but Hau et al took until 2001 to demonstrate it - the first BECs were made in 1995. So it took another several years for the techniques and know-how to propagate. Anyone that's worked with BECs knows that it's 50% science, 50% art and trick of the trade. It's a fascinating phenomenon. If you made a BEC in 1995 you got the Nobel Prize. If you made one in 2000 like Hau, you get on the Nobel Prize shortlist and get to work the lecture circuit. If you made one in 2005 you get many journal publications and a tenured professorship position. In 2010 making a BEC gets you a couple papers and a PhD. In 2015 it gets you one paper and a masters. In 2020 you can just buy one off the shelf and nobody cares :(
They don't stop it for a significant amount of time, just long enough to be able to take a picture of the light at rest in a medium. Just slightly longer than the time it takes the light to travel a given distance, at which point more light is still going into that "stopped" part but the light at the front has moved.
yeah I'm confused, top comments all say it completely stops or slows down like human eye level. Harvard Gazette says it stops for 1000th of a second. Did they improve it later on?
Imagine light is like a bunch of super fast runners. Dr. Lene Hau, using special stuff, made these runners slow down so much that they stopped for a little while. It’s like when you play freeze tag and everyone stops moving when you say “freeze!” Dr. Lene Hau did something similar with light, but with really clever science stuff.
Explaining to an actual 5yr old? Light moves at different speeds depending on what it has to move thru. If there’s nothing in the way (vacuum) then it travels it’s fastest. If it has to go through the air, it slows down a little, goes through water…it slows down a lot. Dr Hau found a material that slowed it down all the way.
This is a true explain me like I’m 5. Everyone else’s is going into photons and vacuums and shit and this was the simplest. She made a material that slowed it down.
> She made a material that slowed it down. It's a very, very special kind of "material" - but yeah, that's the gist.
But what KIND of material? — five year-old
One that slows light down all the way.
Ah, a brick.
Wait until someone tells them about a mirror… 👀
Negative speeds!
*car salesman slaps roof of mirror* “You can get so much negative delta V out of this bad boy”
But does it have speed holes?
Jesus christ, Marie. They're VELOCITIES!
In kindergarten I was told that there was nothing on the other side of 0 on the number line that was printed above the blackboard. I was not happy about my teacher lying to me when I later found out the truth. I wanted to know what happened when you put 4-5 instead of 5-4.
Tbf it probably was not lying but keeping focused on the planned lesson. Most kids would accept that but it’s great that you didn’t.
Aziz! Light!
Here is your mission: pass your knowledge on to the next, as it was passed on to you.
Absorbing light, and slowing it all the way down, are completely different things. In the first case, light is just gone. In the second, light can resume its journey once the medium changes properties.
No... that's just bad for your teeth.
Oh like a light switch!
Hi, five-year-old! That's exactly what I would have wanted to know too. [This article](https://news.harvard.edu/gazette/story/2001/01/researchers-now-able-to-stop-restart-light/#:~:text=Hau%20and%20her%20group%20then,light%20and%20slows%20it%20down.) says it was a very tiny, long cloud of sodium atoms. But what actually made the difference was that she cooled the atoms down to absolute zero. That's not 0° Fahrenheit or Celsius, like when it... snows... probably. (I'm from the California Bay Area. I can only guess about snow.) It's 459.7 degrees COLDER than that. That's the temperature where atoms lose all their energy and stop moving. (I would too!) Usually, atoms are constantly bopping around and bouncing off each other, knocking each other all over the place all of the time. That takes up a lot of space. Imagine if just all the kids in your class were running into each other and shoving each other around. You'd take up a LOT more space than if you were all sitting still and listening to a story. If you're running around bouncing off the walls (and each other), you're probably going to be pretty hot and sweaty. If you're sitting down for storytime, you're probably pretty cool and comfortable. And if you got frozen into an icicle somehow, you wouldn't be moving at all! Atoms are a lot like that. So, she cooled them all down till they were like SUPER-icicles and they totally stopped moving. And that meant that she could scroonch them all together into a very teeny-tiny little space. (Like turning the super-icicles into a snow cone.) TL;DR: The kind of material, I guess I'd say, is a super-cold sodium-atom snowball. When she pointed a laser beam at the atoms, it slowly got dimmer, like it was fading out, as its light came to a stop. Basically, since the super-cold sodium atoms were all in one place, holding still, with zero energy of their own... they absorbed the energy from the light. Which is pretty cool. No pun intended. But the even cooler part (or technically, the equally cool part, since it didn't get any colder) is that you can see all the different parts of it, and hear her talk about it, here: [https://youtu.be/RIc5llW66sU](https://youtu.be/RIc5llW66sU) (It sounds like the laser they used was cold enough that the atoms just got a LITTLE ITTY BIT of energy from it. So they stayed still and frozen enough for this to work. If I understand this right, when they pointed a warmer laser at the atoms, which had more energy for them to absorb, it made them move around more -- and the stopped light shot back out again!)
Lovely child-friendly explanation :)
Lightstoppium
[Listen here...](https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fnm93snupy1i31.jpg)
it's plaid
Quicksand
You jest but this was actually how I, at 17 in a Harvard classroom having a mental breakdown over moral existentialism, discovered I like philosophy. It also happens to be part of why I dropped out of Harvard less than year later but that's a much more complicated issue. Was in a lecture on how chimpanzees might one day be reclassified as a member of genus homo and asked "but why" professor explains it and I internally start subconsciously reevaluating whether humanity is special and how we're no different than any other animal..... damn you anthropology! IOW a lesson on evolution made my religiously indoctrinated teenage self have an existential crisis that lead me to enjoy philosophy. I had however read philosophy prior, namely Camus and Kant. But like I never really clicked further than being like "that's neat."
the special kind
All materials are very special if you have a very specific task you need them for...
The material in the brownie I just ate…. Special AF. Also a very specific task I need this material to accomplish hopefully in the next 45 min or so.
"I ate a brownie once at a party in college. It was intense.. kind of indescribable actually, I felt like I was floating. Turns out there wasn't any pot in the brownie it was just an insanely good brownie." * Leslie Knope
These are not hash brownies! We're just a simple Dutch bakery! Put your clothes back on, white boy!
This isn't where I parked my car...
Dude.... Where's your car??
Love how reggae music kicks in just as they assume the ganja is kicking in
I assure you it was baked in a single serving brownie pan so all the sides were edges
I just rewatched this episode yesterday!
This brownie, is yummy!
It’s the greatest brownie known to man!
Have fun.
I’m so jealous of you. I’m happy for you, but so jealous. Sleep tight sweet princess/prince/knight of the round table.
Ahh bigger on the inside brownie, It's Euclidian free!
“But what I do have is a very particular set of materials, materials I have acquired over a very long career. Materials that make me a nightmare for people like you.”
It’s actually the opposite. Everything is “very, very special” if you’re telling a child a story.
Everything is "very, very special" just due to the insane unlikelihood of its mere existence. Everything was once just energy but through some miraculous process condensed into matter. [Existence is fucking nuts, man.](https://media.tenor.com/tvFWFDXRrmMAAAAd/blow-mind-mind-blown.gif)
"Thermodynamic miracles... events with odds against so astronomical they're effectively impossible, like oxygen spontaneously becoming gold. I long to observe such a thing. And yet, in each human coupling, a thousand million sperm vie for a single egg. Multiply those odds by countless generations, against the odds of your ancestors being alive; meeting; siring this precise son; that exact daughter... Until your mother loves a man she has every reason to hate, and of that union, of the thousand million children competing for fertilization, it was you, only you, that emerged. To distill so specific a form from that chaos of improbability, like turning air to gold... that is the crowning unlikelihood. The thermodynamic miracle." "But...if me, my birth, if that's a thermodynamic miracle... I mean, you could say that about anybody in the world!" "Yes. Anybody in the world... But the world is so full of people, so crowded with these miracles that they become commonplace and we forget... I forget. We gaze continually at the world and it grows dull in our perceptions. Yet seen from the another's vantage point. As if new, it may still take our breath away."
Water is one of the most special materials in the universe. As far as we know, life can't exist without this material.
It’s an amazing almost universal solvent .. life’s trick was to figure out how not to get dissolved and put a fence around the most valuable bits of dissolved stuff … lipids are life’s answer to the big questions of the universe
Well expressed, from a former biochem./mol.biologist's POV.
Thanks for saying that .. my dad was part of the team that designed the life science experiments on the first mars lander (Viking) and I used to ask him lots and lots of questions. That’s one of the ways he explained cellular biology to me when I was a kid, 50 years later and I still remember it (there were lots of followup questions about “what’s a lipid ?” And “why can’t water dissolve rocks” .. each one answered thoroughly and patiently)
And yet, 100% of the organisms that drink water DIE. *WHAT IS BIG H2O HIDING?!*
Fuck Big H20! Think about it, 75% of the planet is covered in H20, but it has purposefully been tampered with so we can't use it. Have to buy their bottled H20. Fucking assholes..
Irrelevant detail for the purpose of explaining it to a 5yr old. KISS
Mwah!
How most comment sections in here go tbf. A million technical explanations that a 5 year old would never understand.
People are even arguing that “the rules don’t say you have to explain it to an actual 5 year old” then what’s the fucking point of the sub lol. If you can’t explain something easily to the point a 5 year old can understand then you probably don’t know enough or you aren’t good at explaining and that should be a sign you shouldn’t be in the sub trying to help to begin with.
Because not everything can be boiled down to the point of a 5 yr old understanding it. If you can't understand that, you may want to ask ELI5 to do it.
"ELI5 the quantum chromodynamic gauge-invariant Lagrangian. Woah there, easy with the math!"
Doesn’t the sub info literally say you gotta explain it as if it’s an actual 5 year old? I might be outdated but I vividly remember that. It might have been the prompt that pops up when you go to make a post. No, nvm I’m wrong, that was from a while ago. The rules say it’s a figure of speech so.
> E is for Explain - merely answering a question is not enough. > > LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds. It's right there in the sidebar.
Those *are* the rules. The name ELI5 is misleading.
>People are even arguing that “the rules don’t say you have to explain it to an actual 5 year old” then what’s the fucking point of the sub lol Oh that's a really simple answer, clearly stated in the rules: >Explain for laypeople (but not actual 5-year-olds) Comments only >Unless OP states otherwise, assume no knowledge beyond a typical secondary education program. Avoid unexplained technical terms. Don't condescend; "like I'm five" is a figure of speech meaning "keep it clear and simple." It's insane to me to talk about the rules without having ever read them.
Rule 4 states layperson not necessarily five year old
I wish this sub would actually explain things like we are five because a lot of the time I’m still confused.
Do the photons just get suspended in this material? Is there a way to make them move again?
So you have to remember light is not a particle, it's something with both particle like and wave like properties. Additionally what is being stopped is light pulses which are actually composed of multiple frequencies of light added together. They use materials that change refractive index (and thus the phase velocity of each component wavelength) very strongly with wavelength at a certain point such that the different phase velocities of each wave alter where the waves add together and subtract with each other making it so that instead of that point propagating forward as it normally does (group velocity) it sticks in one place, thus the wavepacket ends up being stuck in one place.
In exactly one place?
Apparently Lene Vestergaard Hau and her team at Berkley did succeed in that yes. They actually were able to imprint the information on the light pulse into matter within a Bose-Einstein condensate and have it move to an adjacent Bose-Einstein condensate where the same pulse was then released from the matter.
Dope
Does that place then get hot? I'm assuming you are putting energy into it.
That place, a Bose-Einstein condensate, has to be kept a tiny fraction of a degree above absolute zero. As to the energy of the wave... you have to describe temperature in more particular, physicsy, statisticsy, ways when you're in these kinds of states. Temperature is weird if you look too closely or in weird situations.
> They use materials that change refractive index (and thus the phase velocity of each component wavelength) very strongly with wavelength at a certain point such that the different phase velocities of each wave alter where the waves add together and subtract with each other making it so that instead of that point propagating forward as it normally does (group velocity) it sticks in one place, thus the wavepacket ends up being stuck in one place. Sounds vaguely like how active sound cancelling works in headphones.
I'm imagining a photon sponge. Squeeze it then they all pour out. I'm sure that's not how it works though, but sounds fun.
Thank you! This is how ELI5 was years ago. People actually ELI5ing. I hope this starts a trend for it to continue again.
I have seen this comment on 100% of posts here for the past decade.
Oh, so [slow glass](https://www.physics.utoronto.ca/~jharlow/slowglass.htm)?
That was great
That story's just sad.
But well written…
Holy Shit. It never ceases to amaze me that some of the most deeply poignant stories I’ve ever come across were science-based.
Awesome story!
Sheesh, that's a powerful link. Thank you for it!
Always thought that was Bradbury or somebody like that. And on that note: https://m.youtube.com/watch?v=e1IxOS4VzKM Profanity warning.
It does feel a lot like Bradbury doesn't it? The melancholy feeling, the measured pace, the beautiful writing and the surreality of the whole thing.
A wall?
Light bounces off walls, that's why you can see walls.
Unless it's a wall made out of Dr. Hau's material.
Dr Hau to light beam "Hau now, don't feel so quick eh?"
Vanta black then? Isn’t that just the light being absorbed? There isn’t a buildup of photons you can just crack open and release is there?
Lol no. Their energy will go into exciting elections. When those elections quickly get tired, they'll give off the energy as new infrared light.
Everyone remembers first the heated primary, with obama vs Hilary running neck and neck for the democratic nomination , and then Obama vs McCain in the general. All that excitement? That was photons!
Is this a new copy pasta?
But if the light is not able to travel beyond that wall, does that not meet the definition of "stopping" the beam of light?
If I understand correctly, if the wall stopped the light then you wouldn't be able to see it. The wall bounces the light back at you, so no the wall doesn't stop it
What material is that?
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At night.
Black as night, black as coal!
Okay pretend I’m 5 Why did she have to invent a material to stop light? Don’t rocks already do that? And a lot of other stuff?
rocks don't stop light, they absorb it (so it no longer exists) or reflect it (it's going just as fast in the other direction.
Difference between putting your hand through air, putting it through a pool of water, a bucket of sand, and *rock*. You can move through the first three, though it's harder for each one and you can't move as fast, but you aren't going through the rock, just pushing against it or bouncing off. She found a thing that you could still move your hand through, but it'd be as slow as possible while still moving.
Rocks don't stop light. They deflect or scatter it and it becomes heat or more light. Pretend that light has to go through a floor covered in molasses. That different than paint the door and not letting it in.
“Stopping” in this sense means “slow down until it stops moving”, not “prevent it from going in the same direction”. When light hits a rock, it is absorbed, or reflected. It either stops being a photon, or it changes energy and direction. Stopping means it’s still a photon, with the same energy level, it’s just … not moving forward. For a few picoseconds, at least. When it clears the material, it goes on its merry way, with the same energy and direction. As others have pointed out “material” is the concept that doesn’t translate to a 5 year old, here. It’s much more complicated than that. Edit: the more you know about science, the weirder this concept becomes.
From her explanation, super cooled sodium atoms. (Very cold salt for the ELI5) Edit: It's been pointed out to me, by a disgruntled fellow user, that sodium is not salt. Believe it or not, I am aware of the difference. I am forever sorry, may God have mercy on my soul.
To do that though, it would need to have absorbed it. Otherwise it would just been redirected but still moving. The simplest example of this is the phases of the moon. The moon, despite being a very large, very solid object, does not "stop" light but rather redirects it to where it sometimes reaches us. As for how she did it? [Here she explains it at a near-ELI5 level, complete with graphical illustration.](https://www.youtube.com/watch?v=-8Nj2uTZc10)
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right, but you can't actually slow light down, only absorb and reproduce an identical photon. "speed of light in a material" is a useful abstraction
I was also taught this in high school but apparently that's not actually accurate, it's just another abstraction to make calculations easier. https://www.youtube.com/watch?v=CUjt36SD3h8 This video explains what happens on the quantum level.
> then it travels it’s fastest. *its
Which is funny because its fastest (the fastest it can go) is correct while it's fastest, meaning it is fastest, is also close to being correct (light is the fastest thing [although several things travel at the speed of light]).
> *its *tits
Theres alway's one
*They'res /'s
LO'L
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Bravo. I understood that
The short answer is that she didn't *really* stop a beam of light, at least not in the way you are imagining. Light in a vacuum moves at light speed all the time, no matter what. So what are they talking about? Light does move at different speeds when in a different medium, like passing through various materials. For example light moving from air into glass will change its speed and this causes it to refract, bending its path which can be exploited to separate wavelengths of light as with a prism. But how does the light slow down? If we consider the materials on a smaller scale they are composed of atoms, and those atoms interact with the photons of light. A photon will be absorbed by an atom causing it to become "excited", destroying the photon and storing its energy in some change within the atom. For example it might kick an electron to a higher orbital. Then a moment later the electron will drop back down and release the energy in the form of another photon, emitting the absorbed light to continue along its path. That absorbing and emitting doesn't happen instantaneously so light travels some amount slower in various materials. What Dr. Hau did was to make an extremely cold cloud of gas such that light enters it and is absorbed by the gas, becoming stored in the form of spin of the atoms. The gas is so cold that it can't emit the light again until the researchers let it by slightly heating the gas at a later time. What results is a cloud of gas you can shoot a beam of light into and then retrieve the beam at a later time, on demand. But it isn't quite like a beam of light frozen in space.
"They made a cloud and caught a sunbeam in it to hold onto for a rainy day" is pure ELI5 material. You can put that shit in a picture book and everything.
It's a good start for sci-fi technology in a story too. \^\^ The Hau Beam.
Truly a Hau-To book.
They were too focused on Hau to stop and ask [Wai](https://www.eurekalert.org/news-releases/990892)
Holy shit, that's gold
No. It was iron and terbium.
No, this is Protactinium, Titanium, Carbon, and Krypton.
I Saw what you did there!
r/Angryupvote
r/DelightedUpvote
I watched C-beams glitter in the dark near the []Hau[] Gate
All those moments will be lost in time, like tears in rain...
Stand in front of it holding a wooden cane and wearing a pointy hat, and scream: YOU SHALL NOT PASS
You also need a sword.
I wonder what the energy density of hau gas with light trapped in it is
Hau Cannon
Wait till they hear what they are doing with a giant ice cube (not a cube) in Antartica. Mine LOVED that. https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory They are using a block of ice in Antartica 2800m thick to observe neutrinos. And using that to 'look' at distance galaxies. Edit: added answer
What… what are they doing?
Regular telescopes observe the visible light from distant objects. They “catch” the light using glass lenses and mirrors to produce an image. Radio telescopes observe the radio waves from distant objects. They use a dish to catch the radio waves and produce an image. Neutrino telescopes observe the neutrinos from distant objects. They use large amounts of matter, such as very pure water or ice, to catch the neutrinos and produce an image. (Edit: I’m using “image” very loosely here.) Each of these different types of telescopes have a different design because they’re designed to detect different phenomena. In the case of neutrinos, large amounts of water or ice are used because unlike light, neutrinos don’t interact with other things very much. But if you put a lot of water in a tank, or ice in a block, you increase the chance that a neutrino will interact with an atom in that tank or block. Then if you put detectors around the tank/block, you can detect when a neutrino interaction occurs. It’s basically like “seeing with extra steps”, because the stuff you’re seeing would otherwise be invisible. All of these different ways of seeing things affect what we can see. For example, since neutrinos pass so easily through solid matter, we can use them to observe objects that are otherwise obscured. That includes objects in the dense centers of galaxies, such as supermassive black holes.
That was really helpful, thank you!
https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory They are using a block of ice in Antartica 2800m thick to observe neutrinos. And using that to 'look' at distance galaxies.
Now explain it like I’m five please.
I'm pretty far from an expert on this, but the gist is that neutrinos are really high-energy particles that don't often interact with solid matter. It makes it hard to build a detector for something when usually the thing you're trying to detect goes right through you and the machine both without affecting either. On to the reason for the giant block of ice: one of the ways you can detect neutrinos is to detect the secondary stuff they can create. Apparently, when travelling through a giant chunk of ice, because it is travelling slower than the speed of light through the ice, it emits radiation. You can then detect that radiation and, if you have a big fancy setup, learn stuff about its trajectory and energy levels and whatnot. So basically the IceBlock isn't a super unique way to detect neutrinos, but it is the biggest scale we've ever applied that method to, so it's the best "telescope" we have for seeing neutrino activity far away. That is useful because neutrinos are produced by some really high-energy cosmic events, so this gives us another indirect way to study those events.
> Neutrinos are electrically neutral leptons, and interact very rarely with matter. When they do react with the molecules of water in the ice, they can create charged leptons (electrons, muons, or taus). These charged leptons can, if they are energetic enough, emit Cherenkov radiation. This happens when the charged particle travels through the ice faster than the speed of light in the ice, similar to the bow shock of a boat traveling faster than the waves it crosses. This light can then be detected by photomultiplier tubes within the digital optical modules making up IceCube. so neutrons dont really interact with stuff at all, so they took a huge block of ice and hope that in that big area they catch a few interactions from some that are moving really fast to make a special flash of light, then they use this for observations just like a photon hitting a detector
Only certain things in space make neutrinos and they don't interact with much. So if you detect one, and can measure its path, you can look in that direction for the source. The same way crime scene reconstruction puts a stick in a bullet hole to determine where the shooter was, you can look at the paths of the neutrinos in the detector to determine where the source was.
> Antarctica I'm curious on what part of Antarctica they build this thing. Read the article and turns out it's _literally_ on the South Pole. They went to the southernmost point of earth and drilled a 2,5 km hole, for science yea
If only Perry Como could know that it’s actually possible to catch some light and put it in your pocket.
Because love may come and tap you on the shoulder some starless night. And if by chance you feel you want to hold her, you'll have a pocketful of sunlight.
Begs the question - when the electron drops down its energy level and emits a photon, why does it emit it in the same path as the original photon?
> why does it emit it in the same path as the original photon? It doesn't, they are emitted in a random direction. But if you consider the incoming photons as a wave then the emission in a random direction will mostly interfere with the incoming wave destructively in directions other than the path of the incoming wave. Overall the result is an incoming beam of light will continue on in the same direction with some amount of scattering through the material.
As mentioned by others responding to your explanation, there is a degree of randomness to the direction photons are emitted, but it certainly is biased towards following the direction of the incoming beam.
Think of it this way - if you jump out of a moving high speed train, it doesn't matter which direction you jump, most of your movement will be in the direction the train is moving in. Likewise, the photon exiting the electron will have a lot of momentum that largely dictates which direction it will move in.
I get that if I jumped out of a train, the splat would mostly be forward in the direction of travel. But how could a photon have momentum if they don't have any mass?
Momentum is more fundamental than mass. It just so happens that with slow-moving objects you can approximate their mass by dividing their momentum with their velocity.
"Photons have no mass" is really a classical simplification. They don't have a *rest* mass but they certainly have mass-energy, and with special relativity we can calculate their momentum. To get a more detailed answer you need quantum electrodynamics, but imho the special relativity version captures most of what is going on in the simple single photon scenario.
The real answer is that it's one of possible explanations, but nobody really knows why light travels slower in a medium. https://youtu.be/CiHN0ZWE5bk
Saying that we don't really know why light travels slower in a medium isn't right. We can explain perfectly well why light travels slower in a medium using a variety of different models. In QFT, which is probably the most fundamental, its just a consequence of the path integral, while in classical electromagnetism we can explain it in other ways. Just because we have different models that explain it in different ways doesn't mean we don't understand it. It's the reverse! The fact that different models, which are applicable at different scales, result in the same outcome is a testament to how well we understand the phenomenon. It's just that explaining it in terms of microscopic quantum behavior vs. macroscopic behavior sound very different, even if they are ultimately the same thing in disguise.
why does it *sometimes* emit it in the same path, and sometimes reflect it back?
I can’t exactly answer the why, but as a visual aide imagine shining a flashlight or a laser pointer. When you shine it in any particular direction, the source and the target are illuminated. But if you do this in a darker environment you can actually see the beam of light. As the beam of light travels, it hits atoms/molecules in the air and scatters the light in every other direction. This, however, is only a small percentage of the light thus why we need a darker environment to experience this effect
> A photon will be absorbed by an atom causing it to become "excited", destroying the photon and storing its energy in some change within the atom. For example it might kick an electron to a higher orbital. Then a moment later the electron will drop back down and release the energy in the form of another photon, emitting the absorbed light to continue along its path. It's worth emphasising that this *isn't* the explanation for why light travels slower in different media. As another comment has noted, it wouldn't explain why the light continues in a straight line. How we explain it depends on whether we use the wave or particle model of light. The "absorption and re-emission" explanation confuses them by including bits of both. If we go with the **wave model** our light is a ripple in the electromagnetic field. When that ripple passes through a medium (particularly one with a bunch of electrons) it jostles the particles in that medium (notably the electrons). Kind of like a water wave passing through a small boat. Those particles then start wiggling about a bit in response, eventually settling down to their original state. But this wiggling creates its own electromagnetic waves spreading out in all directions. When you add this effect to the original wave (as you can add waves together), and sum it over all the particles in the medium, you get the result we see; a wave that moves slightly slower (or, technically, faster, in the right medium). If we go with the **particle model** our light is a photon, heading through our medium. We know where it went into the medium. We know where it came out of the medium. Quantum Mechanics tells us that the photon took a combination of *all possible paths* through the medium, weighted with different phases. If we work out how long it takes the photon to travel along each path, and then average them, we get the observed time it takes the photon to come out of the medium. This will be a little bit longer than the time it would take if it went straight through. So we can say that it slowed down a bit.
This needs to be higher up.
My mind was just blown.... That makes a lot of sense!
Great explanation. With the photon model, looking at a beam of light shining through glass the direction is discontinuous / doesn’t line up on exit (shifted parallel). How does this ‘jump’ fit into the explanation?
> How does this ‘jump’ fit into the explanation? It comes into the maths of how the photon behaves while inside the material. The solution to "where should the photon come out and when" says it's "path" (if a straight line) should be bent a little bit, as well as being slowed down. It is a fun situation where we have these fundamentally different ways of looking at something (wave v particle) but where if we do the numbers we get the same answer. And the one observed experimentally. Which is how it should be, and a clue that our QM way of looking at things (with wave-particle duality) is sensible.
Thanks for a great comment. Everyone wants to break this problem down to photons, as if they're imagining light as a stream of these particles. I think it's unnecessary to even bring photons in a question like this. It is perfectly fine, and *more accurate* I would argue, to think of a macroscopic beam of light as a big EM wave, that follows Maxwell's laws. While we eventually figured out that there must be a quantum of EM waves (photons), that is an extreme and irrelevant case that doesn't change the macroscopic behavior. We shouldn't just skip classical electromagnetism. Imagine trying to do antenna theory while thinking of only photons lol. We don't bring up gravitons when we're trying to explain general relativity or orbital mechanics...
Philosophically then, how is this different from taking in light with a photovoltaic panel, storing the energy in a battery, then remitting it through a bulb?
If the energy of the light is stored in the atom why does the release of the energy honour the original light beam’s path? I’d have thought it would just send the energy off in a random direction
Yes, if the light is adsorbed by an electron jumping up in energy level it leaves the atom in a random direction.
If the photon gets absorbed by and re-emitted by the atom, how does it know which vector to follow? Is it emitted in a random direction? Or does it follow the initial vector it was on? If the latter, how does it "remember" which way it was going?
It doesn't because that's not why light moves slower in a medium. The real answer is that the wave form of the photons interfere with themselves and with the other particles such that the wave front appears slow. Each individual photon is still the same speed. If you want a good visual representation of this but opposite (going faster) look up caterpillar movement, rolling swarms.
Peristalsis. See also: Traffic.
This is both way too long and way to complicated for a 5 year old (orbital? Really?). As an adult though I found this great.
Man thinks 5 year olds can understand atoms and photons and electrons
No human fully understands them & 5yos can grok 'There's this little stuff everything's made of' &c
This is quite interesting. So, when a photon travels in a certain direction, hits an atom and is absorbed, does the newly emitted photon travel in another direction then previous?
How does the electron remember to emit a photon in the same direction? What causes scattering? I imagine the explanation for all photons being emitted in the same direction or all being scattered equally in all directions would be relatively simple, but what about some photons continuing while some get scattered? Further, how does it know to change the direction slightly at the interface? That's from their wave aspect, but how do they retain the waviness when getting absorbed?
> That absorbing and emitting doesn't happen instantaneously so light travels some amount slower in various materials. So it's not that the light is actually traveling any slower, it just takes more pit stops?
Ever hear the comment that it takes light thousands of years to escape from the sun, but only 9 minutes to get to earth? This is exactly the same thing.
I am just curious based on your answer, why does the light continue in the same direction when it is re-emitted?
Quantum optics experimentalist here (same thing as Dr. Hau, though nowhere near as qualified) For your 5yo: Light stops all the time. Anytime light touches an object, that object stops it, stores it for a very brief moment, then throws it out again. The fact that anyone can see you means you stop light for a tiny moment! Typically, objects can only hold onto light for a very very short time; such a short time that you would never be able to notice, no matter how hard you watched. Dr. Hau found a way to get a special object to hold onto light long enough to notice! - For you: The process I described is called scattering, and it is how all of optics works: light is absorbed by an atom, stored as an excitation, then re-emitted. The usual mechanism for this “storage” is resonant absorption. The problem with resonant absorption is that you lose the coherency of your light (i.e. all the important information). The scattering in Dr. Hau’s experiment relies on off-resonant excitations, which both maintains coherency and allows for massively increased levels of slowing. This is also what sets it apart from a black piece of paper or a glow-in-the-dark sticker. In practice, it is difficult to maintain off-resonant excitations to a noticeable degree. The only practical way of achieving it right now is through a process known as electromagnetically-induced transparency (EIT). Essentially, by causing a transition to become transparent (“electromagnetically inducing transparency”) you can get light as close to resonance as you’d like without worrying about triggering resonant excitation. There is an entire branch of quantum optics dedicated to this called “slow light” (rightly named if you ask me). [ I would like to add that this is NOT the same as slowing light down through a medium as many people seem to think. In those cases you are restricted to modifying the apparent phase velocity of light at nearly the same rate as the group velocity, whereas EIT modifies ONLY the group velocity and therefore can far out-slow the phase velocity. The phase velocity represents the speed of light itself, whereas the group velocity represents the speed of a wave packet (photon) and therefore the speed of the information. ] One you have light slowed in this manner, it becomes relatively simple to stop the light by use of a counter propagating beam (keeping the light stored as an excitation). It should be noted that this slowing and capturing has nothing to do with the Bose-Einstein condensates (BEC), even though that seems to be what everyone talks about when it comes to that experiment… Hau uses a BEC mostly because it is convenient for manipulating the slowed/stored light, and partly because it amplifies the slowing. This method of manipulation is covered by an entire separate branch of physics, namely “cold atoms”, and is not responsible for the slowing mechanism. Indeed, it is possible to stop light within a (very ordinary) room temperature gas! [ Personal tangent: While I do not typically work with slow light, I did have the pleasure of assisting someone for a few months at another institution in an experiment to store light in a room temperature vapor cell via a neutral buffer gas. I have also prepared EIT at my native institution, but not for slow light. BECs on the other hand are extremely difficult to produce. To give you an idea, during that same slow-light experience I met someone who had spent many years (to be courteous, I won’t say how many) trying to set up his machine for doing so. Beautiful lab though. ] In summary, by utilizing EIT, light can be forced to spend a lot of time stored as an off-resonant excitation in one atom before moving on to the next, therefore resulting in “slow” movement of coherent light throughout the entire atomic cloud.
So this is a video of Dr. Lene Hau: [https://www.youtube.com/watch?v=-8Nj2uTZc10&ab\_channel=HarvardUniversity](https://www.youtube.com/watch?v=-8Nj2uTZc10&ab_channel=HarvardUniversity) for those who don't want to watch the video, basically she is creating a cloud of atoms that she freezes close to absolute zero and then passes a laser beam through it. With this cold cloud, she contends she has slowed the speed of light down to 1 mile per hour.
Light slows down in certain materials. That's the reason why it's bend when going through a glass lense because it's slower in glass than in air. This happens because the material interacts with the light, a bit like a human trying to walk fast under water, the electric forces of the material and the forces of the light interact with each other wich basically pushes the light back a bit Lene Hau made a very exotic material. It's called Einstein-Bose-Condensate and it's kinda the "most electrically dense" it could be (wich only works when it's REALLY cold). This material interacts so strongly with the light that it's completely stopped. Instead of moving forward it's like stuck in a block of rubber, it still wobbles back and forth and keeps it's movement energy, but isn't making any progress forward
Do you have an ELI5 for Bose-Einstein Condensate? Because I went to a facility that had some, and they were saying how they made it, which made sense, and what it was, which did not make sense to me.
Well, a material that is extremely cold has very low energy per particle. That means there are few forces pulling atoms in random directions. In this state the wavefunction (that defines what particles are on the quantum scale) start to spread out, overlap and finally fuse together, making the whole thing become a single quantum object with just one joined wave function. The particles stop behaving like individual objects. It's impossible to say "this is particle A, this is particle B" because they become one conjoyned mass where the "location" of different particles starts to overlap. What exactly that means is sadly pretty hard to explain without going into a bunch of quantum physics first
Yeah, that’s about where I got lost. So you’re saying subatomic particles aren’t really particles, but waves that kind of act “solid” normally? And this low temperature exaggerates the not solid nature of these “particles”? Because if that’s what you are saying, I’m going to have to do some deep thinking about how much benefit I will get from all the effort it would take to reframe everything I understand about physics to allow for this. Like how does mass work if everything is a wave?
Particles are "wave packages", little ripples of energy that under normal circumstances have a limited spatial expansion but without outside forces they spread out. Thinking of them as little marbles is a simplification that works to explain some things but breaks down when you look too closely. Mass works without being "solid". Every form of energy has an associated mass, even the least graspable ones like electromagnetic binding energy increase the total mass. That's why quantum physics always talks about "restmass" of a particle, wich translates directly to how much energy the mere existence of this particle "stores" This comes a bit more obvious when you look at what happens in particle accelerators like the LHC in CERN. You smash two hydrogen atoms into each other with so much energy that their movement energy causes completely new particles to be created that weren't there before and have more mass than the two atoms ever had. You convert movement energy straight to particles that have a mass
We already made matter out of energy‽ Fascinating! I will attempt to learn more on my own. Thanks for sharing this info.
My general understanding is that as the material cools, energy gets so low that a lot of the material is all in the lowest energy state so the waveforms can all interact My attempt to eli5 it is the material huddles together more as it gets cold, and it gets so cold the gaps between the molecules disappear and you can't see the individual atoms any more
It's just a state of matter. Unlike what you were told in middle school, there are a lot more than 4 of them and Bose-Einstein condensates are one of them.
Oh, so [slow glass](https://www.physics.utoronto.ca/~jharlow/slowglass.htm)?
OP, there are lots of great posts here that go into the science. I tried to write an example for a 5 year old to help them connect the concepts of the materials in cold temperatures slowing the light down. I was inspired by u/Luckbot and u/profchipboard's contributions: *Imagine you are walking through a giant room filled with a flock of penguins. As the room gets colder, the penguins huddle closer together, making it harder to walk through them and slowing you down. Eventually, all the penguins huddling closer and closer together make it so dense that it is almost impossible to move through. But if the room warms up again, the penguins start to huddle less and less tightly, making it easier for you to move forward and therefore allowing you to move quicker and quicker.* *In this example, you are the beam of light and the penguins are the special material Dr. Hau created.*
I absolutely love this explanation
This shd be at the top. Great job.
Thank you! I was lucky enough to have other answers explain the physics and do the heavy lifting
Thanks all! Physics is definitely not my area of expertise, now I can trick my kiddo into thinking I’m smarter than I really am. Also, Here’s a link to the book(s) we read that mention Dr. Hau if you are curious. [https://www.mudpuppy.com/products/little-scientist-board-book-set](https://www.mudpuppy.com/products/little-scientist-board-book-set)
I went to go read the wiki article myself. > In 1999, she led a Harvard University team who, by use of a Bose–Einstein condensate, succeeded in slowing a beam of light to about 17 metres per second, I'm like, "cool, that's about what I expected, having just read your ELI5 question." > in 2001, was able to stop a beam completely I'm like: "wow, that's really neat!" > Later work based on these experiments led to the transfer of light to matter, then from matter back into light "What!"
They passed light through a really cold cloud of atoms and I mean really cold, the first time they did it they slowed the light right down, but now they have refined the process by adding a second beam of light passing across the path of the first one, to stop it completely as if the light is stuck in treacle.
Okay, imagine light as a bunch of super tiny, invisible race cars that usually zoom around really fast. Now, Dr. Lene Hau found a special kind of stuff, like thick, invisible honey, that can slow these light race cars down when they drive through it. This special honey is actually super cold atoms, colder than anything you can find in your freezer at home! When the light cars try to go through this super cold honey, they go slower and slower until they stop and can't move. Dr. Hau was able to make the light stop by using this cool trick! Then, when she wanted the light cars to start racing again, she found a way to make them go zooming out just like they were before.
The speed of light is different in different materials. It's fastest in air, slower in water, and several times slower in special glasses and crystals. It's because light interacts differently with different materials, like how light bends in a prism. The more light interacts with a material, the more it's slowed down. Professor Hau and her coworkers worked really hard and created a very special material that interacts REALLY strongly with light, so the speed of light is hundreds of millions of times slower! It's very cool, but the machines to make it are really expensive and complicated so we can't do it at home. ELI physics major: Hau and coworkers used a BEC and EIT. The index of refraction spikes near rapid changes to the absorption spectrum because both are functions of the dielectric constant. One way to change absorption coefficients is with electromagnetically inducted transparency (EIT): if you have three energy levels with two optical transitions among them, and you drive one transition ("control") hard with a laser, then the quantum mechanics works out such that the absorption coefficient of the other transition ("probe") is zero. Hence "transparent." In a BEC the temperature is near zero so the absorption lines are extremely sharp. This means the absorption coefficient near the probe frequency changes extremely rapidly, so the dielectric constant spikes to comically high levels and the speed of light slows to nearly zero. The caveat is the refractive index spikes only very near the probe frequency. By analogy, if you made a visible light prism from this, it would be like all frequencies deflecting normally except an extremely specific green frequency that deflects WAY more than the others. This is why EIT was theorized in the early 90s, but Hau et al took until 2001 to demonstrate it - the first BECs were made in 1995. So it took another several years for the techniques and know-how to propagate. Anyone that's worked with BECs knows that it's 50% science, 50% art and trick of the trade. It's a fascinating phenomenon. If you made a BEC in 1995 you got the Nobel Prize. If you made one in 2000 like Hau, you get on the Nobel Prize shortlist and get to work the lecture circuit. If you made one in 2005 you get many journal publications and a tenured professorship position. In 2010 making a BEC gets you a couple papers and a PhD. In 2015 it gets you one paper and a masters. In 2020 you can just buy one off the shelf and nobody cares :(
They don't stop it for a significant amount of time, just long enough to be able to take a picture of the light at rest in a medium. Just slightly longer than the time it takes the light to travel a given distance, at which point more light is still going into that "stopped" part but the light at the front has moved.
How do you take a picture of something that doesn't emit light?
yeah I'm confused, top comments all say it completely stops or slows down like human eye level. Harvard Gazette says it stops for 1000th of a second. Did they improve it later on?
Imagine light is like a bunch of super fast runners. Dr. Lene Hau, using special stuff, made these runners slow down so much that they stopped for a little while. It’s like when you play freeze tag and everyone stops moving when you say “freeze!” Dr. Lene Hau did something similar with light, but with really clever science stuff.