Where Grey Matter meets Dark Matter

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Episode 28 - 11 July 2010
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Normally, people think that to cool something you should probably take it out of the light and put it in the shade. For extreme cooling, you could put it in interstellar space, lightyears away from any stars. Surely that's about as cold as it's going to get? Not so. The coldest things in space are about 1 kelvin (1 degree above absolute zero), which is quite balmy compared to the coldest things on Earth.

In order to get really chilly, you have to get unthinkably silly - blast the thing with a laser. Then you've got laser cooling.


Blast it with a laser!

We’ll get to the lasers in a minute. First, we need to think about what we mean by temperature. So what is it? Well, it's (roughly speaking) the speed the particles are moving at. So a hot gas has fast moving particles, while a cold gas has slower ones. “Aha!” you might think. “So if you can slow down the particles, you lower the temperature!” Bingo, Bucko.

Now, the trick with using lasers is that an atom thinks that light comes in particles called photons. To us, light is a fairly innocuous thing, which couldn’t push the class nerd over. You might even have heard that it has no mass. This is true, but that doesn’t mean that it can’t affect anything. Even though it has no mass, ol’ Einstein told us that it nevertheless has momentum. So what happens to the momentum of two things when they collide?

Let’s put it another way: If you fire a bunch of ping pong balls at a stationary bowling ball at high speeds, what happens? The bowling ball starts moving backwards. It takes a whole heap of ping pong balls to do anything noticeable, but it still has an effect. Firing photons at a mirror is more like throwing ping pong balls at a 500m building. But firing photons at an atom can have an effect, if you use enough of them. Here's a demo where you can cool your own atoms.

Now we have the means to slow down particles, which is the mechanism needed to lower temperature. But it needs to be fine-tuned.

Firstly, not all light can bounce off a given atom — quantum (from the Latin word for 'amount') mechanics tells us that an atom will only accept packets of energy of certain sizes. So what’s the ‘size’ of a photon? The colour, or frequency, of the light, is directly proportional to the amount of energy each photon has – so blue light has more energy than red light. Also, gamma rays, X rays and ultraviolet light have more energy than infrared light, microwaves, and radio waves. So we need to pick light of the right colour to be able to interact with the particles that our gas is made of.

Except it's not as simple as that. We also have to worry about the Doppler effect – when a police car goes by you hear the pitch of the siren change once it passes you. As it's coming towards you it's trying to catch up to the sound waves it's making, so the waves end up bunched together. and when it's going away from you it's moving away from the sound, so the waves spread out. This means the frequency (or pitch) of the sound is higher at first and lower after it passes. It's the same with light, except that for light the frequency means colour.

So, say you’ve got an atom coming towards you. You work out what colour light you need, find a laser of that colour, and shine it. The atom starts slowing down. But then it stops slowing down, and it keeps coming. The frequency of light it sees is lower, so the light is redder, so it has less energy, and suddenly, it can't absorb the packets any more, because they're too small. The solution? Slowly increase the frequency of light. Eventually, It will be enough to interact with an atom moving towards the laser, but low enough to be ignored by atoms which are still or moving away from the laser. Phew.

“But wait!” you scream. “I thought you said in the last episode that you need a particular material to make light of a certain colour. How do you change the frequency of a laser?”. Good question. Magnets. Now shut up and listen.

So now say you've got a collection of atoms, all moving in random directions at random speeds. You start the laser off at a fairly low frequency, so only the fast moving atoms get affected, and slowed down. Then you slowly increase the frequency to slow down more and more of the atoms. To get them to stop completely you do this from six different directions – front and back, left and right, top and bottom. Then all the particles will stop, somewhere in the middle. To make sure they stop in the exact middle, you put a magnetic field through it, which causes the atoms to change their preferred energy packet size, and if you do it the right way, you can ensure that the only atoms not absorbing photons are the ones right in the middle. Eventually the James Bond ones will escape your “magneto-optical trap”, and the rest will end up stopped in the middle.

Once again, that’s not the whole story. It's impossible to stop the atoms completely. The kick an atom gets from emitting a photon is proportional to the amount of energy in the photon. This means that the process can slow the atoms down almost completely, but not quite. They're still, on average, 1/10,000th of a degree above absolute zero (no motion). How much colder can it get?

At this point, to make it colder, an effective technique is evaporative cooling. The (now very cold) atoms are bumping into each other, transferring energy and momentum. Every now and again one gains a lot of energy. When this happens you let it fly out of the mix, and then what's left over has a lot less energy than before. You're literally using these individual atoms to take the energy away.

You might be wondering how a whole bunch of atoms can sit in the same spot. But that's one of the wonders of ultra cold things – Normal physics stops applying and more than one atom can occupy the same space, and have the exact same properties. What you get is a “super atom” - a particle which is large enough to see through a microscope, which still does quantum things. This particle is called a Bose-Einstein Condensate. To get a BEC, you need the atoms to be at around 10^(-8)K (~ 1/100,000,000th of a degree above absolute zero). Bring a coat.


Refs:


BETA:

Imagine a shark big enough to bite through the Golden Gate Bridge, and an octopus ginourmous enough to engulf an oil rig. Now imagine these two mighty leviathans battling it out for supremacy of the world's oceans. Sounds awesome. The film 'Megashark vs Giant Octopus' aims to depict this noble contest of epic seafood, and it does it in style.

As far as real beasts go, the biggest shark in history was the megalodon (Carcharodon megalodon or Megasharkus maximus) which died out about 1.5 million years ago. It's hard to know exactly how big they got since sharks are mostly cartilage (which doesn't fossilise) but 'they' reckon these monsters grew to somewhere between 16 and 25 metres in length, and weighed somewhere around 100 tonnes. Equals trouble.


Look out, old bean, he's right behind you! Source: The Jesse Earl Hyde Collection,
Case Western Reserve University (CWRU) Department of Geological Sciences.

In comparison the world's largest living fish species, the whale shark, only grows to about 12 metres and weighs about 30 tonnes, while the great white shark is only around 6 metres and weighs a paltry 2 tonnes. The whale shark is also just a filter feeder, meaning that it presents no danger to anything of any consequence. The Megalodon on the other hand is at least as big as a humpback whale with the attitude of Jaws (probably). And they both clearly have a taste for aviators.

As for octopi, the largest specimens we have are Enteroctopus dofleini and Haliphron atlanticus, which are about 70kg with an arm span of 4 - 5 metres. This toy is no match for even a real megalodon, let alone the humungasaurus in the film, or a 20,000-30,000 tonne oil platform. Then there's huge squid (they're almost octopi aren't they?); the biggest squid ever caught, of the species cleverly called Colossal squid (Mesonychoteuthis hamiltoni), was about 10 metres long and 500kg when it was caught (although it shriveled up later). Admittedly huge, but still, that's nothing compared to a good old fashioned megalodon.


Source: National Geographic.

Some other giant predatory sea-creatures we've had (in the real world) include the plesiosaurs (the New Zealand Mauisaurus got to about 20 metres long), mosasaurs (Tylosaurus about 17.5 metres) and our modern sperm whales (which grow to about 18 metres and 70 tonnes)


Blue Whale Skeleton.

As for biggest animals at all ever on earth ever: the blue whales. These rampaging behemoths grow to more than 30 metres long and 180 tonnes in pure fearsome mass. Regularly tearing ocean going vessels in two, they consume the entire contents, including all the shuffleboard biscuits. Actually, blue whales are one of the most inoffensive animals on the planet, having practically no enemies due to their size (except for humans who are their enemies because of their delicious size). So the megalodon could probably beat a blue whale (and they did co-exist so it probably happened) but the lame-o octopus is best suited as dinner for a more competent creature.

And for all of our 9 year old listeners, we also talk about dinosaurs.


Refs:


Oops. Mistakes we shouldn't have made but did:

F=ma is the force, p=mv is the momentum. We know, we know.


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