Starts With A Bang! » Physics http://startswithabang.com Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 http://wordpress.org/?v=2.3.2 en The Most Energetic Mystery in the Universe http://startswithabang.com/?p=1672 http://startswithabang.com/?p=1672#comments Wed, 25 Mar 2009 20:05:43 +0000 ethan http://startswithabang.com/?p=1672 When we look out at galaxies, we find the most energetic particles we’ve ever found anywhere in the Universe coming from their centers. Why?

Because as far as we can tell, all galaxies, at their centers, have huge, supermassive black holes! When matter (like a star, globular cluster, intra-galactic gas, etc.) gets too close to one of these black holes, it gets ripped apart, and settles into a disk around the black hole. This disk is called an accretion disk:

Like everything in a strong gravitational field that moves, these particles radiate (give off high-energy photons), fall in towards the center of the black hole, and sometimes get accelerated and shot out of the galaxy!

For a galaxy like ours, with a black hole a few million times as massive as our Sun, we can get extremely energetic particles out: up to 1018 eV, which is 70,000 times more energetic than the LHC!

But our black hole is kind of a commoner — a weakling, even — when you look at other galaxies. There are huge galaxies out there, such as active galaxies and quasars, where instead of a few million times as massive as our Sun, their black holes are billions or even tens of billions times as massive as our Sun:

Well, in theory, the energies of these particles can be thousands or even tens of thousands of times higher than what our galaxy can produce! We’re talking about energies of 1022 eV, which is not only insane, it’s impossible!

Why? Because there’s a maximum energy that particles traveling through the Universe can have. There’s a bath of leftover light from the Big Bang permeating the entire Universe: the CMB. If you smash a particle with too much energy into one of these CMB photons (which is unavoidable as you travel millions of light years), it causes these high-energy particles to slow down until they’re below the “speed limit”. (Okay, it’s an energy limit, but it’s really close enough.)

And unlike cops that pull you over, the light that fills outer space slows you down until you’re below the cosmic energy limit: 5.7 x 1019 eV. Well, do we see a cutoff there?

Maybe. The AGASA experiment says no, there is no cutoff, but the Pierre Auger Observatory says yes, there is one. Who’s right? On one hand, we’ve definitely seen events where we’ve measured more energy than should be allowed. It may mean our theories need revising, or it may mean that there’s something super-energetic happening in our own backyard that we don’t know about. Or — on the other, more boring hand — perhaps we’ve just done a bad job of measuring things at very high energies. Whatever the case, explaining these events that exceed the cosmic energy limit of the Universe is, in fact, the most energetic mystery in the Universe!

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A Little Sun in Your Life… Dire Consequences? http://startswithabang.com/?p=1651 http://startswithabang.com/?p=1651#comments Fri, 20 Mar 2009 20:18:05 +0000 ethan http://startswithabang.com/?p=1651 I really get a kick out of reading The Straight Dope. What started as a weekly column in a Chicago newspaper has grown into a nationwide phenomenon and a small empire, and is often full of fascinating questions and extremely well-researched and knowledgeable answers.

But doesn’t anyone there know to contact me if you’ve got an astrophysics question? Today’s column, which will be nationally syndicated, declares that they cannot answer the following question:

If the pink grapefruit sitting in my fruit bowl spontaneously turned into a grapefruit-sized sun, what would happen to my flat, London, and the rest of the world? If I put it somewhere safe, could I enjoy not paying for central heating? Or would it end life as we know it by melting through my floor, into the African textile shop, through the subway system, and finally to a fiery chasm in the middle of the earth where it would make all volcanoes erupt and kill everything, before coming out the other side and changing the way all the planets spin?

Well, I may be no Cecil Adams, but I can certainly answer this one. Let’s take a look at how our Sun actually works, and then scale it down to be grapefruit-sized.

The Sun is a giant ball of mostly hydrogen gas. It’s extraordinarily massive — about 300,000 times as massive as Earth — and tremendous powerful. The Sun gives off what most people would call an unfathomable amount of energy, but in scientific notation, it’s about 4 x 1026 Watts, which is at least fathomable, although it’s absolutely tremendous. This means that even at the surface of the Earth, 150 million kilometers away, that means we have 129 Watts of solar energy striking us over every square foot that receives sunlight.

But what’s truly exciting, at least for me, is the way the Sun creates its energy. To understand it, we have to go down to the tiniest atomic levels, and look at the hydrogen atoms themselves:

Because the pressure at the center of the Sun is so high, due to the gravitational pressure of having 300,000 Earths pushing in at the core, the nuclei of these hydrogen atoms, protons, get pushed together with a tremendous force. The force is so large that it causes these nuclei to fuse together, in a process called nuclear fusion. With a little math, I can figure out that in order to create the amount of energy the Sun gives off, it has to fuse together about 3.6 x 1038 atoms of hydrogen every second! That little atomic reaction, happening over and over, trillions of times every nanosecond in the Sun’s core, is what produces all the heat, light, and energy we’ve ever received from the Sun.

Now, the person who asked this question wanted to know about having a little stable grapefruit-sized Sun.

Well, here’s a big downer for you: this thing ain’t gonna be stable. If you want to have nuclear fusion going on at the core of this grapefruit-sized ball of hydrogen, you’re going to need a tremendous amount of pressure to push the atoms at the center together. There are only two ways to handle it that we know of, and neither one of them will give you an answer that you’ll like, although they’re both fascinating.

The first way is to artificially increase the pressure, like lasers (shown here) would do. Practically, you wind up getting less energy out than you have to put in to increase the pressure to obtain fusion, which is another disappointment. Scientists working on this call it inertial confinement fusion, and so far, it has never yielded more energy than it took to get it going. So this way looks like a dud. But there’s another way to get nuclear fusion…

You can increase the pressure tremendously — albeit for a very short time — by setting off a small explosion around the hydrogen core, compressing it and causing ignition. There’s a problem with this way, too. The resulting fusion reaction is uncontrolled and runs away, igniting everything. This is commonly known as a hydrogen bomb.

Either way, you give off a tremendous amount of energy, your initial “grapefruit” gets blown apart like a super-powerful exploding grenade, and depending on how much of the hydrogen in there actually managed to fuse would determine what happened. If you scaled down the Sun so that the grapefruit worked on exactly the same scale, you’d only get about 100 million atoms fusing together, or about 100 microJoules of energy. Enough energy to push the hydrogen gas away, but not even enough energy to light a match. But, if you managed to fuse the entire grapefruit into helium, you’d get the energy equivalent of a 160 kiloTonne explosion, or about 11 times the energy of the atomic bomb dropped on Hiroshima, in your fruit bowl.

Without the entire mass of the Sun to insulate this nuclear explosion from the rest of the Solar System, this grapefruit-sized ball isn’t going to last long. Either way, you’re better off getting a heat lamp for your desk, and paying your electric bill.

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Unsolved Mysteries? When like charges attract! http://startswithabang.com/?p=1643 http://startswithabang.com/?p=1643#comments Wed, 18 Mar 2009 21:29:52 +0000 ethan http://startswithabang.com/?p=1643 Attraction and repulsion are two of the simplest concepts in electricity and magnetism: like charges repel and opposite charges attract. (Two steps forward, two steps back.) For electricity, this is how positive and negative charges work:

Two positive charges will repel each other, two negative charges will repel each other, but one positive and one negative charge will attract one another. Simple enough, right? Except, everything is made up of atoms, which have positive and negative charges all throughout them. Certain materials are excellent conductors, which means that positive and negative charges are relatively free to move throughout a material. So if I bring a negatively charged rod close to a neutral conductor, the following happens:

The positive charges line up on the side closest to the rod (because they’re attracted to it) and the negative charges line up on the side farthest from the rod (because they’re repelled). Since the “opposite” charges are closer and the “like” charges are farther, this means that the force from the opposite charges is slightly stronger, and so overall, the negatively charged rod attracts the neutral conductor.

Now, here’s the weird thing: what if you charged up the metal with some negative charges? Would it repel the rod, since negative charges repel? Or would it still attract the rod, since the “opposite” charges are closer than the “like” charges? Well, the answer is both! Check it out!

What I’ve graphed here is Force vs. distance for a negatively charged rod brought close to a negatively charged conducting piece of metal. When the distance is large, they repel each other. But when you bring them close enough, the fact that the opposite charges are closer becomes more important than the fact that there are more like charges, and they attract! There’s even one perfectly balanced point where the force is exactly zero!

And of course, that’s just for electricity. Do magnets do the same thing?

Absolutely! North repels north, south repels south, but north and south attract each other. Magnets make powerful magnetic fields; the stronger the magnet, the stronger the field. What’s really neat, though, is that if you apply a strong enough magnetic field to some materials, like iron, they magnetize, too!

Well, if you bring a weak iron magnet’s North pole close to a strong magnet’s North pole, what do you think will be more important? The fact that the weak magnet is made of iron, and can be magnetized, or the fact that the North pole repels the other North pole? Let’s go down to the molecular level to find out…

Weak magnets have little tiny molecular magnetic moments pointing in many directions. Overall, though, there will be more pointing in one direction than any other, and that’s how your material is “magnetized.” But if you bring a strong magnet close enough, it applies a strong enough magnetic field, and will re-magnetize the weak magnet material to attract the strong magnet! So, is it the same deal as the electric charges? Let’s have a look:

It’s exactly the same! Far away, they repel, close in, it magnetizes and attracts, and yes, there’s a point right in the middle where it balances perfectly, and the force is exactly zero!

So the next time someone tells you that like charges repel each other, you’ll know the exception to the rule!

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Random Ethics Question: Project Paperclip http://startswithabang.com/?p=1574 http://startswithabang.com/?p=1574#comments Tue, 03 Mar 2009 20:13:19 +0000 ethan http://startswithabang.com/?p=1574 Sometimes, when I come home from a long day at work and need to unwind, I start reading BBC News, and occasionally the news archives are more interesting than the actual present news.

Last night, I came across a really interesting read from 2005, about Project Paperclip, which was the US Government’s plan to bring the former Nazi scientists to America and use their knowledge and expertise to further the scientific and military enterprises we had going on here, and also to deny the Soviets that knowledge.

But this required giving amnesty to Nazis (and sometimes even former SS officers) like Werner Von Braun, Arthur Rudolph, Kurt Debus, and Hubertus Strughold. The atrocities that these men and their subordinates were responsible for are well-documented, and include the death of 20,000 slave laborers producing Von Braun’s V-2 missiles, freezing inmates and putting them into low-pressure chambers, and performing human experiments at Dachau and Auschwitz. Truly, these are some of the most despicable things that human beings have ever done to one another.

And yet, the Germans had an incredible amount of scientific, aeronautic/aerospace and military knowledge that we did not. Some examples? Supersonic rockets, nerve gas, jet aircraft, guided missiles, stealth technology, and hardened armor. All in 1945. So what was the ethical thing to do? What was the smart thing to do? And overall, what was the right thing to do? US Air Force Major-General Hugh Knerr’s opinion was the prevailing one:

If we do not take the opportunity to seize the apparatus and the brains that developed it and put the combination back to work promptly, we will remain several years behind while we attempt to cover a field already exploited.

So, for better or worse, about 700 Nazi scientists were put to work for the United States.

The technological advancements and scientific breakthroughs were numerous and swift, and over the next 25 years, Werner Von Braun had masterminded the Apollo program and the Moon landings, Arthur Rudolph had designed the Saturn V rocket, Hubertus Strughold designed NASA’s life support systems, and countless other technologies such as Cruise Missiles, the B-2 Stealth Bomber, and scramjets came as outgrowths of Nazi research. So as a counterpoint to one of the worst atrocities in human history, we also have one of the greatest achievements in human history:

That’s the story of what we did and one of the steps we took to get there. Did we do the right thing? Did we do the just thing? Did we do the smart thing? For me, the answers are yes, no, and yes. And I, for one, am glad I didn’t have to be the one to make that decision. What do you think?

And this last week, so much happened in space news that we not only have a carnival, we have a carnival sideshow as well; enjoy!

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A Myth from Your Dentist? http://startswithabang.com/?p=1566 http://startswithabang.com/?p=1566#comments Mon, 02 Mar 2009 21:41:16 +0000 ethan http://startswithabang.com/?p=1566 One of the beautiful things about facebook is reuniting and catching up with old friends that you haven’t seen in a very long time. Caroline was one of those people for me, someone I knew in high school and thought was awesome, and just lost touch with when we went off to our separate colleges. Well, Caroline has a few “scientific” interests that she shared with me:

That’s right, folks. She’s clearly interested in the science of clean teeth. Specifically, she wanted to know about Ionic Toothbrushes, and whether they really do anything useful or whether it’s solely a marketing ploy. (Although, personally, I’m pretty sure that Caroline is much more likely to fall for the McConaughey marketing ploy.) She says:

I knew someone who used an ionic brush to brush their teeth by opposite polarity, because plaque has a + charge. Then i told my dentist friend who discussed it w/ his collegues, and they agreed the only way to rid of plaque was by mechanical removal. Is the ionic brush a scam?

So, I did what any reasonable person would do. I went online to look for an explanation of how it works. Here’s what I found:

Is it clear yet? No? Looks like I’ll have to give it the ol’ SWAB scientific treatment here, with a little bit of science you can do in your office. The idea is that plaque is stuck to your teeth via the electromagnetic force, which is true. Plaque is mostly made of bacterial cells, and as you may have learned, all cell membranes have a positive charge on their outer surface.

You can test this! Take two pieces of scotch tape, and tape them down to the desk/table. Lift them up quickly, and they’ll rip electrons up off of the table, giving them both negative charges.

You can either move them close to each other, and watch them repel one another. Or, you can bring one of the pieces close to something with a cell membrane, like your hand. It will attract your skin, which tells you that the cell membranes on your skin are positively charged!

So now that you’re convinced that plaque bacteria (above, and eww) are positively charged on their surface, and they stick to your tooth enamel, how do we get it off? Well, brushing your teeth is a great idea, it certainly helps, but most people aren’t thorough enough to do a good job of cleaning their teeth with mechanical power alone. So what do we do? We put negative ions in your mouth, to help get the plaque to let go from your teeth. People don’t like the idea of ions in their mouth, so we give it a different name: fluoride. Which is great for not only toothpastes and mouthwashes, but also as an additive to water in general, so that when you drink tap water, it helps clean your teeth!

So now, what about this magic ionic toothbrush? The idea is that electrically charged things will be more likely to stick to whatever has a bigger charge. So if your tooth is negatively charged, it attracts plaque. If the toothbrush head has a bigger negative charge, the plaque will go to it instead of your teeth. Simple idea, right?

The big question, of course, is how effective is it? You still need to brush your teeth, of course. But it doesn’t work nearly as well as a fluoridated toothpaste does. The charge stays localized on the toothbrush, instead of covering your mouth like a sudsy, fluoridated toothpaste would. But if you have no toothpaste, the ionic brush will, in principle, work better than a regular brush on its own. Take note that this means the explanation of how the ionic brush works on Caroline’s site is not right.

But if you are, like me, someone who brushes their teeth with toothpaste every day, will this help? Well, their advertising says “up to 48% more plaque removed” followed by an asterisk. What does that asterisk mean? It doesn’t say. My recommendation is that if you want to use less toothpaste (or you can’t use it, for some reason), this is a good option. But otherwise, save your money and just use toothpaste (and drink your fluoridated water) like everyone else with healthy teeth.

On a personal note, I’ve still got all 32 teeth and have only had one cavity in the past 10 years. Brushing with a fluoride toothpaste, flossing every day, and drinking 3-4 litres of tap water a day are my only secrets. I don’t believe anecdotes as proof, but there it is anyway; anyone know anything more about whether this is more effective than a placebo?

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Can you slow time down? http://startswithabang.com/?p=1514 http://startswithabang.com/?p=1514#comments Thu, 19 Feb 2009 02:04:33 +0000 ethan http://startswithabang.com/?p=1514 We’ve talked before about how you can make time around you speed up. Go into a spaceship, travel close to the speed of light, and come back to where you started. When you land your spaceship, you will have aged “normally” for you, but everyone who remained on Earth (even your identical twin) will be much older.

This is because the speed of light is the same everywhere and at all times in the Universe. If you travel fast, and by fast I mean close to the speed of light, time for you has to slow down relative to time for people who are at rest. So when you move close to the speed of light, travel back to Earth and come to rest here, you’ll be younger than people who’ve been on Earth the whole time. You are effectively speeding the time of everyone around you up.

Well, reader Tony writes in and asks the following related question:

If I was in a rocket and set a course and speed that countered all motion, including Earth, Solar, Galactic, and Universe, would time then speed up?

Well, the Earth moves around the Sun, the Sun moves around the Galaxy, and the Galaxy hurtles through space. So could you, if you found out how quickly everything was moving and counteracted it, could you effectively slow the time of everyone around you down?

The answer appears to be yes, but not by nearly as much as Neo here can. The problem is, for any measurement of time to make sense, people need to be in the same place at two different points in time. So whatever you do, you need to come back to the place you started at. If you want to do this for the planet Earth, it’s only the Earth’s movement that you can counter.

The Earth moves around the Sun at roughly 30 kilometers/second (67,000 miles per hour), which is pretty fast, but only about 0.01% of the speed of light. So if you sat in Earth’s orbit and turned on your rocket thrusters so that you stayed in exactly the same position while the Earth continued to go around the Sun, time would continue to pass slightly quicker for you than for people still on Earth! They’re moving at 30 km/s, while you’re there, stationary, at 0 km/s. When the Earth finally catches you, though, one year later, what does your clock say?

Your clock is off. By 0.16 seconds. Yes, that’s right, you spent an entire year and you found that your clock was off by 0.16 seconds thanks to all of your efforts. Want to know what’s even worse? Time didn’t slow down on Earth relative to you. After all that, you’re still 0.16 seconds younger than Earth. Turns out that because you left Earth and returned to Earth, the only velocity that matters is your velocity relative to Earth.

So I’m sorry, Tony, because your question is a good one and made a lot of sense. But it turns out the Universe doesn’t work that way, and that if you leave Earth and return to Earth, you can’t make time pass faster for you than it does on Earth. Pretty sad, but hopefully now you know not only what the answer is, but a little bit about why.

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A Physics Riddle http://startswithabang.com/?p=1497 http://startswithabang.com/?p=1497#comments Fri, 13 Feb 2009 20:31:34 +0000 ethan http://startswithabang.com/?p=1497 Yes, it’s Friday the 13th. I don’t care. Yes, it’s also the day before Valentine’s Day. Still don’t care. What’s neat about today? The website I go to for daily puzzles and brain teasers had a physics riddle today. I think it’s super neat. Here’s the riddle:

You have two spheres. They have the same outside dimensions, the same volume, and the same mass. The outside part of it, the parts that you can see, are made of the same material. In every way you can tell, the two spheres are identical. You have one piece of information that lets you know that they’re different, though: one is hollow inside. How do you determine which is which?

(Hint: The answer has to do with physics, not asking some apocryphal mountain-guru.)

Before I give you the answer, let me tell you some things the solution has nothing to do with:

  1. Electricity. If the outsides are made of the same material and you’re only measuring things outside of the spheres, you can’t tell whether there’s any difference inside.
  2. Buoyancy. Even if one is hollow inside and the other one is solid, if they have the same mass and the same dimensions, that means they have the same density, and hence will sink/float at the same rate.
  3. Bouncing them. Okay, you’re on the right track; there are internal differences between a hollow sphere and a solid sphere. The problem is, for some materials, the solid version will bounce higher, while for others, the hollow version will bounce higher. So there’s no solution here.

But there is a solution, and it turns out that it’s pretty simple after all. Know what you need? A ramp. And that’s it. Why? Because even though their masses, volumes, densities, etc., are both the same, their mass distributions are different! This means that when you do something like roll them down a ramp, the one that has more mass in the center gets down the ramp faster, and the one with more mass around the edges takes longer to get down the ramp. Take a look at this simulation for an illustration:

And this has applications to everything from balancing your car’s tires to understanding the rotation of the planets. Pretty neat, and a pretty simple solution to a riddle that could otherwise be very annoying!

Now get out there and enjoy your Friday the XIII!

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Is the Universe a Giant Hologram? http://startswithabang.com/?p=1486 http://startswithabang.com/?p=1486#comments Wed, 11 Feb 2009 22:21:39 +0000 ethan http://startswithabang.com/?p=1486 Some days the questions I get are easy, and some days I get questions from our longtime reader, Ben. This past week, there have been reports all over the news that our world may be a giant hologram. Let’s take a look at what’s going on.

In Hanover, Germany, there’s an experiment called GEO600. These are two perpendicular lasers, and they shoot out for thousands of feet, get reflected, and come back to their original location to make an interference pattern.

Now the reason this is important is because gravitational waves cause ripples in space in a certain way. These perpendicular lasers are particularly sensitive to what gravitational waves do, and the interference pattern will shift in a very particular way if gravitational waves pass through them. This is the same idea that’s behind the upcoming LISA mission.

Now, GEO600, like every laser interferometer we’ve ever built, has not seen any evidence for gravitational waves. But it has seen something that it can’t explain, and that’s always interesting for an experiment.

It found some extra noise, above and beyond what can be predicted/explained by things like the vibrations of the Earth, temperature fluctuations, or instrumental noise. What does this look like? Whenever you do your experiment, you do your best to understand what noise you expect to see, and then you look for deviations from this. GEO600 saw something like this:

So there are two possibilities now: either there’s a source of noise they haven’t figured out, or something physically interesting and novel is causing this. Now, historically, whenever experiments are done, it’s almost always unexpected noise that causes something like this to happen. But once in awhile, there really is a new effect that we have going on.

It’s very important to state, clearly and unambiguously, before we go any further, that this may simply turn out to be noise. This may not be a physical effect at all, and that no other similar experiments (such as LIGO) see these effects.

But if it is a physical effect, Craig Hogan of Fermilab has come up with an extremely interesting possible explanation. He says that this excess noise could be a sign that our Universe has an extra dimension to it. How does this work? Let’s think of a hologram:

A hologram has all the information you could ever want about the dimensions of a 3-D object, but it has it all in two dimensions. For instance, you could tell some object’s (or someone’s) length, width, and depth just from looking at a 2-D hologram. All of the information is encoded in there.

Well, our Universe may be the same exact way. We know about our 3 space dimensions and our 1 time dimension. But we may have more dimensions of space than we know about; many interesting theories have them. One possible consequence is that these extra dimensions could cause extra “blurring” of our 3 regular space dimensions at very small lengths.

Now, this is very interesting, because the noise we see in the GEO600 experiment causes the laser light to blur on scales of about 10-16 meters and below. This is smaller than the size of a single proton, but amazingly, our technology is sensitive enough that we can detect it! But is this blurring due to extra dimensions? Let’s see what the people connected with the experiment say about Craig Hogan’s idea:

However Danzmann is cautious about Hogan’s proposal and believes more theoretical work needs to be done. “It’s intriguing,” he says. “But it’s not really a theory yet, more just an idea.” Like many others, Danzmann agrees it is too early to make any definitive claims. “Let’s wait and see,” he says. “We think it’s at least a year too early to get excited.”

The longer the puzzle remains, however, the stronger the motivation becomes to build a dedicated instrument to probe holographic noise. John Cramer of the University of Washington in Seattle agrees. It was a “lucky accident” that Hogan’s predictions could be connected to the GEO600 experiment, he says. “It seems clear that much better experimental investigations could be mounted if they were focused specifically on the measurement and characterisation of holographic noise and related phenomena.”

So it looks like this is worth further investigation, but it’s way too early to draw any definitive conclusions. But it’s a possibility, and for something as grand as this, for something that would forever change the way we view our Universe, I think it’s worth investigating further, and so does the entire GEO600 team.

What do you think?

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The Physics of Geckos http://startswithabang.com/?p=1459 http://startswithabang.com/?p=1459#comments Mon, 02 Feb 2009 20:12:15 +0000 ethan http://startswithabang.com/?p=1459 Sure, geckos are cute little lizards. They’ve even been made famous in households across the country by certain car insurance companies. But geckos are pretty miraculous in a way that would make even Spider-Man jealous:

Geckos can climb almost anything. Pretty much any surface, however slick, sheer, or wet, can be climbed upon by a gecko, whether completely upright, inverted, or even upside-down. If you live in a tropical climate, you may even notice geckos running rampant on your ceiling:

But unlike common adhesives, such as tape or suction cups, geckos work in a far more sophisticated way; let’s find out how.

The key lies in the way the gecko’s feet work. Unlike other animals, each toe of a gecko’s foot contains hundreds of thousands, if not millions, of tiny, tiny hairs known as setae. These setae are only a tenth of a millimeter long, or about half the size of a typical paramecium.

What’s even more amazing is that each single seta, as you can see above, branches into about 1,000 different endings, each one ending in a tip known as (no joke) a spatula. These spatulae are so small that they can’t be seen with visible light; it takes ultraviolet light just to see these things, which measure in at just about 0.2 microns apiece.

So, on average, each one of a gecko’s four feet contain about a billion of these microscopic spatulae. But what makes them so special? Van der Waals forces. The way these work is pretty simple, and it works similarly to how you can rub an inflated balloon on your cotton shirt, stick it to the wall (or your hair), and have it stay, held up by static electricity.

Well, Van der Waals forces work the same way, except the edges of the spatulae get a random charge, either + or - on the tip. When they come near a surface, it causes a charge separation on the surface, and this makes a small, close-range, temporary attraction, like so:

When you get many of these together, they can form a lattice, and result in an extremely large force. The charges will change over time, but as long as they all change together, the attractive force will remain strong, and thus the gecko will remain stuck to the wall.

There’s one exception, though. What if there were a material that didn’t allow charges to separate? What if there were a material that was immune to forming these induced charges? Well, there is, and it’s teflon. Its molecular structure is so stable (thanks to the carbon-fluorine bonds) that the gecko’s spatulae can’t alter the electric charges at all, and hence there are no Van der Waals forces. (Alright, there are only very small Van der Waals forces, insufficient to allow a gecko to stick to it.) Teflon is the only known thing that a gecko cannot stick to.

And so what can we learn from this? Well, if you can reproduce this, if you can artificially build something with setae and spatulae on it, it could stick to just about anything, too, right?

Might as well see for yourself:

And that’s the physics of geckos. Better get those tiny hairs growing if you want to end up like Spider-Man, kids!

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Supernovae and Dark Energy: Part II http://startswithabang.com/?p=1438 http://startswithabang.com/?p=1438#comments Wed, 28 Jan 2009 20:56:19 +0000 ethan http://startswithabang.com/?p=1438 On Monday, we told you what the Universe is doing, and it’s expanding faster than we can explain. This mysterious expansion’s cause is unknown, and until we figure it out, the name we give to it is called dark energy. But we do know how to measure what the expansion rate is, and despite what the occasional misinterpretation says, it’s extremely simple and straightforward to do.

Let’s make a simple analogy: start with a 100 Watt light bulb.

Now, you can put that light bulb anywhere in the Universe, and you’ll know how far away it is. How? Because you know how intrinsically bright it is, and you can measure how bright it appears, you can calculate how far away it must be. It’s as simple as this:

Well, guess what? This works all over the Universe. You find something that you know its intrinsic brightness, you measure how bright it appears, and you can figure out how far away it is! Well, there’s one special type of object that is the same everywhere in the Universe: a type Ia supernova. They all work the same exact way. Let’s show you:

Start with a white dwarf star. Many stars (including our Sun) will end up like this. When all the nuclear fuel of a star is used up, the core simply collapses and the star sheds its outer layers. What we see at first is a planetary nebula:

but the hot gas of the nebula dissipates after several thousand years. The white dwarf star, at the center of many of these nebulae, remains behind. This is the fate of our Sun. However, unlike our solar system, which only has one star, many star systems have two or more stars in them. If one of those stars is a white dwarf and the other one isn’t, something very neat can happen:

The white dwarf star, if it’s close enough, because it’s so dense, can start stealing mass from the other star! It can do this for a long time, but not indefinitely. As the white dwarf gets more and more massive, the pressure in its center increases. At some point, when the white dwarf reaches a mass of about 40% more than our Sun, the pressure gets too great, and starts to destroy the atoms in the center of the star:

And the collapsing atoms release a tremendous amount of energy, resulting in a type Ia supernova explosion!

Because it’s the same exact physics every time, these have the same brightness every time, and we can use them just the way we use a 100-Watt light bulb! You measure their brightness, figure out their distance, and for the last part, you measure how quickly they’re moving away from us! From this data, you can figure out what type of Universe we live in: open, closed, flat, or accelerating. Guess which one we see?

Closed is the red line, open is the green line (which is far enough away from the data at about 2-3 Gpc that it doesn’t work), flat is the black line, and the accelerating ones are purple. Which one works best? The accelerating one, definitely. We can make more complicated models, but they all need something to explain this. And that’s how we know that there’s dark energy in the Universe. Any questions?

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