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January 2013 Archives

How to Dip Your Hand in Liquid Nitrogen

Sure, if you want to... but only if you're comfortable with the possibility of losing a hand if you mess up.

Here's what happens to a basketball when you freeze it in liquid nitrogen:



Maybe you've seen someone violently shatter a rose, a racquetball, or a banana in the same manner. What do you think would happen to your hand if you dipped it in a bucket of liquid nitrogen?

The answer is given by something called the Leidenfrost effect. Don't have liquid nitrogen in your house? Neither do I, and I'm a physicist who uses the stuff all the time. Luckily for you, the effect can be observed right in your own kitchen for no cost.

Put a sauce pan on your stove and turn it on. Once the pan starts to get hot, sprinkle some water on the surface. The drops should stick and quickly boil away. Leave the pan on the heat for several more minutes, then throw water on it again. If the drops of water dance and slide across the surface, you've just observed the Leidenfrost effect.

Chefs and home cooks alike already know this trick. If the water stays on the pan for a few moments, skimming across the surface, your pan is extremely hot, ready to go. Why, though, does a slower boil indicate a hotter pan -- shouldn't it be the other way around?

When a cool drop of liquid lands on a hot surface, heat is transferred to the drop from the surface. A large surface, with a temperature greater than the boiling point of the liquid, will quickly boil the droplet into a gas. However, when the surface is much, much hotter than the boiling point, something different happens.

An extremely hot surface transfers heat so quickly to the droplet that the bottom is instantly boiled. The boiled steam wants to rise, and so it pushes up on the underside of the droplet. The water is lifted up off the bottom of the pan, and floats on a cushion of steam. This tiny hovercraft can then scoot about on the pan surface. The steam beneath the droplet conducts heat up from the pan slowly; the droplet lives much longer before boiling away.

So here's the trick. Liquid nitrogen boils at -396 degrees Fahrenheit (-196 C). When you dip your hand (Briefly! Very briefly!) into liquid nitrogen, your hand is far far hotter than the chemical's boiling point. This causes the Leidenfrost effect to come to your rescue. The liquid nitrogen surrounding your hand boils instantly and is pushed away from your skin by the cushion of steam (nitrogen gas). This gives you an instant of protection before your hand is horribly burned.

Don't believe me? Here's a demonstration:



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January 2013 Archives

Why is Stephen Hawking so Famous?

How much do you know about Stephen Hawking? How about David Gross, Jacob Bekenstein, Fred Hoyle, Kip Thorne, Yakov Zel'dovich, John Wheeler, or Roger Penrose? All of these physicists are as accomplished in similar work as Stephen Hawking, and some of them even more so. Why is Hawking the only famous one?

Hawking NASA.gif

Hawking's mental capability is the first reason, though he was actually not a standout student in high school and early college. He was smart, but didn't invest much time in his studies. He coasted through undergraduate work at Oxford on intellectual reputation while spending most of his time drinking and having fun. "Towards the end he was working as much as three hours a day" a friend teased. Nevertheless he went to study with the pre-eminent cosmologist in Britain at the time.

Despite a first year in grad school of paying for his previously lax habits, Hawking quickly picked up theoretical cosmology, even upstaging his publicity-mongering advisor. Despite (perhaps even finally motivated by) a diagnosis of A.L.S., he began working extremely hard and produced a string of important discoveries in theoretical cosmology. These papers are the base of a reputation that has since grown into far wider realms.

Quantum mechanics governing the atomic world and General Relativity governing the vast distances of space do not get along. Hawking's theory of black hole radiation (building on work by Bekenstein) was among the first to unite some elements of the two in a famous finding. He collaborated on ideas and work with many other prominent theoretical cosmologists, including all of those mentioned above. Eventually he was awarded the Lucasian Chair of Mathematics at Oxford, a post once held by none other than Isaac Newton. These sorts of accomplishments, though, are only par for the great physicists named earlier. Why is Hawking the archetypal image of an astrophysicist?

We've all heard of his cleverly titled A Brief History of Time. Along with Carl Sagan, Hawking helped introduce the lay public to the fascinating ideas and theories of how the universe has evolved and how it works on the grand scale. Topics such as the Big Bang, the age and expansion of the universe and black holes are explained, with only one equation being mentioned. He has written several more books, covering topics that fascinate everyone: time travel, wormholes, dark matter and Einstein's theories. Hawking was among the first to make the case for these ideas not just to other academics, but to anyone who could read. 

Hawking's iconic image is the other major factor in his popularity. The man with the vast brainpower and minuscule muscle power. A person whose only ability is to think, but who is gifted with tremendous intellectual capacity to do it with. Both tragic and triumphant, beholden to a medical condition and surmounting it to achieve greatness.

Hawking now spends most of his time essentially engaged in public relations for science. He meets with presidents and premiers and makes public statements. With unparalleled media coverage and esteem he can make statements on topics of philosophical importance that grab headline attention. His talent and intellect are unimpeachable. His ability to tell stories, as well as the story of his own life, however, are what set him apart from his peers in the public consciousness.