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« spec fic: a little history | Main | down to earth »


It is 'Hola mis amigos'.

Gracias. We have duly made the correction. We were forever mixing French and Spanish during our Argentine stay, much to the confusion of the local inhabitants....

Just dropped by from a science link at DKos. Great site!

Another DKos migrant, bookmarking, intent on returning. Very nice blog.

I've come here from Kos, and I am impressed by your blog. I started college as a science major, switched to philosophy, and took three semesters of History of Science. I still love reading about science and scientists.

As Ahnold says "I'll be back." But before I leave, I want to discuss this bit:

"We experience this every day: heat cannot flow from a colder to a hotter body; a cup of coffee cools at room temperature, rather than the other way around."

Perhaps this is a quibble, but we do not experience that heat "cannot" flow fom a colder to a hotter object. We experience only hot cups of coffee cooling to room temperature.

Based on that, we may justifiably report that heat has not flowed from the colder body to the hotter. By inductive reasoning, we may predict that heat will never flow that way.

But our experience of hot coffee and induction do not prove the impossibily of heat's flowing from cooler to warmer bodies any more than our experience of swans' being white and a similar inductive leap prove the impossibility of swans' being black.

In 1697 in western Australia, Dutch explorers discovered black swans; today, Starbucks' customers all over the world await that "tiny, infinitesimal chance" that their cooling cups of java will warm themselves.

Neal is absolutely right, and had my brain been a little less fried from an all-night inter-continental flight, I might have expressed that aspect a bit more clearly. That is the fascinating thing about Boltzmann's statistical approach to entropy. His stance that the heat is only most likely to flow from a hotter body to a cooler one was considered quite radical, since implicit in those statistics is a tiny possibility that the opposite could happen. However, I should point out that this kind of statistical fluctuation is so incredibly rare that you'd need to span a time scale that is roughly the age of the universe -- if not more -- in order for this to happen. (Whether this is also true of black swans, I can't say.) Starbucks hasn't been around nearly that long -- although they're mushrooming all over the world on every street corner -- so I hope those customers are prepared for a pretty long wait. :)

String theory not only hasn't produced predictions our experiments could check, it hasn't produced any predictions whatsoever. But more to the point, here are some fun things about the statistical arrow of time that you might enjoy as well.

van Kampen wrote a paper called 'Ten Theorems about Quantum Mechanical Measurements' which is available in a stripped down form in a collection of his writings for laymen called 'Views of a Physicist.' The main example from it goes like this: imagine the two slit experiment. In normal discussions, if you don't 'measure' whether the photon goes through one slit or the other, you get interference. If you do measure it, you don't. And then there is handwaving about collapse of the wave function. van Kampen puts in some physical reality: say we stick an atom in an excited state in one of the slits, but which cannot deexcite unless something distorts its electric field (like a passing particle). If the particle we shoot at the slits goes through the slit with the atom, the atom deexcites and a photon goes whizzing off. It is statistically possible for the photon to return and the atom to reexcite, just horrendously improbable. What we measure is the photon. So the states we have to talk about are not just our incident particle, but the whole system. The space of states divides into two: those in which the photon did go flying off, and those that don't, and it so happens that which slit the particle goes through is entangled with it, so we get a measurement of where it is. As with many paradoxes in probability, the problems only arise when you try to hack up the system into pieces inappropriately. Classically we can, just as the probability paradoxes go away if everything is deterministic, but in the quantum world things don't decompose the same way. Incidentally, collapse of the wave function in this case becomes a theorem: it's the proper way to decompose the spaces given the presence of a measuring apparatus.

In a similar vein, Landau and one of his students proved that in relativistic quantum mechanics, every measurement has a minimal space-time interval which it can occupy. I don't have the reference to hand, but if you look in volume 4 of Landau's Course of Theoretical Physics it's there in the first chapter, or I could dig it up if anyone's interested.

Maxwell's ideas about his demon were made precise in the 20th century, largely by Landauer from IBM's research labs. In fact, it costs entropy kT ln 2 to destroy a bit of information (and by "destroy" I mean mix off into the heat bath until it's irretrievable). These arguments apply to biological systems equally well: enzymes are generally lovely examples of Maxwell demons.

And finally, on the cosmology side, it has nothing to do with origins, but I ran across a pretty little paper about a year ago (again, reference not to hand, I am not in my office, I can dig it out if anyone cares) which says that although the entropy of the universe is increasing, it is not increasing as fast as the maximum possible entropy of the universe, which would just be one big black hole.

And speaking of prefaces with senses of humor, as well as Boltzman... Poor Ludwig also features prominently in one of the best lead-ins into a textbook I have come across - David Goodstein's classic reference on statisical physics, *States of Matter* .

"Ludwig Boltzmann, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics... Perhaps it will be wise to approach the subject cautiously..."

Makes you wanna dive into eh?

Welcome back. The recently published book of Richard Feynman's letters mentions that he and his son Carl had discussed the possibility of two time dimensions. Amusingly Feynman says that he couldn't visualize the resulting space-time but that Carl could.

Another fascinating perspective on time comes from Carver Mead's recent _Collective Electrodynamics_. Mead claims that the obvious way out of the infamous EPR paradox is that time runs backwards sometimes. Mead introduces the idea by picking at Feynman's discussion of electromagnetism in the _Lectures on Physics_. Mead's book is a bit technical but well worth reading for anyone who is comfortable with Jackson's _Classical Electrodynamics_.

The Price book sounds like a winner. I'd have time to read it if weren't for all these dumb blogs.

N. Peter Armitage

Funny but unfortunate that you mention Feynman and Carver Mead in the same comment. The two did not get along as Feynman had to correct Mead a little too often, specially about the issue of reversible computation (which Mean hasn't still understood, so it seems). Mead's book isn't that technical, just worthless. The idea that time-reversibility can "pacify" the EPR paradox is not his but originates with Olivier Costa-de Beauregard and was recently taken up, a lot more constructively, by Aharonov and Vaidman. This, by the way, is also well handled in Price's book which Jennifer so wisely picked and reviews above. If I were you I would sell Mead's book on Amazon and use the proceeds to by me Price's.
Have a nice summer.

Can somebody explain what does "mercurial mood swings" mean?

really nice psot.

Very entertaining, witty, likeable and inspiring blog.

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    Physics Cocktails

    • Heavy G
      The perfect pick-me-up when gravity gets you down.
      2 oz Tequila
      2 oz Triple sec
      2 oz Rose's sweetened lime juice
      7-Up or Sprite
      Mix tequila, triple sec and lime juice in a shaker and pour into a margarita glass. (Salted rim and ice are optional.) Top off with 7-Up/Sprite and let the weight of the world lift off your shoulders.
    • Listening to the Drums of Feynman
      The perfect nightcap after a long day struggling with QED equations.
      1 oz dark rum
      1/2 oz light rum
      1 oz Tia Maria
      2 oz light cream
      Crushed ice
      1/8 tsp ground nutmeg
      In a shaker half-filled with ice, combine the dark and light rum, Tia Maria, and cream. Shake well. Strain into an old fashioned glass almost filled with crushed ice. Dust with the nutmeg, and serve. Bongos optional.
    • Combustible Edison
      Electrify your friends with amazing pyrotechnics!
      2 oz brandy
      1 oz Campari
      1 oz fresh lemon juice
      Combine Campari and lemon juice in shaker filled with cracked ice. Shake and strain into chilled cocktail glass. Heat brandy in chafing dish, then ignite and pour into glass. Cocktail Go BOOM! Plus, Fire = Pretty!
    • Hiroshima Bomber
      Dr. Strangelove's drink of choice.
      3/4 Triple sec
      1/4 oz Bailey's Irish Cream
      2-3 drops Grenadine
      Fill shot glass 3/4 with Triple Sec. Layer Bailey's on top. Drop Grenadine in center of shot; it should billow up like a mushroom cloud. Remember to "duck and cover."
    • Mad Scientist
      Any mad scientist will tell you that flames make drinking more fun. What good is science if no one gets hurt?
      1 oz Midori melon liqueur
      1-1/2 oz sour mix
      1 splash soda water
      151 proof rum
      Mix melon liqueur, sour mix and soda water with ice in shaker. Shake and strain into martini glass. Top with rum and ignite. Try to take over the world.
    • Laser Beam
      Warning: may result in amplified stimulated emission.
      1 oz Southern Comfort
      1/2 oz Amaretto
      1/2 oz sloe gin
      1/2 oz vodka
      1/2 oz Triple sec
      7 oz orange juice
      Combine all liquor in a full glass of ice. Shake well. Garnish with orange and cherry. Serve to attractive target of choice.
    • Quantum Theory
      Guaranteed to collapse your wave function:
      3/4 oz Rum
      1/2 oz Strega
      1/4 oz Grand Marnier
      2 oz Pineapple juice
      Fill with Sweet and sour
      Pour rum, strega and Grand Marnier into a collins glass. Add pineapple and fill with sweet and sour. Sip until all the day's super-positioned states disappear.
    • The Black Hole
      So called because after one of these, you have already passed the event horizon of inebriation.
      1 oz. Kahlua
      1 oz. vodka
      .5 oz. Cointreau or Triple Sec
      .5 oz. dark rum
      .5 oz. Amaretto
      Pour into an old-fashioned glass over (scant) ice. Stir gently. Watch time slow.