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Sunday, May 18, 2008

Speaking of Persistent Popular Misconceptions...

which we were in the ID thread, what's up with the persistence of the idea that Einstein's formulation of E=mc2 led 'to the development of the atomic bomb and nuclear power', as I just read a book called A Perfect Mess? The energy released from atomic fission and fusion is due to the fact that the elementts in the middle of the periodic table tend to have lower potential energy than those above or below them -- I think Iron-56 is the trough -- so that converting a very light or very heavy element to one closer to the middle releases the energy difference. E=mc2 has no more to do with the process than it does with the release of energy from a fire -- which is to say it is useful in calculating the energy released, but not for understanding the basic process. In some alternate universe, the atomic bomb could have been developed before E=mc2 was discovered. And you don't need to study advanced physics to figure this out-- just have access to Wikipedia.

7 comments:

  1. I'm not doubting the truth of your post, but the physicist quoted in Wikipedia seemed to give a bit of a non sequitur:

    "...The theory of fission is what physicists call a non-relativistic theory, meaning that relativistic effects are too small to affect the dynamics of the fission process significantly."

    E=mc^2 doesn't really have anything to do with weird stuff happening when things go near the speed of light. This guy is making it sound as if E=mc^2 is irrelevant to fission, when (as Gene points out) it's entirely relevant, just not essential.

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  2. I'm with Gene on this: the "packing fraction graph"--very steep at the beginning, troughing at iron, and gently upward to unanium and beyond--is quite famous.

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  3. I'm with Gene on this:

    I'm not sure if this is meant to distinguish your post from mine, Wabulon. If so, let me repeat, I am not doubting anything Gene wrote.

    I was just saying that in the Wikipedia article on this topic, in the section on this it seemed part of the proof was the physicist's quote that I reproduced above. So my only point is that that particular quote has nothing to do with E=mc^2. (So the fault is with Wikipedia.)

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  4. One last thing, just in case a real physicist strolls by and thinks I'm completely full of it:

    E=mc^2 is also useful for relativistic applications. In particular, when an object's velocity goes up, so does its kinetic energy, according to KE=0.5*mv^2.

    So this is an intuitive way of seeing why things can't go faster than light. As the object's velocity increases, so does its kinetic energy, but then so does its mass (according to E=mc^2). That means you have to apply ever greater force to get the same acceleration, because the mass of the object is apparently getting higher and higher.

    In the limit, the object would have an infinite amount of energy (/mass) when v=c, and so that's why you can never accelerate something to the speed of light.

    BTW I am sure the above is a little bit off, because photons have v=c and they don't have infinite energy. There's some deal with rest-mass (for photons it is zero) versus relativistic mass. But I think what I said above is correct in spirit.

    Aaaaanyway, all I was trying to point out originally was that in the Wikipedia article, they made it sound as if E=mc^2 clearly had nothing to do with atomic reactions, because of the low velocities involved. And that by itself is not a good argument.

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