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(Guardian)   Everything you ever wanted to know about Einstein's famous equation, but were too insecure to ask. Bonus: article notes equation correction   (theguardian.com ) divider line
    More: Spiffy, Einstein, kinetic energy, special relativity, equations, nuclear power stations, natural sources, atomic bombs, electrical charges  
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3503 clicks; posted to Geek » on 05 Apr 2014 at 9:27 AM (2 years ago)   |   Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»



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2014-04-05 12:20:52 PM  
3 votes:
And now the rant. First, the equation did not "give birth to the atom bomb". Radioactivity had already been discovered by some guy who left uranium salts sitting on top of a photographic plate, and that led to many experiments which classified the types of radiation (alpha, beta, gamma, ...) and gave us a model for an atom as a dense nucleus surrounded by an electron cloud. The neutron was discovered experimentally in 1932 by James Chawick and neutron-induced fission was discovered later that decade by Otto Hahn et al. Leó_Szilárd was one of the first people to realize that the fission process could lead to an energetic chain reaction.

Einstein's involvement in the atom-bomb development was political, not technical. His "E=mc2" did help to provide a theoretical basis for the phenomena which had been measured, but honestly nobody gave a shiat if the fission fragments weighed slightly less than the uranium which had gone into the device. All they really needed was the observation that lumps of U-235 got very hot when you whacked them into each other.

Rant part 2 - the article didn't even mention the long form of the equation, E2 = p2c2 + m2c4, where 'm' is rest mass and 'p' is momentum (for example the momentum of a massless photon).
2014-04-05 09:46:23 AM  
3 votes:
1kg of "stuff" contains around 9 x 10^16 joules, if you could somehow transform all of it into energy.

I love how these articles always contain some 'what if you could completely convert mass to energy' number.  You can't. You still have to obey all other conservation laws - boson number, charge, spin, etc - so you really don't convert 'mass' to energy in nuclear fission/fusion.  What you release is some of the binding energy between particles as they change to a more stable arrangement.

It's easiest to understand if you look at the binding energy per nucleon curve for elements, inverted.

img.fark.net
Everybody wants to get to the most stable form, iron.  So you're not really destroying particles and converting them to pure energy - you're just taking the particles you have and getting them to snuggle closer together.
2014-04-05 04:22:18 PM  
1 vote:

Ivo Shandor: the long form of the equation, E2 = p2c2 + m2c4, where 'm' is rest mass and 'p' is momentum (for example the momentum of a massless photon)


The full form allows this handy representation of how the terms are related: (Source)

img.fark.net
2014-04-05 09:52:22 AM  
1 vote:
Didn't find TFA particularly interesting or informative.

One of my favorite explanations of E=mc2 comes from the minutephysics YouTube channel: Einstein's Proof of E=mc2
 
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