Tuesday, September 27, 2005

E = mc2 Explained

How would 10 top physicists two Nobel Prize winners among them describe Einstein's equation to curious non-physicists? Listen online by selecting Play All or choose individual clips below. PBS NOVA as some real cool podcasts. Check them out.

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Davie said...

What I can't get my head round is, C= the speed of light, thats a velocity right? So its like so many miles/second, kilometers per second etc. How can one multiply a number by a velocity, its like trying to multiply an apple by the square of a pear, to me anyway. (my maths are limited) Also I must assume Mass is in kilogrammes? and energy in joules?... Can someone please help me out on this... perhaps with a worked example.. I'm a bit thick, Thanks, Davie.

John Blake said...


Meanings of the formula

This formula, which was discovered by Albert Einstein, proposes that when a body has a mass (measured at rest), it has a certain amount of (very large) energy associated with this mass. This is opposed to the Newtonian mechanics, in which a massive body at rest has no kinetic energy, and may or may not have other (relatively small) amounts of stored energy (such as chemical or thermal energy). That is why a body's mass, in Einstein's theory, is often called the rest energy of the body. The E of the formula can be seen as the total energy of the body, which is proportional to the mass when the body is at rest.

Conversely, a single photon travelling in empty space cannot be considered to have an effective mass, m, according to the above equation. The reason is that such a photon cannot be measured in any way to be at "rest" and the formula above applies only to single particles when they are at rest. Photons are generally considered to be "massless," (i.e., they have no rest mass or invariant mass) even though they have varying amounts of energy.

This formula also gives the quantitative relation of the quantity of mass lost from a resting body or an initially resting system, when energy is removed from it, such as in a chemical or a nuclear reaction where heat and light are removed. Then this E could be seen as the energy released or removed, correponding with a certain amount of mass m which is lost, and which corresponds with the removed heat or light. In those cases, the energy released and removed is equal in quantity to the mass lost, times the speed of light squared. Similarly, when energy of any kind is added to a resting body, the increase in the resting mass of the body will be the energy added, divided by the speed of light squared. [Source: Wikipedia.org ]

Also Davie, Click on the "read more" link and listen to the PBS NOVA podcast. I found it very interesting as well. Thanks for your post to my blog- J. Blake