MadSci Network: Physics
Query:

Re: How does electron action account for instantaneous photon velocity ?

Date: Sat Jun 5 13:13:17 2004
Posted By: Phil Marsden, Post-doc/Fellow
Area of science: Physics
ID: 1085166158.Ph
Message:

The question of how a photon can be emitted instantaneously when an electron de-excites is more of a philosophical one than one of physics, so I am inclined to say that I am not qualified to answer the question. However, since physicists were traditionally scholars of natural philosophy, I will give it a go.

First the simple answer:

How can a photon be emitted instantaneously? It just does!

Now let us get serious:

In the history of the physics of particles travelling very fast (so- called relativistic particles), a brief review of which can be found here, everything was very confused around the turn of the 20th century. People were still debating how light travelled through the vacuum and at what speed it did travel. In 1905 an Einstein (almost definitely Albert - I will leave you to look up this debate on Google) proposed the idea that the speed of light was always constant, regardless of the velocity of the observer. This is, of course, contrary to "classical" physics where velocities simply add together or subtract from each other.

More information on Special Relativity can be found all over the net:

http://science.howst uffworks.com/relativity.htm

http://www2 .slac.stanford.edu/vvc/theory/relativity.html

http://theory.uwinn ipeg.ca/mod_tech/node132.html

Anyway, in Einstein's picture of the Universe photons always have constant velocity (in vacuum). There is no room in the theory for acceleration and would be tricky to put in, since the particles of light are defined as being massless. However, this is beside the point since there have been many many tests of the this theory. Next year we can celebrate 100 years of testing the theory with, as yet, no evidence to suggest that it is wrong. That is not to say that we should not keep on testing it and, maybe one day, we will see that photons are accelerated by electrons, but today, as we understand the universe, photons are emitted instantaneously.

So now, how do we rationalise this? When we think about photoemission we often think about it as a scattering process. An electron scatters with a photon emitting or absorbing it. Scattering processes are convenient since they happen very quickly. In this case, certainly faster than we can measure and our measurement limit is something like 10s of attoseconds or maybe better. 1 attosecond = 0.0000000000000000001 seconds.

After all that, maybe you get the idea. We cannot really rationalise it, we can only say that, within the framework of our current understanding, we know that it happens really fast - so fast, in fact, that it is probably instantaneous. Who knows right now? That is why we continue to research with higher and higher precision instruments, higher energies in our particle accelerators, lower temperatures, fewer photons, fewer atoms... Physics is as exciting now as it has ever been - maybe more so!


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