Saturday 26 June 2021

Wave packets

I have started to read Misner, Thorne and Wheeler's great tome on gravitation. It starts quite differently from Carroll by emphasizing a coordinate free view. (Pythagoras and Euclid proved a lot without coordinates which were popularized René Descartes about 2000 years after them.) In section 2.5 MTW claim that the pattern of surfaces (named $\widetilde{{k}}$) generated by a matter wave (aka De Broglie wave) provide the simplest example of 1-forms which we know are covariant vectors or vectors with indices downstairs. I need to revise my quantum mechanics and find a book on the internet by JD Cresser. I think it is a bit hand-wavey but it helps me understand a bit more. The first step is generating a wave packet from a regular sine wave (or similar). The wave packet will be a particle.

Cresser starts with a cosine wave with a wave number 5 so the equation is $\psi=\cos{5x}$. In this case the wave number is the number of waves that fit in a length $2\pi$. Sometimes it is then number per unit length. He then takes another wave with wave number 5.5. The '2 sine waves' image shows them both. You can see a patter emerging. That becomes clearer if you add them and divide by 2 as shown in '2 waves added'. It is the famous beat that is heard if two nearby notes are played at the same time. He then adds more, all with wave numbers in the range 4.75 to 5.25 and claims that by adding even more you will get a wave packet. That is not true! If you add in more and more waves the picture remains the same, there are always small bulges and the large bulges just move off into the remote distance. The range shown is for $x=\pm65$. On the 9 wave picture there are large bulges at about $x=\pm100,\pm200\ldots$ and on the 17 wave picture at about $x=\pm200,\pm400\ldots$ (I checked three of those). Then something in his text suggested that I should add waves in a larger range and that does produce a wonderful wave packet.

With 999 waves and $k$ in the range 4 to 6 the picture improves and with a slightly larger range, 0 to 10, it is a pretty decent wave packet (although I think there might be another spike well off the chart - try it yourself!).

Numbers and wave generator at Wave packets.pdf (4 pages)