Consider a radio wave of wavelength λ=100m. If the electromagnetic waves are streams of photons, then a radio wave has to be a stream of photon. How big can be a photon of this wavelength?
For a wave-packet to be of a certain wavelength λ, the wave-packet has have a length L, s.t. L/λ >> 1. But, how long is a photon in this wave? A photon cannot be of a length ℓ
Hello,
In the "old war" between particles versus waves, waves won. Nothing is a particle. There is no dualism. How big are photons is therefore, not a question that makes sense in this formalism. The particle momentum and its wavelength are still connected by de Broglie's equation, not its size, but just holds for the apparent particle that we see.
Cheers, Ed Gerck
So, we conclude that a photon behaves like a particle only when the wave-packet is very small?
Hello Sofia,
I would say that one is able to always find an experiment where a photon does not behave as a particle, notwithstanding its wavelength.
How about any signal, not just a photon? In fact, any signal, periodic or not, that has finite energy can be represented as a composition of classical waves based on a single function, the sine (Fourier). The photon is not a classical wave, while the sine itself has infinite energy, but whatever you measure (e.g. a photon) is a signal, has finite energy, and must, therefore, have a Fourier representation in terms of a single function, the sine.
Cheers, Ed Gerck
@Ed Gerk,
Ed, the perfect sine is an idealization. We don't have such things in our experiments. The signal form closest to the perfect sine is the laser beam (in quantum optics terminology, the coherent wave). Such a beam can maintain the sine form for a long time (years ago, somebody told me that it may be a couple of milliseconds, but is the the technology advances all the time).
About the photon, you see, a coherent wave doesn't have a wel-defined number of photons. Though, each photon in it behaves in the same way as the wave-packet. We can, of course, prepare very dilluted coherent waves with an average of 1 photon per wave-packet.
Best regards from me!
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