Physics Question #328

Benoît Mailhot, a 16 year old male from the Internet asks on February 1, 1998,

Are light and matter the same thing? Can you get matter from light?

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The answer

Ian Keith Affleck answered on January 31, 1998

Light consists of particles called photons, from the point of view of quantum physics. Other types of matter are made of different constituent particles: electrons and nuclei (composed of protons and neutrons, in turn composed of quarks). These are certainly different objects than photons.

In particular photons have zero mass while these other particles are massive. (There are other particles that appear to be massless. Neutrinos were thought to be massless, but now they appear to have mass. This has only been shown for 1 or 2 of the 3 types of neutrinos but it seems likely that they are all massive. Are there any other massless particles besides photons? Perhaps only gravitons which are very difficult to observe.)

While light and matter are clearly different from each other, they interact with each other in various complicated ways. For example, light can be emitted by atoms. This basically means that an atom can emit a photon. The study of the interactions between light and matter is an important field of scientific research with important technological applications.

Ian Keith Affleck answered on June 25, 2003

Reader Dale Tucker asks: In your opinion, would it be possible for a photon to carry information about the atom of its origin?

Affleck: Yes, a photon definitely carries a certain amount of information about the atom which emitted it. Photons of particular frequencies (or correspondingly energies or wave-lengths) are emitted by particular types of atoms. This is used extensively in astronomy. The material making up a distant source of photons (such as a star) can be identified by the frequencies of the photons. One can also sometimes deduce the velocity of the atom by the frequency shift of the photon. (The cosmological red-shift is an example of this.) Furthermore, a given photon frequency corresponds to a transition between particular initial and final states of the atom so the quantum states that the atom was in immediately before and immediately after emitting the photon can also be deduced from the frequency. Specification of these states generally involves specifying the spin of the atom among other things.

[Editor: What about the concept of "Entanglement"--the notion that particles' can somehow "know" to flip their spin, when their twin does so even though it might be light years distant. Is one carrying information about the other?]

Affleck: Yes, entanglement certainly applies to photons- in fact that is the context in which it is perhaps most often discussed. The famous proposal of Einstein, Podolsky and Rosen to observe an apparent paradox of quantum theory was based on the emission of a pair of photons, going in opposite directions with an "entangled" wave-function.

When you measure the spin of one photon you instantaneously discover the spin of the other (possibly very distant) photon by "collapsing the wave-function". Saying that one photon is "carrying information" about the other one may not be quite the right way of looking at it however. I would prefer to say that they are correlated. Each photon has equal proability of being in the spin up or spin down state and is actually in both at once in a certain sense. However, the correlation implies that if a measurement is made which determines the spin state of one photon then the (opposite) spin state of the other is instantly determined.

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