Engineering: Ask the expert

 

If you have a question that only an expert can answer, then send it to us and it could be published in the magazine. And if it is published you could win an Amazon voucher. Mark Alldridge from Poole in Dorset asked this month’s winning question.

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“As I understand it, a photon is a ‘massless’ particle. But I have read somewhere recently that a photon only has zero REST mass. Does this mean that a photon DOES have mass? - How can this be? My understanding is that anything that possesses mass can never attain the speed of light.”

Expert, David Saxon, Kelvin Professor of Physics at the University of Glasgow in the UK provided an answer:

A: That’s a very good question. That’s in the territory where we have to pay attention both to relativity and quantum mechanics at the same time, which means our intuition is not a very good guide. A photon does have zero mass; you can never bring a photon to rest. Whoever observes it always thinks its travelling at the speed of light. So the idea of a rest mass does not apply to a photon. So that sounds very simple until we introduce quantum mechanics and there comes an interesting BUT…

BUT… Heisenberg’s Uncertainty Principle says that we can never measure things precisely – if we measure position well, we’ll measure momentum badly; if we measure time well, we’ll measure energy badly, and this allows some wriggle room…

So at the level of elementary particle physics where we are looking at the interior workings of some reaction, particles appear with masses which are not the ones that we normally associate with free particles, and in fact, the quantity occurs in the equation as the square of the mass that can be either positive or negative. 

When things live for a very short time… we call them ‘virtual particles’ because they don’t escape from the reaction – they’re not like free particles which we can see travelling through space or weigh in a bucket… when you’re dealing with a virtual particle, Heisenberg’s Uncertainty Principle allows them to cheat on the mass and indeed the quantity which occurs in the mass squared and in the Virtual Particle that can be positive or negative. It’s an introduction to the wonders of quantum mechanics.

Do you have a science or engineering question for an expert from one of the UK’s institutions to answer? Maybe it is about your school science projects or something you have read. Your interest could be rewarded.

Email editor@bescenta.co.uk and not only could your question be published along with an answer from an expert, but you could win a £30 Amazon voucher for your efforts. One winning question is chosen per issue, and don’t forget to include your name, age and where you’re from.

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