Why options? Why photons?

Hello 🙂
It’s been an interesting second week. My second course for this term (Nuclear and Particle Physics) has begun and feels like we have already had much much more than a week of it… It’s one of those courses that quickly sail through lots of results from equations we can’t derive or understand or do and hmm. I’m not it’s biggest fan so far (although it has been useful in telling me how much of special relativity I need to revise from last year). I think the ‘nuclear’ section might contain a bit more information that is more accessible– we shall see.
To get the rest of my complaining over early on in this blog, it turns out that after carefully selecting my course options for the next two years and discussing them with my tutor it turns out that one of the courses I wanted became almost instantly really oversubscribed, and apparently it is a first-come-first-served waiting list (on which I suspect I am about 40th). I spent most of today raging about this– the problem is that if I can’t do this course next year then I can’t do another course I want next year that leads on from it unless I potentially do them both in tandem (if they don’t clash) and that means I have to take a fourth year option this year and uggghhh– as I said before the options are really complicated so this is quite frustrating.
I am in a better position than some of my friends who have more than one course they want full before they had even checked their email to find the options form was open– and who have more than one course leading on from the ones they wanted this year. I am going to see if I can register as non-attending or something similar but still watch the lectures online and take the exam. The reason the courses are full is that they were assigned to one of the smaller lecture theatres and fire safety things mean you can’t over-fill the rooms which is completely understandable and works most of the time very well I think. This year the courses have been re-jigged and I suppose there are going to be some oversights about how many people want to take a certain course– it’s just frustrating when it happens to your one!
Anyway, that is the last of the negative stuff. Last weekend I went it Birmingham to visit my friends and see McBusted (because I am really really cool) which was amazing! I also came back with some pretty useless (but really pretty) clothes and thankful that my degree doesn’t involve exams in handling pigeons and horses (one of my friends is doing veterinary medicine– top tip: apparently if you let go of the pigeon in your exam and it starts flapping around the room, that at is an automatic fail.)
I wanted to end this blog with a bit of a brain-teaser that one of my tutors brought up in tutorial today, namely this question: ‘can a photon be emitted if it isn’t absorbed?’
He asked the question of the different models of light that we knew about, starting with Fermat’s theorem– that light always takes the shortest path between two points– where it doesn’t simply make sense to ask the question as the two points are absorption to emission, so if there is no absorption there is no path.
Nowadays you can measure the recoil of the particle that emits the photon as its momentum is conserved, so you might think instinctively the answer would be: well yeah, obvs you can– go ahead and emit photons all you like!
However, photons travel at the speed of light, so according to Special Relativity time has stopped for them. To a photon, its point of emission and absorption are connected– they both happen instantaneously. So then what would happen if it never reached anything– was never absorbed?
QED (quantum electrodynamics) is the current theory describing particles with electric charges and photons and though we are starting to mention it in lectures I definitely don’t understand it enough to be able to give you a definite answer on what it says about this. After looking into it though, I’m not even sure that the experts could agree either way!
Initially I struggled to find anything online about this topic at all, let alone a peer reviewed source but eventually I managed to stumble across this paper (http://arxiv.org/pdf/1306.4310.pdf) entitled ‘what is a photon?’ It is really clearly and simply written and well worth a read (as well as being short and containing none of that horrible maths needed for a lot of this topic.)
It talks about an interpretation of photons simply as an interaction between an absorber and an emitter and thinking about photons as two different fields, one moving forward and one backwards in time. This sounds horrible and crazy but turns out to predict interactions as we see them.
That’s not to say it doesn’t cause some crazy statements such as ‘The sun would not radiate if it were alone in space and no other bodies could absorb its radiation’ and that at certain time scales there is a ‘complete interdependence of the past and future’. Hmm.
Anyway, as I said I don’t think this is a commonly accepted theory but it is pretty convincingly explained by the paper and also the opinion of Dr.Allen, the tutor who started this discussion.
Aha, you might think. That is certainly a little unsettling, but really come on now– your photon is always going to be absorbed, no matter where you point it. That is a good point, but in these days of an increasingly expanding, potentially infinite universe there is no guarantee that a beam of light pointing into space will ever ever ever reach something to absorb it. Dr. Allen seemed more than a bit annoyed at the universe’s (rather rude) decision to go on expanding faster and faster.
Looking into this has already taken far far too long out of my revision, so I might have to leave it there for now– but I am determined to come back to it later, even if that later is in two years once I have covered (hopefully if the options allow) Quantum Field Theory. Have a good weekend everyone and fingers crossed that the universe is planning on falling back together into a Big Crunch after all.

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