Gravitons

Since beginning my occasional series “The Big Questions” in Sky At Night Magazine, I have been contacted by people asking me to explain other enigmas of physics. Many of them are already extremely well informed, so it’s a real challenge to give an accurate and satisfying answer – a challenge that I find myself enjoying. Writing for the magazine has been an elevating and refreshing experience in many ways.

It would be wrong to mention Sky at Night this week without paying tribute to Sir Patrick Moore, whose inspirational life came to an end on Sunday. I won't attempt an obituary, as many have been published this week, far better than I could manage. Brian May, writing in The Guardian, put it particularly well: "the world has lost a priceless treasure that can never be replaced". But I will just add my ha'peth, as one of the legion of scientists who owe their earliest interest in astronomy to Sir P. I would like to express my gratitude for his enormous enthusiasm and thought-provoking presentation. Unfortunately, unlike the presenters of the TV programme, contributors to the magazine do not tend to meet, working instead by email and telephone. Nevertheless I feel fortunate to have my name appearing in a few of the same issues as his.

Returning to the point, one of the readers recently e-mailed me with a really deep question. It's a bit more technical than my usual blog entries, but I know it's something that a lot of physicists wonder about. So, with my correspondent's permission, I’ll reproduce his question and my answer here in full.

Subject: Gravitons

Dear Dr Evans

As a Sky at Night reader I have seen and enjoyed your first two articles, so look forward to the others.

Separately, in the last year or so, I have make making an effort to understand, or at least, get a better feel for Einstein’s relativity.

I have been able to (sort of) follow the steps that lead to Special Relativity and the how the problem of gravity (instantaneous action at a distance and so on) was resolved by the development of General Relativity simplistically as a geometric description – curved space-time.

What still does confuse me however is that elsewhere gravity continues to be described as a force with attempts being made to combine it with the electromagnetic, strong and weak forces into a theory of everything. In this context  the graviton is described as the force carrier for gravity (analogous to the photon, W & Z bosons, and the gluons for the other forces) on the way to the development of a quantum theory of gravity.

I suppose the question is whether gravitons are reconcilable with general relativity, or whether this means that the latter in itself is still not a complete description. I have looked at various websites to try and get some light thrown on this but without much success – references to stress energy tensors, and so on don’t mean anything to me, and I doubt are ever likely to!

It would be useful to know from someone working is this field what the current thinking is – definitely not in terms of any technical description, but just a summary that could be understood by the interested non specialist as to the status of the various theories.

If you are able to find time to respond, that would be useful, but quite understand if not.

Regards

Brian Radesk             

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Dear Brian,

Thanks for your message. I'm glad you've been enjoying my articles.

It's a very interesting question that you ask. The short answer is that General Relativity hasn't been overturned. It has been built upon. Or, more accurately, it is being built upon, since there isn't yet a complete quantum version of it.

First, I should say, gravity is a force. I mentioned in my last article that "in a sense" there is no force, only geometry, but that's really just a way of understanding the nature of this particular force. Look in detail at any type of force, and you'll find an alternative way of describing it. So there is no contradiction between General Relativity and descriptions of the "force" of gravity.

Gravitons, like photons and the other "gauge bosons" (force carriers) are very confusing, and almost universally misunderstood. They are called "particles", but things that are called "particles" in quantum field theory (the "standard model") are not really particles at all in any sense that we would normally use the word. You might have heard of the idea of wave-particle duality in quantum mechanics, where an object like an electron sometimes seems to behave like a particle, and sometimes like a wave. In fact, all quantum mechanical "particles" behave almost entirely like waves. The only thing that they have in common with classical particles is that, under certain very specific conditions, they can be countable.

Take the theory of electromagnetism, for instance. This was all worked out by the Victorians, culminating in James Clerk Maxwell's theory that unified electricity and magnetism by describing them both with four elegant equations. That theory predicted the existence of electromagnetic waves, including light. 

Subsequently, those waves were found to exist with only a discrete set of possible amplitudes (or energies). So you can count the amount of energy possessed by those waves in discrete steps. One of these countable increments in the energy of an electromagnetic wave is called a photon. You see, it's not really much like a particle. And the existence of photons doesn't over-throw Maxwell's theory of electromagnetism.

Similarly, Einstein's field equations of General Relativity have solutions describing gravitational waves. Applying the principles of quantum mechanics, people expect those waves only to be allowed to exist with a discrete set of possible energies. These countable excitations of gravitational waves are called gravitons, but we don't yet have a full self-consistent mathematical description of them. Whatever the correct theory of quantum gravity turns out to be, you can bet it will incorporate the field equations of General Relativity.

I hope this helps.

Best wishes.

Mike Evans.

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Dr R Mike L Evans

Lecturer

Department of Applied Mathematics

University of Leeds, LS2 9JT, UK

@PhysicsBloke