This is a genuine question on my part – and I would be grateful for any answers!

The inspiration for asking the question comes from Genius: Richard Feynman and Modern Physics – my current “listen while running” book – along with Feynman’s own description of radiation in QED – The Strange Theory of Light and Matter.

Feynman argues that there is no radiation without absorption: in other words a tree that falls in an empty forest does indeed make no sound (if we imagine the sound is transmitted by photons, that is).

This sounds like a gross violation of all common sense – how could a photon know when it leaves a radiating body that it is to be absorbed?

But then, general relativity comes to our rescue – because in the photon’s inertial frame the journey from radiator to absorber is instantaneous.

But how can a body that exists for no time at all, exist at all?

Then again my assumption in asking this question is that time is in some sense privileged as a dimension of spacetime. This is a pretty deep controversy in theoretical physics these days and I am not qualified to shed much light on it – but let us assume that a body can exist with a zero dimension in time but real dimensions in space, can we then have bodies which have zero dimensions in space but a real dimension in time? If so, what are they?

###### Related articles

- Entangled toy universe shows time may be an illusion (newscientist.com)
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- And Let There Be “Molecules” of Light (blogs.scientificamerican.com)
- New form of photon-based matter is essentially a lightsaber (geek.com)
- Scientists create never-before-seen form of matter (americankabuki.blogspot.com)
- Fascinating Friday – Feynman (thecomplainerscompendium.wordpress.com)

## 2 responses to “In what sense do photons exist?”

Hi Adrian,

I would say that under the contemporary mainstream view, this is the kind of question that is either largely ignored or dismissed as philosophy.

I recently wrote a paper in which I gave four arguments based on the special theory of relativity which suggest that photons (or anything that travels at c in space) do not enjoy the same status of existence as massive objects (mass being inextricably associated with a nonzero proper time). In particular, I raise the possibility that photons may be considered as objects that do not exist in spacetime, but in a lower-dimensional analog.

To understand why something that might exist in a lower-dimensional analog of spacetime should be considered to exist “outside” of it, you have to first remember that the geometry of Minkowski spacetime is very different from the Euclidean geometry, but it is the latter which largely shapes our intuitions. In particular, in Euclidean geometry, time is an independent parameter that applies not only to 3-space but also equally well to 2-space. In Minkowski spacetime, on the other hand, proper time is proportional to the interval, the spacetime analog of the Euclidean concept of distance (It cannot really be called “distance” because it can be negative-definite). But since intervals of the Minkowski 2+1 analog belong to the set of intervals that have 3 constituents (2 spatial, 1 time) whereas the intervals of Minkowski 3+1 spacetime belong to the set of intervals that have 4 constituents, the two sets of intervals, and therefore the two sets of proper times, are disjoint. It is in this sense that one can consider an object that exists in a lower-dimensional analog to exist “outside” Minkowski spacetime.

If you are interested to learn more, you can read the paper here:

Click to access 1306.0076v2.pdf

Though the physical basis of the arguments is uncontroversial, the interpretation given to them is, as far as I know, new and might be considered highly controversial. I tried to write it accessibly and hope it helps satisfy your curiosity and inspire to ask new questions.

Best,

Armin

PS: a follow-up paper which examines the same question from the perspective of quantum theory and also presents some criteria for physical existence is in preparation.

Thanks. I’ll have a look at your paper, though my SR/GR is rusty – it’s over 25 years since my BSc 🙂