Can light exist if it never interacts with anything?

Question

I've heard some physicists mention that photons do not experience time. From the perspective of the photon, it begins and ends instantaneously.

With all the photons shooting out into empty space, it seems likely that the majority of them will never interact with anything, and therefore, will never have an end point.

So if a photon never has an end, can it ever really exist in the first place? Alternatively, is the universe arranged in such a way that all photons must reach an end point eventually?

Answer 1

I suspect your question will be closed. However, I will have a stab at clearing up your confusion. You have posed two questions. The first is whether something can only exist if it has 'an end'. I have three objections to such a thought. The first is that we are made of fundamental particles, such as protons and electrons which, as far as we know, do not have 'an end' and yet we clearly exist, so that suggests that the possession of 'an end' is not a requisite for existence. My second objection is that the question implies a weird circular causality, by suggesting that the presence or absence of something that happens later (ie an end) could determine something that happens earlier (ie something coming into existence). My third, and most obvious, objection is that countless trillions of photons clearly do have 'an end' and clearly do exist.

Your second question is whether the Universe is arranged in such a way that all photons will eventually be absorbed. The answer is that we don't yet have a single proven model for the eventual fate of the Universe, so we don't know what will happen to all the photons that are ever created.

Answer 2

It is not necessary for a photon to be absorbed or scattered to have an effect on the universe. Photons have momentum, and hence gravitational effects. The gravitational effect of a single photon is obviously very small (for any photon with energy we are familiar with. However, a vast sea of space filled with photons that will never collide with matter, could, due to its vastness, exert a theoretically detectable effect.

Answer 3

This question reminds me of the ideas about time proposed by Julian Barbour. Barbour proposed that in a physically meaningful sense all physics that happens is happening simultaneously, and that the directionality of time that we experience is an apparent phenomenon.

There is a logical implication of relativistic physics that I think is related to the thing you are asking about.

If you have an emitter of light (for example a star) and a detector of light (for example a telescope on Earth), the measured frequency of the light arises from the relative velocity between the two. That is: the frequency of the received light as measured by the telescope's spectrometer is not exclusively due to the velocity of the emitter with respect to some luminiferous Aether (since it is assumed there is no Luminferour Aether), nor exclusively due to velocity of the detector.

It is upon detection that the frequency of the propagating light is unambiguous. The information that determines what frequency the spectrometer will detect is a function of the relative velocity between emitter and detector. As long as the progating light isn't detected the status of the frequency of the propagating light is ambiguous.

(The above problem does not arise if a luminiferous Aether is assumed. With a luminiferous Aether there is a frequency event twice; upon emission and upon detection, both determined by the velocity of the emitter/detector with respect to the luminiferous Aether, and while in transit the light itself is in an unambiguous state. I guess this was one of the reasons the 19th century physicists opted to assume a luminiferous Aether.)

One way out of the ambiguous frequency conundrum would be to stipulate that in order to be propagating at all light must be both emitted and received.

Your question is phrased in terms of 'from the perspective of the photon', and from there you give a line of reasoning to suggest that in order to exist at all the photon must be both emitted and received.

I don't think the 'from the perspective of the photon' angle is meaningful. Mathematically: yes: for a photon the amount of proper time that elapses collapses to zero.

In mathematics, the point where an entity collapses to zero is like reaching a point where you are dividing by zero. As we know: whenever you introduce division by zero you effectively introcude infinite ambiguity. So I recommend against reasoning 'from the perspective of the photon'.

That is why I gave another line of reasoning that suggests that in order for light to be propagating at all it must be both emitted and received.