# How to identify if a photon comes from the sun?

Is there any way to know whether a group of particles is generated from the sun rather from an artificial source?

• No, seriously, you must make sure that there is no other sources between you and the Sun. According to the Maxwell equations, all charges and currents are a source of the electromagnetic field, so to get photons from the Sun you must remove/supress the other possible sources. In other words, remove the "noise". A lot of work to do. – Vladimir Kalitvianski May 18 at 9:02
• @Vadim: Nowadays, there is a QR code. – user21820 May 19 at 9:22
• @Vadim: Exactly; so we can tell apart those from the sun and those from alien star systems. =P – user21820 May 19 at 9:30
• @user21820 Do you slice them open and count the rings? – Barmar May 19 at 14:16
• I would check their meme usage. If they are outdated by 8 minutes or so, they are probably from the sun. If they are outdated by anything more than that, then they came from facebook. – T. Sar May 19 at 18:31

Photons are identical particles characterized by energy and a direction of propagation. If you see just a photon, without any other information, from these two properties, you cannot distinguish a solar photon from one coming, let's say, from a tungsten filament at a temperature of $$5800\,\mbox{K}$$ (the surface temperature of the Sun).

On the other hand, if you see many photons coming from the big glowing spot in the sky, you can reliably tell that they come from the Sun.

• In principle same-wavelength photons are identical, and blackbodys radiate in pure black body radiation - in practice though you have information about the distribution of photons from the sun that makes it deviate from a pure black-body spectrum; e.g. from absorption in our atmosphere and things like that. So that, if you got a box and captured some sun-photons, and got another box and captured some 5800 K tungsten filament photons; their spectroscopic data will show up different – Joshua Lin May 18 at 19:54
• Could photon spin be used?? – marshal craft May 18 at 20:03
• @JoshuaLin If you do that you are making some assumptions such as assuming there arent other filters or sources involved. If you can make these assumptions then you arent basing your decision on the photons themselves and might as well just go with the "big glowing spot" method. Its pretty reliable. – Matt May 18 at 22:09
• @Shmuel Newmark: Not unless the tungsten filament is really close, so that the number of photons it emits is enough to measurably distort the emission curve (as shown in another answer). But that is a statistical measurement of a great many photons, and says nothing about the source of any individual photon. – jamesqf May 19 at 4:19
• Isn't measuring a direction of propagation pretty hard, especially for a single photon? This further complicates the problem of actual experiment, unless the sun and the other source are not both exposed to the detector at the same time. But I imagine pretty impossible to distinguish the sun and a star next to/behind on the sky emitting photons having same energy arriving at the detector would be even though classically you could expect a different angle. For multiple photons, one could obtain a distribution and infer there are two sources, for single photon it would be quite impossible IMO. – luk32 May 19 at 10:57

There is no physical difference between the photons emitted by the sun and those emitted by an artificial source, so there is no way you can look at a photon and know for sure what generated it. However, sunlight has specific characteristics which means that you can usually distinguish it from e.g. electric lights.

For instance, you can look at the spectrum (frequencies) of the group of photons and see if they match the spectrum of the sun. This is because any black-body radiates light at different frequencies with specific intensities depending on the temperature of the black-body. The temperature at the surface of the sun is about 6000 K, and so it radiates light with this black-body profile.

There is no physical principle that prevents an artificial source from perfectly emulating sunlight however, and in that case there would be no way to distinguish the two.

For a single photon, you can only calculate a probability based on the photon's energy (and polarisation) and the sun's spectrum.

For a group of photons, you can check for properties that are inconsistent with a chaotic (natural) origin. For example, the polarisation pattern of sunlight is well known, and your group of photons can be checked against the expected distribution.

For a group of photons, you can also test for non-classical properties, i.e. for the presence of quantum light. For example, if you detect a group of photons with perfectly equal temporal spacing (i.e. anti-bunching, second order correlation function $$g^{(2)}(\tau) \leq 1)$$, the light source is likely to be artificial. Various tests exist for coherent light (laser light), and other quantum light properties such as squeezing, Fock states, and entanglement.

Imagine a quantum harmonic oscillator. You transfer some energy to the oscillator, so its energy level increases by one quantum. You transfer some more energy to the oscillator and its energy level increases by another quantum. Which quantum is which? How could you even tell?

In quantum field theory, such as quantum electrodynamics, fields are Fourier-decomposed into an infinite sum of harmonic oscillators. When the field receives energy through an interaction, one of these quantum harmonic oscillators gains a unit of energy. When the field in question is the electromagnetic field, we call that unit of energy, that unit excitation a "photon".

These unit excitations have no identity. You cannot tell which one is which. I was inclined to say that when you capture a strong beam of sunlight, chances are most of the photons you capture came from the Sun but even that is not true: it would presume that the photons in question did have an identity in secret. (Perhaps a more apt analogy would be warming a cup of water by a degree then by another degree, and then asking which degree came from where.)

Another relevant example, I think, comes in the form of solar neutrinos. Fusion reactions deep inside the Sun produce electron neutrinos. However, due to neutrino mass mixing, excitations of the neutrino fields that began their existence as electron neutrinos may be captured by a terrestrial detector as muon neutrinos. So where did a specific neutrino come from? The Sun? It didn't even emit muon neutrinos. Somewhere else? No, there are no other sources of neutrinos. Again, the answer is that just like photons, neutrinos have no identity either. What the detector interacts with is the neutrino field in its excited state, not some specific miniature cannonball fired by the Sun.

• Given that the neutrino field was excited by the sun, and that the energy of this excitation travelled in a localized way from the sun to the earth, I think de Broglie would say it travelled pretty much exactly like a miniature cannonball. The fact that neutrino cannonballs have an oscillating flavor makes them pretty, but no less particle-like. – Matt May 20 at 12:02