How Do We Track Photons Through Space?

When a particle moves through spacetime, how do we know it is the same particle and not some excitation that is passed from place to place?
Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. Image credit:  NASA/JPL-Caltech

Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

We don’t! This is a really interesting feature of our universe, and it comes from the observation that all subatomic particles are described by a few key properties, but are otherwise completely and utterly identical. Electrons appear to be identical to all other electrons. All photons (if they carry the same energy within them) are identical to all other photons of that energy. Protons are identical to other protons, and neutrons are identical to other neutrons.  All of these particles are distinguished from each other by their mass, electric charge, and a property called their spin. However, each and every single electron in the Universe has the exact same mass, electric charge, and spin. There are no other measurements we can do to distinguish a given electron from another.

We could think of it along these lines: let’s say I give you a ping pong ball, and tell you that this one is special because it’s yours. But then we throw that ping pong ball into a bag full of other balls which look just like yours and mix them up. It’d be quite difficult to tell if the one I pull out of the bag next is the one I initially gave you or another one. If that newly drawn ball is identical in all measurable ways to the original one I told you was yours, there’s really no way to tell if it’s the one I originally handed to you or not.

Photons have one extra parameter that can distinguish them from each other and it’s the amount of energy they’re carrying. This energy corresponds to the color of the light - the more energy, the further to the blue the light appears, and the less energy, the harder to the red it falls. I can distinguish a blue photon from a red one as it hits my camera because of this difference in energy, but the mass, electric charge, and spin of those two photons are the same.

If the photons have the same energy when they arrive, then I’ve run out of ways to distinguish them.

Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft are green; and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red. Image credit: NASA/JPL-Caltech/GSFC/JAXA

Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft are green; and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red. Image credit: NASA/JPL-Caltech/GSFC/JAXA

So if I dump a bunch of photons into my metaphorical bag, and they all come out again, there’s no way for me to tell if my favorite photon came out first or last. The closest astrophysical approximation to this simple setup is light which strikes the surface of the Sun and is then absorbed. That photon is now mixing with a huge number of other photons created within the depths of the Sun, and I have no way of flagging that particular photon to distinguish it from the flood of other, identical photons which are streaming outwards away from the Sun.

The energy that a photon carries isn’t a fundamental property of the photon in the way that its electric charge (which is neutral) and its spin are fundamental properties. Fundamental properties cannot be changed, no matter what happens to these photons in the course of bouncing around the Universe. So the energy of a photon, not being a fundamental property, canbe changed. And this energy often is changed, making our attempt to keep track of individual photons even more difficult. The photons that stream from the Sun and onto the surface of the Earth deposit some of their energy into the matter of the Earth, heating up the ground. That heating process depletes the energy remaining in the photon, and so the photon which reflects away has changed the amount of energy that it carries with it. So, if I see photons streaming into a region of space where they must interact with other objects, the identities of individual photons are even more scrambled than they would have been while they were streaming freely through space.

Your idea of energy excitations passing from place to place is precisely the right one for fundamental particles - nothing we can measure will tell me which electron is my favorite.

 

Remember to vote for Astroquizzical, which has been nominated for Canada's top 12 favorite science blogs!

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

Are The Photons That You See The Same Ones That Left The Sun?

We are taught that light behaves both like a particle and like a wave. We think of the photon as the particle form of light. Question: When a photon hits your eye as you look at the sky, or directly at the sun (through a really dark glass), is that the same photon that left the sun? Or does light operate more like electricity? The electron that powers my desk computer is just the result of a series of bumping electrons that eventually push a local electron through my power supply.
The sun at dusk. Image credit: Oliver Herold, CC BY 3.0

The sun at dusk. Image credit: Oliver Herold, CC BY 3.0

Originally posted at Forbes!

It depends on what that photon has been through – and in this case I mean that literally. Let’s start with the simple case, which in this case means I’m going to temporarily put you in space with invincible eyeballs to stare at the sun. In this case, the photon of light has left the sun, and travelled through the vacuum of space, and hit your eyeball. The photon that hits your eye is the same as the photon that left the surface of the sun.  This is pretty straightforward, as there isn’t anything that could have interfered with the beam of light making it to your eyes.

Now, we can remove your eyeball invincibility, and give you back your dark glasses – welder’s glass is best for this sort of thing. Now the light has a bit of a barrier to make it through. The difference is not that the photons are pushing each other along like a current to make it through the dark glass, but that the vast majority of the photons don’t make it through the glass. That opacity is what makes it safe for you to view the sun this way; you’re not blasting your eyeballs with too much light. But the photons which do make it through should be the same as the ones that left the sun, something like eight minutes earlier. If I also put you back on the surface of the earth, there will be effectively no difference in how the photons behave. The atmosphere plays very little role in this story; by nature of us being able to see through it, it doesn’t affect visible light very much.

Glow in the dark bird figurine. Image credit: wikimedia user Lưu Ly, public domain.

Glow in the dark bird figurine. Image credit: wikimedia user Lưu Ly, public domain.

However, the constancy of photons from the sun doesn’t hold for all visible light photons that might make it to your eye. A photon of light can be absorbed by a material (say, dust, or a glow-in-the-dark dinosaur), and that material will temporarily gain the energy that had been held in the photon of light. That energy usually gets dumped back out again in the form of a new photon, and depending on the material that absorbed the light, that new photon can be in the visible. A glow-in-the-dark dinosaur, to stick with that example, absorbs ultraviolet light, and then radiates it back out in the visible over a period of time.

So if you’re looking up at the night sky or the daytime sky, those photons making it to your eye should be the exact same photons that left their respective stars. However, if you’re looking at something which is glowing under a blacklight, or a glow-in-the-dark toy, those photons are different from the ones that left the blacklight.

Have your own question? Feel free to ask! Or submit your questions via the sidebar, Facebook, twitter, or Google+.

Sign up for the mailing list for updates & news straight to your inbox!