How are photons actually created?

366 views

(Searched and although this has been asked before I still can’t find an answer that makes much sense to me)

So how does a photon come into existence, and how can it instantly be travelling at C? I turn on a light bulb and photons are instantly created and travelling at C…but created from what, exactly? The light bulb filament is in a vacuum, but every time I turn on the light bulb new photons are simply created out of nothing (with no “fuel” to draw from)?

I guess I’m struggling to understand how heating the filament results in billions (?) of photons in a constant stream travelling at C, when a millisecond earlier there was just an inert metal thread in a vacuum.

In: Physics

Hi /u/ixtechau!

Our everyday-experience gives us the intuition that “stuff” should be conserved, i.e. that the amount of particles should stay the same during physical processes.

However, this intuition is simply wrong. There is no rule to say that the number of particles has to be conserved.

The wire is heated up by the electric current, and this thermal energy is given off in the shape of photons. Thus, energy is conserved, but the number of particles is not.

So if you’re familiar with the whole mass/energy equivalence thing (E=mc^2 etc) then you can start to see how energy can be converted to matter (setting aside that photons are massless; the principle still holds). Generally when you get photon emission, it is caused by some particle (say an electron) transitioning from a higher-energy state to a lower-energy state. That difference in energy must be liberated somehow, and photon emission is often the result. Conversely, absorption of photons can promote particles up to higher energy states – this is the basic principle of the photoelectric effect.

In an incandescent light bulb, what you are actually seeing is a conversion from thermal energy to electromagnetic energy; so in some sense the vibrational energy of the particles in the filament is converted (in some fraction) to photons of a range of energies.

When an electric charge changes its direction of travel (accelerates), there’s a change in the local electric field which produces a magnetic field. As that magnetic field grows and then shrinks, it produces a corresponding electric field. These two interweaving fields are self-perpetuating and will continue to travel in the initial direction of the electric charge as fast as they possibly can, which happens to be light speed. Though it’s nothing more than a magnetic and electric field going back and forth, it’s distinct enough that we’ve given it a name: The photon.

When that photon is produced by the charged object’s acceleration, it takes a little bit of that objects energy away from it. That’s the “fuel” that you’re wondering about. Every light bulb, no matter how it works, is made of atoms or molecules. And, if you remember your high school chemistry, you’ll know that atoms and molecules have a dense nucleus with negatively charged electrons going around them.

In the case of an LED bulb, the electrons are given a tiny bit of energy that makes them go around the nucleus faster than usual. As the electrons go around the more energetic orbital, they accelerate, making them want to release that energy as a photon so that they settle into a more stable orbital that won’t allow them to lose any more energy.

With older incandescent bulbs, the atoms are simply heated up, making them jiggle in place as they collide with one another. Because they’re made of combinations of charged particles, they’ll release a photon every time they change direction from a collision, again removing a little bit of energy from the system.

This, by the way, is why particle accelerators have to be built larger to make the particles go faster. A smaller loop makes the particles accelerate more sharply, causing them to release photons and lose energy. Larger loops generate less acceleration, making the energy loss less pronounced.

[removed]

You seem to tripping over a concept of vacuum as “nothing.”

Modern physics has fields present at every point in space, even in a true vacuum. One of the fields in question is the electromagnetic field. A photon is an excitation of the electromagnetic field, basically a wave/ripple in the field.

Another misunderstanding in your comment is that “there’s no fuel.” This isn’t combustion. Rather than fuel, there is energy. Energy can change forms. In this case from electrical energy to light (photons). Electrical energy is dumped into atoms of the filament causing excitation of the electrons. Electrons dropping from their excited level release quantized energy “packets” which we perceive/understand as photons.

If that’s insufficient and you still get hung up on the mechanics of the specific transitions mentioned above you’re going to have to ask a particle physicist, or really dig into quantum physics, but I guarantee that will leave you with more questions rather than less. I’ve yet to meet anyone who had less questions after a dive into quantum physics.

Oh, and your “billions (?)” is many orders of magnitude too low unless you’re talking about an extremely short time period.

>but every time I turn on the light bulb new photons are simply created out of nothing (with no “fuel” to draw from)?  If there’s no “fuel”, how come you get an electricity bill?

There’s two ways a photon can be emitted:

1) An accelerating charge. Note that changing direction and going back and forth are ALSO acceleration. It’s a change in velocity (direction and speed) not just speed itself. So any time you have vibrating charges (like heat vibration in a hot filament), a charge going in a circle (like in particle accelerators) or a charge colliding with an obstacle (that’s how an Xray lamp works), you’re going to get emission of photons.

2) A particle “dropping down” an energy level. Particles can have more energy than they would like, and in many systems the only way for them to get rid of it is to emit a photon. I honestly don’t know HOW it works, I’m not sure if there strictly is an explanation, but we know that they do and the rules governing when emissions can and cannot happen.

As for why they immediately go at C… Because that’s their nature. Photons are massless and massless particles travel at C.

And yes, I know “photons in a medium don’t travel at c”, but that’s a more complex problem that has to do with charges in that medium, and behaviours of waves, and how they interfere with other waves… and some physicists decided to just stopped calling an electromagnetic wave in a medium a “photon” and call it a “polaron” instead, because it has different properties and therefore isn’t the same particle. But that’s honestly well beyond and ELI5.