Do photons follow a sinusoidal path?



Does a single photon moving through space in an overall straight line actually trace out a sine wave, turning left and right as it goes?

In: Physics

It goes in a straight line. The sine wave you often see associated with them represents the fact that light is both a particle and a wave.

They are waves – they do not follow waves. It is an oscillation in the direction of a field, not an oscillation in position. They move in a straight line.

Here’s my best attempt at explaining Quantum Field Theory to a five-year-old:

Think of a large pool filled with water. If the water is very still, you can barely see it’s there. However, when the water is disturbed (or “excited” in the case of a field), you can observe the ripples as they move through the water.

If you create a very large disturbance in the water and the ripples splash against something, it can create individual droplets that appear visually separate from the rest of the water.

Now imagine that droplets are more or less likely to be created at certain points in the ripple (I.E. closer to the centre of the ripple, or at the peaks rather than troughs).

The droplets aren’t guaranteed to appear at any point, but the *probability* of them appearing at certain locations is higher than others.

The ripple represents the “wave” portion of light. The sine wave that we represent light as is a *wave function*. Like the ripples in the earlier example, a photon (our metaphorical droplet) is more likely to be found at certain points in the probability wave (ripple).

Just like the droplets only appearing when the ripple splashes against something, photons only exist at the moment that they are observed. Imagine this as a ripple in the photon field, where photons only pop in to existence when the wave “splashes” against something (like other particles).

Hope I did okay!

Imagine that you have two magnets, you turn them so that the negative poles are facing each other and then put them next to each other. If you push one magnet towards the other then the other magnet will get pushed as well – even though the magnets haven’t touched. This is obviously because you have two magnets repelling each other. What we experience as light is a very similar process.

An atom is technically electrically neutral because the positive charge of the nucleus is cancelled out by the negative charge of the electrons orbiting it. But that’s only really true on a very simple level. The reality of the situation is that atoms have a very, *very* slight negative charge.

This is caused because all of the electrons are on the outside, while all of the protons are on the inside. Because all atoms have a negative charge on the outside, they repel another. If the atoms are very close to another then that repulsive force is quite large, which is why atoms don’t merge with one another. The force drops off extremely quickly such that it is extremely extremely minor if there is any meaningful separation between two atoms – but its still there.

This means that if any atom, anywhere in the universe moves towards you then you will experience a very, very slight force pushing you away. Ordinarily this is so small that its imperceptible. This also causes a weird “pushing” force when an atom moves away from you. In essence, every atom in the universe is pushing you, so if one atom moves away then the slight decrease in force that you experience from that atom results in you moving towards it because every other atom in the universe that’s on the opposite side of you is still pushing with the same amount of force.

What we experience as light is when you have a large number of atoms all moving back and forth very quickly. Most atoms are vibrating back and forth very rapidly – you may have heard that most objects at normal temperatures give off infrared light as heat. The reason they do this is because at normal temperatures, the atoms in most objects are vibrating back and forth about 10,000,000,000,000 times every second. That causes the atoms in your body to vibrate by the same amount and that vibration is what we perceive as infrared light.

When people draw out a lightwave what that wave is representing is the average amount of net motion in light emitting object at any given point in time. So imagine a wave that goes from 1 to -1. At 1, the atoms in the object are travelling away from you at a speed of 1, which is their maximum speed. At -1, the atoms in the object are all travelling towards you at a speed of 1. In between 1 and -1 the atoms are either slowing down, because they’re traveling towards a nearby atom that is pushing them back, or they’re speeding up because they’re traveling away from a nearby atom that, again, is pushing them back. At 0 the atoms have stopped so there is no net movement.

In other words, that lightwave is telling you the *quantity* of photons. At 1 and -1 there are a lot of photons and so they’re able to have a large effect on you. At 0 there are none and so there is no effect on you. The photons themselves simply travel in a straight line and have no idea whether there are any other photons or anything else.

Every photon is identical to every other photon – the only difference is the direction that they’re travelling in. We perceive photons as having different effects and energy levels because we can’t actually perceive or observe individual photons – all that we can observe is the aggregated effect of large numbers of photons over time.

No, a photon does not move in a sinusoidal path.

A photon travels in a straight line from source to destination, and the “sine waves” that you are thinking of are simply a *representation* of the electromagnetic field that the photon is made of.

The oscillation of the electromagnetic field describes the *properties* of the photon (frequency, polarization, and velocity), not its actual physical movement.

Part of the difficulty in this is that a photon is *both* a particle and a wave at the same time. It was a massive logical leap when it was made, and it’s still counterintuitive today, but it is fact and has been verified time and time again.