What lets photons travel?

Question

Electricity flows from a point of higher level of potential toward a point of lower potential. Liquids flow from a point of higher level toward a point of lower level. I want to know, in the same manner, how do we explain the flow of photons in a beam of light from its source of origin, say, from a candle or from the sun?

Answer 1

Photons, like any other kind of particle, have energy and momentum. They keep going in a straight line because, unless they encounter matter to interact with, there is nothing that can change their energy and momentum; these quantities are conserved. Basically, Newton’s first law applies even to photons when travelling through a vacuum.

Put differently, photons don’t need anything to “let them” travel. They travel in a straight line (through curved spacetime) until they interact with matter. The photons of the cosmic microwave background have been traveling for 13.8 billion years.

Answer 2

Electricity flows from a point of higher level of potential toward a point of lower potential.

That is true in some cases, but not in all. In a battery electricity flows from lower potential to higher potential. Also, in capacitors and inductors the direction of the flow of electricity with respect to potential changes over time.

The movement of electricity is governed by the laws of circuit theory. While in some cases the result is as you described, that is not the general law.

Liquids flow from a point of higher level toward a point of lower level.

This is also true in some cases but not all. For instance in a pump liquids flow from lower level towards a higher level, and splashing liquids often reverse their direction with respect to height.

The movement of liquids is governed by the laws of fluid mechanics. While in some cases the result is as you described, that is not the general law.

in the same manner, how do we explain the flow of photons in a beam of light from its source of origin, say, from a candle or from the sun?

We do not describe them in the same manner at all. None of the flows described above were generally described in that manner.

Instead we describe them using the relevant laws of physics. In the case of photons, their flow is governed by the laws of quantum electrodynamics.

Answer 3

Waves! Light travels outward from a source in the form of electromagnetic waves. Imagine throwing a stone in a pond, waves travel outwards from the source but eventually die out due to attenuation. Similarly, an interaction that produces electromagnetic waves in the visible spectrum (~400nm to 700nm) is perceived as visible light. These waves travel outward from the source radially in all directions. In vacuum, there is no attenuation of electromagnetic waves so they can travel very large distances.

Answer 4

Electricity flows from a point of higher level of potential toward a point of lower potential.
Liquids flow from a point of higher level toward a point of lower level.

This is true but misleading. It would be more accurate to say:

Electricity accelerates from a point of higher level of potential toward a point of lower potential.
Liquids accelerate from a point of higher level toward a point of lower level.

If you think about electricity flowing in a wire then there are two forces acting on the electrons. The potential difference is trying to accelerate the electrons while the resistance in the wire is trying to decelerate them. In a circuit these two forces balance out so the electrons move at constant speed i.e. we get a constant current. Likewise for water flowing in a pipe: the pressure difference is accelerating the water while the viscous drag is slowing it down. Again, at equilibrium the two forces balance and we get a constant flow rate.

If you have ever squirted water from out of a hosepipe then you'll know that when the water leaves the pipe, so there is no longer any pressure difference acting on it, the water doesn't just stop. It carries on moving. Although fewer of us will have done the same experiment with electrons, if you use a potential difference to accelerate electrons in a vacuum then fire them off into space they will keep going without needing a potential difference to keep them going.

And this explains why the photons emitted by your candle or the Sun keep moving. You need energy to create the photons but once they have been created no energy is required to keep them moving.

Photons are actually a bit of a special case because you can't accelerate a photon since it always moves at the speed of light. If yu're interested this is discussed in the question Does a photon instantaneously gain $c$ speed when emitted from an electron?

Answer 5

Electricity can essentially be reduced to charged particles being attracted (repelled) by particles of opposite (same) charge. The flow of liquids can be boiled down to any particle with mass that is subject to a gravitational field.

In both these situations there are potentials (or force fields) involved that actively contribute to the motion of the respective particle. On the other hand, if you were to take any particle and were to give it a push in the vacuum (in an empty universe), they would just go on and on without interaction. I think one could describe the creation of a photon and its subsequent travelling through space as the latter. Photons are massless and thus travel at the speed of light. This also means that gravitation has a more discreet effect on Photons than on particles with mass and the effect of earths gravitational field on them would be much less obvious than a river flowing down a mountain. But, for example, photons emitted behind our sun can be seen on earth (under the right circumstances) because the suns gravitational field is large enough to bend their motion (or rather spacetime).

So, to summarize, I would say that there is no 'natural potential' that photons are drawn to. However, photons are subject to gravitational fields and they can interact with other particles and get annihilated (for example atoms). But they are not drawn towards any 'drain' like the examples you gave. And as someone else pointed out, their interaction with matter is best approximated by quantum field theory.

Answer 6

Photons are particles. They just need kinetic energy and momentum to travel. Note that if they had been waves they would have required a medium to travel.

Answer 7

I think perhaps the answers are expressing the concept of particles and energy moving from higher potential to lower potential... neither is really relevant for photons.

Photons travel due to the relationship between the electric and magnetic fields as expressed in Maxwell's equations. In some sense, they travel because of the harmony and balance between a time-varying electric field inducing a magnetic field which in turn generates the corresponding electric field. The interplay between these two, through the cross product of the electric and magnetic vectors, results in a corresponding electric and magnetic field that is "ahead" of the existing fields.

This is the true nature of the "motion" of light:

1. The electric field induces the magnetic field
2. The magnetic field induces the electric field
3. The flow of energy follows the right-hand-rule in cross products causing the energy to advance