B What Keeps Photons Traveling Across the Galaxy?

[Policetac]

TL;DR Summary

How can a photon of light literally travel across the galaxy?

How can a photon of light literally travel across the galaxy? Where does it get the energy to do so and what then keeps it going? Why isn't yhe frequency and energy depleted? Does the coldness of space have anything to do with it?

 

[Vanadium 50]

The source powers it.
Why does it keep going? What's stopping it?

 

[berkeman]

Welcome to PF.

TL;DR Summary: How can a photon of light literally travel across the galaxy?

How can a photon of light literally travel across the galaxy? Where does it get the energy to do so and what then keeps it going?

Much farther than across the Galaxy in fact; it can travel across the Observable Universe, no?

And paraphrasing V50, "Fire a bullet into the vacuum of space, and what stops it?"

A more on-point question might be, "How can the radiated EM from so distant a source (billions of light years away) still have enough energy to be detected by us?" Part of the answer is that the initial radiated energy was so intense (like from suns) that it contained zillions of photons, so per the inverse square law of intensity attenuation, after light years of expansion, the EM coming from those intense sources still has enough intensity/photons to be detected here.

 

[Nugatory]

Why isn't the frequency and energy depleted?

Electromagnetic radiation, which is what light is, loses energy by interacting with matter (the "lost" energy is absorbed by the matter, heating it up). Intergalactic space is a very close to total vacuum, so no matter to interact with and no loss of energy.

 

[malawi_glenn]

TL;DR Summary: How can a photon of light literally travel across the galaxy?

How can a photon of light literally travel across the galaxy? Where does it get the energy to do so and what then keeps it going? Why isn't yhe frequency and energy depleted? Does the coldness of space have anything to do with it?

Neutrinos can also travel across the galaxy, other particles as well.

 

[Hyperfine]

TL;DR Summary: How can a photon of light literally travel across the galaxy?

How can a photon of light literally travel across the galaxy? Where does it get the energy to do so and what then keeps it going? Why isn't yhe frequency and energy depleted? Does the coldness of space have anything to do with it?

You have gotten good answers posted above. I would like to add one additional comment directed specifically at the following part of your question: "... what then keeps it going?"

For the moment, think about light as an electromagnetic plane wave as illustrated below (public domain image courtesy of NASA and Space Telescope Science Institute). The light wave propagates in the x-direction as shown. The electric field (yellow) oscillates in the y-direction and the magnetic field (blue) oscillates in the z-direction. The electric and magnetic fields are always perpendicular to one another, and perpendicular to the direction of propagation. Notice how the fields oscillate together. The important point is that changes in the electric field drive changes in the magnetic field, and vice versa. The significance of those changes is that the light drives itself--it self-propagates, it keeps itself going.

 

[sophiecentaur]

You have gotten good answers posted above. I would like to add one additional comment directed specifically at the following part of your question: "... what then keeps it going?"

For the moment, think about light as an electromagnetic plane wave as illustrated below (public domain image courtesy of NASA and Space Telescope Science Institute). The light wave propagates in the x-direction as shown. The electric field (yellow) oscillates in the y-direction and the magnetic field (blue) oscillates in the z-direction. The electric and magnetic fields are always perpendicular to one another, and perpendicular to the direction of propagation. Notice how the fields oscillate together. The important point is that changes in the electric field drive changes in the magnetic field, and vice versa. The significance of those changes is that the light drives itself--it self-propagates, it keeps itself going.

View attachment 323029

10/10 for not introducing photons into this.

 

[Policetac]

Thank you to everyone. Now I need to process it all.

 

[PeroK]

Thank you to everyone. Now I need to process it all.

Although technically not strictly relevant to the photon, you might like to read about Newton's first law of motion.

 

[sophiecentaur]

Although technically not strictly relevant to the photon, you might like to read about Newton's first law of motion.

Newton's opinion of mass and how it's affected by forces did not stretch to particles of zero mass. So I don't think it's particularly relevant here.

 

[sophiecentaur]

Thank you to everyone. Now I need to process it all.

I'd encourage you to concentrate on the classical notion of waves and avoid letting quantum mechanics muddy your waters. If you want to get a grasp of what happens at great distances then start with shorter distances - just a few thousand light years and sensible Energy densities - and assume you're dealing with a continuous quantity.
This will not be selling you short in any way but it will give you the best chance of overall progress. Promise yourself you won't get involved with photons until you have the basics sewn up.

 

[PeroK]

Newton's opinion of mass and how it's affected by forces did not stretch to particles of zero mass. So I don't think it's particularly relevant here.

Perhaps not, but it can't hurt to know Newton's first law.

 

[vanhees71]

It's unavoidable to learn classical physics, i.e., Newtonian mechanics, special theory of relativity and classical electrodynamics first, before thinking about photons. Photons are in no way particles, and even massive particles are rather described by quantum fields in relativistic QFT than as point particles.

To answer the question in the subject of this thread, "what powers light?", can be answered easily within classical (relativstic) physics, i.e., Maxwell's electrodynamics. First one should note that "light" are electromagnetic waves, i.e., time-dependent electromagnetic fields transporting energy, momentum, and angular momentum through free space. The source of electromagnetic fields are electric charges and currents.

A charge at rest has an electrostatic field around it, and this transports no energy, momentum, and angular momentum. Since a charge moving with constant velocity is equivalent to a charge at rest seen from another inertial frame of reference, also a charge moving with constant velocity does not emit electromagnetic waves, but in addition to the electric field you also have a magnetic field.

If the charge is accelerating, it emits electromagnetic waves, which transports energy and momentum. Since energy and momentum are conserved, these are taken from the moving charge, and to keep it in (e.g., oscillatory) motion you have to keep it going by putting in more energy to compensate for the loss by radiation, and that's what "powers electromagnetic waves".

 

[russ_watters]

Although technically not strictly relevant to the photon, you might like to read about Newton's first law of motion.

Yeah, I was thinking the same thing and held back an earlier joke for fear of a pedantic response:

Why does it keep going? What's stopping it?

Definitely not Newton's first Law!