# Light is the only thing in existence

ABHoT
To my layman-ey thinking light is the only thing in existence you can define by the fact that it is only ever moving, can never be stationary. Is there anything else in existence sharing this property? Otherwise this makes light very, very weird in a unique/special way?

Homework Helper
Welcome to PF!

Hi ABHoT ! Welcome to PF!

Any such thing would have to have zero rest-mass.

We used to think that about neutrinos, but recent experimental evidence has changed our minds.

The graviton, if it exists, should do.

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Everything in the universe is always moving and never stationary. After all, every object has a non-zero temperature, and that means its atoms are always vibrating.

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ah, but any individual non-zero-rest-mass particle, eg an electron, is always stationary in some frame.

ABHoT

I can tell I have a lot more learning to come. If mass affects the time light takes to travel is there a mass or planetary situation or something somewhere that is freezing light in place? Does that mean the speed of light isn't constant?

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Gravity can effectively delay light but it certainly can't stop it (the local speed of light, ie in any local frame, is always c). The closest to stopping light is that gravity near r = 3M round a black hole can make light orbit a large number of times before carrying on … see the PF Library on photon sphere …​

Accordingly, a photon with L/E = 3√3M can orbit on the photon sphere ($r\ =\ 3M$) with period $6\pi\sqrt{3}M$, or can approach the photon sphere, circling ever closer either just outside or just inside it with approximately the same period, but never quite reaching it.

Lens and mirror effects:

Similarly, a photon with L/E slightly greater than 3√3M may circle the photon sphere a number of times before returning to distant space.

So a black hole can act as a lens giving rise to n ring-shaped images of a background star, each ring corresponding to light which has circled 1,2,3,..n times around, for some positive integer n (which depends on the distance beyond the black hole).

And it can act as a mirror giving rise to n ring-shaped images of a foreground star, in the same way.

These effects are too faint to be observed, but the "zeroth ring", in which light is focussed without circling the black hole at all, has been observed, and is known as gravitational lensing

ABHoT

My list of things to learn is getting a tad long and deep and I feel like I may never catch up :)
Will google gravitational lensing. Will be ages until I have read up on all this.
Does light only come from matter? The difference between a light bulb switched on and off is just temperature (or electricity?) If a star wasn't 'on fire' would it give off any light?
I am wondering where the energy is going. Emitting light must cost the matter its energy(?), but being hit by light doesn't give you any so where is it going?

Homework Helper
Does light only come from matter? The difference between a light bulb switched on and off is just temperature (or electricity?) If a star wasn't 'on fire' would it give off any light?

A star with no nuclear fuel left would give off light according to its temperature (same as a planet does).

Basically, everything gives off a range of light according to its temperature (google "black body radiation").

eg red stars are cooler than blue stars.

A light bulb gives off light because it is hot.

In addition, some things give off light at certain exact frequencies because their molecules are vibrating (eg sodium lamps).
I am wondering where the energy is going. Emitting light must cost the matter its energy(?), but being hit by light doesn't give you any so where is it going?

No, being hit by light does give you both energy and momentum.

qwe

that's why lasers fry and sun burns. think like a physicist everywhere you go :)

ABHoT

If you switched on a powerful light bulb in deep space and 5 miles out created a tightly packed sphere made out of human observers all looking inwards, they would all see the light. If you then moved out 10,000 light years away and created another much larger tightly packed sphere of human observers they would all also see the light too (there was nothing blocking the view at that part of space), although it would be sometime later.
In what way has the light changed? Is it less dense? This is really weird to me. Is the light dimmer? Is this because the 'signal' has degraded, dissipated because of distance, lost energy, or is it more 'spread out' because the 'sphere' is larger?

If you shone a powerful flashlight instead, one that is more directional and less sphere-like, would it still spread out in the same way within its line?

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or is it more 'spread out' because the 'sphere' is larger?
Correct, because the photons have spread out over a larger area, the flux per square area is lower.
If you shone a powerful flashlight instead, one that is more directional and less sphere-like, would it still spread out in the same way within its line?

Any beam, even the best made laser fired in a vacuum, is subject to dispersion. It will certainly disperse less than a spherically emitting body, but it will not remained confined to a single line. It will probably spread out in a cone-like fashion, although I'm not 100% sure of this.

ABHoT

Thanks. Does light emit at different 'rates'? It sounds to me like the photons dispersion at the circumference or 'vertically' are determined by the size of the sphere, but the horizontal/radius photons density would be determined by how many photons per second (or whatever) are being emitted. Particle-y on the one axis, wave-like on the other?

Actually this begs a question, when measuring flux over an area how deep do you measure? One photon thick?

When photons from two different light sources collide do they change direction? Do more than one photon ever try to occupy the same physical space? Do they pass through each other or miraculously always miss each other?

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Gold Member

It's all relative, as Einstein noted. Photons do not collide, they pass right through each other - much like dark matter particles. Food for thought. Reinventing the universe is a dirty job. You have an enormous amount of observational evidence to refute.

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Calimero

Thanks. Does light emit at different 'rates'? It sounds to me like the photons dispersion at the circumference or 'vertically' are determined by the size of the sphere, but the horizontal/radius photons density would be determined by how many photons per second (or whatever) are being emitted. Particle-y on the one axis, wave-like on the other?

Actually this begs a question, when measuring flux over an area how deep do you measure? One photon thick?

When photons from two different light sources collide do they change direction? Do more than one photon ever try to occupy the same physical space? Do they pass through each other or miraculously always miss each other?

It seems like you are mixing light flux (given in lumens) with illuminance (given in lux, which is lumens/m^2). Anyway, you don't count the photons. Basically you are measuring how much energy is transfered from the source to the detector in the unit of time. If source varies with time faster then detector can detect it, you will get average result.

Photons are bosons, and they can occupy same quantum state, according to the Pauli exclusion principle. That means that they are free to move 'through each other' without colliding. If that were not the case our world would look a lot different.
Anyway when speaking about light duallity, and when considering photons as particles it is wrong to picture them as little blobs of matter, because they are not that. Photons are smallest possible 'packages of energy'.

ABHoT

Thank you-thank you-thank you. I can't believe I understand something about quantum physics! even something small.