## What gives light its speed? - by Victor T. Toth - Quora Question Review

This document contains a review of the answer by Victor T. Toth on the question in Quora: "What gives light its speed?"
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### Reflection

What gives light its speed?

This question has (at least) three meanings.

First, why is light traveling at all?

Second, why is it traveling at its particular speed?

Third, why is this speed an invariant, same for all observers?

So then, the first question. Why is light traveling?

You must first take a step back. In any reaction, or collision, elementary particles are created, including photons. All these particles, including photons, move through space from point to point, in time. You also could define that, as if photons have a speed.
OK, we have electricity and magnetism, both already known to humans at the dawn of civilization. But by the late 18th, early 19th century it became evident that the two are related. This relationship was ultimately formalized by Maxwell (the famous Maxwell equations). Maxwell's efforts had an unexpected result. Electric fields changing in time induce magnetic fields; magnetic fields changing in time induce electric fields. As it turns out, this can happen even in empty space, far from any sources of electricity and magnetism. And in empty space, this mutual back-and-forth between the two fields takes the form of a plane wave, which travels in space at a set velocity.

The second question: Why 299,792,458 m/s?

There is no real good answer on the question: why is light travelling at its specific speed. This is human made definition. In different part of the universe, most probably it is different.
Well, we define our units this way. Indeed, the meter is defined as the distance that light travels in 1/299,792,458 seconds. Using different units, e.g., the so-called natural units that theoretical physicists prefer to use, the speed of light is just 1. It is for this reason that physicists don't consider constants that have some units of measurement attached (like meters per second, miles per hour, furlongs per fortnight) truly fundamental. The only constants that really matter are dimensionless (meaning, no units of measurement attached) ratios. This is for instance why physicists go on about the fine structure constant, which is a dimensionless measure of the strength of electromagnetism. Being able to derive its value from fundamental principles would be one of the holy grails of theoretical physics! (Or demonstrating, through astrophysical observations of distant objects, that its value is not constant after all.)

Finally... why is the speed of light constant, i.e., the same for all observers?

This are two questions: A) Is the speed of light constant? B) Is the speed of light constant for all observers? If you consider the Sun, the Sun emits at every moment, we call that event A, millions of photons and these specific photons 'travel' through space in spheres with the Sun at its center, assuming that they travel in all directions with the same speed.
We can also consider a comet, which also travels through space around the Sun. At the same moment, event A, the comet also emits photons and these specific photons also 'travel' through space in spheres with the comet at its center? Is this correct ? No that cannot be possible. The photon-sphere is centered around the point of emission were the comet was at the moment of event A.
What this means the photon-sphere of the Sun also is not centered around the present position but around a position in the past.
Now consider an observer. What has an observer to do with the speed of photons, i.e. the position and time of events? Observers have nothing to do with the physical behaviour of photons. In fact the only thing they should do, is to define a method such that the time and the positions of events can be established as accurate as possible. The events the position of the Sun and the trajectory of the comet, during a certain period.
One way to explain this has to do with permeability and permittivity. Permeability is the ability of a medium to sustain magnetic fields; permittivity is the ability of a medium to sustain electric fields. Both fields can exist in a vacuum, so vacuum as a medium has non-zero permeability and permittivity. And the speed at which the aforementioned plane waves propagate is just the inverse square root of these two. But then... if two observers, moving relative to each other, measured different values for the speed of light, that would mean that the properties of the vacuum would be different for these two observers. I.e., the properties of the vacuum would be observer-dependent. To the best of our knowledge, this is not the case. So instead of predicting something that's not observed in nature (vacuum's properties depending on who's looking) we modify classical mechanics, turning into a theory, special relativity, that predicts the same vacuum for all observers. And this means the same speed of light for all observers.
To measure the events of the position of the comets, you should use only one reference frame.

### Reflection 1 - Question Review

The why question "What gives light its speed?" cannot be answered. It is a fact of nature. It is related to the question: Why are there protons, neutrons and electrons. The physical reality is that there are protons, neutrons, and electrons, including photons and gravitons. They all move. They all have a speed.

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Created: 1 June 2023

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