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Comment by vchak1 | Hacker News Reader

What is so special about the speed of light? As a thought experiment, if everyone on the planet was blind, would c have been replaced by the speed of sound?

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The first very special thing that was observed about the speed of light is that it is NOT relative. That is, if I fire Alice light beam at you from a moving train, while Bob fires a beam at you from a platform, both beams will reach you at the same time. Sound does not behave the same way, light was the first thing that we observed like this.

This was a gigantic problem, an experiment contradicting one of the most fundamental laws of nature as we knew them at the time - Galileo Galilei's principle of relativity.

Note that this observation has nothing to do with our eyes's ability to perceive light. The same observation will not happen with sound waves; and it will hold even for frequencies of light that we can't directly observe with our bodies, such as radio waves.

As others note, it was later discovered that this is not a special property of light itself. It is in fact a special property of the universe, and it applies to any particle without mass; the photon happens to be the only massless particle that we can directly observe, so it was the one which gave the name to the physical quantity.

I recently learned this and it blew my mind because it had never occurred to me.

Another way to put it is that when you’re in a moving bus and you throw a ball towards the front of the bus, the ball is moving the speed you threw it plus the speed of the bus, but, when you shine a light, the photons from the light are moving the same speed as someone off the bus! That seems super weird.

Exactly. Now build a clock by bouncing the light between two mirrors, and think about how the clock looks like in the bus, and outside of the bus. -> special relativity.

> That is, if I fire Alice light beam at you from a moving train, while Bob fires a beam at you from a platform, both beams will reach you at the same time.

That's wrong. Simultaneity is ill-defined in relativity.

The correct example is, "if Alice fires a light beam at you from a moving train and Bob fires a light beam from you from the platform, you will measure the Alice photons as going equally fast as the Bob photons.

Absolute simultaneity is ill-defined, but simultaneity in my frame of reference does exist.

However, you're right, your formulation is more precise, and actually possible to measure.

As chilinot said, it’s the speed of any massless particle in a vacuum. A massless particle has nothing slowing it down, so it moves at the maximum possible speed. It’s actually the propagation speed of cause and effect, or put another way, how long it takes for a quantum event to affect whatever is in the adjacent point one Planck length away. It’s simply how quickly these things ripple forward when there is nothing slowing these ripples down.

Every other massless particle moves at the same speed. “Sound” is not a massless particle, it’s propagation of the compression of matter, so therefore moves much, much slower and would not replace c. It has nothing to do with what we can observe and rather to do with its properties. Personally, I find referring it to as “the speed of light” is confusing since it rarely has anything to do with light/photons other than that light happens to move at that speed.

> or put another way, how long it takes for a quantum event to affect whatever is in the adjacent point one Planck length away.

Said like that, it reeeeeaaallly makes you think "we live in a simulation", doesn't it ^^ ?

I watched this recently[1] and the whole relativity thing made me think that its kinda like how in games often physics is processed as local clusters (for parallelism) and it made me think that reality appears to be simulated in this local clusters too. Relativity exists because that way each local cluster is independent and can be simulated in parallel, sharded across the servers! :)

Hell, why not take it a step further and say that the simulation is relative to an observer (which can be an inanimate thing, of course) as an optimisation because why bother simulating what isn’t seen or interacted with by an observer?

[1] https://www.youtube.com/watch?v=fHRqibyNMpw

It is the speed of cause and effect. If something changes at location X, that cannot cause anything to change at location Y faster than the distance between X and Y divided by the speed of light.

Light (in a vacuum) goes as fast as the speed of cause and effect. And it just so happens that the speed of light is pretty easy to measure (as compared to other possible things).

So if we were all blind, we would still be affected by this speed.

The speed of light is just the speed of any particle without matter. We just use "speed of light" since its simpler to understand and talk about rather than saying "speed of matter-less particles".

The speed of sound is the speed of air-molecules bouncing into each other (~300m/s). Since air-molecules have matter, they dont travel at the speed of light.

To be pedantic, a particle without mass. Matter also includes things without substance like physical fields. So a photon is matter.

At least this is what I was taught in school many years ago...

To be pedantic, a particle without rest-mass...

Yes, that was a typo on my part.

The difference between sound and light is that sound needs a medium. This medium breaks the symmetry: There is a special system, the one in which the bulk of the medium doesn't move. For light, you don't have that. All reference systems are equal, independent of the speed they move with respect to each other. Light appears at the same speed in all of them. That's not possible with Newtonian velocity addition.

The experimental "proof" of the frame-independence comes from the michelson morley experiment. (Scare-quotes, because you can't prove things in physics, only disprove)

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