## Why does light move at the speed it does?

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Arkham
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### Why does light move at the speed it does?

What is the underlying property that determines that light travels at 299,792,458 m/s in a vacuum? That is, why does it move at the speed that it does?

scarecrovv
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### Re: Q about the speed of light

I don't think anybody knows. The speed limit is c, and that's all there is to it. There must be some fundamental properties of the universe which underlie all others. As far as I know, c is one of them. It's not turtles all the way down.

Now some physicist is probably going to come along and show that I'm wrong.

cpt
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### Re: Q about the speed of light

I think that Maxwell would argue that it's the permittivity and permeability of free space that underlie the speed of light in a vacuum, since light is an electromagnetic wave and those determine how magnetic and electric fields behave in a vacuum.

LaserGuy
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### Re: Q about the speed of light

Arkham wrote:What is the underlying property that determines that light travels at 299,792,458 m/s in a vacuum? That is, why does it move at the speed that it does?

Well, our system of units are totally arbitrary, right? Saying that the speed of light is 299,792,458 m/s requires you to know what a meter is, and what a second is (The metre is defined in terms of the speed of light, in fact, not the other way around). So we can say, well, relative to our arbitrary standard, light moves this fast. But that isn't even a good measurement, because our standard isn't fixed--if I believe you are moving very quickly and I'm not moving at all, then your metre will be shorter than mine--even though when we both measure the speed of light, we get the same answer (but don't agree on anything else).

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### Re: Q about the speed of light

Arkham wrote:What is the underlying property that determines that light travels at 299,792,458 m/s in a vacuum? That is, why does it move at the speed that it does?

It has to travel at some speed.

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Arkham
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### Re: Q about the speed of light

LaserGuy wrote:
Arkham wrote:What is the underlying property that determines that light travels at 299,792,458 m/s in a vacuum? That is, why does it move at the speed that it does?

Well, our system of units are totally arbitrary, right? Saying that the speed of light is 299,792,458 m/s requires you to know what a meter is, and what a second is (The metre is defined in terms of the speed of light, in fact, not the other way around). So we can say, well, relative to our arbitrary standard, light moves this fast. But that isn't even a good measurement, because our standard isn't fixed--if I believe you are moving very quickly and I'm not moving at all, then your metre will be shorter than mine--even though when we both measure the speed of light, we get the same answer (but don't agree on anything else).

OK, but why does light travel at a descrete and not infinate speed? So if we define the meter due to the speed of light, then lets use another measurement. Why does the speed of light travel in 7.931517 × 1014 cubits per fortnight?

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### Re: Q about the speed of light

From a certain point of view, light does travel at infinite speed. That point of view being the photon's own point of view since it will travel any distance in zero relative time. The "speed of light" is a poor name for the fundamental constant c, really. It'd be better to be called the speed of the universe. Anything that has no mass has to travel at this speed.

A better question would be: why do relativistic effects happen such that "infinite speed" appears to be finite speed to an outside observer?

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LaserGuy
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### Re: Q about the speed of light

Arkham wrote:
LaserGuy wrote:
Arkham wrote:What is the underlying property that determines that light travels at 299,792,458 m/s in a vacuum? That is, why does it move at the speed that it does?

Well, our system of units are totally arbitrary, right? Saying that the speed of light is 299,792,458 m/s requires you to know what a meter is, and what a second is (The metre is defined in terms of the speed of light, in fact, not the other way around). So we can say, well, relative to our arbitrary standard, light moves this fast. But that isn't even a good measurement, because our standard isn't fixed--if I believe you are moving very quickly and I'm not moving at all, then your metre will be shorter than mine--even though when we both measure the speed of light, we get the same answer (but don't agree on anything else).

OK, but why does light travel at a descrete and not infinate speed? So if we define the meter due to the speed of light, then lets use another measurement. Why does the speed of light travel in 7.931517 × 1014 cubits per fortnight?

Well, among other things, if the speed of light were infinite, it would probably mean that photons carry infinite energy. Matter would also have infinite energy, and nuclear bombs would destroy the universe.

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### Re: Q about the speed of light

Arkham wrote:OK, but why does light travel at a descrete and not infinate speed? So if we define the meter due to the speed of light, then lets use another measurement. Why does the speed of light travel in 7.931517 × 1014 cubits per fortnight?

To some extent, the answer boils down to "because it does". The question "why" implies some intentionality behind the causation, and looking for that in the fundamental principles of the universe doesn't really belong on a Science board.
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### Re: Q about the speed of light

rflrob wrote:The question "why" implies some intentionality behind the causation,

News to me.

elasto
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### Re: Q about the speed of light

Science never really addresses 'why' questions. That's for the realm of philosophy and religion. Science deals with 'how' questions.

If Bob punches Larry, and Larry asks Bob 'why did you punch me?' - he's not expecting a response of 'well, some electrical impulses travelled between neurons using mechanism A, down my nerves using mechanism B, triggering a cascade of changing energy states in my muscles using mechanism C' - that would better suit the question 'how did you punch me?'

'Why did you punch me?' is expecting a response more along the lines of 'because I was angry with you because of situation Z.'

When science asks 'why does X happen?' it's really asking 'how does X happen?' - and when it comes to the fundamentals like 'why does light move at speed c?' or 'why does an electron have the charge e?' - when you rephrase it as the more accurate question 'how does light move at speed c?' it becomes more obvious there isn't going to be a reason. There might be a reason why it's finite rather than infinite, but beyond that it's going to be arbitrary unit selection.

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### Re: Q about the speed of light

Arkham wrote:What is the underlying property that determines that light travels at 299,792,458 m/s in a vacuum? That is, why does it move at the speed that it does?

The underlying principle is the definition of the metre — the metre is defined as the distance light in a vacuum travels in 1 / (299 792 458 seconds).

Quite apart from that, the 'reason' light travels at these speeds (as far as 'reasons' go for things that appear fundamental) is Maxwell's Equations. When you work with them you end up with the equation of motion for a wave, travelling at 1 / Sqrt(μ0ε0), which are empirically observed constants. This is equal to c, and was also an important step in discovering relativity — the equations gave you the speed of the EM wave is c, but didn't say with respect to what.
As it turns out, with respect to every frame.
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### Re: Q about the speed of light

SlyReaper wrote:The "speed of light" is a poor name for the fundamental constant c, really. It'd be better to be called the speed of the universe.

I like to think of it as “the speed of time”. It is the unit of conversion between lengths in the time-direction and lengths in the space-directions.
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dainbramage
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### Re: Q about the speed of light

cpt wrote:I think that Maxwell would argue that it's the permittivity and permeability of free space that underlie the speed of light in a vacuum, since light is an electromagnetic wave and those determine how magnetic and electric fields behave in a vacuum.

That wouldn't explain why particles which don't interact electromagnetically are restricted to the same speed, though.

The best answer we can really give right now is that the universe has a speed limit, which light always travels at because it has no mass. Why does the universe have a finite speed limit? We don't know.

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### Re: Q about the speed of light

Arkham wrote:OK, but why does light travel at a descrete and not infinate speed?

An infinite speed would, at least in some senses, violate the principle of locality. Put another way, if the speed of light was infinite, the number of things that could effect a particular thing would also be infinite. That could lead to the forces acting on that thing being infinite, or at least indeterminate, which would be a bad thing.
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### Re: Q about the speed of light

For one thing, the speed of light is more fundamental than the electric and magnetic properties of free space. It isn't really the speed of just light, after all. We just call it that for historical reasons. A better answer might be that photons are massless, and as such, must travel at that speed, which is really a property of space-time. As for why this speed has that particular value, well, there's no good answer to that so far, except perhaps that it was randomly set by quantum fluctuations when the Universe was created. See http://en.wikipedia.org/wiki/Multiverse

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Rococo
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### Re: Q about the speed of light

I've only had light relativity (pun intended?), but maybe I can give a slightly better answer than "because."

First, a fact about space-time: everything travels through it at the same overall speed, which of course is c. An object in its rest frame can be said to be moving through space at zero speed and moving through time at the speed c. As a massive object starts to move (again, relative to some frame), it 'diverts' some of its speed through time to speed through space instead. Light moves entirely through space and not through time at all (although note that from the photon's viewpoint space is a flat plane, so the whole thing really breaks down). Mathematically, this gets expressed in the invariant length of the 4-vector (http://en.wikipedia.org/wiki/4-velocity).

An intuitive way to visualize this is to think of pure time movement and pure space movement as two orthogonal axes, the x and y on a graph, and the velocity vector as pointing at some angle between 0 and 90 degrees. This suggests that velocity should be thought of as an angle. And in fact, there is a concept that fits this very nicely called Rapidity (http://en.wikipedia.org/wiki/Rapidity). Rapidity is like velocity, but it happens to go from 0 for something that isn't moving at all, to infinity for light, and add linearly in between. So from a certain perspective, the reason velocity ends at some finite number is really just because we usually measure the hyperbolic tangent of the sensible indication of how fast something is moving.

Fine, so why does light have infinity rapidity? Well, maybe the answer is 'because it takes no energy to accelerate it, so it must have the maximum rapidity.'

I couldn't find an easy way to prove this, but I strongly suspect the following to also be a valid answer: 'Magnetism is really a relativistic phenomenon, and because of this the force it causes, and thus the energy density its fields produce, are only equal to those of electric fields at the speed of light. Light is ultimately propagated through electromagnetic induction, so if the magnetic field carried more or less energy than the electric field, the energy carried by the wave would either decay or exponentially grow, violating energy conservation either way.'

Of course every 'why' leads to another, and some of the reasonable follow-ups to this would be:

-Why does spacetime have a hyperbolic geometry (or, why is time different than space?)
-Why does the electromagnetic force communicate with massless particles?
-Why is speed through spacetime constant?

I can't give a answer to any of these, of course, but in my opinion they are deeper, more fundamental questions than the one we started with, so I'll call that progress.

Arkham
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### Re: Q about the speed of light

Thank you all.

cpt
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### Re: Q about the speed of light

dainbramage wrote:
cpt wrote:I think that Maxwell would argue that it's the permittivity and permeability of free space that underlie the speed of light in a vacuum, since light is an electromagnetic wave and those determine how magnetic and electric fields behave in a vacuum.

That wouldn't explain why particles which don't interact electromagnetically are restricted to the same speed, though.

To this, I think Maxwell would say "Particles? Light is a wave, silly. Also, what is this 'Relativity' everyone speaks of?"

ftfs
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### Re: Q about the speed of light

have another look at mr maxwells equations

you will find that 'Relativity'

is built in

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### Re: Q about the speed of light

But special relativity was only really developed after (and from) Maxwell's equations, hence the comment about Maxwell not grokking the term in its modern usage.
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### Re: Q about the speed of light

ftfs wrote:have another look at mr maxwells equations

you will find that 'Relativity'

is built in
Inasmuch as the equations imply a constant speed for light, sure.

Inasmuch as [everything else in special and general relativity], less so.
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### Re: Q about the speed of light

ftfs wrote:maxwells equations

you will find that 'Relativ-

-ity' is built in

FTFY. It was so close to a Haiku already that I couldn't resist.

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### Re: Q about the speed of light

Very nicely said, Rococo, especially coming from someone who claims to be a novice in relativity.

eSOANEM wrote:But special relativity was only really developed after (and from) Maxwell's equations, hence the comment about Maxwell not grokking the term in its modern usage.

Precisely. The constancy of the speed of light is pretty obvious from Maxwell's equations, but at the time, c was interpreted as being constant with respect to the electromagnetic aether. It took almost another 30 years to arrive at the Lorentz transformation, which arose through attempting to make the Maxwell equations invariant when transforming from the reference frame of the aether to a reference frame that were moving with respect to the aether. And then it took almost another 15 years before Einstein had the brilliant realization that trying to explain the constancy of the speed of light via arguments from electromagnetism was essentially a red herring, and that the constancy of c wasn't caused by the electromagnetic nature of light after all, but instead arose due to the nature of the geometrical relationship between space and time, and that the relationship between electric and magnetic fields is a manifestation of this more fundamental geometrical principle.

This was a major change of perspective. It must've been really hard for physicists who'd spent a large part of their career working on this problem to have Einstein's solution thrust upon them, and I'm sure many of them were unable to make the transition to Einstein's new approach. Even someone as brilliant as Poincaré (a veritable polymath who was a master at hyperbolic geometry, and the person who first realized that the Lorentz transformations form a group) never fully came to terms with the true meaning of relativity. At least, that's the impression I gained years ago from reading Poincaré's essays on this topic (admittedly in translation).

It's hard to shake off the influence of history, and we continue to use the phrase "the speed of light" to refer to c more than a century after the theory of special relativity was published. So it's not surprising that most people who haven't learned the fundamentals of special relativity have the impression that c is intrinsically tied up with the "mysterious" nature of light, and it generally takes a while for students of special relativity to get over that and to grok that SR is really all about the geometrical relationship between space and time (in a flat spacetime).

Arkham
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### Re: Why does light move at the speed it does?

So if I'm understanding this right, The speed of light is c, while many other things are also set to c as well.

The SoL does not define C, C defines what the SoL is, as well as the speed of gravity and any other massless particle. We just call it the speed of light due to historical nomenclature.

Thus, C is really the universal constant.

Ok, so then what I guess I am not getting is, why is C the speed that it is? Why that specific number, and not any other arbitrary value? Does it have to do with the mass energy equivalence principal?

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### Re: Why does light move at the speed it does?

Arkham wrote:Thus, C is really the universal constant.

Ok, so then what I guess I am not getting is, why is C the speed that it is? Why that specific number, and not any other arbitrary value? Does it have to do with the mass energy equivalence principal?

Not really. The value of the speed of light depends on your choice of units, in standard relativistic units (and planck units), c=1 (ignoring units) because it is the unit of speed. Because c has units, (of speed), it can be made to have any finite value simply by changing your units and so its value has no great significance to the nature of the universe.

The truly universal constants are those without units. The value of these constants do not vary from unit system to unit system and so they do play a role in describing the structure of the universe, as such, these are the real universal constants.
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### Re: Why does light move at the speed it does?

Arkham wrote:So if I'm understanding this right, The speed of light is c, while many other things are also set to c as well.

The SoL does not define C, C defines what the SoL is, as well as the speed of gravity and any other massless particle. We just call it the speed of light due to historical nomenclature.

Thus, C is really the universal constant.

Ok, so then what I guess I am not getting is, why is C the speed that it is? Why that specific number, and not any other arbitrary value? Does it have to do with the mass energy equivalence principal?

It has to be some speed. It just happens to be that one. There's no "why" behind it, it's a fundamental constant. The mass-energy equivalence is based on that constant; if the speed of light were doubles, mass would simply contain 4 times as much energy. It's like asking why we live on Earth, and not some other arbitrary planet in the universe. We just do.

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### Re: Why does light move at the speed it does?

eSOANEM wrote:in standard relativistic units (and planck units), c=1 (ignoring and has no units) because it is the unit of speed conversion between lengths in one direction and lengths in other directions

Fixed.
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Arkham
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### Re: Why does light move at the speed it does?

I know science isnt aroudn to satisfy me, but i just find the answer "it just is" to be highly unsatisfying.

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### Re: Why does light move at the speed it does?

Richard Feynman wrote:Do not keep saying to yourself, if you can possibly avoid it, "But how can it be like that?" because you will get "down the drain," into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.

Feynman was talking about Quantum Mechanics, but fundamentally all of Nature is like that.
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### Re: Why does light move at the speed it does?

Arkham wrote:I know science isnt aroudn to satisfy me, but i just find the answer "it just is" to be highly unsatisfying.

The thing is, if the speed of light was googleplex m/s, or 6 furlongs per fortnight, you'd still be asking why it wasn't a different speed. There's usually a "why" for most stuff in physics, but when you follow the chain back, you eventually hit the fundamental constants. Those constants are called fundamental because there's no "why" behind them.

So if that is an unsatisfying answer, consider that 3e8 m/s is probably one of the few speed-of-light values that result in a habitable universe. If the speed of light were higher, nuclear reactions in stars would burn that much hotter, and the star would burn itself out that much quicker, reducing the probability of sentient life capable of asking these questions ever having a chance to evolve. Lower the speed of light, and those nuclear reactions may not occur at all, again making the universe uninhabitable. A low speed of light would also create black holes out of things that in our universe wouldn't be black holes. Such a reality could be pretty hostile to life. The speed of light also governs the speed of other massless bosons which would have weird effects on the strong nuclear force, possibly gravitation if the graviton turns out to be massless, etc.

So although the details haven't been worked out, it would seem there is a bracket of possible values for c which would allow for sentient beings such as ourselves to exist. So if the speed of light were outside that bracket, it would be a moot point because we wouldn't be around to wonder about it.

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### Re: Why does light move at the speed it does?

What's being referred to here is known as the Anthropic Principle, and it isn't philosophically satisfying, but it does seem reasonable if there are a large set of possible values that are all actualized in some manner, although some values may be preferred, if something like natural selection among fundamental constants holds.

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### Re: Why does light move at the speed it does?

Qaanol wrote:
eSOANEM wrote:in standard relativistic units (and planck units), c=1 (ignoring and has no units) because it is the unit of speed conversion between lengths in one direction and lengths in other directions

Fixed.

I was trying to make my comment also apply to Planck units (where it does have units, because it is the unit) as well as standard relativistic ones.
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### Re: Why does light move at the speed it does?

SlyReaper wrote:
Arkham wrote:I know science isnt aroudn to satisfy me, but i just find the answer "it just is" to be highly unsatisfying.

The thing is, if the speed of light was googleplex m/s, or 6 furlongs per fortnight, you'd still be asking why it wasn't a different speed. There's usually a "why" for most stuff in physics, but when you follow the chain back, you eventually hit the fundamental constants. Those constants are called fundamental because there's no "why" behind them.

So if that is an unsatisfying answer, consider that 3e8 m/s is probably one of the few speed-of-light values that result in a habitable universe. If the speed of light were higher, nuclear reactions in stars would burn that much hotter, and the star would burn itself out that much quicker, reducing the probability of sentient life capable of asking these questions ever having a chance to evolve. Lower the speed of light, and those nuclear reactions may not occur at all, again making the universe uninhabitable. A low speed of light would also create black holes out of things that in our universe wouldn't be black holes. Such a reality could be pretty hostile to life. The speed of light also governs the speed of other massless bosons which would have weird effects on the strong nuclear force, possibly gravitation if the graviton turns out to be massless, etc.

So although the details haven't been worked out, it would seem there is a bracket of possible values for c which would allow for sentient beings such as ourselves to exist. So if the speed of light were outside that bracket, it would be a moot point because we wouldn't be around to wonder about it.

I was wondering about that, and considering that there could be multiple universes that are unstable due to changes in specific parameters, such as the weight of the electron or the speed of c.

So really, the best guess I can come up with for the speed of c in this universe is that c is the maximum transmission speed of information from one 'grid' point to the next that are 1 planck length long. But i know nothing so thats probably just as accurate as pink nerve gas farting dragons.

 which of course it is, since no information can move faster than light. What i mean to say is that due to the 'density' of the quantum foam, things are can only push through it so fast. The fabric of the universe itself is the friction on the speed of C.

It makes more sense in my head.

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### Re: Why does light move at the speed it does?

There's one property of the speed of light that's always confused me a little (which, admittedly, isn't all that hard!), and that's how it has the same value to an observer, no matter what relative (to the light being measured) speed they are travelling at.
Example;
Spoiler:
Car A and car B are both travelling along the motorway, A at 60mph, B at 70mph. To a stationary person at the side of the road, car B is clearly travelling at 70mph, but to the occupants of car A, car B has a relative speed of 10mph.

Now, carry this example across to an experiment measuring the speed of light - the experiment while stationary measures the speed of light to be 3x108m/s. Fair enough. Now, place this experiment onto something travelling 21000m/s (quick Google search tells me this is the speed of a space probe) and it still measures the speed of light to be 3x108m/s. The light, we can assume, comes from the same source, which is stationary relative to the moving experiment (for example - it came frm the sun, with the experiment moving away from the sun at 21000m/s).
Can anyone offer me an insight into how this works?

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### Re: Why does light move at the speed it does?

Time dilation and length contraction exactly make up for the difference in velocity.
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### Re: Why does light move at the speed it does?

Dr.Buck wrote:Can anyone offer me an insight into how this works?

In addition to what Gmal said, velocities don't add linearly which is why I see 0.9c+1c (the velocity of light given off by a ship moving towards me at 0.9c) as still being 1c.
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### Re: Why does light move at the speed it does?

That this is so is actually the foundation of special relativity. It takes a pretty special mind - or really a number of special minds - to look at Maxwell's equations (which tell us how electricity and magnetism behave), and from there realise that you can show that an oscillating electric charge can create a pair of perpendicular electric and magnetic fields that propogate; and from there notice that this results in the fields propagating at the speed that's already known to be the speed of light, and realise that this means light actually is propagating electromagnetic fields; and then from there realise that the equations hold true no matter what inertial reference frame you're in, so that changing your coordinates doesn't change how fast you perceive the light to be moving; and from that you wind up at all the craziness that is special relativity, which tries to make sense of that fact and as a result shows that two observers in differently moving reference frames will perceive distance and time to be different.

Here's a description of Maxwell's contribution to that craziness.
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scratch123
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### Re: Why does light move at the speed it does?

I remember reading something once about fundamental constants like the speed of light being in simple ratios relative to one another. It was somewhere on wikipedia but I can't seem to find the link. Anyone know what I am talking about?

thoughtfully
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### Re: Why does light move at the speed it does?

You might be thinking of Planck Units.

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery