Why does light move at the speed it does

[PeterDonis]

If someone asked you why a bicycle was slower than a 2000 horsepower drag racer, would you say it is because of units?

As has already been said in this thread, ratios of speeds are dimensionless numbers so questions about them are questions about physics, not units.

 

[FactChecker]

As has already been said in this thread, ratios of speeds are dimensionless numbers so questions about them are questions about physics, not units.

And a ratio of 1 is equality, so c = 1/√μ₀ε₀ is saying something. Steering the discussion into units is just distracting.

 

[PeterDonis]

a ratio of 1 is equality

A ratio of two speeds, yes.

so c = 1/√μ0ε0 is saying something

Huh? This isn't a ratio of two speeds.

 

[Nugatory]

Many questions about magnitude are dependent on units. That does not make them all meaningless questions.

It would be easier to respond with a concrete example. What is a question about magnitude that meets all of the following criteria?

1) The answer is dependent on units.
2) The question is not, even on on closer examination, better framed as a question about the ratio of two magnitudes (so that the units cancel).
3) The question meets your standard for being "not ... a meaningless question".​

I submit that any such question will turn out to be about how we're defining our units, not about how the universe works. For example:

And a ratio of 1 is equality, so c = 1/√μ0ε0 is saying something.

It is. It's saying that we've defined the quantities $\mu_0$ and $\epsilon_0$ in such a way that if we ever find that the inverse square root of their product is not equal to $c$ then we need to find a more accurate meter stick; the one that we're using doesn't conform exactly to the standard definition of the meter. Compare this behavior with the fine structure constant and you'll see the difference.

 

[nitsuj]

There was allot of mention about the fine structure being the reason "the speed of light is what it is". are all fundamental forces not constrained to c? Do all those forces have their independent physical reason that by chance constrains them to c?

wiki says that electromagnetic and strong force have been confirmed to be mass-less.

 

[nitsuj]

It would be easier to respond with a concrete example. What is a question about magnitude that meets all of the following criteria?

1) The answer is dependent on units.
2) The question is not, even on on closer examination, better framed as a question about the ratio of two magnitudes (so that the units cancel).
3) The question meets your standard for being "not ... a meaningless question".​

I submit that any such question will turn out to be about how we're defining our units, not about how the universe works.

lol Why are the loud parts of movies too loud, and the quite parts too quite? Natural units please.

 

[mfb]

This thread makes me wonder if there is an anthropic argument as to why the speed of light is as many orders of magnitude away from our everyday experience as it is. If there is any link between the chemistry that makes humans possible, the speed of light, and the values of things like the fine structure constant, then maybe there is a more satisfying answer for the OP. Something along the lines of - 'The only values of the fundamental constants that can support chemistry that enables Life As We Know It puts the value of c roughly where we observe it'.

Chemistry as we know it does not allow macroscopic things to move fast (a significant fraction of the speed of light) in a natural way. The energy involved in chemical reactions is of the order of binding energies, binding energy divided by mass is of the order of $$\alpha^2 \frac{m_e m_p}{(m_e+m_p)^2} \approx \alpha^2 \frac {m_e} {m_p} = 2.90 \cdot 10^{-8}$$ where $\alpha$ is the fine structure constant, $m_e$ is the electron mass and $m_p$ is the proton mass. Note how it is dimensionless - it is small compared to 1, and its value is the same in every unit system. It tells you chemical reactions don't have sufficient energy to accelerate something to relativistic speeds in a realistic way.
The motion of humans is driven by chemical reactions. We have to be slow - or, in other words, the speed of light has to appear very fast to us.

 

[MistySaa]

Why does light move at the speed it does, why not half the speed or double?

My two cents would be, 'because that's the upper limit of how fast something can go in this deeply intricately functional universe, and that's exactly the speed it must be. And doesn't need to be faster'. Relative to other things moving, it's very very very fast, and it's the limit for how fast something can go. And light travels as fast as something can possibly go.

If the upper limit was, say, half that speed, perhaps the universe would break down, wouldn't be able to function as it does.

But light wouldn't travel at that speed, because, let's say, the 'laws of physics' spontaneously arise interdependently, because that's 'the only way it can be'. Like an intricate expanding origami taking full form, every part leaning on each other as it opens. Or pops into existence.

I might be making some technical errors here, i.e. about the specific generation of the laws of physics, but it's meant to give perspective to the issue anyway.

 

[Dale]

And a ratio of 1 is equality, so c = 1/√μ₀ε₀ is saying something. Steering the discussion into units is just distracting.

It is saying a tautology in SI units and it is literally a meaningless statement in many other unit systems where not all of the terms are even defined. You simply cannot avoid a discussion of unit systems in this context.

I will not debate the concept of slower or faster with you. If someone asked you why a bicycle was slower than a 2000 horsepower drag racer, would you say it is because of units?

As @PeterDonis mentioned the ratio is dimensionless, so it holds for any system of units. I also mentioned this above in the discussion about the ratio of the speed of light to the airspeed velocity of an unladen swallow. There are unit-independent ways to look at this, but they all trace back to the fine structure constant, not c.

 

[Dale]

There was allot of mention about the fine structure being the reason "the speed of light is what it is". are all fundamental forces not constrained to c? Do all those forces have their independent physical reason that by chance constrains them to c?

wiki says that electromagnetic and strong force have been confirmed to be mass-less.

There is only one speed which is invariant. So any massless particle will go at that invariant speed, regardless of its other properties. If that speed is infinite then you have Galilean relativity, and if it is finite then you have Einstein's relativity.

If it is finite (as experiments show it is) then it sets a natural scale of speed. Hence natural units set it to 1. That applies for all interactions, not just EM.

However, when you dig into questions about the speed of light and get people to really identify what they are interested in it is typically anthropomorphic things like how many heartbeats would it take for me to walk the distance that light can travel in one heartbeat. These things are governed by chemistry and therefore the fine structure constant.

If they were interested in questions of nuclear physics and related time and distance scales then the other coupling constants would describe the meaning of the speed of light.

 

[PeterDonis]

There was allot of mention about the fine structure being the reason "the speed of light is what it is".

For a "B" level thread that's about the best we can do. Getting into the details of the Standard Model, as your questions do, probably deserves a separate thread at the "I" or even "A" level.

 

[Grinkle]

@nitsuj I enjoyed the book below.

https://www.amazon.com/dp/0224061356/?tag=pfamazon01-20

It will give you enough background to know if you are interested enough to pursue more math-based discussions. You can probably find a lot of freely available background on the internet, and I now see a much less expensive Kindle version of the above title available, too.

 

[nitsuj]

For a "B" level thread that's about the best we can do. Getting into the details of the Standard Model, as your questions do, probably deserves a separate thread at the "I" or even "A" level.

I just meant to highlight the relationship between energy, mass and momentum. Seems a simply put way to explain why a thing, such as light goes the speed that it does. Not to further probe the mechanics different fundamental interactions, which would of course, determine the speed of said thing to go c. My reasoning is based on "a photon isn't mass less because we can't move with it." in other words that mass is fundamentally different than energy.

 

[nitsuj]

@nitsuj I enjoyed the book below.

https://www.amazon.com/dp/0224061356/?tag=pfamazon01-20

It will give you enough background to know if you are interested enough to pursue more math-based discussions. You can probably find a lot of freely available background on the internet, and I now see a much less expensive Kindle version of the above title available, too.

It looked interesting so read some reviews most which said it is good, though one I read pointed out silly errors...as a layman its kinda tough to find a good science read that doesn't fill the head with incorrect poorly conveyed concepts.

for the price it looks like a safe bet!

 

[Arkalius]

I feel like this question is really just reducible to asking why fundamental constants have the magnitudes that they do, instead of different ones. This starts to lead down a path of metaphysics and philosophy. There may not be an objective answer to those questions, or at least, it may not be an answer we are capable of discovering. Some like to imagine there was a supernatural intelligence fiddling with metaphorical dials, fine tuning our universe into a particular state. To me, the only honest answer to the question we have right now is "we don't know".

 

[Chris Miller]

What I am saying is that why does it not travel at 150000 m/sec or 600000 m/sec ? Why 300000 [k]m/sec [in a vacuum]?

It's a function of Hubble's constant and the size(/curvature) of the universe would be my guess (/hypothesis).

 

[PeterDonis]

It's a function of Hubble's constant and the size(/curvature) of the universe would be my guess (/hypothesis).

Please review the PF rules on personal speculation. There is nothing in any of our physical theories that would suggest this.

 

[Mike5000]

I love how you all remain kind and gentle in your responses to challenging questions already asked and answered. Thanks for providing such inspiration for being patient with others.

 

[ImStein]

I think some care should be taken in describing fundamental "constants" of the universe (e.g. speed limit c, or the fine structure constant).

As I use the terms:
"universal" means the same value throughout the universe (at all locations in a given simultaneity)
"invariant" means the same value to all inertial observers (at a given location across all simultaneities)
"absolute" this one is tougher, but I take it to mean "unexceeded" (often relating to an asymptotic limit)
"constant" means the same value past, present and future

Of the four, as they relate to speed limit c, I would consider the last to be least certain, especially in the very early universe.

 

[bob012345]

Why does light move at the speed it does, why not half the speed or double?

It's a simple straight forward question which was obscured by the several answers regarding units which is nowhere implied in this question. As an electrical engineer, my answer is the impedance of free space.

 

[PeterDonis]

units which is nowhere implied in this question. As an electrical engineer, my answer is the impedance of free space.

You do realize that the impedance of free space has units, right? And that we can choose units in which such a thing as "the impedance of free space" doesn't even exist?

 

[bob012345]

However, when you dig into questions about the speed of light and get people to really identify what they are interested in it is typically anthropomorphic things like how many heartbeats would it take for me to walk the distance that light can travel in one heartbeat. These things are governed by chemistry and therefore the fine structure constant.

Assuming one beat per second and thus 300,000km, it would take almost 7years to walk at 5km/hr assuming no sleep. About 7 years worth of seconds or 220 million beats. But my heart beats faster as I walk so using that metric, the distance would be proportionally less. Walking at the same speed, the number of beats is invariant.

 

[bob012345]

You do realize that the impedance of free space has units, right? And that we can choose units in which such a thing as "the impedance of free space" doesn't even exist?

You can express it in any units you like but it's a physical value that is the same just like the speed of light. I interpreted the original question to mean invariant of whatever value in your chosen set of units, why isn't it half or double. That went beyond the definitions of units. I used the impedance of free space in the same sense. If you shoot me you can describe the bullet with any system of units you like, but when it hits me it's going to do an invariant amount of damage.

 

[PeterDonis]

If you shoot me you can describe the bullet with any system of units you like, but when it hits me it's going to do an invariant amount of damage.

Yes, and if you want to understand that invariant amount of damage in terms of physics, not arbitrary choices of units, you can't look at quantities with units; you have to look at dimensionless numbers like the fine structure constant or ratios of quantities.

This has already been discussed in some detail in this thread.

 

[bob012345]

Yes, and if you want to understand that invariant amount of damage in terms of physics, not arbitrary choices of units, you can't look at quantities with units; you have to look at dimensionless numbers like the fine structure constant or ratios of quantities.

This has already been discussed in some detail in this thread.

Which is what the impedence of free space is. The ratio of two constants of nature. The speed of light is a constant of nature regardless of the fact that it isn't dimensionless. It's only the representation that's arbitrary. It has nothing to do with units.

 

[PeterDonis]

Which is what the impedence of free space is. The ratio of two constants of nature.

The ratio of two constants with different units, i.e., a ratio that has units itself. Not a dimensionless ratio. When I said "ratio" I meant "dimensionless ratio"; I assumed that would be clear from the previous posts in this thread.

The speed of light is a constant of nature regardless of the fact that it isn't dimensionless. It's only the representation that's arbitrary. It has nothing to do with units.

I'm sorry, but simply continuing to repeat your contrary position without addressing what has already been said in this thread is pointless.

 

[bob012345]

The ratio of two constants with different units, i.e., a ratio that has units itself. Not a dimensionless ratio. When I said "ratio" I meant "dimensionless ratio"; I assumed that would be clear from the previous posts in this thread.
I'm sorry, but simply continuing to repeat your contrary position without addressing what has already been said in this thread is pointless.

Well, I wasn't planning posting here today until got an invitation by email about the site and the various topics asking me to participate. Perhaps you should ask yourself why a person would take up the invitation only to have someone like you always trying to put them down with oneupmanship comments and closing off discussions? Yes, certain points were discussed before, but the point you miss is this, they weren't discussed by me. So please, quit trying to end discussions after inviting my participation. That's more pointless. I didn't come her to be bullied by you.

 

[PeterDonis]

certain points were discussed before, but the point you miss is this, they weren't discussed by me

But you are not discussing them. You are simply repeating your position without addressing them. That is what I drew your attention to.

quit trying to end discussions

I'm not trying to end any discussion. I'm trying to get you to contribute to the discussion. You aren't posting in a vacuum; you are coming to a discussion that has already taken place.

 

[Roger Chase]

Why does light move at the speed it does, why not half the speed or double?

It's actually the speed of cause and effect (causality). You can visualize this if you think about it from the changing E&M field view, that is, a changing Electric Field causes a changing Magnetic Field in the direction of propagation. That changing Magnetic Field in turn causes a changing Electric Field, again in the direction of propagation, etc ad infinum. The speed of that cause and effect is the speed of light "c"

 

[bob012345]

But you are not discussing them. You are simply repeating your position without addressing them. That is what I drew your attention to.
I'm not trying to end any discussion. I'm trying to get you to contribute to the discussion. You aren't posting in a vacuum; you are coming to a discussion that has already taken place.

Sorry, the 'bully' charge was too strong but at the moment that's what it felt like.

 

[PeterDonis]

It's actually the speed of cause and effect (causality).

This just rephrases the OP's question (instead of "speed of light", "speed of causality"). It doesn't answer it.

You can visualize this if you think about it from the changing E&M field view

This can help to understand how light propagates, but it doesn't explain why it propagates at a certain speed--or why that speed happens to be the same as the "speed of causality".

The previous discussion in this thread addresses those issues.

 

[PeterDonis]

This thread has run its course and is now closed.