B Why does light move at the speed it does

[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.