Why doesn't light move at an infinite speed?

[PAllen]

I believe I heard Feynman suggest this possibility in an interview, but it's interesting to consider the possibility that at some time in the future, our physical laws could be viewed within a kind of evolutionary context. Is it conceivable that the laws of physics as we observe them now are the "evolutionary" product of some earlier conditions in which the governing physical laws were different?

I don't mean that last bit as a rhetorical question. I'm an ignorant (by comparison) undergraduate, and I'm curious as to whether that is, in any sense, a viable question for future physicists.

See Lee Smolin's Fecund Universes hypothesis for an instantiation of this idea. Unfortunately, since it actually makes testable predictions, it has run into difficulties.

http://en.wikipedia.org/wiki/Lee_Smolin

 

[nitsuj]

A great question has been asked by OP. I don't think that I've seen a convenient answer yet.
The speed of light could be 600000km/s and yet we would arrive at a similar theory of relativity.

What is clear to me is, that by saying object A will always travel at speed X seen by any observer from within any frame of reference absent of gravity, it follows that every other object will travel below that speed X if you want to preserve causality and logic.

I can only guess, that the speed of light is somehow connected to fundamental properties of space itself.

I wonder what Einstein had to say about this if anyone has a link to it. He certainly asked this question himself.

A great guess,

And imo is the kinda fundamental question the op is asking.

It has to do with length & time, what they "are" / how they're defined.

And it is "fundamental" to spacetime, the name is literal. Perhaps Lengthtime would be more clear.

Infinite speed? can something be infinitely short in length, or an infinitely small amount of time*? This is all the same rhetoric as infinite speed. And to the point that has already been made, it's either infinite or it's not. Basically causality. either cause precedes effect or it doesn't.

Also a fair way to conclude an FoR for a photon is nonsense.

 

[thetexan]

I also don't understand why. Why 186000mile per second? Why could it not have been 1000000 miles per second. Is there something that physically stops a photon at 186000? What is the limiting factor?

tex

 

[arindamsinha]

My thoughts on this are as follows:
- Light (or broadly energy) has a physical existence in space (though not necessarily mass), and is therefore quantized in some way
- A quantum can travel very fast, but the very fact that it has to 'travel' means it cannot be at two places (origin and destination) at the same time
- If light had infinite speed, then the same quantum of light would have to be everywhere in the Universe at the same time (having bounced of something or the other and been redirected infinitely) - since travel time is zero, origin and destintation cannot be distinguised

This is why light (or anything) must have a finite speed.

 

[arindamsinha]

I also don't understand why. Why 186000mile per second? Why could it not have been 1000000 miles per second. Is there something that physically stops a photon at 186000? What is the limiting factor?

tex

That one is because of the way we have defined the units - meter (or mile!) and second. Define meter and/or second differently, and the speed would be different.

In some scientific units, the meter and second are treated as equal, and in those the speed of light is 1.

And, of course, as long as it is finite, it must have SOME specific value based on any set of units.

 

[Austin0]

Did you read the FAQ I mentioned,
https://www.physicsforums.com/showthread.php?t=511385

Did it answer your question? If not, what experiment - if any - would answer your question?

Interesting link.
" In these units, the speed of light equals 137.0359991. But this number is simply the inverse of the fine structure constant, defined as e2/ħc, where e is the fundamental charge and ħ is Planck's constant over 2π." This is a pretty amazing "coincidence"
But of course the fine structure constant is equally quantitatively inexplicable so no real help.

Units may be arbitrary but the real numbers are directly if not absolutely exactly correspondent to the real world. So it would appear that this relationship is as fundamental as pi=3.14 Aren't the permittivity and permeability of vacuum essentially the attributes of spacetime that determine the speed of light??

 

[tiny-tim]

the speed of light is fundamental to space-time

it is the ratio between space dimensions and the time dimension

the minkowski metric is (cdt)² - dx² - dy² - dz²

logically, speed should be dimensionless (no units),

and we should put c = 1, to put the four dimensions on an equal footing​

but then a car going at what we call 108 kph, or 30 m/s, would have a speed of 0.0000001, which is really inconvenient!

so, for convenience, we define distance so that our speedometers show sensible numbers

(btw, µ_o, the permeability of the vacuum, is also defined for convenience …

it's 4π 10^-7 henries/metre …

the 10^-7 is there only to avoid all those zeroes, eg going into a shop and asking for a 0.0000003 amp fuse!)

 

[nitsuj]

the speed of light is fundamental to space-time

it is the ratio between space dimensions and the time dimension

the minkowski metric is (cdt)² - dx² - dy² - dz²

logically, speed should be dimensionless (no units),

and we should put c = 1, to put the four dimensions on an equal footing​

but then a car going at what we call 108 kph, or 30 m/s, would have a speed of 0.0000001, which is really inconvenient!

so, for convenience, we define distance so that our speedometers show sensible numbers

(btw, µ_o, the permeability of the vacuum, is also defined for convenience …

it's 4π 10^-7 henries/metre …

the 10^-7 is there only to avoid all those zeroes, eg going into a shop and asking for a 0.0000003 amp fuse!)

That's a great way to put it. We when think of speed as laypeople, it's how fast is this compared to that. which is completely different from what c is.


If something moves it must exists at all "points" along it's path (some amount of time & for some length), that is "appear" continuously. To go faster than c is to "teleport" or not exist for a time/length.

 

[TrickyDicky]

Thus it was said that "the velocity of light in our theory plays the part, physically, of an infinitely great velocity."
-http://www.fourmilab.ch/etexts/einstein/specrel/www/

Why didn't you bring up this quote by Einstein when I was arguing this very same thing in the locked thread about SR versus galilean relativity and the twin paradox one.
If I had known this was stated by Einstein in his seminal paper on SR I could have saved me a lot of posts explaining precisely that the velocity of light in SR plays the part , physically, of infinitely great velocity (in classical mechanics).
Now your position then strikes me as purely cynical if you already knew that Einstein quote.

 

[Alesak]

I've found this interesting link: Why the speed of light is invariant.

What they seem to say is that finite speed of signal(here light) is necessary for universe to allow discrete space and time(ie Planck length and Planck time). More precisely, their postulate says it is possible to observe events apart at least Planck length or happening at least Planck time after each other.Intuitively, I understand speed of light as exchange rate between time and space. If you want to move faster through the space, you need to pay for it with time! (meaning you experience less time)

 

[Mordred]

Definetely an interesting article

 

[harrylin]

Why didn't you bring up this quote by Einstein when I was arguing this very same thing in the locked thread about SR versus galilean relativity and the twin paradox one.
If I had known this was stated by Einstein in his seminal paper on SR I could have saved me a lot of posts explaining precisely that the velocity of light in SR plays the part , physically, of infinitely great velocity (in classical mechanics).
Now your position then strikes me as purely cynical if you already knew that Einstein quote.

You were not arguing that same thing but a misunderstanding of that same thing. After that thread was locked I offered you to continue our conversation in private which you thought not needed. The issue was your claim that in classical mechanics "light speed is infinite", "it takes no time for light signals to travel". In Maxwell's electrodynamics on which SR is based, light propagates at c=3E8 m/s.

 

[haushofer]

I've found this interesting link: Why the speed of light is invariant.

What they seem to say is that finite speed of signal(here light) is necessary for universe to allow discrete space and time(ie Planck length and Planck time). More precisely, their postulate says it is possible to observe events apart at least Planck length or happening at least Planck time after each other.


Intuitively, I understand speed of light as exchange rate between time and space. If you want to move faster through the space, you need to pay for it with time! (meaning you experience less time)

You don't experience less time. Everything is normal in your frame of reference. It's only when you compare your clock with that of twin you could see a difference, depending on the relative movement between you and your twin.

 

[Alesak]

You don't experience less time. Everything is normal in your frame of reference. It's only when you compare your clock with that of twin you could see a difference, depending on the relative movement between you and your twin.

Of course, but speed also is only when you compare (change of) position to your twin.

It's all really weird anyway. Any of you more experienced guys "feel" you undestand SR very clearly?

 

[TrickyDicky]

You were not arguing that same thing but a misunderstanding of that same thing.

Not at all, but if you thought so that adds further reasons for you to have mentioned the Einstein quote.

The issue was your claim that in classical mechanics "light speed is infinite", "it takes no time for light signals to travel".

I made a lot of qualifications to your simplifying quotes. I said that in classical mechanics the velocity of light c o f SR acts (conceptually) as if it was an infinite velocity. How is that different from the Einstein quote apart from using the word "act" instead of "play the part"?

In Maxwell's electrodynamics on which SR is based, light propagates at c=3E8 m/s.

I specifically referred to classical mechanics not Maxwell electrodynamics, the discrepancy of the latter with classical mechanics was precisely the starting point for Einstein to develope SR.

 

[Fredrik]

Any of you more experienced guys "feel" you undestand SR very clearly?

Yes, I have to say that I do. There are still problems that I find difficult, like the one in the thread bcrowell started the other day. It involves an indestructible submarine moving at a relativistic speed in indestructible water in a very large tank that's accelerating in a direction that's perpendicular to the motion of the sub in the tank. What kind of masochist even comes up with these problems? But when it comes to basic stuff like the twin paradox, it all feels very simple now.

 

[Milan Nikolic]

My thoughts on this are as follows:
- A quantum can travel very fast, but the very fact that it has to 'travel' means it cannot be at two places (origin and destination) at the same time
- If light had infinite speed, then the same quantum of light would have to be everywhere in the Universe at the same time (having bounced of something or the other and been redirected infinitely) - since travel time is zero, origin and destintation cannot be distinguised

This is why light (or anything) must have a finite speed.

What can we say about the wave function? How serious are the results of experiments by which a quantum entity is in every point of the universe?

 

[ZapperZ]

At this point, it is worth reminding people of the PF Rules, and our rules prohibiting speculation based on unverified ideas.

If you are presenting your "thoughts", and it is based mainly on "tastes" or guesses, rather than valid physics or peer-reviewed papers, then it should not be posted here.

Zz.

 

[harrylin]

Not at all, but if you thought so that adds further reasons for you to have mentioned the Einstein quote.

He said the same as we did, and you did not understand it; how could it help? If you now understand it thanks to that, then I should have mentioned it - but apparently that is not the case.

I made a lot of qualifications to your simplifying quotes. I said that in classical mechanics the velocity of light c o f SR acts (conceptually) as if it was an infinite velocity. How is that different from the Einstein quote apart from using the word "act" instead of "play the part"?

It is the other way round from what you mean with that: in classical mechanics the limit speed is much faster than the speed of light (infinite), while in SR it is equal to the speed of light. And I had also answered that here:
https://www.physicsforums.com/showpost.php?p=4112328&postcount=21

I specifically referred to classical mechanics not Maxwell electrodynamics, the discrepancy of the latter with classical mechanics was precisely the starting point for Einstein to develope SR.

Maxwell's electrodynamics was used with classical mechanics. I interpret the thread-locking to mean that the moderators do not want us to elaborate more on that topic on this forum, and it is only somewhat related to this thread; thus from me just one last elaboration here.
For disambiguation, let's give the limit speed its own symbol, L.

1a. In classical mechanics there is no limit speed: L=∞
1b. In Newton's light theory, the speed of light V is relative to the source: V<∞
1c. In Maxwell's electrodynamics, the speed of light V is a vacuum constant: V=c
2. In SR the speed of light V is a vacuum constant that equals the limit speed: V=c=L

I can be reached by email.

 

[RedSonja]

Virtual particles may propagate at speeds larger than c, but relativity prohibits that any information is transferred at velocities larger than c.

 

[Alesak]

Yes, I have to say that I do. There are still problems that I find difficult, like the one in the thread bcrowell started the other day. It involves an indestructible submarine moving at a relativistic speed in indestructible water in a very large tank that's accelerating in a direction that's perpendicular to the motion of the sub in the tank. What kind of masochist even comes up with these problems? But when it comes to basic stuff like the twin paradox, it all feels very simple now.

Screw LHC, let's do this!

 

[TrickyDicky]

It is the other way round from what you mean with that: in classical mechanics the limit speed is much faster than the speed of light (infinite), while in SR it is equal to the speed of light.

Ok, if you can't see this is equivalent to what I wrote I guess there's no need for further discussion.

 

[Milan Nikolic]

If the expansion of the universe overcome the speed of light for a given photon, then what happens to the photon? I guess it's still difficult to answer this question, but I think that may already give some assumptions.

The answer may lie in the dark entities.