Register to reply 
Why does the speed of light have a maximum speed? 
Share this thread: 
#1
Mar1810, 04:28 PM

P: 419

As the title says  Why does the speed of light have a maximum speed limit? In the outer space the medium that transmits lights is as close as possible to what we call "nothing", though not really nothing in the physicists sense. The nature of space should determine the rate of transmission through it, right? Does empty space being close to a perfect vacuum limit the speed of light? Objects exist in space, so space must also be an object, and it must in some measureable way excert influence on the speed of light.
Is there a hypothesis on the reason why the speed of light has an end limit to its speed and what limits it? Or is its speed considered a "given" for now? 


#2
Mar1810, 04:34 PM

Sci Advisor
P: 2,687

The constancy of the speed of light in a vacuum is one of the postulates of SR. As a postulate, it's not proven from something more basic (think of a math postulate). This postulate has been confirmed time and again by experiments, however.
So well has this fact been proven, that the SI unit of meter has been redefined to be the distance light travels in 1/299792458 seconds. 


#3
Mar1810, 04:44 PM

P: 419

So there is no meaningful answer why the speed of light is close to 300 000km/sec^2 in vacuum or what limits it to that speed? 


#4
Mar1810, 04:59 PM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,345

Why does the speed of light have a maximum speed?
The most meaningful answer in Physics: experimental evidence!



#5
Mar1810, 05:09 PM

Sci Advisor
P: 2,687

Well, in physics we can always go around and "explain" things like this, but in the end you have to take some things as postulates and let the experimental evidence show you that they are true. I mean, I could say that light travels at 300,000km/s because the permittivity and permeability of free space take on specific values, but you can always just ask me "why are they these values?"...and then we just go on a loop.



#6
Mar1810, 08:02 PM

P: 597

So the important fact that light travels at a fixed velocity is derivable from known theory, which is how Einstein saw it; his 1905 E=mc^2 paper contains a footnote: "The principle of the constancy of the velocity of light is of course contained in Maxwell’s equations". The specific value of this constant is what then isn't really derivable from anything without reference to some experimental input. 


#7
Mar1810, 08:19 PM

Sci Advisor
P: 2,687

Yes, but then I can equally well ask "why are those two constants the way they are?". I think whether we take the permittivity and permeability or the speed of light to be what they are depends on which theory or which way we construct the model. Whatever model we choose to use must be based on several postulates from which all the deductions can be made. Those postulates can then only be verified by experiment. We can take different postulates and derive each other (like Newtonian F=ma to Lagrangian principle of least action), but in the end experimental evidence reigns king over physics.
But I guess my point is more on the side of the philosophy of science than science itself. All I'm trying to say is that eventually one just needs to take the experimental evidence as the "proof" of the pudding. Not everything can be derived from something else without eventually going in a loop. 


#8
Mar1910, 10:31 AM

P: 603

Light travels at the speed it does because that is that rate at which energy is exchanged between the electric field and magnetic field. Or stated alternately, that is the rate polarization takes place in a vacuum.
Specifically it is determined by the constants: vacuum permittivity and vacuum permeability expressed as: [tex]c = \frac{1}{\sqrt{\varepsilon \mu}}[/tex] All else regarding speed limits is pure conjecture. 


#9
Mar1910, 12:25 PM

Emeritus
Sci Advisor
PF Gold
P: 5,585




#10
Mar1910, 12:54 PM

P: 61

But then something has to give right? Yes that is correct. From our perspective an object may not look like it is going faster and faster as it approaches c but we will see its mass increase. From the objects perspective it does not see its mass increasing but it does see itself going faster and faster. I can take a spaceship going very close to c and go from one end of the galaxy to the other in a few seconds. The galaxy will have aged many years in the process though. That limit is some totally random constant, trying to understand why it is this constant as apposed to another is pointless. The important thing is this constant's relation to other physical constants. Certain relations may provide a universe that can sustain life while other relations won't. Everything clear now? 


#11
Mar1910, 10:38 PM

P: 4,513

Given the measurement properties of spacetime called the metric, you can obtain one unique velocity that is nonzero. If you follow paths where parts of the path have a velocity over the unique velocity, you get all kinds of physically questionable things happening. You could go back and meet yourself in the past. You could have perpetual motion machines. You could cool water down by boiling it. Stuff like that. The speed c is just a property of spacetime geometry. 


#12
Mar2110, 10:05 AM

P: 597




#13
Mar2210, 03:37 AM

P: 603




Register to reply 
Related Discussions  
Using light to see the world requires speed of light to be maximum?  Special & General Relativity  23  
Idea for light speed or possibly faster than light speed travel?  Quantum Physics  1  
Finding maximum speed, maximum acceleration, and maximum resisting force for pendulum  Advanced Physics Homework  8  
How do we know the maximum speed of light?  General Physics  1  
Does the speed of a wave in a spring depend on the maximum speed of each coil?  Introductory Physics Homework  3 