That's begging the question, in the original sense of the term.
While it has long been observed that hypothetical changes in the speed of light would be difficult to measure if certain other quantities changed with it in lockstep (and by "long" I mean 100+ years, going all the way back to the original debates around relativity), it is still meaningful to ask if the speed of light has changed relative to the quantities that it seems to related to.
If that turns out to be the case, it will mean that our definition is wrong, not that the speed of light isn't changing. Our definition is a constant because we believe it to be a constant. If we're wrong, it will need to change.
It's been defined as a constant to make calculations/measurements via physics/maths easier/possible, but that doesn't mean defining it as a constant is correct, or necessarily a good idea.
I could define Pi as 3 and it would be CONSTANT. We'd then argue over what shape a circle looked like, but my maths would certainly be easier than yours due to lack of irrationality.
I take your meaning, and it seems literally correct word-for-word, but it's hard to know what to make of it: a quantity is constant or it is not, and definitions do not change it.
Perhaps the relevant point is that the speed of light is not a number, but a dimensioned (if that's a word) quantity; and now the question becomes: is the dimensioned quantity (either its numerical value, or the meaning of the units) changing? I agree that it's hard to know how to make sense of the notion of units changing, though.
> the speed of light is only guaranteed to have a value of 299,792,458 m/s when measured by an inertial observer in a vacuum.
This is the average speed of photons in vacuum, right? Because according to the quantum field theory, photons should have a probability of hitting virtual particles that popped into existence, which would slow it down a little.
It actually does. Kind of. (But it doesn't have anything to do with why it can be refracted.)
Light has no rest mass. But, light can never be at rest.
Light does however, have momentum. And when it is reflected/refracted, it will transfer some of that momentum to whatever it hit.
Also remember that forces act on pairs of things. Light can be deflected by gravity (which will change it's momentum); therefore it must produce a gravitational field (which will in return change the momentum of whatever deflected it). And of course gravity is proportional to mass.
Light which is moving towards and object will gain momentum, like a falling rock would. Only instead of moving faster (since it's at a fixed speed), it shifts towards higher frequencies. If you look at the equations slightly differently, you get gravitational time dilation.
If you take a large-scale view, you get really good results by using a mathematical model that says that the light is behaves like a wave with a certain speed, and that the speed is slower in water.
If you take a small-scale view, you get decent results by using a model that says that individual photons have a small chance of being absorbed by the water molecules and then new photons are radiated again.
Supposedly there is a quantum mechanical view which applies on all scales and gives even more accurate values, but I have never seen it worked out for something as complicated as the interaction between light and a surface of water molecules. Both of the others I have worked out myself back when I was a physics student.
It still moves as a wave and has a wavelength. That wave is electromagnetic and interacts with the electrons and protons in the atoms of whatever it's traveling through. The medium it's refracting through acts like a viscus fluid compared to the vacuum.
Yes. Refraction takes place because photons are slowed down through interactions with matter. If a photon is absorbed briefly and re-emitted, this can be taken to represent a reduction in velocity, even though the photon travels at c when between atoms.
In a convex lens, photons take longer to pass through the thickest part, which creates a concave wavefront that naturally converges on a focal point:
Photons don't need to have mass for this to happen, because they aren't being deflected like billiard balls, they're being slowed by their interaction with atoms.
> Yes. Light is slowed down in transparent media such as air, water and glass.
[...]
> Special Relativity
> [...]
> The speed of light does not vary with time or place.
These two different ways of talking about speed of light caused megabytes of meta bickering on the Wikipedia page for speed of light, and a case for Arbitration, with topic bans and warnings. (Also the suggestion that you can measure the speed of light in SI units, because after 1983 the SI units use light to define them.)
The SoL page shows some of the problems of WP. Articles should have a simplistic lead (with caveats), a general introduction, and then higher level discussion of the oddities as currently understood. The WP page ignores all that, and leaps in at the deep end which means the article isn't much good for anyone.
I hope this clarifies the appartent contradiction. Light can't help but move at any other speed than c = 2.997 x 10^8 m/s. It can't go faster, nor slower. So how does the speed of light appear to slow down in certain circumstances? In a vacuum, the speed of light is always c. There is nothing to interfere with the propogation of the individual photons (little atoms of light) as they move through the vacuum. However, in a material such as glass the individual photons of light are absorbed and reemitted many trillions of times by the molecules making up the glass. The photons do a "stop over" and don't move at all, the velocity of these photons is zero (actually the photons temporarily don't exist except as energy absorbed by the atoms in the glass). Between lattice points or individual atoms the photons travel at velocity c. The combination of stop overs (absorbtion and reemission events) and free propogation gives the appearance light is travelling at a slower overall velocity than c.
[+] [-] kazinator|11 years ago|reply
Einstein does not mean "speed" when he uses the word "velocity" here or anywhere. He means "a vector quantity consisting of a speed and direction".
[+] [-] qwerta|11 years ago|reply
https://en.wikipedia.org/wiki/Metre#Timeline_of_definition
[+] [-] jerf|11 years ago|reply
While it has long been observed that hypothetical changes in the speed of light would be difficult to measure if certain other quantities changed with it in lockstep (and by "long" I mean 100+ years, going all the way back to the original debates around relativity), it is still meaningful to ask if the speed of light has changed relative to the quantities that it seems to related to.
If that turns out to be the case, it will mean that our definition is wrong, not that the speed of light isn't changing. Our definition is a constant because we believe it to be a constant. If we're wrong, it will need to change.
[+] [-] partomniscient|11 years ago|reply
I could define Pi as 3 and it would be CONSTANT. We'd then argue over what shape a circle looked like, but my maths would certainly be easier than yours due to lack of irrationality.
[+] [-] JadeNB|11 years ago|reply
Perhaps the relevant point is that the speed of light is not a number, but a dimensioned (if that's a word) quantity; and now the question becomes: is the dimensioned quantity (either its numerical value, or the meaning of the units) changing? I agree that it's hard to know how to make sense of the notion of units changing, though.
[+] [-] cLeEOGPw|11 years ago|reply
This is the average speed of photons in vacuum, right? Because according to the quantum field theory, photons should have a probability of hitting virtual particles that popped into existence, which would slow it down a little.
[+] [-] evanb|11 years ago|reply
[+] [-] huuu|11 years ago|reply
[+] [-] tbrownaw|11 years ago|reply
Light has no rest mass. But, light can never be at rest.
Light does however, have momentum. And when it is reflected/refracted, it will transfer some of that momentum to whatever it hit.
Also remember that forces act on pairs of things. Light can be deflected by gravity (which will change it's momentum); therefore it must produce a gravitational field (which will in return change the momentum of whatever deflected it). And of course gravity is proportional to mass.
Light which is moving towards and object will gain momentum, like a falling rock would. Only instead of moving faster (since it's at a fixed speed), it shifts towards higher frequencies. If you look at the equations slightly differently, you get gravitational time dilation.
[+] [-] mcherm|11 years ago|reply
If you take a large-scale view, you get really good results by using a mathematical model that says that the light is behaves like a wave with a certain speed, and that the speed is slower in water.
If you take a small-scale view, you get decent results by using a model that says that individual photons have a small chance of being absorbed by the water molecules and then new photons are radiated again.
Supposedly there is a quantum mechanical view which applies on all scales and gives even more accurate values, but I have never seen it worked out for something as complicated as the interaction between light and a surface of water molecules. Both of the others I have worked out myself back when I was a physics student.
[+] [-] snarfy|11 years ago|reply
[+] [-] lutusp|11 years ago|reply
In a convex lens, photons take longer to pass through the thickest part, which creates a concave wavefront that naturally converges on a focal point:
http://arachnoid.com/example/index.html#Lens_Example
Photons don't need to have mass for this to happen, because they aren't being deflected like billiard balls, they're being slowed by their interaction with atoms.
[+] [-] gus_massa|11 years ago|reply
[+] [-] dnautics|11 years ago|reply
[+] [-] jcromartie|11 years ago|reply
[+] [-] newguy101|11 years ago|reply
[+] [-] jbb555|11 years ago|reply
[+] [-] nl|11 years ago|reply
[1] http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...
[+] [-] jackgavigan|11 years ago|reply
[+] [-] DanBC|11 years ago|reply
> Yes. Light is slowed down in transparent media such as air, water and glass.
[...]
> Special Relativity
> [...]
> The speed of light does not vary with time or place.
These two different ways of talking about speed of light caused megabytes of meta bickering on the Wikipedia page for speed of light, and a case for Arbitration, with topic bans and warnings. (Also the suggestion that you can measure the speed of light in SI units, because after 1983 the SI units use light to define them.)
http://en.wikipedia.org/wiki/Wikipedia:Arbitration/Requests/...
The SoL page shows some of the problems of WP. Articles should have a simplistic lead (with caveats), a general introduction, and then higher level discussion of the oddities as currently understood. The WP page ignores all that, and leaps in at the deep end which means the article isn't much good for anyone.
[+] [-] MisterMashable|11 years ago|reply
[+] [-] RivieraKid|11 years ago|reply
That's just an assumption.