Is The Speed of Light Everywhere the Same?

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html

by jc123 • 12 years ago

47 54 12 years ago

math.ucr.edu

newguy101• 12 years ago

I heard this said on NPR this afternoon. I thought I misheard the guest. I guess I didnt! Interesting

DanBC• 12 years ago

Does the speed of light change in air or water?
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.

RivieraKid3• 12 years ago

The speed of light does not vary with time or place.

That's just an assumption.

sampo2• 12 years ago

> The speed of light does not vary with time or place.
That's just an assumption.

Supported by evidence. (And no counter-evidence is known.)

MisterMashable2• 12 years ago

If we take relativity at face value, the speed of light is c = 2.997 x 10^8 <<<locally>>>. <<<Globally>>> the speed of light can be anything. Cosmologists talk about the universe expanding faster than light during the inflationary phase, which sounds confusing because we're told nothing can move faster than light. While it is true that the speed of light must propogate at only one velocity for all freely moving frames of reference, the speed at which space itself expands can be anything! Relativity does not restrict the speed at which space itself expands or contracts. TL;DR The maximum local speed of anything moving through space in a freely moving frame of reference is c. The speed at which space itself expands or contracts is not limited in any way by Einstein's theory and is consistent with observations of the the inflationary universe in which the universe expanded at faster than the speed of light.

MisterMashable2• 12 years ago

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.

imaginenore2• 12 years ago

Why do the molecules reemitt the light in the same direction as they absorbed it?

tfgg• 12 years ago

This argument and explanation is constantly coming up on the subreddit askscience. I don't violently oppose it as much as some do, since I think it's somewhat helpful, but I would like to say that my preferred interpretation is that a `photon' isn't really a well defined object when inside strongly-coupled materials. For example, can you really sensibly talk about a freely propagating radio wave when its still within a quarter wavelength of an antenna? When atoms are spaced tenths of a nanometre apart and visible light has a wavelength of hundreds of nanometres it seems a bit silly to talk about it propagating freely in a material.

When you go through quantum field theory, photons are defined in terms of freely propagating particles, not interacting with other fields. When the disturbance in the EM field propagates into, e.g. a dielectric material, and strongly couples to various nuclear and electronic excitations, it is better described in terms of a massive quasiparticle, the polariton, which is a hybrid of the photon, phonon and electron fields and, being massive, propagates at less than c. You can, of course, describe it in terms of perturbations to the free photon corresponding to various types of virtual absorption and re-emission, but it's a bit misleading to think of it being physically absorbed and re-emitted with little stopovers. If anything, the classical model of a continuously interacting medium interacting with the EM wave creating a coherent response wave which interferes with and appears to slow down the EM wave is more instructive.

dnautics• 12 years ago

It is possible that the speed of light is anisotropic, thus leads to a theory formulated by frank tangherlini

jcromartie• 12 years ago

And also creationist nonsense.

huuu• 12 years ago

Does anyone know why refraction occurs when light doesn't have mass/weight?

mcherm• 12 years ago

The light interacts with the water molecules.

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• 12 years ago

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.

gus_massa• 12 years ago

I'm trying to understand the motivation of your question. Why the mass/massless of photons is related to the refraction?

tbrownaw• 12 years ago

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.

lutusp4• 12 years ago

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:

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.

nl• 12 years ago

Surprised no one has mentioned Vernor Vinge's wonderful, awesome, mind exploding Zones of Thought novels[1].

[1] http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...

jackgavigan• 12 years ago

tl;dr: No.

cLeEOGPw• 12 years ago

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.

evanb2• 12 years ago

Ward-Takahashi identities prevent, in some sense, the virtual particles from screwing up the speed of light.

qwerta• 12 years ago

Modern definition of distance is derived from speed of light. So by definition speed of light is CONSTANT.

https://en.wikipedia.org/wiki/Metre#Timeline_of_definition

jerf2• 12 years ago

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.

partomniscient4• 12 years ago

Indeed, and we too will also need to change ourselves accordingly.

partomniscient4• 12 years ago

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.

TeMPOraL• 12 years ago

I think it works from the other direction. You don't get to say Pi=3.14 and then suddenly circles; the value of Pi is defined by whatever works for circles. Likewise, the speed of light is derived from whatever makes space and time one thing, thus you can't just assign a value to it. The value comes from the underlying structure of theory of relativity.

jerf2• 12 years ago

"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."

Actually that argument was had and ended a long time ago; you can set pi to 3 and you get a thing called a spherical geometry. See, for instance, http://mathforum.org/library/drmath/view/55021.html . Between the word "spherical geometry" and what you find in that link you'll have the keywords to continue digging if you want to.

astrodust• 12 years ago

Suddenly there are 1.90986 people in an average marriage.

JadeNB• 12 years ago

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.

lutusp4• 12 years ago

It's best to separate two elements here -- the measured speed of light, and the relationship between space and time. The relationship between space and time is easily expressed in closed form:

t' = t √(1-v^2/c^2)

That's pretty clear and easy to interpret. So measurements of c may be messy and subject to controversy, but the geometry expressed in the above equation, and what it tells us about spacetime, is much more clear.

kazinator• 12 years ago

A curvature of rays of light can only take place when the velocity [Einstein means speed here] of propagation of light varies with position.

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".

jbb555• 12 years ago

Surely the only answer possible to this is we don't know because we've not been everywhere?

Is The Speed of Light Everywhere the Same?

Related Stories