7 ms·

Is The Speed of Light Everywhere the Same?

47 points by jc123 12y ago

newguy101 12y ago
I heard this said on NPR this afternoon. I thought I misheard the guest. I guess I didnt! Interesting
DanBC 12y 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/Case/Speed_of_light 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.
- RivieraKid 12y ago
  > The speed of light does not vary with time or place. That's just an assumption.
  - sampo 12y 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.)
    - RivieraKid 12y ago
      > Supported by evidence. (And no counter-evidence is known.) That was true for Newtonian mechanics for a long time too.
      - JoeAltmaier 12y ago
        Not for very long. There were things unexplained by Newton starting the first day, right?
    - alphydan 12y ago
      There is plenty of evidence, but it would require to change some of our cosmological models (so it's not without controversy). A physicist that has explored the issue in detail is Joao Magueijo: https://en.wikipedia.org/wiki/Jo%C3%A3o_Magueijo https://en.wikipedia.org/wiki/Jo%C3%A3o_Magueijo For a good review of the VSL Theories (Varying Speed of Light Theories) and the evidence behind them see http://arxiv.org/abs/astro-ph/0305457 http://arxiv.org/abs/astro-ph/0305457
      - sampo 12y ago
        > There is plenty of evidence According to your second link: "The evidence is currently slim".
  - MisterMashable 12y 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.
    - RivieraKid 12y ago
      Also, the speed of light in a different part of the universe could be different.
    - weavie 12y ago
      I like to think of it as the speed of information. The quickest any information can move from point A to point B is c. (Even if those points are themselves moving.) The universe can expand faster than c because there is not actually any information moving between the center and the perimeter.
- MisterMashable 12y 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.
  - imaginenore 12y ago
    Why do the molecules reemitt the light in the same direction as they absorbed it?
    - jplewicke 12y ago
      They don't -- they are re-emitted in random directions that are statistically related to the original direction: http://en.wikipedia.org/wiki/Scattering http://en.wikipedia.org/wiki/Scattering .
    - dhimes 12y ago
      On average it's a requirement of the conservation of momentum. But note that sometimes photons come out in a different direction (scattered), including to opposite direction (reflection). In these cases the momentum is absorbed by the medium.
      - antoko 12y ago
        You're either very confused, or you have a much deeper understanding of physics than me. I love that it could be either. Does the conservation of momentum apply to things with no intrinsic mass?
dnautics 12y ago
It is possible that the speed of light is anisotropic, thus leads to a theory formulated by frank tangherlini
- jcromartie 12y ago
  And also creationist nonsense.
huuu 12y ago
Does anyone know why refraction occurs when light doesn't have mass/weight?
- mcherm 12y 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 12y 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 12y ago
  I'm trying to understand the motivation of your question. Why the mass/massless of photons is related to the refraction?
- tbrownaw 12y 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.
nl 12y 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_series http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...
jackgavigan 12y ago
tl;dr: No.
cLeEOGPw 12y 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.
- evanb 12y ago
  Ward-Takahashi identities prevent, in some sense, the virtual particles from screwing up the speed of light.
qwerta 12y 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 https://en.wikipedia.org/wiki/Metre#Timeline_of_definition
- jerf 12y 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.
  - partomniscient 12y ago
    Indeed, and we too will also need to change ourselves accordingly.
- partomniscient 12y 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 12y 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.
kazinator 12y 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 12y ago
Surely the only answer possible to this is we don't know because we've not been everywhere?

47 points by jc123 12y ago

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

DanBC 12y 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/Case/Speed_of_light 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.

RivieraKid 12y ago
> The speed of light does not vary with time or place. That's just an assumption.
- sampo 12y 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.)
  - RivieraKid 12y ago
    > Supported by evidence. (And no counter-evidence is known.) That was true for Newtonian mechanics for a long time too.
    - JoeAltmaier 12y ago
      Not for very long. There were things unexplained by Newton starting the first day, right?
  - alphydan 12y ago
    There is plenty of evidence, but it would require to change some of our cosmological models (so it's not without controversy). A physicist that has explored the issue in detail is Joao Magueijo: https://en.wikipedia.org/wiki/Jo%C3%A3o_Magueijo https://en.wikipedia.org/wiki/Jo%C3%A3o_Magueijo For a good review of the VSL Theories (Varying Speed of Light Theories) and the evidence behind them see http://arxiv.org/abs/astro-ph/0305457 http://arxiv.org/abs/astro-ph/0305457
    - sampo 12y ago
      > There is plenty of evidence According to your second link: "The evidence is currently slim".
- MisterMashable 12y 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.
  - RivieraKid 12y ago
    Also, the speed of light in a different part of the universe could be different.
  - weavie 12y ago
    I like to think of it as the speed of information. The quickest any information can move from point A to point B is c. (Even if those points are themselves moving.) The universe can expand faster than c because there is not actually any information moving between the center and the perimeter.
MisterMashable 12y 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.
- imaginenore 12y ago
  Why do the molecules reemitt the light in the same direction as they absorbed it?
  - jplewicke 12y ago
    They don't -- they are re-emitted in random directions that are statistically related to the original direction: http://en.wikipedia.org/wiki/Scattering http://en.wikipedia.org/wiki/Scattering .
  - dhimes 12y ago
    On average it's a requirement of the conservation of momentum. But note that sometimes photons come out in a different direction (scattered), including to opposite direction (reflection). In these cases the momentum is absorbed by the medium.
    - antoko 12y ago
      You're either very confused, or you have a much deeper understanding of physics than me. I love that it could be either. Does the conservation of momentum apply to things with no intrinsic mass?

dnautics 12y ago

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

jcromartie 12y ago
And also creationist nonsense.

huuu 12y ago

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

mcherm 12y 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 12y 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 12y ago
I'm trying to understand the motivation of your question. Why the mass/massless of photons is related to the refraction?
tbrownaw 12y 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.

nl 12y 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_series http://en.wikipedia.org/wiki/Vernor_Vinge#Zones_of_Thought_s...

jackgavigan 12y ago

tl;dr: No.

cLeEOGPw 12y 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.

evanb 12y ago
Ward-Takahashi identities prevent, in some sense, the virtual particles from screwing up the speed of light.

qwerta 12y 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 https://en.wikipedia.org/wiki/Metre#Timeline_of_definition

jerf 12y 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.
- partomniscient 12y ago
  Indeed, and we too will also need to change ourselves accordingly.
partomniscient 12y 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 12y 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.

kazinator 12y 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 12y ago

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