Is the Speed of Light Constant? Exploring the Scientific Debate

In summary, it has been suggested by a notable scientist that the speed of light is constant, meaning that it will be measured as the same regardless of who is doing the observing or what speed they are moving relative to each other. However, the speed of light may vary within the Heisenberg uncertainty principle, and accelerating influences like inflation and vacuum energy/quintessence may cause variations in light speed in vacuum. When traveling through a medium, light will have a different speed, but the ultimate speed limit for information propagation remains constant at the speed of light in vacuum. The constancy of light applies only to vacuum, as the laws of spacetime transformation do not change with different media.
  • #1
Selnex
It has been suggested by a notable scientist that the speed of light is constant. However does this mean that the speed of light is the same regardless of what medium it is traveling through? I am nearly certain I have heard that the speed of light is not the same in water as it is in air. More importantly perhaps, there was that experiment done to attempt to prove that ether in space does not exist and I think that experiment was dependent on the conceptual notion that light would travel at a different rate in ether as it would in a vaccum.
 
Science news on Phys.org
  • #2
Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.

JMD
 
  • #3
When it is said that the speed of light is constant, it is meant that it will be measured as the same regardless of who is doing the observing or what speed they are moving relative to each other.

Two observers are moving with respeect to each other. A beam of light passes both of them. Each will measure that light beam as traveling at the same speed with respect to themselves.
 
  • #4
The speed of light may also vary within the Heisenberg uncertainty principle. Since the vacuum contains fleeting virtual particles, fundamentally the speed of light is constant there only in the average over quanta.

Accelerating influences like inflation and vacuum energy/quintessence may cause variations in light speed in vacuo.
 
  • #5
Originally posted by nbo10
Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.

JMD

basically that's just a restatement of the principle of inertia as applied to light. there probably isn't a real vacuum in space so light speed is never constant.
 
  • #6
Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.
HUGE misconception here. Matter, so most mediums (media?) is almost entirely empty space. Light travels at C through these empty spaces. But when it hits an atom, it is absorbed (or reflected). When absorbed, there is a delay before it is re-emitted (if it is re-emitted). This delay is responsible for the APPARENT speed of light being less than C when traveling through a medium. In actuality, whenever light exists as light it is ALWAYS traveling at C.
 
  • #7
Note - The coordinate (i.e. non-local) speed of light varies in the presence of a gravitational field.

Pete
 
  • #8
Hi,
No Russ. I have to disagree with you. Though your explanation is partially acceptable, it is not relevant in this case.
When one is talking about the constancy of the speed of Light, he is talking about the ultimate speed of propagation of information by any means. That is the heighest attainable speed. In vacuum light travels with a speed of c, which cannot be attained by any material particle. Whereas, when light is traveling through some medium, it is traveling with a lesser speed, but still the speed limit for information propagation is c, and not the speed of light in that medium. Hence, it is possible for a material particle to have speed greater than the speed of light in the medium. The phenomenon of Cherenkov radiation is a good example of it.
A better explanation of the original querry can be given as:
Einstein's theory of relativity does not directly say anything about the speed of light. It does say that there is an ultimate limit to the speed by which information can be propagated. (i.e. the only invariant speed in the spacetime transformation is finite, unlike infinite in Newtonian relativity.)
It is an experimental fact that the speed of light in vacuum does not depend upon the frame of referrance ...so it must be invariant and hence the symbol "c" (which just denotes invariant speed of the transformation) can be successfully identified with the speed of light in vacuum.
This is nothing to do with the fact that light travels with different speeds in different media, the laws of spacetime transformation does not change with the media and hence the value of "c". It is just a coincidence that "c" and speed of light in vacuum are the same.
 
Last edited:
  • #9
It's not a coincidence; Special Relativity quite clearly mandates that a particle travels at light speed if and only if its rest mass is zero. Since light has zero rest mass, it travels at light speed always, not just in a vacuum.

I agree with Russ, the "slowing" of light in materials is an apparent effect. We measure when light enters the object and when light comes out of the object with no reason to believe we're observing the exact same photon.

Hurkyl
 
  • #10
A while ago there were some researchers who suggested that C (max) may have changed over the life of the universe.
 
  • #11
Russ makes the matter seem to be composted of billiard balls which swallow upt and spit of BB's which are the photons. I've never pictured matter that way. The atom is not like a biiliard ball. Instead picture matter made of of a smeared out cloud out electrons over a lattice of ions.

EM wise (i.e. for a light*wave*) inside matter there is a preferred frame of referance and in that frame of referance the permiability of matter "u" and the permitivity of matter "e" is differnt than empty space. The wave velocity in matter (for a homogeneous and and istotropic medium) is v = 1/sqrt(ue). For matter v < c.

The constancy of light applies to vacuum.

Pmb
 
  • #13
Hurkly - What's your point? You're just quoting someone who says the same thing as someone above. That was answered by an undergrad - it's just basically repeating the opinion mentioned above.

It's not very meaningful to say that a photon is emitted by an atom and reaborbed by the atom its already attached to, especially since the wave function of the electrons spread over the surrounding atoms.

However I'm of the opinion that neither view is an exact explanation.

Pete
 
  • #14
Originally posted by Adam
A while ago there were some researchers who suggested that C (max) may have changed over the life of the universe.

I have heard about that, too and been wondering how rapidly this proposed change is believed to be occurring?
 
  • #15
Lurch

I don't know, sorry.
 
  • #16
No Russ. I have to disagree with you. Though your explanation is partially acceptable, it is not relevant in this case.
This may just be a question of semantics, but I don't think so. The problem is that if you fire one photon at a time through a medium, you will NOT get a constant C. Depending on the path each photon takes and the number of times it is absorbed and re-emitted, its apparent speed will be different. Thats why the apparent C through a medium is actually an AVERAGE apparent C.

Russ makes the matter seem to be composted of billiard balls which swallow upt and spit of BB's which are the photons. I've never pictured matter that way. The atom is not like a biiliard ball. Instead picture matter made of of a smeared out cloud out electrons over a lattice of ions.
Electrons are tiny and located far away from the nucleus of the atom (compared to their size). Thats a lot of empty space. The billiard ball analogy doesn't fit because billiard balls are solid. Atoms are mostly empty space. Also, you are thinking about solids. Gases are mediums too. I read somewhere that on its trip through the atmosphere a photon of blue light is only scattered about once before getting to the ground. Thats a long way to travel in a medium without hitting something.
 
  • #17
russ said
Electrons are tiny and located far away from the nucleus of the atom (compared to their size). Thats a lot of empty space. The billiard ball analogy doesn't fit because billiard balls are solid. Atoms are mostly empty space.
That anaglogy is flawed. A photon does not move around inside an atom. A photon does not move around inbetween electrons either.

Do you know what is meant by the term "electron cloud"?

Pete
 
  • #18
Originally posted by nbo10
Yes light does have different speed's when traveling through a medium. The speed of light in a vacuum is always constant.

JMD
Look at that!
Is it because the vacuum is all alike?
Would you say that the velocity of light is also constant is same kind of waters?
 
  • #19
I had came across last night the Ewald-Oseen Extinction Theorem, which I suppose is your "more complete answer".

When a light wave hits an opaque surface, it interacts with surface molecules causing them to vibrate which generates an electromagnetic wave that fully cancels the primary wave inside the opaque surface.

For a transparent medium, the cancellation is not total and the primary (precursur) wave indeed reaches the other side of the medium at the full speed of c, although extremely weak because the interaction with the molecules of the medium scatter the energy around... eventually the energy gets across and the amplitude of the wave on the far side matches that on the near side, taking a time consistent with a wave of velocity c / n (n = index of refraction).


The catching and releasing model would then be a good approximation to what actually happens.


Hurkyl
 
  • #20
basically that's just a restatement of the principle of inertia as applied to light. there probably isn't a real vacuum in space so light speed is never constant.
No, it's not the principle of intertia. The speed of light is referred to as c because it is indepedent of the movement of your particular frame of reference. If you yourself are moving at .5 c relative to somebody on earth, light will still appear to be moving at c relative to YOU as well as on EARTH! That is what is unusual about lightspeed, and is where all the time dilation and length contraction effects originate.
 
  • #21
Do you know what is meant by the term "electron cloud"?
Yes, the electron cloud is the probability function of where around the atom you find electrons.
That anaglogy is flawed. A photon does not move around inside an atom. A photon does not move around inbetween electrons either.
You seem to be implying that the electron cloud is an impenetrable barrier to the passage of light and that any light entering the cloud gets captured. It is not. In fact, the electron cloud doesn't even always surround the entire atom. Sometimes light travels right through an atom without being absorbed and sometimes it doesn't. This often depends on the wavelength. Thats what transparency is.
 

1. What is the speed of light?

The speed of light is a physical constant denoted by the letter "c", which stands for the speed of light in a vacuum. Its value is approximately 299,792,458 meters per second.

2. Is the speed of light constant?

According to Einstein's theory of special relativity, the speed of light is constant in all inertial frames of reference. This means that the speed of light is the same for all observers, regardless of their relative motion.

3. How was the constancy of the speed of light discovered?

The constancy of the speed of light was first observed and measured by Danish astronomer Ole Rømer in the late 17th century. He noticed that the observed time between eclipses of Jupiter's moon Io varied depending on the distance between Earth and Jupiter, leading him to conclude that light has a finite speed.

4. Are there any exceptions to the constancy of the speed of light?

While the constancy of the speed of light has been confirmed by numerous experiments, there are some theories that suggest it may not be constant in all circumstances. For example, some theories in quantum physics suggest that the speed of light may vary in extreme conditions, such as in the early universe.

5. Why is the constancy of the speed of light important?

The constancy of the speed of light is a fundamental principle in physics and has many important implications. It forms the basis of Einstein's theory of relativity, which has revolutionized our understanding of space and time. It also plays a crucial role in modern technologies such as GPS and telecommunications.

Similar threads

Replies
4
Views
2K
Replies
9
Views
2K
Replies
13
Views
2K
Replies
48
Views
5K
Replies
19
Views
1K
  • Optics
Replies
2
Views
1K
  • Optics
Replies
7
Views
1K
  • Special and General Relativity
Replies
1
Views
612
  • Special and General Relativity
Replies
1
Views
602
Back
Top