I read your answer that light does not accelerate and it seemed too make sense but if you turn on a light bulb the photons emmited must accelerate to the speed of light in air since they were not moving before, don’t they?
Those photons didn't really exist before. There aren't a fixed number of photons. They can be destroyed and created. The energy, momentum, and angular momentum they have were there beforehand, but not in the form of photons.
(republished on 07/29/06)
Follow-Up #1: photon acceleration?
It is incomprehensible to me how a photon of light can go from 0 to 67 million miles per hour instantaneously. What is the motive force that propels the photon? Where does the energy come from? Why is c "limited" to 186,000mps? Why is the speed of light finite? What prevents the light from traveling even faster than it does? I realize that the answers to these questions are unknown, but I would be interested to hear the speculation of the physicists in the scientific community on this subject.
Wichita, Kansas USA
We've addressed part of that question , about 'acceleration', above.
The energy and momentum just come from other particles and fields.
I think it would be fair to say that there is not any known reason that the universe has a finite speed limit rather than an infinite one. However, at least the mathematical description with a finite limit is logical and self-consistent.
(published on 08/21/07)
Follow-Up #2: energy and light speed
Assume visible light traveling in vacuum and entering a transparent glass with refractive index 1.5. Because the glass is transparent, the light gets transmitted and comes out from the other side. Prior to entering the glass, the velocity of light was c. Just after the vacuum-glass interface, its velocity is c/1.5 (therefore there is deceleration!). After it comes out of the glass-vacuum interface, its velocity is again c (now there is acceleration from velocity c/1.5 to c). Where is the energy source for driving this deceleration and acceleration of the photons?
- B K Chandrasekhar (age 53)
That’s a really interesting question. It turns out that a blip of light has the same energy in the glass as it had outside. There is a universal quantum relation between energy and frequency, E=hf, where h is Planck’s constant. The frequency doesn’t change in the glass, so the energy per quantum doesn’t change, and neither does the number of quanta.
The velocity-dependent energy formula (mv2
/2) familiar from classical physics only applies to particles with rest-mass traveling at much less than the speed of light.
In the vacuum, you can classically describe the light energy as entirely consisting of electric and magnetic field energies. In the glass, the field energy is reduced but the energy is still there in the kinetic energies of particles (mainly electrons) oscillating in response to the fields.
(published on 09/01/07)
Follow-Up #3: photon basics
I was taught in school that Photons are both a particle and a wave, and that photons have a specific mass. You are saying this is not true? If not, then why are our schools teaching it and if photons do have mass then how do they get around the acceleration part? If they are just energy and are "already there in another form" Then how does electricity effect metal or gas to transform that energy that is "already there" into photons/light coming from a light bulb.
- Nathan (age 25)
Casa Grande, Az, USA
I don't think that what you were taught in school is really wrong. Quantum objects (photons are an example) show wave-like behavior in some circumstances and more particle-like behavior in other circumstances.
You can treat a photon as having a specific mass, its energy divided by c2
, so long as you're careful about the meaning of 'mass'. Here you'd mean the ratio of the momentum to the velocity. Some people use 'mass' just to mean rest-mass, and the rest-mass of photon is zero.
The key point is that photons do not exist in fixed numbers. They can be created and destroyed. They do not sit at v=0 and then somehow accelerate up to v=c. When they exist, they are always traveling at speed c.
The conversion of other forms of energy to electromagnetic (photon) energy can be described in some cases simply as driven by the acceleration of charged particles, just like in the classical theory of electromagnetism. In quantum electrodynamics there are other specific rules governing the likelihood of energy present in charged particles getting converted to EM field energy, and you need these more subtle rules to predict, for example, how long a hydrogen atom typically spends in an 'excited state' before it falls to the lowest-energy state, emitting a photon.
(published on 01/26/09)
Follow-Up #4: Speed of deflected light
According to theory of relativity ,When light rays travels around the massive object,their path gets deflected.My Question is that whether this deflection causes some deceleration of photons or not ? and how ?
- Amol Jagtap , age 25
Pune , Maharashtra , India
No, the speed of light remains the same. Only the direction is changed. It's like riding in a car on a circular race track at constant speed.
You might also be wondering what happens as light directly approaches or leaves a massive object. It would seem like its speed would change, but as far as any local observer sees, it always travels at the speed of light. There's no way to patch this up without using a different geometry of spacetime than we intuitively believe. The new geometry is called General Relativity. Mike W.
(published on 07/27/09)
Follow-Up #5: photon momentum in glass
In follow up question #2, how momentum of photons is conserved when light enters glass and again when it leaves it? Does photons become more massive as they enter refractive medium which slows its speed?
- Indrajit Kuri
New Delhi, India
This is a great question. It's a matter on which there's not quite unanimous consensus, but I'll give my own view, which seems in accord with recent experimental evidence and also is probably the closest to a consensus view. (See http://focus.aps.org/story/v22/st20
Let's say that the glass has anti-reflection coating, so we don't have to worry about any photons being reflected. The wavelength λ of the light in glass is reduced. Using the quantum relation p=h/ λ, the momentum p per photon is increased.
How can that be? As the light enters the glass, there's an attractive electromagnetic interaction between the light and the glass. The glass is pulled back toward the direction the light came from. This tiny motion of the large glass mass has the necessary p to keep total p conserved. An opposite recoil occurs as the light leaves.
(published on 08/12/10)
Follow-Up #6: photon momentum in glass
It is a well known fact that speed of light decreases as it passes through any refractive medium like glass. But in your reply you say that 'the momentum p per photon is increased' as the light enters the glass. If photons mass is unchanged, shouldn't then the speed of light increase in glass? if momentum of photon increases. Isn't it contradictory?
- Indrajit Kuri
It's a little tricky to talk about the "photon's mass", but if you were to assign an inertial mass (p/v) to the photon, it would be larger in the glass. You can think of the bare photon as being dressed in a disturbance of the electrons in the glass, with some inertial mass attributed to that disturbance.
(published on 08/24/10)
Follow-Up #7: How far will light travel?
light emitted from a star for example, how long and how far will it travel if it encounters nothing on its path? What happens to light in a black hole when it cannot escape?
- Prabhakar (age 47)
If a photon doesn't bump into anything it will travel forever. The age of the universe is thought to be 13.7 billion years old. This number comes from astrophysical observations and is a consequence of the so-called 'Big Bang Theory'. We can observe light coming from this time by looking at the Cosmic Microwave Background (CMB). So that light has been happily traveling for 13.7 billion years.
(published on 07/29/11)
Follow-Up #8: Speed of Light vs Speed of Sound
This "light speed thing" has been buzzing around in my mind.Something about the "speed limit" idea does not set right with logic as i know it.Everything i have read about photons seem unique to other particles. In spite of what I know about evolution and adaptation to the environment, it seems too coincidental That the two most important,sound waves and photons have exact speeds. How would our perception of sight ever work well or evolve properly if photons did not behave exactly the way they do ? Yet it is accepted at least right now that electromagnetic waves do not propagate in any kind of "substance" as sound does. Could it be possible that there is some kind of "medium" here and in space that we are not aware of yet that might limit it's speed? And if we do find it someday all of this would make total sense.
Another interesting thing about light (and owning plenty of LED flash lights)is that the question comes up, Does a higher watt plash light produce more photons or photons that carry more energy. The same question seems more amplified in the case of different powered lasers.
- Chris (age 57)
Glen Cove , NY 11542
Your last couple questions are much simpler to answer than the rest so I'll address them first. Two similarly designed flashlights, emitting light with different power, will basically just emit different amounts of approximately the same energy photons. Similarly, lasers of the same color but greater power simply emit more photons than those of lesser power. However, lasers of different color emit photons of different frequencies. The higher the frequency, the higher the energy of the photon. Frequency follows color, from lowest to highest, in standard ROY G BIV rainbow order. This means a blue photon carries more energy than a red photon, so a red laser emitting the same power as a blue laser will emit more photons of less energy.
I'm not sure I understand your other questions exactly but you're definitely concerned with the constancy of the speed of light, and a similar phenomena with sound.
The first thing I need to correct is your assumption that the speed of sound is exact. This isn't true. The speed of sound varies greatly from one material to another. For instance, the speed of sound in water, approximately 1480 m/s, is around 4 times greater than that in air at about 340 m/s. Things like solid metals often carry sound even quicker, iron for example at about 5100 m/s. But even in air the speed of sound is not constant, factors that affect it include humidity, temperature, and pressure among others.
In most materials the speed of sound is defined as
Where P is the bulk modulus (or bulk elasticity for gasses) which is basically a measure of stiffness for a given material, and ρ is the mass density.
Next, the speed of light isn't actually constant either. When we say the phrase, the speed of light, we're usually talking about the speed of light in a vacuum. This value is accepted to be constant, but light does slow down when it propagates through matter. It slows by a factor called the Refractive Index, which typically increases with a materials density. Light however can act as a particle on its own and doesn't need any sort of matter to be present in order to propagate so we say it has no medium.
It used to be thought that light traveled through a medium, the so called "luminiferous aether", in the same way sound travels through air or some other material. Without getting too much into it, experiments like the Michelson-Morley experiment
have shown strong evidence against the medium and in support of Einstein's special relativity.
Now if light didn't work exactly the way it did, well the universe as we know it today wouldn't exist. But if in its place were something just a tiny bit different and in that quasi-universe there evolved something a bit like the animals we have here on earth then they would likely evolve a similar sense of sight to some animal currently on earth. Evolution is a process that rewards the ability to pass on genetic code. Pretty much all matter interacts with light in one way or another, and things that want to eat you, or you want to eat, are no exception. So being able to see your food/predators is very beneficial in living long enough to pass on one's genes.
So to put it in a sentence. Evolution follows physics, not the other way around.
Thanks for the question,
(published on 05/01/12)
Follow-Up #9: photons in water
leading up to my question are thoughts regarding what I have read previously. From what I have read it would appear that a photon has no RESTING mass because it is only observed in motion. This sounds like photons cease to exist if they were "stopped" . It has also been explained that in science if there is an observation then more of the same observations could be applied.
One observation is that photons can be observed moving slower than their limit speed when they pass like through water and they still exist. The first question is if photons are slowed down do they loose some of their mass or intensity? If farther experiments could be done to farther slow down photons would photons as they are increasingly slowed start to increasingly loss mass (fade) until they almost do not move and virtually disappear until they stop and do disappear ? is there a a mathematical formula relating to the speed of the photon and it's mass?
- chris (age 58)
Glen Cove , NY,Nassau
I've moved your question to follow-up ones that lead into it better.
As photons go into materials, they become something a little different, dressed in excitations of electrons. Assuming no reflection, the net energy E of the whole packet stays essentially the same, since the frequency f doesn't change and the relation E=hf is universal. However, the wavelength λ becomes shorter, so the momentum p of the packet goes up, since p=h/λ is also universal. (The momentum conservation is maintained by a slight recoil of the whole material.)
So since p goes up and v goes down, if you define inertial mass as p/v, it goes up by a factor (c/v)2
, where n is the index of refraction. In a typical material with electric but not magnetic susceptibility at optical frequencies, this is just a factor of ε, the electric susceptibility.
(published on 08/02/12)
Follow-Up #10: Does light need a medium?
Why is it that in this discussion of light and its speed you are stating that light travels through a vacuum and it does not need a medium to do so when we know that there are gravitational fields practically everywhere, not to mention the 97% Dark Energy & Dark Matter, which light must travel through ?
- George Mavros (age 68)
NYC, NY. USA
You can measure the effects of gravitational fields on light. They're usually very small, except near black holes. There's no indication that somehow light would stop propagating where the field goes to zero. (For insiders- yes we know that the gravitational field doesn't have a unique value, but for now let's use a conventional coordinate frame where it's about zero in most regions.)
As for dark matter, we know where it is because of its gravitational effects on visible matter. Light propagates just fine through regions where it's dense and where it's very sparse. Other than the weak gravitational effects, light and dark matter don't influence each other.
What about dark energy? Since at least in the current epoch, it seems to be everywhere, we can't make the same sort of argument we made about dark matter. Conceivably a full understanding of dark energy and a deeper understanding of light could both end up pointing to some Calabi-Yau geometry of some underlying string field, which I guess you could call a "medium". That is not only way over my head but also beyond the current state of the art.
(published on 09/03/12)
Follow-Up #11: Is there an ether
Thank you Mike. My understanding is that light is traveling through a medium at all times. The existence of a perfect vacuum seems inconceivable to me as you yourself state even weak gravitational fields exist everywhere, and as we have imaged through telescopes, within galaxies, clusters of galaxies and super galactic clusters. In fact as far as we can observe today everything in the presently detectable universe is connected at least gravitationally. I strongly suspect that Dark Energy & Dark Matter, indirectly detected and proposed theoretically today, might have a close relationship to gravity. The final point I would like to make is that, to my understanding,a perfect vacuum or 'nothingness' cannot exist.
- George Mavros (age 68)
NYC, NY. USA
George- You've misread a bit of what we wrote.
In a standard coordinate frame, gravitational fields are very weak in most places. In some places they are zero. You can pick other coordinate frames which shift the places where the field is zero. None of that does anything to the transmission of light.
Because of the large variability in the density of dark matter, we can conclude with a lot of confidence that it's essentially irrelevant to the transmission of light.
We've discussed elsewhere that the question of whether or not there can be a perfect vacuum relies too much on semantic choices to be worth pursuing, at least at our current level of understanding. Maybe once a quantum theory of gravity is developed that will change.
Perhaps more importantly, what we mean by exploring these issues is not to just look for some satisfying words but rather to look for some mathematical structures that tell us something correct about what to expect to see in the world.
(published on 09/04/12)
Follow-Up #12: motion and gravity
I'm not quite sure how my questions really relate to the other questions I've read here about photons and such, but hopefully you might be able to shed light on them anyhow. My first question comes from the understanding (from Brian Greene's The Elegant Universe, pg. 52) that a muon traveling at 99.9 percent of light speed becomes 22 times more massive than a resting muon. Would this not create its own small gravitational field? If so, would larger objects also gain extra gravitational force by traveling faster? Provided the later question is yes, could we suggest that gravitational fields only exist because of motion? I have more questions in this line of questioning but I think I'll wait answers in case my future questions become moot.
As I understand it, according to Einstein's Special Relativity, time moves differently for those of us moving slower than those of us moving faster. Based on that idea, how can we understand or even approximate the age of the universe based on CMB since our movement in space is relative to the earth-sun-galaxy-other galaxies? How would we know if everything we observe visually and technologically isn't traveling through empty space at some immeasurable velocity?
- Rodger (age 27)
Mesquite, TX. U.S.
Yes, the motion of the muon (in our reference frame) adds to its energy and hence its contribution to the gravitational field, again in our reference frame. In its own frame its at rest, and hence its field is not enhanced in that frame. The situation is slightly more complicated because the momentum also contributes to the gravitational effects. At any rate, there is also a rest mass/energy for most familiar particles, and that rest energy is by far the most important source of gravity under familiar circumstances.
Relativity doesn't say "time moves differently for those of us moving slower than those of us moving faster" because it denies that the terms "moving slower" and "moving faster" have any objective meaning, except within a particular choice of reference frame. Nobody is moving in their own frame. It is true that the time intervals assigned to events differ between frames moving with respect to each other, but which way the differences go depend on how the events are moving with respect to the frames.
We can assign a velocity to everything we see without changing the laws of physics describing things. Nevertheless, the most obvious convenient choice is to set the average velocity of everything within range of us to zero. The easiest way to do that is to say that the CMB coming in from all directions is equal in the best frame. Using that natural frame, our galaxy is in motion at about 600 km/s. (see http://ned.ipac.caltech.edu/level5/March05/Scott/Scott2.html
(published on 03/11/13)
Follow-Up #13: Many cosmic question
If neutrinos are heavier and slower than photons, why do neutrinos go straight through objects while photons can be refracted and slowed down in different mediums? Why did neutrinos arrive from Supernova 1987A before the photons, after traveling for 168,000 years? Shouldn't the photons have had plenty of time to catch up and pass the neutrinos if they travel slightly faster? Is the speed of light actually constant or does it change with time as universal gravity lessens as the universe expands? Did the universe start with a slow bang and gradually but very slowly speed up as universal gravitation lessens? Do photons have mass? Can entrophy decrease inside black holes? Thanks for any answers.
- Kevin Henriksen (age 42)
Q: "If neutrinos are heavier and slower than photons, why do neutrinos go straight through objects while photons can be refracted and slowed down in different mediums?"
A: Neutrinos don't participate in electromagnetism. The interact only via gravity and the weak nuclear force. Those are both far too weak to cause the sort of slowing that affects photons.
Q: " Why did neutrinos arrive from Supernova 1987A before the photons, after traveling for 168,000 years? Shouldn't the photons have had plenty of time to catch up and pass the neutrinos if they travel slightly faster?"
A: No. They were catching up, but not nearly enough to make up for the initial photon delay in getting out.
Q: "Is the speed of light actually constant or does it change with time as universal gravity lessens as the universe expands?"
A: So far no measurement has indicated that the fundamental constants are changing.
Q: "Did the universe start with a slow bang and gradually but very slowly speed up as universal gravitation lessens?"
A: Is there a theory along those lines? Does it give anything like the extraordinary detailed fit to the data (e.g from the Planck satellite) given by the standard inflationary Big Bang theory?
Q: "Do photons have mass?" A: see http://van.physics.illinois.edu/qa/listing.php?id=22874
Q: "Can entrophy decrease inside black holes?"
(published on 08/22/13)
Follow-Up #14: photon physics
Thank you very much for your quick responses. A few more questions: Why were photons delayed in getting out of Supernova 1987A? With mass constant and the universe expanding, wouldn't universal gravitation lessen over time? How do you know what is happening entropy-wise inside black holes? Aren't black holes re-organizers of matter? How do polarized sunglasses block 50% of the light and polarize the other 50% no matter what angle they are tilted to the incoming light? Is there a way to record the temperature of a photon?
- Kevin Henriksen (age 42)
Let's start with one I know:
"Is there a way to record the temperature of a photon?: A photon has no temperature. Temperature describes a probability distribution for being in states with different energies. "A photon" sounds like it's in a single definite state.The only single state that has a definite temperature is the state of lowest energy, with T=0, which has zero photons.
" Why were photons delayed in getting out of Supernova 1987A? ": I guess the photons had to work their way out through a plasma of charged particles. They would scatter and be absorbed/emitted many times, unlike the weakly-interacting neutrinos. Something like that happens with photons, but not neutrinos, coming from the Sun.
"With mass constant and the universe expanding, wouldn't universal gravitation lessen over time? " : The gravitational tension pulling the whole universe back together would (and did) lessen over time. However, unless there's some completely new physics not yet understood and not evident in the behavior of galaxies, that has no effect on the more familiar symptoms of universal gravitation- the attractions of stars, galaxies, etc.
"How do you know what is happening entropy-wise inside black holes? " I think it's fair to say that, at least at the moment, we don't quite know what's going on entropy-wise inside a black hole. (see the many recent arguments about black hole firewalls) Viewing the black hole from the far outside, several lines of argument (beyond my ability to reproduce) say that its entropy is A/4, where A is the horizon area in Planck units. (see https://en.wikipedia.org/wiki/Black_hole_thermodynamics, http://www.scholarpedia.org/article/Bekenstein-Hawking_entropy) I'll update this answer if I can find a good explanation.
"Aren't black holes re-organizers of matter?" Yes, but so is everything else interesting, e.g. you. The question is whether, as viewed from the outside, black holes do something special, that is change quantum states in a "non-unitary" way, in which a single state turns into a probabilistic mixture of states, not just a more complicated-looking single state. The current consensus leans toward answering "no, it's unitary" but the confidence on that answer has recently dropped. If the answer changes to "yes, it's non-unitary", then maybe we'll also have to re-evaluate whether less exotic systems (e.g. you) are also somewhat non-unitary. That would mean an enormous revolution in basic physics.
"How do polarized sunglasses block 50% of the light and polarize the other 50% no matter what angle they are tilted to the incoming light? " : A better way to think of this is that the incoming light was already a mixture (usually about 50/50) of the two orthogonal linear polarizations. The polarizer absorbs one but not the other. (See https://en.wikipedia.org/wiki/Polaroid_(polarizer)) In practice, they don't work perfectly, and will transmit a little of the blocked polarization and block a significant amount of the transmitted polarization. Those percents will actually depend some on the whether the polarizer directly faces the light or is tilted. Perhaps what you mean is how can they work the same even as they're turned at different angles, still directly facing the light. Here the key idea. Because the equations describing the wave propagation are linear, you can get the behavior of the whole wave by summing the behaviors of any parts you break it into. You can take the same wave and break it into orthogonal polarization components along any two right-angle axes. That's because the electric fields are vectors, and you can express vectors in any basis set that you choose. So you just pick a basis where one axis is along the direction the polaroid absorbs and the other is at right angles to it.
(published on 09/12/13)
Follow-Up #15: photon energy and frequency
In photoelectic effect, it is said that the energy of the electrons emitted increase with the increase only with the frequency of the photon. Here, how can a photon have a frequency? By definition frequency is the number of oscillations of produced in a second. As far a photon is concerned where does the oscillation come from? What oscillates in a photon? Also, how such an oscillation decide the energy of the photon?
- Mohan (age 61)
chennai, tamil nad, india
This is one of those simple-sounding questions whose answer will have to go deep into the unfamiliar world of quantum mechanics. As a result, you will probably find it unsatisfying. If some reader can think of a better way to convey the ideas, we'd love to hear it.
Q: "What oscillates in a photon? "
A: A photon, like any other quantum state, has a "phase", a complex number that rotates in the complex plane. The frequency is the frequency of that rotation.
Q: "Also, how such an oscillation decide the energy of the photon?"
A: This one is easy. That quantum frequency is exactly the same thing as energy, always, for everything, not just photons. They're just two different names for the same thing. Often different units are used when the name "energy" is used and when the name "frequency" is used. The unit conversion factor is called Planck's constant.
So the interesting question becomes: what does that abstract quantum frequency have to do with the more familiar frequencies you measure, such as the rate at which a classical electrical field might oscillate? All those classical frequencies come from beat frequencies between quantum states with different frequencies (or, you could say, energies). As they go in an out of phase, interference terms between them change, so "things" move around and classical fields change. It's easier to visualize this idea with states that represent the fuzzy positions of particles rather than photon states. It turns out that electrical field oscillations come from beats between states with different numbers of photons. That sounds like it means that a state with one photon (or any other definite number) would not have an oscillating electrical field. And that conclusion is indeed right- a state with a definite photon number has an unchanging distribution of possible fields centered on zero.
(published on 11/11/13)
Follow-Up #16: photon mass in matter
I hope my questions aren't stupid but i really love physics and i have hundreds of questions in my mind without an answer. So, please be patient if my questions are dumb.
I read an answer saying that a photon has zero rest mass since it moving at the speed of light. I also know that MIT managed to slow down the speed of light of a photon to be just few hundred miles per hour. If mass decrease when speed increase and vise versa, then now after MIT researches we can measure the rest mass of a photon..based on that we can measure a weight of a photon too.
- Hisham Ragheb (age 32)
Bolkely, Alexandria, Egypt
Hi Hisham- Good question. I've placed it as a follow-up to some related questions.
The weight of a photon is just given by ghf/c2, where g is the gravitational field, h is Planck's constant, f is the photon frequency, and c is the speed of light. That weight just depends on one property of the photon: energy, hf. That doesn't change as the photon goes into some material and changes into a different sort of particle, moving more slowly.
(published on 11/22/13)
Follow-Up #17: understanding why light travels at c
Are there any significant counter-arguments within the physics community regarding the idea of emitted photons (light) instantaneously assuming the speed of light? In other words is it completely accepted that there can be no acceleration phase because photons in a vacuum ONLY exist at speed “c”? And lastly, am I right in saying that the strongest argument for the fact that an acceleration phase does not exist is this: for a particle that can travel at the speed of light to fit with Einstein’s relativistic equations eg momentum = (mass * velocity) * Lorentz factor, it must be assumed that that particle (photon) has zero rest mass (otherwise it would have an infinite momentum), and therefore the important consequent assumption being that the particle therefore can only exist at “c”? In other words, its because everything just “fits” and makes sense mathematically if we just think of photons as having to solely exist at the speed of light?
- Patrik Amethier (age 18)
Yes, the arguments go exactly the way you surmised.
(published on 03/06/14)
Follow-Up #18: speed of light and ether
What evidence are you using that would suggest light moved anywhere? For example, you don't measure resistance in metal and say something travelled at the speed of temperature.
- Charlie (age Cur)
Toronto, Ontario, Canada
I'm not sure what you mean by "the speed of temperature", but probably this is what you're getting at. In many familiar waves (plucked string, water ripples,...) the wave travels at some speed but the stuff that is doing the waving (string, water,...) isn't going anywhere fast. You're thinking that although electromagnetic waves travel at c, the "stuff" that's waving isn't traveling, and you want to call that stuff "light".
The key problem with that is that what we mean by light is the wave itself. We know when and where it starts, and when and where it arrives. So you can measure the speed. It's "c".
Now you can claim that the light wave is in some underlying stuff, which people used to call "the luminiferous ether". Many experiments looking for this ether failed to find it. Still, if you're willing to have it obey some surprising properties (relativity), you're free to claim it exists. So then you'd have something connected with the light that's not traveling at c, just as the electrons in a wire don't travel as fast as the electrical signal. It doesn't matter, because the speed of light doesn't mean the speed of that hypothetical ether but rather the speed of the light itself.
(published on 03/23/14)
Follow-up on this answer.