Does light always travel at lightspeed?

Discussion in 'Physics & Math' started by proycon, Apr 8, 2002.

  1. proycon Registered Member

    Hey..I'm new to this forum and I've got a question about the speed of light..

    I was wondering about the velicity of light, c.... which can't be exceeded due to Relativity...BUT...can light go slower than c?

    i.e. Does light go actually slower when travelling through a medium? Or does it merely interact with the matter and therefore 'come out' later..

    It's just something I was wondering...
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  3. (Q) Encephaloid Martini Valued Senior Member

    Light is comprised of photons. Photons have energy which is dependent on their frequency. The photons, when traveling through a medium other than a vacuum, slow down. The speed depends upon the photons frequency (vibrations per second) of the oscillations of the electric/magnetic fields in the photons and the medium it travels through. If the photons are not absorbed by the medium, the light is said to refract. Refraction of light does not change the frequency of the photons, therefore does not change the energy of the photons. The ratio between the speed of light in a vacuum with that of the speed through the medium is called the 'refractive index.'

    What actually changes the speed of light as it travels through the medium is the distance between consecutive peaks of the vibrations of the oscillations of the electric/magnetic fields in the photons. The distance gets shorter.
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  5. c'est moi all is energy and entropy Registered Senior Member

    Light Freakssssss

    if seems that the questions about light keep on coming ... a lil more and we'll have to open a special thread for all light issues
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  7. nanok Registered Senior Member

    wow, very interesting! are there any theories about how to increase the frequency??? or is that just a stupid question with no answer??? but the way I think is if you can decrease it why not increase it???
  8. SpyFox_the_KMeson Doctorate of Yiffology Registered Senior Member


    I believe Richard Feynman actually theorized that light could travel at faster or slower speeds, but the potential drops off for that light. It's basically just a mathematical trick, so light (in a vacuum) travels at lightspeed. By the way, if you haven't read QED by Richard Feynman, I highly reccomend it.

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  9. Fukushi -meta consciousness- Registered Senior Member

    now this is something I'm not shure of

    After they went trough the atmosphere (for example) and they slowed down,....and they leave the atmosphere,....

    do they speed up again when they re-enter the vacume of space?

    In my humble opinion: I don't know,...but I tend to say no, in fact: maby they (the fotons) do not always travel at light-speed in a vacume,....wouldn't that be plausible? and will it account for dark energy/mass?

    If there's courtacy for this in any means I want it !!! hehehe,....

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  10. thed IT Gopher Registered Senior Member


    Indeed, as light passes into a new medium it immediately speeds up, or slows down. It's why refraction happens.

    A niftier affect is Cerenkov Radiation.
  11. James R Just this guy, you know? Staff Member

    Light slows down in a medium because photons are being continually absorbed and re-emitted by atoms in the medium. There is some delay between absorption and re-emission. When travelling between atoms in the medium, light travels at its maximum (constant) speed of 299792458 metres per second.
  12. Fukushi -meta consciousness- Registered Senior Member


    a simple answer will do pls:

    yes: fotons speed up again when they leave the medium that slows them down,...
    correct? cause your explanation of it does not clarifie things really,...

    I knew that fotons slow down,...(depending on wich medium) but are you shure they speed up again? That's all I want to know,....

    could someone answer in an understandable yes or no?

    And has anyone a clue why this fotons speed up again when they leave the medium? what is their propulsion: or is it rather an exotic property of the fotons: like : pp (PolyPropulene: it bends to its original form after warming it up)

    Last edited: Jun 3, 2002
  13. Alpha «Visitor» Registered Senior Member

    Light doesn't speed up or slow down. It always travels the same speed. It just appears to go slower in a medium because it interacts with it, bouncing of molecules and such, making it's path longer in a medium. Once it gets back into the vacuum there is no medium so it doesn't seem to be going slower any more.
  14. Fukushi -meta consciousness- Registered Senior Member

    Thank you Alpha !

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  15. Ender Registered Senior Member

    Concider this

    I haven't even seen any real proff for PHotons. However if they did exist, they would slow down.

    But would they speed up is the real question?

    No, because they would need something that is traveling faster than light in order accelerate it?!?!?!

    Wow I really just confused myself!
  16. huh??? Registered Senior Member

    No, and you don't need some fancy Bose-Einstein Codensate to figure that out. All you need is a prism or a magnifying glass. To speed it up, well, there are experiments "prooving" light can travle faster than c, but this is because the 'information' can travle faster, not the actual photon itself. This can be observed through the quantum tunnling/teleportation effect.

    Anotherway to think of it is because light is a wave, it's speed depends on the medium through which it travles (sound travles faster throught steel than water, and travles faster through water than air). This can bee confusing, however, since it suggests that a vacuum is very, very rigid.

    By the way, what does YIFF mean?
  17. Deja vu Registered Senior Member

    Here is an interesting recent article from Light has been slowed and now speeded up in labs.

    No thing goes faster than light
    Physics in Action: September 2000

    The observation of a light pulse leaving a gas-filled chamber before it had even arrived sparked a media frenzy, yet the laws of physics have remained intact.

    Nothing can travel faster than light. Despite a recent raft of reports in the media, this statement is as true now as it ever was. Nonetheless, experiments over the past 20 years have been forcing us to re-examine what we mean by the word "nothing". In the latest experiment, a group of researchers at the NEC Research Institute in Princeton, US, observed the peak of a laser pulse leave a small cell filled with caesium gas before it had even entered the cell (L J Wang, A Kuzmich and A Dogariu 2000 Nature 406 277). Apparently, the peak of this pulse is simply not the kind of "thing" to which Einstein's famous law applies.

    At almost 300 000 km s-1, the cosmic speed limit, c, is one of the most widely known constants in physics. A massive object needs infinite energy to reach c, while massless particles like photons always carry their energy at precisely the speed of light. More importantly, the relativistic notion of simultaneity makes it clear that no information can travel faster than light without throwing all our concepts of cause and effect into disarray. Relativity teaches us that if two space-time events are separated so that they cannot be connected by any signal travelling at c or less, then different observers will disagree as to which of the two events came first. Since most physicists still believe that cause needs to precede effect, we conclude that no information can be transmitted faster than the speed of light.

    Nevertheless, velocities greater than c can be observed. Suppose a lighthouse illuminates a distant shore. The rotating lamp moves quite slowly, but the spot on the opposite shore travels at a far greater velocity. If the shore were far enough away, the spot could even move faster than light. However, this moving spot is not a single "thing". Each point along the coastline receives its own spot of light from the lighthouse, and any information travels from the lighthouse at c, rather than along the path of the moving spot. Such phenomena are described as the "motion of effects", and are not forbidden by relativity.

    Long-held theories

    Figure 1

    In optics, the possibility of superluminal velocities was with us throughout the 20th century. The overall velocity (or "group velocity") of an optical pulse passing through a medium is determined by the way the refractive index varies for the different frequencies that make up the pulse. Since the peak of the pulse occurs when all the frequencies add up in phase, the peak can be delayed by a large amount if each component experiences a very different refractive index (see figure 1a).

    When the energy of the optical pulse differs from the energy difference between two electronic energy levels in the atoms of the medium (i.e. when the light is far from resonance), the refractive index increases with frequency. This "normal" dispersion reduces the group velocity below c. Roughly speaking, an atom may temporarily absorb a photon, even though the light is not exactly at resonance, and re-emit it some time later, thus slowing down the light.

    However, the behaviour of the light pulse is very different closer to the absorption line, where the refractive index decreases with increasing frequency. This behaviour leads to so-called anomalous dispersion in which the sign of the delay changes, which means that the group velocity can exceed c. This problem was treated in a classic analysis by Arnold Sommerfeld and Léon Brillouin, who pointed out that the strong absorption and distortion that occur at the resonant frequency generally make the group velocity a meaningless concept. They demonstrated that neither information nor energy can travel faster than light in this region. Throughout most of the 20th century, this was usually accepted as the last word on superluminal group velocities.

    However, the field was revived in 1970 by Geoffrey Garrett and Dean McCumber, then both at Bell Laboratories in the US. They showed that it should be possible to observe an undistorted Gaussian pulse with a group velocity exceeding the speed of light, or even with a negative group velocity, provided the pulse has a narrow bandwidth and the region though which it travels is short. This effect was dramatically confirmed in an experiment by Steven Chu and Stephen Wong, then also at Bell Labs, in 1982 (Phys. Rev. Lett. 48 738).

    Although Sommerfeld and Brillouin's conclusion - that neither energy nor information travels faster than c - remains valid, the group velocity is not entirely meaningless. The smooth Gaussian waveform is reshaped by the absorber, leading to a peak at precisely the time predicted by the group velocity. As for the energy, most of it is absorbed by the medium, and the sensible conclusion is that the transmitted energy comes from the leading edge of the incident pulse, which never travels faster than the speed of light.

    Conventional wisdom slowly began to adapt to the idea that superluminal group velocities need not imply that the pulses are extremely distorted, as long as most of the energy in the pulse is absorbed. This absorption makes it possible for the velocity of the energy propagation, like the velocity of the information, to remain less than the speed of light regardless of the superluminal speed of a peak.

    Experimental breakthroughs

    Over the past ten years, similar superluminal effects have been studied in connection with quantum-tunnelling experiments. In such experiments, the transmitted energy is once again quite small (R Y Chiao and A M Steinberg 1997 Progress in Optics XXXVII 347).

    In contrast, the NEC team creates a region of anomalous dispersion in a nearly transparent medium. Wang and co-workers do this by pumping energy into the caesium vapour to create a kind of optical amplifier. First a laser is used to pump most of the caesium atoms into a particular spin state. Next, two additional pump lasers are used to lend energy to the atoms. These atoms can amplify light from yet another "probe" laser by making an electronic transition in which they absorb "pump" energy and re-emit it into the probe beam. There are two specific frequencies at which such a probe can be amplified in this way. By replacing absorption with amplification, the NEC team can swap the regions of normal and anomalous dispersion (see figure 1b). A region halfway between the two amplification lines appears where there is little loss, amplification or distortion. Here the group velocity becomes negative and nearly constant. Indeed, Wang and co-workers measured a group velocity of -c/310. In other words, a pulse travelling a distance, L, is advanced by 310L/c.

    Figure 2

    The meaning of a negative group velocity is illustrated in figure 2. Within the cell, the peak of the pulse travels backwards relative to the direction it is moving in outside the cell. Long before the incident light pulse reaches the cell, two peaks appear at the far end: one travelling away from the cell at c, the other travelling back towards the entrance. This second pulse travels 300 times more slowly and is timed to meet up with the incident peak. The transmitted pulse travelling at c appears to leave the cell some 60 ns before the incident pulse arrives, enough time for it to travel an additional 20 metres.

    What is shocking is that such an effect has been observed for the first time without a great deal of attenuation, amplification or distortion of the pulse. It appears as though energy has, in fact, travelled faster than light.

    Of course, this is not the case. The effect observed at NEC only works in the presence of an amplifying medium, i.e. a medium that stores energy. In this case the energy is stored in the pump-laser beams. The caesium atoms are prepared in a state that allows them to transfer energy from these beams to the signal beam. The faster-than-light propagation occurs because the pump beams preferentially amplify the leading edge of the incident pulse, lending power to the signal and being repaid by absorbing some of the energy in its trailing edge. (It is important to note that even the dramatic 60 ns advance is only one fiftieth of the width of the pulse.) This is exactly analogous to the intuitive explanation of normal dispersion, except that in this case the atoms temporarily amplify the light pulse rather than absorb it.

    A fascinating suggestion is that this experiment might work even for a pulse composed of only a single photon. However, there has been a good deal of controversy over how to discuss the information transmitted through such a system by a single-photon pulse, and many subtle issues remain.

    Although relativity emerges unscathed from these experiments, our understanding of exactly which velocities are limited (or not) by c continues to evolve. And even though neither energy nor information is transmitted faster than light in experiments like the one at the NEC, it has already been proposed that the effects may one day be useful in compensating propagation delays in electronic systems.

    For the time being, physicists will kept be busy trying to clarify their intuition about relativity and learning how to accurately describe the information carried in real optical or electronic pulses.

    Aephraim M Steinberg is in the Department of Physics, University of Toronto, Canada
  18. Zero Banned Banned

    Oh well, in case you want a simple answer to the original question...

    the constant, c, is the speed of light in a vacuum. Light can travel more slowly depending on the medium. The statement, "nothing can go faster than light" is misleading. It should be corrected to "nothing can go faster than light in a vacuum." In other mediums, like diamond, sound travels more quickly than light.
  19. Boris2 Valued Senior Member

  20. ChaosUltima Registered Member

    Speed of light is an absolute constant, and the photon travels at the speed of light at all times in all frames of reference. You might say it travels slower in other mediums, such as water, or diamond, but in actual fact, if you look into at atomic level, the photon travels at the speed of light into the medium, is absorbed by the atoms in the medium, and is re-emitted again. The longer time the light takes to travel through a medium as compared to vacuum is due to the delay between the absorption and emmission of the photon. What do I mean by that? That means that, in vacuum, the photon does not have any atom "blocking" its path, so it is not absorbed by any particle of matter in vacuum. In vacuum, the speed of light observed is the absolute speed of light. In the medium, however, it might take, for example, half a second longer to travel through, so you see that the speed of light is much lower in that medium. When viewed in molecular level, the light travels from one atom to another at the speed of light, is absorbed for a certain amount of time, then re-emitted and it is again absorbed by another atom. When it exits the medium, the total amount of time taken by each atom absorbing and re-emitting the photon adds up to that extra half second. When it exits the medium, it has no more atoms to absorb it, so it appears to speed up again, but at all times, it keeps at constant speed.
    Since light is a massless object, it does not accelerate. It is just emitted and it just travels at the speed of light when it is emitted.
    Boris, I would like to point out that in the two articles, it seems that the speed of light in diamond is 77000miles/s and speed of sound is 12000m/s in diamond. The speed of sound is not greater than light.
  21. Satyr Banned Banned

    Only if the creator wishes it to.
    If He does not then light can travel at sub-lightspeed making it not light.

    God can also make black into white and white into black without us ever noticing a difference.
    God is weird that way.
  22. max7331 Registered Member

    Trying to understand photons, light and mass

    Although the nature of my question will lead most of you to see I know very little about physics, I still think it's necessary to point it out first.

    So what I am having trouble grasping is Light speed. My question is; Do photons have mass? When travelling at light speed wouldn't the photons increased mass prevent it from travelling at light speed?
  23. eram Sciengineer Valued Senior Member

    Why do I suddenly notice a bunch of new names in this thread?

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