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How can light be a constant?

  1. Sep 12, 2009 #1
    I understand that it makes the relativity theory work nicely, but....

    Whether it's a particle or wave, light must have mupltiple speeds.
    Light particles can be charged or excited which would increase the speed of the particle.
    Light also has different wavelengths, each with different oscillations and travel length.

    All this seem to support that light is NOT a constant.
  2. jcsd
  3. Sep 12, 2009 #2


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    Higher energy photons have larger frequencies (shorter wavelengths), not different speeds. This is determined by the quantum equation E=hf, where E is the energy, h is Planck's constant, and f is the frequency.
    Waves can have different wavelengths but the same speed; wavelength refers to how far apart the peaks of the wave are, speed tells you how fast a disturbance in the form of a wave actually travels (in the case of a wave with a single uniform frequency as opposed to a superposition of multiple frequencies, the speed is just the phase velocity which means it's the speed at which each of the peaks is actually moving).
  4. Sep 12, 2009 #3


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    No you don't understand. Relativity theory was developed because light speed is constant, not the other way around.
  5. Sep 14, 2009 #4
    Well in a way you are right. Light speeds are not constant.

    The speed of light through any given medium is constant (including vacuum).
  6. Sep 14, 2009 #5


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    I don't know if the OP was talking about the speed of light or light itself, as the title and
    I never heard yet of a charged light particle. Do you mean electrically charged?
    By the way, how would you excite a photon?
  7. Sep 14, 2009 #6


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    Most people who say "constant" mean "invariant", i.e. the same in all inertial frames. Only the speed of light in vacuum is invariant.
  8. Sep 14, 2009 #7
    No, most physicists who say constant in this case mean invariant.

    Most physicists though, don't assume an increase in wavelength means an increase in speed, most physicists would consider the rather obvious possibility of frequency decrease...I'm going to guess OP is not "most physicists."

    Just trying to get the point across that light does have different speeds in some cases.
  9. Sep 14, 2009 #8
    But yeah, light was measured to be invariant experimentally. So, no matter how much you like or dislike a theory, if nature kills it, She kills it. Light, nature says, propagates through a vacuum at the same speed regardless of frame of reference.
  10. Sep 14, 2009 #9

    I understand that highly charged photons are released by our sun constantly, but
    Gamma bursts should release light particles at faster than light speed.

    We know through observation that light is affected by gravity and also by heat.
    These two variables lead me to believe that light speed is not a set velocity.
    Especially when dealing with light from far off traveling through various gravitational and temperature changes, while still remaining in a vacuum.

    For example; If light can be sucked into a black hole, then it can also be repelled by an exploding star.
  11. Sep 14, 2009 #10


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    Please explain what do you mean by "charged photons".

    By heat of what? The medium the light is going through?

    I don't know Relativity but from what I've heard light is not "sucked" by black holes. Rather it always follows the same direction which can be curved because the presence of mass. (Anyone correct me if I'm wrong).
    Light is emitted from starts, not repelled.
    I suppose you believe photons to be electrically charged, right?
  12. Sep 14, 2009 #11
    *Example of a charged particle from the sun hitting our atmosphere is the Aurora Borielis.
    All electromagnetic radiation has particle-like properties as discrete packets of energy, or quanta, called photons. The frequency of the wave is proportional to the particle's energy. Because photons are emitted and absorbed by charged particles, they act as transporters of energy. The energy per photon can be calculated from the Planck–Einstein equation:
    where E is the energy, h is Planck's constant, and f is frequency.

    *I stated that the medium was a vacuum, the heat & gravitational forces would come from nearby stars.

    *Mass does produce gravity, but gravity is the force that will cause light to bend.
    The gravity of a black hole is so great that any light traveling too close will be drawn into the singularity, never to escape. Reguardless of direction of travel.
    Hence the reason I said the black hole "sucked up light".
  13. Sep 14, 2009 #12


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    This is your conclusion, what is your justification?
    Charged with what? Not electric charge as light carries no electrical charge. You can "boost" light i.e. increase its frequency. It sounds like you are confusing speed with energy or momentum.

    Or maybe you are confusing amplitude with speed. If you are talking about a mass on a spring or vibrating string then yes increasing frequency and energy will "speed up" the transversal motion. But the speed of light is not how it moves transversally but how fast the wave moves through the medium (propagation).

    Imagine a long heavy string stretched out horizontally. You strum is lightly at one end and the pulse moves down the string at a given propagation speed. If you watch a point on the string as the wave passes you'll also see a transverse (up and down) speed of the string itself while the wave goes by. Strum harder and you'll get faster transverse speeds but the propagation speed will generally be about the same.

    With light the transverse "motion" is not in any measurable dimension but rather a more abstract degree of freedom. There's no way to speak of the "transverse speed" only the amplitude of the transverse component (how strong the electromagnetic fields get at that point). (Note you can create an imaginary model of what is happening so that so that the EM field is a motion of some extra dimension [look up Kaluza-Klein theory] but again this gets far away from the usual meaning of speed of light as the speed of propagation.)

    Wavelength multiplied by frequency gives the speed. Double the frequency and halve the wavelength and you get a wave traveling at the same speed. Here you are implicitly assuming frequency is fixed when you assert changing wavelength implies changing speed and assuming wavelength is fixed when you assert different "oscillations" (which I take to mean frequency) implies changing speed.

    Assume instead that the speed is constant and you get that increasing the frequency implies decreasing the wavelength and vis versa.

    You can't argue the speed of light, you measure it. Go out and measure the speed of light you'll see it is a constant (in a vacuum). That's what makes it science and not religion. This has been done to the degree that we really can't measure distances precisely enough to test any further.

    One gets another version of the above three-way comparison. To measure a speed we must also measure a distance and a time. Saying one is constant is only meaningful relative to the other two. Since theory and practical measurement seem to agree with respect to the speed of light in vacuum we now adopt this as a mathematically fixed value and define distances in terms of how long it takes light to travel them. So now basically the speed of light (in vacuum) is defined to be a constant. This is a good definition so long as no dispersion occurs in vacuum which I explain next:

    Now some (actually most) mediums are called dispersive which means that different frequencies will travel at (slightly) different speeds through the medium. This is why for example a prism separates light into different frequencies. The different frequencies refract different amounts. The amount they refract is determined by the difference in speeds as they cross into/out of the medium. Since different frequencies change speeds by different amounts they refract at different angles.

    Dispersion is a problem for making good camera lenses and telescope lenses because you want the "red" "green" and "blue" frequencies to all focus at the same distance. Dispersive lenses will have different focal lengths for different frequencies. (Notice the infra-red mark on camera lenses indicating where they focus relative to visible light.)

    But the degree to which the vacuum is free of dispersion has been confirmed to the limits of current measurements. See the wikipedia article http://en.wikipedia.org/wiki/Speed_of_light" [Broken] for references.
    Last edited by a moderator: May 4, 2017
  14. Sep 14, 2009 #13


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    From wikipedia :
    , although it agrees with your statement, it clearly does not say that photons are charged particles like you previously said.

    Ah ok. So you say that heat by radiation can excite photons? (you exactly said :
    ). I'll wait for someone else to confirm/infirm your statement but as far as I know you won't affect photons by radiating them with other photons.

    I prefer letting others to answer in details your questions. I'm sincerely not qualified enough.
  15. Sep 14, 2009 #14


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    I think you may be confusing protons (which are charged) with photons (which are not charged).
  16. Sep 14, 2009 #15


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    Thanks jambaugh for the good explanation.

    I don't think he is. He considered "photons" as light and having wave-like properties. I think he believes photons are electrically charged particles, like he said twice.
  17. Sep 15, 2009 #16
    As a photon is absorbed by an atom, it excites an electron, elevating it to a higher energy level. If the energy is great enough, so that the electron jumps to a high enough energy level, it may escape the positive pull of the nucleus and be liberated from the atom in a process called photoionisation.
    Conversely, an electron that descends to a lower energy level in an atom emits a photon of light equal to the energy difference. Since the energy levels of electrons in atoms are discrete, each element emits and absorbs its own characteristic frequencies.
  18. Sep 15, 2009 #17


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    What has that got to do with this thread?
  19. Sep 15, 2009 #18

    Vanadium 50

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    That's true. And plagiarized from the New World Encyclopedia. And unsupportive of your claims.
  20. Sep 15, 2009 #19
    Look I'm trying to think outside of the box here, not quibble about semantics.
    Forgive me if I am not up to speed on fermion's and gluon's and everything in quantum mechanics as I have only completed the 7th grade.

    In my opinion physics is common sense, as long as you can identify all of the components and variables.

    We've all been taught that light travels at 186,000 miles per second.
    As does all electromagnetic radiation, including: radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.

    It's so fast that we can really only estimate the speed. Even if there are infinitesimal differences, say between red and blue light, it may not matter in the grand scheme of things. As we can really only guestimate the speed of white light.
    Such is the case with Pi commonly being shortened to two decimal places. You can have round and rounder... but it's close enough for most work.

    Maybe light is only affected by heat while in a medium, and the wavyness or mirage's occur because of refractive changes, which means the effect I witnessed is only optical and not actual.

    Gravity however does essentially "bend" the trajectory of light without a medium or phase change. Does that mean that light could have a mass? Maybe not, but how exactly does gravity affect light?
    Could enough gravity actually pull light in faster, could the force be strong enough to cause light to accelerate?
    Or could a massive stars outbound velocity of light be slowed until it breaks free of the gravitational waves? Sortof like going through a medium, but the medium is actually gravity in a vacuum.

    Conversly, why couldn't a super nova push EM energy or even mass out faster than 186,000 mps?

    It seem that special relativity says that not matter how fast I am going light will still travel 186,000 mps away from me.
    So if could be traveling near 186,000 mps and shine a flashlight in from of me. Would that light be traveling at twice the speed of light to a third party stationary observer?
  21. Sep 15, 2009 #20

    Vanadium 50

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    Son, before you "think outside of the box" you have to know where the box is.

    You've made a bunch of claims that are completely unsupported by evidence and observation. Doesn't it make sense to learn how the universe works first, before going around telling people that have studied this for longer than you've been alive that they are wrong?
  22. Sep 15, 2009 #21


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    "Common sense" isn't very useful in relativity, since we all have the same intuitive ideas about what the properties of space and time should be, and those ideas have been proved wrong.

    We usually call all those things "light". They are just different wavelenghts, but you know that already.

    There's no such difference in any of the current theories (classical electrodynamics, quantum electrodynamics, general relativity). It's of course possible that there's a (very small) difference in the real world, which would then have to be explained by some future theory. (I think loop quantum gravity has already predicted a difference),

    It doesn't affect light directly. It affects the geometry of spacetime, changing what curves through spacetime we have to consider "straight". Light travels along those "straight" lines.

    Something that's traveling along one of those "straight" lines (they're actually called "geodesics") is by definition not accelerating. Proper acceleration is (in special relativity and general relativity) a measure of the deviation from geodesic motion. Note however that a geodesic is a straight line through spacetime, but its projection onto "space" (which is a certain 3-dimensional subspace of 4-dimensional spacetime) is a curved path. "More gravity" would curve that path even more, but that doesn't mean that light can be accelerated. (You can always define a coordinate system on spacetime in which its speed is changing, but that doesn't mean that it's accelerating in an objective sense).

    Because massless particles (light) travels along null geodesics, and massive particles along timelike geodesics. This can be taken as the definition of "massive" and "massless". The energy required to accelerate a massive particle to speed v goes to infinty as v goes to c.


    No. Velocites in special relativity "add up" according to the rule

    [tex]u\oplus v=\frac{u+v}{1+\frac{uv}{c^2}}[/tex]

    assuming that they're in the same direction or opposite directions. Otherwise the formula gets much more complicated. The [itex]\oplus[/itex] notation isn't standard. It's just what I like to use.
  23. Sep 15, 2009 #22
    When it comes to thinking outside the box Einstein gets 10 out of 10.

    You are traveling at just below light speed, as well as any other sub luminal speed, relative to some objects in the universe. You are also stationary relative to others. The word relative is important and necessary.

  24. Sep 15, 2009 #23


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    Dead wrong.
    "Common sense" requires you to have experienced it before, for it to be common (you must have something to compare it to). When is the last time you hung out near a black hole or exploding star are few in a spaceship at the speed of light? Using your daily Earthly experiences to form your "common sense" will reap only tears and heartache.

    Among other things, GPS tracking systems currently in use in cars are dependent on us knowing the speed of light to a very fine degree. They can pinpoint a location within metres - a distance light can travel in a few billionths of a second. And that's just a $100 application. There are applications that cost billions; their precision requirements will be proportionate.

    No. Light is not "pulled" or "sucked" by gravity. Gravity bends space. Light follows straight paths through bent space. No matter how tortuously you bend the space, it will not alter the speed with which light travels through it.

    This is better. Asking questions, not making claims, is the way to enlightenment. Ask more questions. Make fewer claims.

    It would require an infinite amount of energy to accelerate any mass to the speed of light, let alone beyond it.
    The light would travel away from you at exactly c as measured by you. The light would travel away from you just slightly faster than you are moving as measured by an external observer. The difference in observations is due to time dilation due to differing frames of reference. This is Einstein's legacy.
    Last edited: Sep 15, 2009
  25. Sep 15, 2009 #24
    Hi Do you know if the GPS system applies the same speed evaluation for up signals as it does for down signals???

    Does this mean that the time dilation in a lightclock at a low gravitational altitude, say approaching a black hole would be the result of curvature???
    Would this somehow mean that the bending of spacetime would actually increase the distance of the reflection?
  26. Sep 15, 2009 #25


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    While that is technically true it is also true for any measurement. You always obtain an estimate. The real question is how close is the estimate.

    In 1979 the speed of light was measured by Baird and Whitford to be 299792458 m/s with an error (standard deviation) of only 0.2 m/s. That is a error (coefficient of variation) of 0.00000007%. That is a REALLY good estimate!

    In fact, it is such a good estimate that there was much more uncertainty in previous standards of the meter, so in 1983 c was defined to be exactly 299792458 m/s meaning that there is no uncertainty in the speed of light and there was instead a 0.00000007% uncertainty in the length of the meter. Further improvements in our ability to measure the speed of light have reduced the uncertainty in the length of the meter to about 0.0000000025% in typical laboratory settings.

    Differences in speed are usually easier to detect than actual speeds, so these types of experiments typically have even higher accuracy. As of 1999 Schaefer measured them to be equal to within a few parts in 10^21 using 30 keV and 200 keV light.
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