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Thread: Question about the speed of light

  1. #1
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    Question about the speed of light

    Any physics enthusiasts about that could help me with these beginners quandries?

    First question: In the equasion E=MC2, C is the speed of light. How can the speed of light be squared since Einstein's special relativity finds that the speed of light it the speed limit of any matter?

    Second question: if light moves on waves does that imply that light is moving faster than the speed of light because it is not only going the distance of point A to point B at the speed of light, but it is traveling down the dips and up the hills of the wave which should take longer than a straight trajectory?

    Thanks
    Darthe
    "A smooth sea never made a skilled mariner"

  2. #2
    Quote Originally Posted by darthe
    Any physics enthusiasts about that could help me with these beginners quandaries?
    First question: In the equation E=MC2, C is the speed of light. How can the speed of light be squared since Einstein's special relativity finds that the speed of light it the speed limit of any matter?
    Second question: if light moves on waves does that imply that light is moving faster than the speed of light because it is not only going the distance of point A to point B at the speed of light, but it is traveling down the dips and up the hills of the wave which should take longer than a straight trajectory?
    OK, I'll try...
    The speed of light is a number and any number can be squared. You're not increasing the speed in the formula you're just squaring a number.
    Physicists have been debating whether light is a wave or a particle for quite a while. Check out http://www.glenbrook.k12.il.us/gbssc...ht/u12l1a.html
    Light has both wave-like behaviors as well as particle-like behaviors (photons). If they are conceptually like particles riding on a wave like surfers on a wave then they can all be going straight and not going up-and-down like skiers over bumps.
    Google "light wave" and "light particle" for more info.
    Carl
    Last edited by carl; 04-17-2007 at 09:43 AM.

  3. #3
    Quote Originally Posted by darthe
    Any physics enthusiasts about that could help me with these beginners quandries?

    First question: In the equasion E=MC2, C is the speed of light. How can the speed of light be squared since Einstein's special relativity finds that the speed of light it the speed limit of any matter?

    Second question: if light moves on waves does that imply that light is moving faster than the speed of light because it is not only going the distance of point A to point B at the speed of light, but it is traveling down the dips and up the hills of the wave which should take longer than a straight trajectory?

    Thanks
    Darthe
    Darthe, great questions. Let me try to answer even though I am not a physicist.

    How can the speed of light be squared?
    • Saying that something is equal to C^2 doesn't mean that anything is travelling at speeds exceeding C^2. You can just as easily express the equation using square roots, i.e. (E/M)^1/2 = C.

    Wouldn't photons be travelling faster than the speed of light because they have to go from side to side?
    • Light is both a wave and a particle at the same time. One way of thinking about it is that the photon is vibrating or has a "field" of a certain width.

    Wise.

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    Sorry for the delay here, been focusing on traveling to the wound care clinic and then home to play with my granddaughters for a day.

    Thank you both for your answers. Now I totally understand, hehehe. I have started reading Physics Demystified by McGraw-Hill but it is a lot of math so I am plodding through. I just finished A Briefer History of Time by Hawkings and I found it very readable and interesting but I'm not sure that I retained much. I will definately be reading it through a couple of more times. I have gotten to page 25 of Relativity without much comprehension. Maybe I'll set that one aside for a bit.

    I do enjoy watching the youtube videos about relativity. There are many people there explaining different aspects, for instance this one - http://www.youtube.com/watch?v=V7vpw4AH8QQ&NR=1 - is a great little visual about the photon traveling through space and time. But so far this guy - http://youtube.com/watch?v=Gti_55JclpE - is my favorite. Although how he gets these things straight with a joint in his hand is a wonder. I guess he is everyones favorite with 17,970 views.

    Darthe
    "A smooth sea never made a skilled mariner"

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    Senior Member Foolish Old's Avatar
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    Though I find these "big" questions fascinating, contemplating the answers reminds me that they take me out of my educational depth.

    It seems to me that the speed of light is a measure of how long it takes light to travel from point A to point B without regard to what path it took to get there. Is this simplistic thinking?

    I understand that speed is determined by measuring the time it takes to travel a distance, and that a wave has a greater distance than a straight line.

    Still, when we measure how long it takes a runner to dash 40 yards, we don't take into account that, because she may travel from side to side and up and down, she may have run more than forty yards.

    Do we know if light maintains a constant speed during it's entire journey between two points?

    How about the same question applied to ocean waves traveling from shore to shore?
    Last edited by Foolish Old; 04-20-2007 at 07:33 AM.
    Foolish

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  6. #6
    There seem to be some misconceptions about waves.
    Think of a wave in water. When I plunk a pebble in a pond here at position A, a wave will travel to there at position B. The wave has traveled the straight-line distance, A-B. Nothing has traveled the total distance that you might measure on the water's surface where you include the "ups and downs" of the wave, which would be greater than A-B. In particular, no individual bit of water has moved away from its original horizontal position in the pond; it has just moved up and down.

    Water waves are an example where the wave appears as a disturbance of the medium that is perpendicular to the direction of wave travel. Sound waves are an example of a different kind of wave - the disturbance there is not a lateral displacement, but a compression of the medium.

    Light is different yet, in that it does not appear that light waves are a disturbance of any medium. This was a difficult concept to grasp, and originally it was thought that there must be some medium that light traveled through and made waves in - the "ether". However, experimentation has not uncovered any evidence of the ether's existence (Michelson-Morley experiment). Nevertheless, the same equations that describe water and sound waves also describe many of the attributes of light, thus in some ways light may be considered to be a wave.

    In other ways, light can be described in similar terms to those describing particles. The quantum mechanical equations describe a sort of hybrid of wave/particle. In the end, nobody really knows what light is. However, as it can be described amazingly well by the equations, we tend to conflate the description of something with the statement that we know what that something is.

    - Richard

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    Senior Member JeffH's Avatar
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    squared implies an area

    Quote Originally Posted by darthe
    First question: In the equasion E=MC2, C is the speed of light. How can the speed of light be squared since Einstein's special relativity finds that the speed of light it the speed limit of any matter?
    See if this makes sense.

    Remember from geometry that a squared number implies a measurement of a (surface) area.

    An explosion expands in a spherical fashion as little bits (particles) explode outward very quickly - in the case of an atomic explosion the speed of the particles (radiation) is the speed of light.

    The rate that the surface area of the sphere increases is the square of the exploding particles speed.

    So the equation could be interpreted as:
    energy = a bit of mass expanding spherically at the speed of light.

    Jeff
    Doh!

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    Quote Originally Posted by JeffH
    See if this makes sense.

    Remember from geometry that a squared number implies a measurement of a (surface) area.

    An explosion expands in a spherical fashion as little bits (particles) explode outward very quickly - in the case of an atomic explosion the speed of the particles (radiation) is the speed of light.

    The rate that the surface area of the sphere increases is the square of the exploding particles speed.

    So the equation could be interpreted as:
    energy = a bit of mass expanding spherically at the speed of light.

    Jeff
    Jeff! That is very clear, thanks. I think I do see now. And I see that trying to grasp the more advanced concepts of physics without an understanding of basic math formulas can get me in trouble. I was thinking 186k times 186k rather than height times width. I appreciate your clarity.

    Darthe
    "A smooth sea never made a skilled mariner"

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