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Relativity Stumper for you Einsteins

  1. Jan 11, 2008 #1
    It has been said one of the great mysteries of the universe is how light can be both a wave and a particle. Indeed this is lots of fun to ponder. But one thing I can not mesh into the e=mc2 equation is when matter approaches the speed of light, it expands and thus never quite reaches that speed. Well if light is part matter, how the hell does it defy this law and get to be so fast? Do light particles expand as they travel? Please enlighten this freshman physics afficionado...
  2. jcsd
  3. Jan 11, 2008 #2


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    Light is massless and its speed is equal to a fundamental constant.

    It does have a momentum however.
  4. Jan 11, 2008 #3
    I believe you're assuming that for it to behave as particle under some conditions, it must have mass; after all marble, grains of sand or salt all have mass. But whether its a great mystery of the universe, or only puts into stark relief, how our imprint of experience and perceptual apparati limits what we can imagine. I don't want to get too Eastern woo-woo here, but really IMHO the greater mystery is how as humans with 5 senses and a fairly primitive brain, have the arrogance to imagine that all things in the universe should be easily captured by the familiar for which we have words. Infinity and eternity are both good examples to ponder--try as we might these are concepts that are easy to say we can embrace, but much harder to really wrap your mind around. A lot of relativity strikes a counterintuitive chord for which there is no recourse but at some point to trust in mathematics and physical experiment. At a deep level, tho, it is supremely logical. This is the triumph.
  5. Jan 11, 2008 #4
    The relativistic mass of an object traveling at close to the speed of light is really just its kinetic energy, which approaches infinity as speed approaches the speed of light for an object with rest mass. Relativistic mass confuses a lot of people. It's easier to think of an object traveling at a high speed as having a high kinetic energy, which can be turned into mass by e=mc^2. Kinetic energy is a function of rest mass and velocity. Photons have no rest mass, they are never at rest, so they don't have kinetic energy in this sense (although photons can certainly be formed FROM kinetic energy or be turned INTO kinetic energy)
  6. Jan 12, 2008 #5
    The wave-particle dualism of light is an example of a problem in science that has no simple solution. Maybe in the future, a simpler explanation for the properties of light will be found. In the meantime, the idea of a dual concept is not unreasonable in view of our present knowledge. Furthermore, we live with and frequently tolerate dual concepts in other areas of our culture. For instance, there is a fable of the three blind men who were asked to describe an elephant. One grasped the tail and declared the elephant to be a limp rope. Another put his arms around a leg and said the elephant is a tree. The third man felt the hide and decided the elephant is made of sandpaper. To the extent of their observations, each man was right. Similarily, depending on the type observation we make of its nature, light appears to be a wave phenomenomor a stream of particles (photons).
  7. Jan 12, 2008 #6
    Maybe I missed something, but the question seemed to me to be essentially a special relativity question, even though he mentioned wave/particle duality. He was asking why light can travel at the speed of light if it has mass. The answer is that light doesn't have REST mass.
  8. Jan 12, 2008 #7


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    When objects approach the speed of light they don't change in size or even in their "proper mass" (the mass that they get when they measure themselves), what happens is that observers stationary relative to the object see that objects mass is growing by some factor - not getting bigger in size. But light doesn't have any mass at all so even multiplying it by some factor makes it stay at zero.
  9. Jan 12, 2008 #8


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    Not true! For [itex]v\rightarrow c, ~\gamma \rightarrow \infty[/itex].
  10. Jan 12, 2008 #9


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    You're right, I guess you actually have to take the limit.
  11. Jan 16, 2008 #10
    It doesn't have to be either, that is just how we describe it given our current understanding of its properties. The future may invent another descriptive label that better explains it.
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