For Jeff: Special relativity?

>> Tuesday, August 4, 2009

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Jeff King asked: So my question is this can you explain Einstein's special relativity in a way that us mere mortals can understand… because I fail to see how light could pass you at 186,000 mi/sec if you are traveling at 186,000 mi/sec yourself… but yet Einstein's special relativity states “The speed of light in vacuum has the same value c in all inertial frames of reference.” Meaning light or your perception is not changed upon your perspective or you relationship to it. Basically regular theory of relativity…

Whew. Give me something easy. Special relativity.

OK, here's the thing (i.e., the disclaimer). This disclaimer is I'm not a theoretical physicist and not qualified to provide an in-depth explanation of anything quantum-wise. Just the layman's version. And that means it might not be perfect.

You asked about the immutability of the speed of light. Well, here's how I understand it.

The Principle of Invariant Light Speed – Light in vacuum propagates with the speed c (a fixed constant) in terms of any system of inertial coordinates, regardless of the state of motion of the light source.

Now, in your question is a statement that seems to contradict the theory, that light would pass you if you were going the speed of light (and, of course, it would if it was going any direction other than the one you were going). The key elements of this are that light always goes the same speed whether it is emitted from a "stationary" source or a moving source. That does not mean that it would necessarily appear to be the same speed relative to someone else's travel.

What does that mean? That means light does not act like mass. A bullet, if you shoot it from plane going faster than the speed of sound, will go faster than a bullet from a stationary target. The muzzle velocity is "added" (more or less) to the velocity of the muzzle for a final speed that's more than either. But, relative to the speed of the plane, the bullet is going the same speed as the stationary bullet. These are items of mass and are governed by general relativity.

Now, for most of us, we're going slowly enough that a bullet hitting you at 800 m/s and a bullet hitting you at 1143 m/s doesn't sound different (though they impart different energies), but the damage potential is different. Now, if the plane is going close to the speed of light, the bullet can't go faster than the speed of light, according to the theory.

Light is special. It always goes the same speed. If you are going just short of the speed of light and turn on the headlights, the light will be going 186,000 mi/s even if you're going 185,000 mi/s. Why? I've got this quote that might help.

God runs electromagnetics by wave theory on Monday, Wednesday, and Friday, and the Devil runs them by quantum theory on Tuesday, Thursday, and Saturday.
-- Sir William Bragg
Does it? Well that's because it doesn't make sense, not for people working in the macroscopic world. Photons aren't really matter and they aren't really energy. They're both and we don't have a real handle on what that means - just models and theories. We know how light behaves under certain circumstances and how it behaves in other circumstances. We have models that predict that behavior quite well, but not a model that predicts all its behavior. But it doesn't behave like regular things.

Now, when it comes to the speed of light and us regular folks going mundane speeds, the fact that the speed of light is always the same doesn't really mean much. But it means energy evaluation of light is different than it is for matter. Kinetic energy is often the determinant of energy for matter, but all light goes the same speed, yet it has different levels of energy that ends up indicated by wavelength.

Now, you mentioned space time impacts and more - I couldn't fit all that in a blog, but I'm also not sure I'm telling you anything useful. We have a large number of theories to try to explain the way quantum particles, photons, neutrinos, etc. behave. And some of them (or more) could be right. Some of the side effects of special relativity like E=mc2 and time dilation have been demonstrated - but I don't think we truly know why. We've seen things behave counter-intuitively and much of quantum physics is all about finding tools to predict this behavior as much as possible (Heisenberg uncertainty principle notwithstanding). But, when it comes to why and what the real implications are of that behavior, what particles we haven't discerned yet and what that means, how all of this ties with other things we can't explain out there like apparent dark matter and the like, we're just speculating. In a very systematic and educated way, but we're just scratching the surface.

I suspect that, two hundred years from now, we'll probably look back on our understanding of today and feel much like we feel when we look at hygienists and biologists from two hundred years before now.

Want the two cent explanation: light ain't matter and doesn't act like it is...much. Until we understand why, we probably can't go faster.


  • flit

    something in here reminded me of neutrinos again - must be the "particles we haven't discerned yet"

  • The Mother

    I highly recommend Brian Green's book, The Elegant Universe. His insightful analogies really make this esoteric stuff (mostly) understandable.

  • mmfiore

    For another more sensible take on how reality works check out this site. Super
    The Theory of Super Relativity: Super Relativity

  • Stephanie B

    There's a reason I don't bill myself as a theoretical physics. If this is a topic that intrigues you (me, not so much), I urge you to take advantage of the other resources my commenters have provided. They always come through when I'm not complete.

  • Jeff King

    thx for the time for looking into it... i watch a film on it and made my mind hurt thinking about it. i will just say they think this is true and like you said 200 years from now it might be completly dif from now. thx again

  • Xavier Terri

    Teoría Conectada: The best physics theory since 1687

    The new Copernican revolution

    I read Lee Smolin's book ‘Las dudas de la física en el siglo XXI’, 2007, Ed. Crítica. Wonderful. I have seen that Lee Smolin are looking for a new big idea, the fundamental simply idea for the progress and unification of physics. Seems that Smolin got the conviction that both quantum mechanics and GR theorys don’t understand the deep nature of time (page 355). It is right for GR unless.

    Here is what i say:
    No more Lorentz’s Transformations. The new alternatives transformations (’relational transformations’) are deduced on ‘LA PARADOJA DE LOS GEMELOS DE LA TEORÍA DE LA RELATIVIDAD ESPECIAL DE EINSTEIN’, f.i., equations (22) and (23) with “C” and “D” given by (42) and (43) , pages (33) to (36). From them arises the ‘teoría relacional’, an alternative to special relativity.
    The generalitation of this ‘teoría relacional’, the unique possible classic alternative to GR, appears on ‘EXTRACTO DE LA TEORÍA CONECTADA'. 3 are the fundamental equations. (84), (171) and (172), pages (146) and (182). This 3=24 equations are necessary in order to eliminate the Newton-Einstein’s absolute space. There are not absolutes accelerations (neither absolutes velocities, of course).

    The Dark Matter problem is solved in ‘Apéncice C’, page 205 (Exponential factor gets important at large distance from the center).

    What about a “quantum teoría conectada”? I believe that you can say something important about it.

    Xavier Terri
    Terrassa 2009 august 6

    P.D.: Smolin is completely certain when he tell that the great historic mistake comes from Descartes-Galileo ('Las dudas de la física en el s.XXI', pages 355-356). This great historic error is the Principle of Inertia (movement is a relational concept. It is completely false that the movement of ALL free bodies is a straight line. Some of them move, respect the SAME reference system, in a curved way. Spacetime metric defines a specific relationship betwen bodies and reference system.Pure evidence beyond the reason: at night, see the stars). Also, Principle of Inertia leads us to the inertial-non inertial dichotomy. Where is INVARIANCE of physical laws? (pages 45 on 'Paradoja' and 141, eq. (77), on 'Teoría Conectada').

  • Stephanie B

    See, Jeff? Now you know who to ask next time.


    great blog keep up the good work

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