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In school, I've been taught that waves follow the path/shape of a Sine. Often demonstrated by either a projection of a circle or by a swinging pendulum or shaking string.

I guess those are simple models with great academic value. But since we do live in a 3 dimensional space, I assume that in reality the path of light follows a spiral.- (However, I can't seem to find any source that confirms this.) After all water also prefers a spiral path when it flows down the kitchen sink.

My 1st question: does light indeed follow a spiral/corkscrew shape ?

Regardless of that, the speed of light is limitted. But since the light does not travel in a straight line. There seems to be an internal speed which is higher. i.e. If the amplitude and/or frequency are increased, then the net speed is still limitted (to c, the speed of light), but the "internal speed" along the corkscrew shape increases. Well, maybe.

My 2nd question: what is the internal speed limit of an electromagnetic wave?, if there is one

This is the point where my head really starts spinning. What I'm really wondering about now, is that even in that case, the speed of light may not really be broken, even for the speed along the spiral path. The photon has its own clock, he may still perceives that he is moving at the speed of light, everything just appears to move a bit slower from his point of view. ... Or the spacetime may even be "curved" in some special way so that this wave doesn't really break the speed of light ? That's where I'm lost. Is this just nonesense?

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  • $\begingroup$ Also, if you "can't seem to find any source that confirms this" shouldn't that strongly indicate that your assumption is incorrect? $\endgroup$ – Kyle Kanos Jul 16 '17 at 20:06
  • $\begingroup$ Related: physics.stackexchange.com/questions/64513/is-lights-path-a-wave physics.stackexchange.com/questions/93893/… and others I'm too lazy to hunt up. You're far from alone in the misconception expressed in "I've been taught that waves follow the path/shape of a Sine". I've seen presentations of ways to avoid giving student that impression, but the diagram we us as an idiom seem to lead some people astray by their very nature. $\endgroup$ – dmckee Jul 16 '17 at 23:04
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Light follows a straight line. Period. In fact, this is defined to be so in General Relativity.

The sinewaves you are used to seeing are not moving up or down, or left or right. They are representing the strength of electric and magnetic fields as the light beam travels in a straight line. The strengths of these vary sinusoidaly, while the light itself travels in a straight line. It's actually quite a common misunderstanding, because it's so much easier to demonstrate what a wave looks like if you're showing it as displacement (up/down) rather than as a strength of the field.

As a result, there is no "internal speed" of light. It' just traveling in a straight line.

Once you're comfortable with the idea that the variables being plotted are field strengths, rather than simple displacements, we can discuss polarity, which may be of use to you. There's two ways light can be polarized. Linear and circular. This really is just showing how the electric and magnetic fields vary in strength with respect to each other. However, one of the graphs is a spiral, which may be where you got the idea that light moves in a spiral in the first place. Wikipedia has a great visual animation showing the progression of the wave. It shows that light clearly travels in a straight line, even though the graphs of the strengths of the fields vary sinusoidaly.

Linear Polarization
Linear polarized light graph from hyperphysics, showing the field strengths in the "electric field axis" and "magnetic field axis."

Circularly Polarized Light

Circular polarized light graph from Wikipedia, showing how the direction of the electric and magnetic fields can vary over time.

Graphs can fool you. It's always good to understand where they came from, and what the axes mean. The circular polarized graph sure makes it look like light travels in a spiral, but when you dig into the meaning of the axes, you see that that is not true. The "light wave" is actually traveling down the center (where all the arrows are coming from), and the arrows are showing the strengths of the fields.

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  • $\begingroup$ Thank you for clearing that out. Am I correct that the electric and magnetic fields described above are the ones of the Lorentz force ? - Thanks. $\endgroup$ – bvdb Jul 17 '17 at 7:51
  • $\begingroup$ Yes, those are the fields I am referring to. $\endgroup$ – Cort Ammon Jul 17 '17 at 14:04

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