A question about rays of light

<b>phlogistician</b>:

<i>But the cpacitance of a fixed system has a fixed value, so all you are saying is that if we choose U, then that changes the dielectric value for the system, according to whatever units we decide to use.</i>

It doesn't change the physical characteristics of the system, of course. It only changes the numerical values given to them.

<i>The point is, that all of these physical constants are not in isolation, each one has a relationship with another.</i>

Yes, and the basic choices you need to make to uniquely determine a system of units are values of c and u (in vacuo), as I said.

<i>Saying we set an abitrary value for one (U) and saying that value is meaningless rather misses the point that they all interact. Tweak one value, they all change.</i>

I didn't say it was meaningless. I said the choice of u in a vacuum is arbitrary (except that it can't be zero).

<i>Now, if it was possible that the values could be tweaked so U was zero, then would that not be a more 'common sense' value to describe vacuum? Could that be done, ......?</i>

No. As I said before, Maxwell's equations only hold for non-zero values of u.


<b>Greco</b>:

<i>Okay so far, so if if I pointed a laser at an oscillating strong EM field and looked at the laser beam with polarised glasses I should be able to see the laser oscillating at the rate of the EM field oscillations, is that correct?</i>

No, because the wavelengths of the oscillations are too small, and the frequencies are too high for your eye to see the oscillations. You'd need some other kind of detector - such as a photomultiplier tube.

<i>If the answer is yes, next question is, why cant a magnetic field bend a light beam instead of just rotating it. Isnt that what happens inside a CRT? I understand that the beam is made up of electrons but dont moving electrons create EM fields? </i>

The light for a CRT is created when electrons hit the phosphor coating of the screen, depositing energy, which gets converted to light. The steering of the beam involves steering the electrons. Electrons are charged particles, and so are affected by both electric and magnetic fields. Light photons, on the other hand, are uncharged.

 

phlogistician:

<i>Hoorah! We're there! The formula for determining the relationship between the values is fixed by the physical system. The units are arbitrary, but the value of u will never be non-zero for a vacuum, no matter what units we choose, no matter what arbitrary values we choose for c.</i>

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That's what I've said all along.

<i>Does it make sense that _nothing_ has an attribute with a value?</i>

Well, it also makes sense to say that two points in empty space are separated by a distance of 3 kilometres, or that the speed of light in empty space has a non-zero value, so it's not really that surprising that we can associate certain values with "empty" space. But it is a jump from there to draw the conclusion that empty space is a medium.

<i>So, does the fact that _nothing_ has a speed limit of c make sense?</i>

Space doesn't have a speed of limit of c. Objects travelling in space have a speed limit of c. That has more to do with the objects than the space.

<i>Doesn't matter how arbitrary we think the values are, the implications are significant.</i>

That's where we differ, isn't it?