Re: Questions (Space)

Sea Wasp <seawaspObvious@xxxxxxxxxxxxxxxxx> wrote:

Tina Hall wrote:
David Friedman <ddfr@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Tina_Hall@xxxxxxxxxxx (Tina Hall) wrote:

If there isn't anything, there can't be anything that travels. Not
solid doesn't matter, as long as you explain what else it is. Right
now, I don't have an alternative to 'solid'.


What there is is a magnetic and electric field, at right angles to
each other


Do you want me to get a headache?

(I have no idea what you're trying to say, it looks just like those
things that don't explain anything and just lead to frustration. If you
want to talk about this, explain it in a way I understand.)


and to the direction the light is traveling.


The light that travels has to be something. There's something that first
hits the cupboard, then my eye, for example. Or comes from a distant
star.

Energy.

[Tina, what I'm about to add is not a direct answer to your question.
It's a digression, commenting on something the people replying to you
said. Read it if you want, but don't get annoyed that it's not answering
your question, because it isn't supposed to.]

It may be true that energy is travelling, but that's, at some level,
incidental. (And I know, or used to know, about Poynting vectors and
stuff like that: I'm making a more fundamental, possibly sfnal, point.)

Analogy: if I throw steel ball-bearings at you, the ball-bearings are
travelling, and so is their kinetic energy, but the energyy transport is
incidental. It might help (and might not) to compare the situation if I
throw them at different speeds, or bearings made of different stuff.

The point is, that it's the little balls of metal that are the things
that move. If you think of light as made of photons, it is the photons
that move, the energy transport is incidental (because mass equals
energy, like Einstein said).

For a wave motion, *nothing* moves from A to B. If you think of light as
an EM wave (instead of photons), then nothing physical moves. But waves
can still transport energy (compare ocean waves: you can build wave
power generators to extract energy from them, but the water doesn't
travel across the Atlantic: all the actual water particles move in
circles).

So since light is both particle and wave, it both travels and doesn't
travel. At this point, anyone whose head isn't hurting hasn't understood
the problem! :-)

For some other examples of things that move without travelling: the spot
of light on an (old fashioned, cathode-ray tube) TV. The spot of light
"moves" across the screen, painting the picture. But actually, nothing
physical moves across the screen at all (there are electrons travelling
down the length of the tube and hitting the screen, but that's
different.)

Another example is a giant, light-years long, pair of scissors. It's
quite within the realm of possibility for them to cut a giant,
light-years wide, *** of paper faster than the speed of light. The
physical scissor-blades can't move faster than light, and neither can
any part of the paper, but the cutting-point can.

Since FTL travel leads to paradoxes, I sometimes wonder what this would
look like. I guess from the right vantage point, and in a spaceship
travelling with a suitable velocity, it would look as if the giant
scissors were *un*cutting the paper, seamlessly stitching two pieces
back together.

Final example: shine a (bright) laser at the moon: if it is focussed
enough, it will make a spot of light (which you could see in a
telescope). With a flick of the wrist, you can send this spot of light
dashing across the moon's surface faster than light - because the spot
of light isn't physical.

It's even possible to get light travelling faster than light (phase
velocity) but not to transmit energy or informtion faster than light
(group velocity). {Phase and group velocity are keywords to google on,
for anyone who wants to verify I'm not making this stuff up.}

Jonathan
.

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