Re: SCI: slow light

Andrew Trapp (
Tue, 23 Feb 1999 16:32:25 -0600

Hash: SHA1

Allow me to delurk for a bit. Getting back to the start of this thread, not too long ago KPJ <> wrote:

>It appears as if <> asked:
>|However...that leads to a question. If light is "slowest" in a
>|Condesate...and "fastest" in a there anything "less"
than a
>|vacumn? It is possible to slow light it even theoretically
>|possible to speed light up?
>|No clue here...a vacumn is a vacumn right?

>It appears if <> answered:

>|Yes. It is. c in a vacuum is as high as anything can go. If you put
>+ energy
>|into the photon, it simply increases in frequency. This is why the
>|knew they had measured an extremely slow speed in the condensate,
because if
>+ they
>|had just drained energy from the light, it would have changed
frequency. What
>+ it
>|did do was retain its energy at the same frequency, which told them
they were
>|getting a refractive phenomenon.

>If ``we'' could create a specific matter/space configuration with less
>density than vacuum, then ``we'' would need to update the theory to
>handle this new case, as it probably would allow for FTL motion.
>As far as I know, nobody has discovered such a case yet.
>Therefore, currently it appears as if nothing can move faster than c.

Ah, but we CAN create regions of space-time with less density than "normal" vacuum. Ever hear of the Casimir effect? When two conducting plates are placed a small distance apart, the energy-density of the vacuum between them decreases to less than the energy-density of regular interstellar vacuum. This is because normal vacuum is filled with virtual particles, constantly popping into and back out of existence, which is in turn due to the Heisenberg uncertainty principle. The two conducting plates prevent any virtual particles from popping up between them which have modes (wavelengths, essentially) greater than the separation distance between the two plates. As a result you get the vacuum on the outside of the plates applying a measurable force attempting to press the plates together, and this force increases (exponentially?) the closer together the plates get. It is also dependent on the surface area of the plates.

This is directly analogous to having two tanker ships next to each other. Normally, waves press against the hull of a ship about equally from all directions (excluding any surface currents). When one ship pulls alongside the other though, each ship blocks waves that would normally exert pressure against one side of the other ship's hull. As a result the two ships feel an attraction. The closer together the two ships get, the more the waves between them can only travel parallel to the ships and thus the less the force keeping them apart is.

Now as to whether decreasing the energy-density of a vacuum will allow light to travel greater than c I don't know. On the one hand, negative energy densities seem to be a prerequisite for building any kind of nontrivial wormhole. (Although wormholes do not explicitly allow FTL travel.) I also recall it being required for Alcubierre (sp?) engines, which in theory can travel faster than c as measured by outside observers. So it might be a possibility that light can travel faster in a smaller energy-density vacuum. But I am not a physicist or physics major and my knowledge on this subject is mostly limited to what I've read on my own. (Though I have done _some_ research on the Casimir effect and wormholes.)

OTOH a smaller energy density might not have any effect on the _speed_ of light. Photons racing away from a massive source lose energy, resulting in a redshift; but their speed remains the same. Thus it seems to me that if you had a source of negative mass, light speeding away from it would undergo a blueshift, an increase in energy, but there's no reason to believe it would increase the _speed_ of the photons. So how comparable are the effects of negative mass with those of negative energy? About the same as between positive (normal) mass & energy, I would think.

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Andrew Trapp                     +--Atheism         +--Libertarianism <=-----+--Futurism
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