Listings Question About Ping

[Marko Vojinovic]

On Monday 26 December 2011 10:31:02 les wrote:
> On Sun, 2011-12-25 at 17:12 +0000, Marko Vojinovic wrote:
> > Wow, look, another OT thread to contribute to! :-D
[snip]
> > [me loading extension_Relativity... skipping: already hard-coded]
> > 
> > In any discussion related to theory of relativity it always helps to
> > have an expert around --- so you can consider yourself lucky. ;-) Since
> > this part of the thread is already completely OT, feel free to ask
> > whatever you like about relativity, I'll try to respond as long as I
> > don't become too busy with real life stuff... :-)
> 
> Hi, Marko,
> 	This is a great offer.  I have a smattering of knowledge about some
> things, and complete blanks about most in physics.

Glad to help out! :-) If you have many questions, maybe it would be a good 
idea to take it off-list, since otherwise we might get bashed for contaminating 
the Fedora archives etc... ;-)

> 	One of the recent things I became aware of is something called
> ballistic transport of electrons in carbon nano-tubes.  I read somewhere
> (and don't remember where) that electrons in carbon nano-tubes appear to
> exceed the speed of light.  I have made really precise measurement of
> various electrical things, such as 500femto farad capacitors, and
> currents into hundreds of picoamps, along with time measurements
> resolved into the attosecond range, so I realize that there is a lot of
> capability out there, but how is it that the speed of an electron
> through a carbon nanotube can be measured repeatedly at speeds greater
> than light speed?  I have a few ideas, but I haven't read any articles
> on how the actual physical measurement was made.

I am not familiar with the details of the measurment process those folks use, 
but in general --- I would do "something" (like bombing the nanotube with an 
electron gun) to accelerate the free electrons past the speed of light in the 
nanotube, and then try to capture the Cherenkov radiation it emits. You can 
read more about the latter on

  http://en.wikipedia.org/wiki/Cherenkov_radiation

Measuring the Cherenkov radiation should be fairly simple in principle (it's 
just detecting light coming out of the nanotube --- using for example 
photomultiplier detectors, or whatever else is suitable), though the actual 
frequency of that light may vary according to how fast I can make the 
electrons travel through the nanotube. If they move fast enough, the nanotube 
should just "glow", similar to the "blue glow" of a nuclear reactor core.

However, being a "theoretical" physicist, I cannot say whether the above 
"thought experiment" construction would be feasible. Maybe those folks are 
doing it differently (also more efficiently and more precisely, I guess). There 
is usually more than one method to measure any physical quantity, so...
Hence, since I am not familiar where did you read about that result, I cannot 
comment on any actual experiment (and I am not an expert in carbon nanotubes 
either :-) ). But the description above should give you a general idea how 
these things could be measured in principle. :-)

All that said, I should just make a small-and-obvious remark that none of the 
above contradicts theory of relativity in any way. Relativity deals with the 
speed of light in *vacuum*, whereas the speed of light in various materials 
(including nanotubes) can be significantly smaller, and various particles (such 
as electrons) can easily be pushed to outrun it. But both speeds are still 
smaller than the speed of light in the vacuum, in accord with special 
relativity. Just a comment to avoid any possibility of confusion in that 
regard. :-)

Best, :-)
Marko