Hello piclisters,

It's funny for me to make my debut on this list so far off topic, but
since it's been more than a year since I last blinked my LED and I kept
lurking here mainly for the OT discussions, perhaps it's actually
appropriate, especially as my current occupation is studying
particle physics.

On Mon, May 22, 2006 at 07:18:34PM +1200, Russell McMahon wrote:
> 
> And Dave has touched on one of the great beauties that I return to 
> marvel at from time to time. And fail to understand, frame of 
> reference wise or otherwise. viz: All photons are joined together in 
> an instantaneous intercommunicating 'web' throughout the universe. 
> They all dwell in the eternal now. They know no time or essentially 
> space. It is far easier to make slightly (or very) incorrect 
> statements about them which help to illuminate, or confuse, or both at 
> once, than to make categorical statements of fact. Well educated 
> particle physicists (whose ranks I am far from, and will never be 
> counted amongst) may believe differently. They may just possibly be 
> correct. Just possibly. A photon leaving Alpha Centauri Proxima 
> (second closest star to earth, one of the two pointers to the Southern 
> Cross for fortunate Southern Sky viewers, and another leaving Sol 
> (closest star to earth) at the same "time" will arrive at a viewer at 
> the same 'time' as each other and instantaneously, and will never 
> actually 'go' anywhere, as everywhere is everywhere else and there is 
> no here and there and there is no time - all from their own 
> perspective of course, and is there a more valid one? For us there 
> will be a time difference between their arrivals of about 4 years.
> 
> There. Was that poetic enough for such an occasion ? :-).

It was beautiful, but not nearly lyrical enough. Being a photon is as
lonely as it gets - not only is the moment of your birth indistinguishly
intertwined with the moment of your death (and if you happened to be
diffracted by a prism, scattered by an atom, or tunneled through an
optical fiber, than with those as well), the zero time differential will
also leave no way to communicate. The two photons take each zero time to
arrive, but it's not the same time, and even if it was they'd have no
way to know that. That is also the problem with the 'instantaneous
intercommunication web' description - it isn't instantaneous except for
the photons themselves, and even then only for one at a time. From any
other point of view the light isn't going neither quickly nor slowly, but
exactly at the speed of light.

I think the crux of the uneasiness here is the expression 'the point of
view of the photon', since having a point of view implies a process of
perception and cognition which in itself implies a flow of time, time
that the photon doesn't have. And dividing zero distance by zero time
gives you a processor fault, not understanding. The way to approach
this should be to take the limit of what happens when you accellerate
a massive object more and more. Its mass will increase, time will slow,
space will shrink, communications will become more difficult (through
Doppler effect and syncing problems), and on the limit this is the state
of the photon.

> I just know someone is going to complain about at least one part of he 
> 'physics' back there.
> 
> And fwiw and AFAIK and IMHO and I'm happy to be corrected if wrong - 
> re other statements made:
> 
> All massless particles always travel *AT* C, never near it. They can 
> only travel at C and MUST do so to be massless. They all have zero 
> rest mass and their apparent masses are related to their energies in 
> any frame of reference except their own. (! :-)).

Indeed. At the photon's frame its frequency is 0, and since there is
nothing there that could be used as ground reference it isn't even a DC
offset. At its own realm, the photon doesn't exist.

BTW, this talk of the photons frame of reference suffers from another
drawback - a frame of reference defines specific location and velocity,
while the photon may have none to offer. Take a simple dipole antenna,
for example - the waves it produces propagate in all (lateral)
directions equally. If we tune it low enough to emit one photon at a
time, that photon will propagate in the same manner. You can define a
frame of reference that will follow the photon in a specific direction,
but not for all of them at once.

> Just because the speed of light varies in different media it doesn't 
> mean it isn't constant in the way that constant is meant by those who 
> use the term usefully :-). Cerenkov radiation (which even budget 
> equipped amateurs can detect and measure from incoming gamma rays) is 
> the result of photons (usually the ones termed Gamma rays)(photons on 
> speed) desperately adjusting their terms of reference to conform to 
> reality. (ie extra lower energy radiation 'shed' to allow the high 
> energy photon to 'slow down'. ) It's also the cause of the nice 
> actinic blue glow in radioactivity storage pools (as simulated in 
> suitably budgeted movies) as photons adjust themselves to the 
> discovery of the pool.

Cherenkov radiation is _not_ produced by photons, only by electrically
charged particles, if they happen to travel faster than the speed of
light in the medium. This is the electromagnetic equivalent of a sonic
boom - the field created by the charge changes too quickly for the local
atoms to adjust, and they radiate when the return to equilibrium. The
only way photons can generate Cherenkov radiation is if they are
energetic enough to create a pair or charge particles that are fast
enough to exceed the speed of light in that medium.

Wikipedia has a nice page about this with some (surprising) details.
> 
> C and the speed of light are one and the same. Declaring that they are 
> different may make everything make a lot more sense, but reality 
> doesn't know about this :-).

c is shorthand for the speed of light in vacuum, a semi-mythical beast
since a pure vacuum cannot exist. In that sense, light never reaches the
speed of light :-)

> Photons are 'real' particles - but they have no mass so they (like 
> particles which have rest mass) cannot be accelerated to the speed of 
> light BUT with photons they are already there and with the others they 
> can never get there.
> 
> Expansion of the universe or no (shaky ground here) nothing moves 
> faster than C relative to anything else in out universe *in a manner 
> that can be usefully or meaningfully expressed in terms which 'make 
> sense' in our reality*. ie you can (and apparently do) have particles 
> which are coupled instantaneously at any distance and you can make 
> measurements which prove that this has been the case or even that this 
> is currently the case or even that it will be the case BUT you cannot 
> use that knowledge to make any use of the FTL or instantaneity of the 
> coupling.

I don't think the coupling between different particles (I assume you mean
the-effect-formerly-known-as-the-EPR-paradox) can be fairly described as
'instantaneous at any distance'. Firstly, because instantaniety is
undefined for events that occur at different places. Any two events that
are separated in space but close enough in time so that a signal emitted
from one event will not reach the place of the other before the second
event has already occured, can be seen by different observers as if they
happened instantaneously, or as if either happened before the other. In
other words, it's not only that cauality is not allowed to run faster
than light, within our current framework it is unclear what faster than
light action means.

Secondly, the coupling of the two particles does not mean that they
communicate with each other after separation, either faster than light
or not. In a sense the two particles after separation are still some
kind of a mix between all the options they have. When one of them has
been measured, the second simply loses all the options that are not
compatible with that measurement. As a single particle example take the
photon we irradiated from that dipole antenna above. Let's say we
surrounded that transmitting antenna with a sphere of receivers that can
absorb the photon. Once the photon has been detected by one of the
receivers, all the others are prevented from sensing it. The receivers
don't communicate between themselves, but they are 'coupled' all the
same - once the photon hit one detector, it will have never went in the
others' direction in the first place, although until then its
propagation was perfectly symmetric.

This is maddeningly counter intuitive, although perfectly predictable.
The only physicists still bothered by this behaviour today are people
who actively try to understand what's going on there. Among these some
belive that each measurement splits the universe and leaves our
conciousness in the consistent part, while others believe that
underlying physical reality has nothing to do with space and time (which
are but emergant phenomena), and that the coupling are simply a
non-local effect. But most just got used to it and went on.

> Some (or all) flavours of Neutrinos *may* have negative rest mass, and 
> I hope that they do. If they do then they must travel faster than 
> light. There have been various measurements made from time to time 
> that suggest that this may be the case but the results tend to fade 
> away like the grin on the Cheshire Cat when attempts are made to 
> verify observations.

Actually all measurements of neutrino mass have zero inside their error
margins, or close enough to it. Some old experiments measure the square
of the mass seemed to have their entire bars below zero, but that was
probably just underestimation of their error - newer and much more
precise measrements are all consistent with 0.

The only indication that at least some of the neutrinos are massive
comes from neutrino oscillations, but their mass is still too low for
direct measurement.

> Exercise for fun:
> Those with glazed eyes may wish to stop reading about here :-).
> Usefulness of anything beyond here is highly likely to be zero for 
> many values of zero.

> It is easy to plot classical energy of a particle (or starship or ...) 

What formula do you use for this plot? I don't think any I know has
these minima (or else I haven't bothered to trace their behaviour after
v=c... :-)).

Yair.
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