MadSci Network: Physics

Re: What is the length of a photon?

Date: Mon Aug 30 22:02:43 1999
Posted By: Layne Johnson, Undergraduate
Area of science: Physics
ID: 935767266.Ph

Hello, John.

I'll take the second question first.  Yes, the neutrino is the smallest 
particle we've yet identified.  Tachyon is a theorhetical particle 
containing no mass.  If it exists, it would qualify as the smallest 
particle.  But its' existance isn't very well accepted in our community.  I 
like to think it's there, because it is the best answer I have to 
Heisenburg's uncertainty principle.

As soon as quantum particle is observed, it deviates.  It changes 
direction, speed, or both.  The very act of observation causes it to change 
its course.

Heisenburg said we can know the location of a quantum particle, or the 
direction and velocity of its movement, but not both.  Einstein, among 
others, cried "bull!"  Well, "bull" wasn't his exact response.  His exact 
response was to put Heisenburg's principle to the test.

Einstein and his partners figured that if two quantum particles collided, 
they would recoil from one another according to Newton's idea of equal and 
opposite reactions.  Thus, after the initial seperation, the exact location 
of particle #1 could be inferred by the measurable location of particle #2, 
while a measurement of particle #1's direction and velocity could be made 
at the same time.  Indeed, both location and directional velocity could be 

The problem is, Einstein's test didn't work.  Particle #1 wasn't where it 
was supposed to be, or it wasn't headed where it should have been going.  
Quantum particles are funny that way.  Einstein conceded defeat and 
modified his special theory or relativity to incorporate quantum phenomina, 
and renamed it the general theory of relativity.  Of course, the full story 
is much richer than my two paragraph version, but you get the idea.

Now back to tachyon.  This will sound crazy, but it seems to me that 
quantum particles must be in communication with one another.  The 
observation of particle #2 caused #1 to change, although #1 wasn't itself 
observed.  And the bizarre part is this - the distance between #1 and #2 
was great enough, and the time between the location measurement of #2 and 
the velocity measurement of #1 was short enough that communication between 
these two particles would have been faster-than-light speed.  It's almost 
impossible to believe, but it's been verified time and time again by the 
greatest minds of our century, using the most accurate measuring devices 
ever built.

We know that nothing with mass can travel faster than light speed.  But 
tachyon could.  Light speed is a barrier to mass, but not to massless 
particles.  Could a massless particle be emitted by #1 the moment its 
directional velocity changes due to observation?  And could #2 change not 
only as a result of observation, but also as a result of #1's massless 
particle emission?

I believe so, and that's why I believe tachyon is real.  But until it's 
proven, neutrinos are the smallest things around.

I'm going to avoid your first question, becuase I don't know the length of 
a photon.  The Lorentz contraction works for me.  Maybe this is because of 
the nature of a photon.  It isn't really a particle, and it isn't really a 
wave.  Properly speaking, it's a boson, which is a "particle" of energy, as 
opposed to a fermion, or "particle" of mass.  You are correct that l = c t, 
but it is equally correct to say Lorentz is correct, becuase a photon is 
energy, and not mass.  I can't give you a measurement, becuase I can't 
resolve this in my mind, either.  This is the best answer I can give.  If 
it isn't what you need, please ask the question again.  If it is directed 
my way, I'll defer it to another MAD Scientist, and hopefully you'll get a 
more satisfying answer.

Layne Johnson

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