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At 05:29 PM 2/17/00 -0600, Cliff Parker wrote:
>
>Hugh Haskell wrote in an earlier post -- Photons are particles (not like
>electrons or protons, but particles nevertheless)
Photons are not completely like protons, but not completely unlike, either.
>I would like some discussion on this point as I try to clarify my thinking.
>What characteristics are necessarily present in order to call something a
>particle?
>I have listed a few characteristics particles often seem to have and
>thoughts about how each may apply to photons. Comments, clarifications,
>disagreements, and instructions are hereby solicited.
>
>1) Charge - No. Photons like neutrons and many other "particles" have no
>charge.
Agreed.
>2) Mass - No.
>I guess photons are massless since they travel at the speed of
>light. I don't really understand what this means however especially when
>momentum and energy are considered.
Photons have no rest mass. In general, we have
E^2 = p^2 c^2 + m^2 c^4
where m is the rest mass. For photons and such, this reduces to
E = p c
>3) Momentum - Yes. I understand that photons do have momentum.
>Exactly what this means however is unclear to me.
> It must not mean p = mv since photons have no mass.
For particles in the nearly-at-rest limit (p << mc) the foregoing equation
can be expanded to first order to give results tantamount to p=mv. Photons
are never in that limit.
>4) Inertia - I am really baffled on this one. No mass means no inertia but
>photons obey Newton's First Law. How can that be?
Newton's first law is tantamount to conservation of momentum. Photons have
momentum. That's all you need. See above.
>5) Energy - Yes, they can cause change I suppose.
>I used to be more sure of
>this, but that was when I thought I understood what energy was.
>After considering Leigh's thoughts and those of others I'm not
>so sure anymore.
Photons have energy. No problem.
>6) Something that is quantized - Perhaps a particle can be considered
>anything that is quantized.
Maybe. That's one way of stating my item #2 below.
At 08:24 PM 2/17/00 -0500, Hugh Haskell wrote:
>All points here well taken. I was not trying to make any definitive
>statement about particles, just trying to differentiate them from
>maxwellian waves, without letting them get mixed up with the
>"ordinary" particles of everyday life.
Well, what you are trying to do is possible in practice, but it's not
possible in principle! In principle, there is no such thing as a wave as
distinct from a particle, or vice versa. There is only stuff. In some
limits, stuff acts like a classical wave. In other limits, stuff acts like
a classical particle. Note that I said "acts like" not "is".
Now, in practice, we can tell the difference between microwaves and pinto
beans. The differences include the following:
#1b) At ordinary temperatures, the thermal deBroglie wavelength of the bean
is much less than the size of the bean. Therefore we never see diffraction
of beans. However, there exists a temperature (an exceedingly low
temperature) at which bean diffraction must, in principle, occur.
#1m) At ordinary temperatures, microwaves diffract.
---
#2b) At ordinary temperatures, the thermal energy of the bean is much less
than its rest mass. Therefore we never see spontaneous bean-antibean pair
production.
Even protons are not spontaneously produced at ordinary
temperatures. Unlike beans, though, there is a temperature that is high
enough to produce protons yet not high enough to decompose them into
components.
#2m) At ordinary temperatures, an ordinary-sized box has a very high
occupation number in each of a multitude of microwave modes. Note that h/k
is 21 GHz per Kelvin. That means that the microwaves' particle properties
are mostly washed out by the law of large numbers. However, if we consider
lower temperatures, smaller boxes, and/or higher modes, eventually the
particle properties will become noticeable.
OK?