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Re: Just what is a particle?



So far, I don't disagree (much) with anything anybody has written
about this topic. but I think it is important that we remember just
how it got started. It was written in response to a high school
student's question forwarded to PhysShare by the student's teacher.
Everything people have said about particles and photons is
technically correct, but to involve the student to whom I was
responding in this discussion would, I think, just serve to confuse
him or her further, and therefore, I chose to minimize the
wave-particle duality issue in the interests of getting to a concrete
answer to the student's question.

Now that I've said that, you all can go on and debate these issues
until the cows come home, whether or not they leave an interference
pattern on the barn door.

Hugh

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?


Hugh Haskell
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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