Re: Various questions
- From: Gerry Quinn <gerryq@xxxxxxxxx>
- Date: Tue, 17 Apr 2007 19:56:03 +0000 (UTC)
In article <4622065F.6030408@xxxxxxxxxxxx>,
Gerry Quinn schrieb:
In article <461E4183.2010006@xxxxxxxxxxxx>,
In the first place, the Sun is a classical object; it creates lots of
entropy, and the 'environment' is always looking at it.
Every classical object is also a quantum object, composed of myriads
of quantum particles. How does it become classical?
When it's not useful to look at it as a quantum object. Which is
nearly always, in the case of an object as massive as the Sun.
The Environment is also always looking at a single particle.
Thus there is no difference.
So how do we carry out experiments involving quantum phenomena? The
answer is by *not* looking, over some interval of time and/or space.
I also mentioned that the Sun generates entropy - effectively a
continuous blizzard of measurement events.
In the second place, the Sun differs from an electron in that it is
composed of more than one particle, and the interactions of these
particles define a size. The same applies to protons. Composite
objects have a well-defined, if not always exact, size.
And the same allies to electrons; their mass defines an intrinsic
length scale, the Compton wavelength.
So it does, but to say the size of an object is given by the Compton
wavelength is just silly. Apply it to the Sun, and you can see why!
It may be that there are 'size' questions that can usefully be answered
by referring to the Compton wavelength, but it's not a generic way of
An electron does not have well-defined component particles (virtual
particles don't count).
Why don't they count? The dressed stated of an electron (with a size)
is made up of sizeless bare electrons, in a similar way as the hydrogen
atom (with a size) is made of an electron and a proton, except that
the electron invloves inifinitely many bare particles.
Because they don't have any characteristic distance between them that
is a function of their interactions. Atoms in the Sun, quarks in a
proton, or bricks in a house do, because they are real particles.
So I think the only answer to "What size is the electron?" is "Define
what you mean by size."
It is well-defined by the literature. See my theoretical physics FAQ
(Are electrons pointlike/structureless?)
I don't believe "the size of an electron" is well-defined by the
literature. For example, your web page refers to the de Broglie
wavelength, and to the charge radius. Like the Compton wavelength,
neither of these can be considered the obvious definition of size.
By contrast, the definition for a composite object in terms of the
characteric distances between its components is universal, and
corresponds to ordinary concepts of 'size'. So composite particles do
have a well-defined size.
- Gerry Quinn
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