J G Miller wrote:
On Thu, 17 Sep 2009 21:11:53 +0100, Tim S wrote:

The photon is considered to have zero mass *at rest*

This is something that has always bewildered me.

If something has zero mass, does it exist?



Well how much mass is there in the theory of gravity, so does it exists?

You can't look at the quantum world through neoclassical spectacles..it
simply makes no sense.

How do you know something has mass anyway? it resists motion. If its
resistance to motion is non linear, then one way to express that is to
say its mass changes with velocity.

Howver velocity is relative motion, so the question is, how can
something have a different mass depending on whether you are moving at
the same speed as it in the same direction, or more if you are moving in
the opposite direction?

I suppose what Einstein is really saying is that a things apparent mass
changes depending on the speed of it *relative to the observer*. There
is no such thing as absolute mass, there is only rest mass.. and of
course since a photon ALWAYS travels at the speed of light, it aint a
photon if it stops and gives up its energy to something else..



*If* that is the case, then presumably photons can never be at rest,
otherwise they would cease to be.


Indeed. It is considered that they sort of do when they interact with
something.

I suppose the other point of view is that outer space exists yet it
has effectively no mass.

But does it really have zero mass since even outer space
is not a pure vacuum and there are still one or two atoms
per large volume?


Space without anything at all in it, may be said not to exist at all.

Its a philosophically empty concept anyway, since without anything in
it, it cant be observed, measured, or talked about.

However, outer space must be composed of something in certain theories
since in those theories it is argued that it is bent by gravity, and
one cannot bend something which is not there.


Why not?

I am reliably informed that Peter Mandelson is bent, and thank the Lord,
he certainly isn't here, or indeed I suspect. all there, either ;-)

Anyway, you are 'confusing the map with the territory'.

Theories about the world, and indeed our perception of the world, are
not what the world IS. They are diagrams of aspects of it.


(Corrections or further explanation of my misapprehension gratefully awaited.)

Not much to say that greater minds than ours struggle to really pictrure
relativity and quantum physics ..I retreat into philosophy: Its easy for
me. I know absolutely nothing about anything for sure, and that's the
one sure thing. the rest is ideas about things: Those ideas rest on
assumptions many of which are metaphysical, and can neither be proved
nor disproved. They are just intellectual and perceptual sticks hammered
in the ground. Starting points for a suite of schemata that 'describe
the way stuff is'.

For example, in an infinite flat plain, wher am I? If its featureless, I
dont know, and it doesn't matter. Only when I put a stick in somewhere
and use another to measure against it, can I say things like 'I am three
sticks away from the One True Stick' If I put another stick in, I can
say 'I am three sticks away at an angle of 30 degrees to the One True
Stick and the One True Line' etc etc.

Classical physics relies on a simplistic world view in which mass is
conserved, and energy is conserved separately. Eisntein theorised that
this was an approximation that would break down in the limit, into mass
being an form *of* energy, and interchangeable with it. The proof of the
pudding was, amongst other things, that atom bombs and nuclear power
stations worked. The implications were that a would clock spring is a
few femto grams heavier than an unwound one. And that caesium clocks
orbiting the earth would run a tad slow - which they do.

So he had a choice: ditch the classical view altogether or modify it. He
chose to express it in the latter terms, by saying that energy and mass
are interchangeable and part of the same thing. Rather than dreaming up
a new thing and making them both expressions of that. He may have done
us a disservice there.

The finally important thing to understand is that you get into a heck of
a mess using the wrong worldview on a problem. We are born with, or
maybe we just learn, to look at the world in a single way, and call what
we see fact..real..if I kick it, and it hurts its not an illusion is
it? It's there! Or *is* it?..Kant certainly felt that his philosophical
reasonings removed any certainty about space, time, energy, mass..as
actual real world entities..no, he supposed them to be qualities by
which we apprehended the world and divided it into entities whose
relationships could be measured and whose interactions observed..there
was no evidence to suppose that they actually existed in the real world
at all, they existed merely as a common language we all shared (or just
some of us, it appears) to describe the world.


If you actually look at the modern quantum worldview philosophically,
that's pretty much the way it is. Something certainly is somewhere, but
what it is, and where it is is a LONG way from Fred Flintstone stubbing
his toe on a 'massive' rock. And the certainty that if he always kicks
it, it will always hurt..

The important thinmg to realise is that a set of accepted theories about
the world, and yea, even your perception of said world are not 1:1
apprehensions of what is 'really there' They are maps, one drawn by the
intellect, the other by the subconscious, to help you get around it in a
reasonable way. Science is an attempt to precisely define the terms of
all the elements it finds in a particular worldview. The 'what you see
is what it is' worldview. The problem is as it gets deeper and deeper,
it finds the whole thing unravelling, and gets patched with first
Newton, then Einstein in a desperate attempt to preserve the normal
experience of the world and describe the ultra fine or ultra coarse
detail of it in the same terms as you would describe a rock you stub
your toe on. With remarkable lack of success.

It is probably true to say that science at the Quantum level has almost
completely given up trying to provide a coherent picture of the world in
terms a normal person can understand: It has mathematical relationships
that seem to work, but the implications of making them 'true' pictures
of the world are so intellectually destructive that people simply don't
bother any more. Time and space are, it seems, no longer subject to the
same restraints we thought they were. Things can be anywhere and
everywhere at the same time, outcomes may cause events, and the present
arguments are whether there are 10, 12, 13 or more universes of which
this one is just random bits of all the others poking through. People
could go mad..

I console myself with the thought that since I didn't know what the
world was in the first place, not knowing what it is in ever more
complex ways doesn't REALLY bother me at any more.. ;-)


Whilst the average Joe, to judge by things I read on the 'net - hasn't
even really understood the way Newtonian physics says the world is, let
alone relativistic physics, and don't even go NEAR quantum physics. Even
the physicists doing it don't know. I understand it JUST enough to see
where the philosophical problems are. I have no solutions to them. Other
than the metaphysical as outlined here. Namely taht science cabnnot
continue to use a rational materialist worldview in pursuit of
science..which Kant's 'critique of Pure Reason' said what - 300 years
ago? but has been largely ignored or misunderstood (and I wouldn't say I
understand it totally either).