Originally posted by wolram
as you say marcus typical english, i admit iam out of my depth now, i thought gravity was a product of mass, it seems you are saying gravity can exist without mass, is a photon truly masslesss?
great to read all your replies, i think this forum is "simply the best".
swimming out of ones depth is good practice
and does not automatically lead to drowning in my experience
so i try to spend a substantial part of each day out of my depth
I found a good page about mass
http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html
It is in the Usenet Physics FAQ
http://math.ucr.edu/home/baez/physics/index.html
This could well be the world's coolest FAQ, it has things like
"if I go very fast will I turn into a black hole"
You might like what it says about mass. The core notion of
mass is inertia. Indeed the inertia of a thing at rest (because inertia
of a moving thing is ambiguous----the "longitudinal" inertia has been
discovered to be different from the "sideways" inertia.)
When something is moving it has a "longitudinal" inertia and
a sideways or "transverse" inertia. But it no longer has a mass
because mass is a directionless quantity. So the custom is to
assign to each object the "invariant" mass which is the inertia
it WOULD have if it were sitting still. Lorentz discovered this
ambiguity of inertia of a moving object back in 1904 even before Einstein.
YES just as you say, a photon really does not have mass.
It cannot sit still. Only things that can
sit still can have mass. "Relativistic mass" is a confusing
idea, Einstein deplored its use, there is an ongoing attempt to
eliminate the expression from physics and use only the "invariant mass" concept. And so on.
You will see if you look at the FAQ page.
A great paper about mass equals rest mass was in June 1989 Physics Today by Lev Okun.
But this
John Baez online thing in the
Usenet FAQ is adequate.
One does not need to have massful things
in order to have gravity.
There could be a universe with no matter in it,
with nothing but bunches of light sailing around
and that light would still warp space and cause
the rays of light to bend and follow the curved geodesics, and
bunches of light would attract bunches of light.
The equations (GR) that model gravity do not have mass in them
they have *energy density* and related pressures. Energy is
what causes gravity in GR. Energy tells space how to curve and
curved space tells energy how to move.
The guests have come back so I do not have time to edit this.
It is really a draft and much too long. But I must either send it
as it is (with apologies) or erase it.