Re: Dark matter/energy - is it real?

Logan Kearsley wrote:
"Tue Sorensen" <sorensonian@xxxxxxxxx> wrote in message
news:1144357218.287478.207070@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Wayne Throop wrote:
: "Tue Sorensen" <sorensonian@xxxxxxxxx>
: Then I can't see why lensing is proof that light has gravity. Can you
: explain it more carefully?

Light is bent, right?

Yes, since it follows the curvature of space.

Light has momentum, right?

When it hits something, yes. But isn't that a transformation of the
energy that it usually uses to maintain constant light speed? Light has
energy but AFAICS it doesn't automatically follow that it also has
gravity.

No. The fact that light has momentum is a requisite result of special
relativity.
The total energy of a particle is given by E^2 = (mc^2)^2 + (pc)^2, and
relativistic momentum is given by p=ymv (using y because I can't type a
gamma). If mass is set to zero, we find that E = pc. Pc usually equals zero,
/unless v = c/, in which case y becomes indeterminate and the momentum can
have any value. Thus, massless particles like photons must travel at the
speed of light, and have momentum p = E/c.

Well, I'm having a hard time arguing with that! :-) But I would like
some examples of what kind of momentum light can have (as given by its
source).

Momentum is conserved,
right? You know, every action has an equal and opposite rection, right?
Therefore, in this gravitational interaction, the light moves the star,
(a teeny teeny teeny bit) therefore the light *has* gravity as well as
being *affected* by it.

So anything that has momentum also has gravity? Sorry, when it comes to
light I fail to see the logic. Does light's momentum come from the push
of the radiation itself, or from the particles with rest mass that are
generated just as light hits and interacts with something?

Light's momentum comes from whatever emitted it in the first place.

And even of light that hits a star moves it a bit, I still don't see
how that gives the light gravity.

'Hitting' is not under discussion. We're discussing lensing. It is an
observed fact that light bends around massive objects, without ever
impacting them. Since momentum is a vector quantity, that alteration of a
photon's path alters it's momentum, even though it's energy is unchanged.

Maybe I'm a complete ignoramus, but it sounds strange to me. We have a
set velocity, a set energy, but lots of different options for momentum?
Seems like momentum is an additional form of energy added to the
equation... is it essentially the same as kinetic energy?

I admit there are many things I don't yet understand. Doesn't
relativity say something along the lines of increased speed equaling
increased mass? As an object goes faster, its mass increases, always
requiring more and more energy to accelerate further. So that if it is
infinitely close to the speed of light, it will take infinite energy to
make it go faster. But it doesn't seem logical that an object's gravity
should depend on its speed. But this is a different discussion, I
suppose.

In
order to satisfy conservation of momentum and Newton's Third Law, that
alteration of a photon's momentum must be accompanied by an exactly equal
and opposite alteration of the massive body's momentum, which requires that
the photon exert a force on the massive object. Ergo, light must have
gravity.

You will groan now, but I still cannot accept this. We're talking about
momentum exchanges. How does that translate to gravity? How does
momentum have anything to do with gravity?

- Tue

.

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