Speed of gravity

[email removed] (Scott S. Kellogg) writes:
>
> According to the seminar on gravitiation I sat through,
> gravity propogates at the speed of light.

Time for a stupid question:

If gravity goes at the speed of light, and light goes at the speed of light,
how can gravity catch up with light?  If it can't, why can't light escape
from a black hole?

Make that two stupid questions:

What do gravity waves do when they hit a black globe?  If they get absorbed
by the black globe, then you can have a nice perpetual motion machine - have
a small vehicle carrying a black globe which switches on said globe and floats
up, then switches off the globe and falls down, then switches the globe back
on before it hits the ground and floats up, etc.

- --
 "Keyboard?  How quaint!" - M. Scott

 Adrian Hurt			     |	JANET:  [email removed]
 UUCP: ..!uknet!cee.hw.ac.uk!adrian  |  ARPA:  [email removed]

PAVEWAY (address lost due to overzealous editing) writes:

> PS.....    Gravity and Gravity waves....?   'how fast does gravity go?'
> if there is such a thing as a gravity wave  could someone tell me what they
> do and how fast they go?

[email removed] (Scott S. Kellogg) replies:

> According to the seminar on gravitiation I sat through,
> gravity propogates at the speed of light.

Are you sure? I thought that gravity propagated instantaneously. As
far as I know, they're still working on finding a gravity wave/particle.
If they do, it'll be a big step towards the "Grand Unified Theory"
that theoretical physicists get all starry-eyed about.

- -- Tim Soholt ([email removed])

[This was sent to traveller-request, so I forwarded it on to the
distribution list.  Watch your To: address! -- James]

As far as I can remember, current theories call for a charge carrier
particle for all four forces.  The carriers of electromagnetism (the
photon), the strong nuclear force (the pion) and the weak nuclear
force (the intermediate weak boson) have been found.  All have zero
rest mass and all have a set velocity of c, 2.998x10^8 m/s.  The
reason that they are affected by gravity is that they have a mass
due to their speed -- zero mass and infinite (relativistic) speed
yields any number for relativistic mass you want... ^_^  Since they
have mass, black holes can haul them back, and they end up orbiting
inside the event horizon if they enter it.

No one has found the fourth, the 'graviton', because it would be very
difficult to build a graviton detector.  (Can you think of a
substance that isn't affected by gravity?)  However, it is also
postulated to have zero rest mass and a velocity of c.  I believe
that it is not supposed to be affected by itself (i.e. gravitons
can escape the effects of black holes), perhaps because it can't
make sense any other way.  I'm not quite sure, but I assume that
since black holes are postulated to have an effect outside their
event horizon, gravitons are not affected by gravity.  (Perhaps they
have no mass?  Hmmm.)

Responding to Adrian Hurt's second 'stupid' question, as he calls
it, I believe that black globes block the electromagnetic force
exclusively.  All electromagnetic radiation, and matter (which
carries electric/magnetic charge -- I think that since neutrons spin
they carry some magnetic charge, correct me if I'm wrong) are
stopped by a black globe.  The two nuclear forces are insignificant
to anything inside the field.  Gravity, however, affects it.  I
think I remember something in the High Guard rules involving a
squadron with black globes jumping in so as to glide around a planet
on a hyperbolic course, unglobe, and attack the ground
installations.  In order to do that they'd need to be afffected by
gravity.

Responding to Tim Soholt, I don't believe that gravity propagates
instantly.  Einstein believed that information could not travel
faster than the speed of light, and information in an Einsteinian
sense can be encapsulated in gravity waves (either gravitons of a
particular wavelength/frequency/energy or ripples in the fabric of
space-time, whichever you prefer) if you have a large enough mass as
a transmitter.  I believe that Einstein considered that and came up
with the answer that gravity travels at the speed of light because
it fit the rest of the model well.

Jack Dietz
// [email removed]

Greetings!

I'm very new to TML and hadn't intended to post anything yet until I had
more of a "feel" for the flow of discussion on this group.  But, when I
saw the comments below, I just had to respond!  Byt the way, my
background is in physics and I design/build particle detectors for a
living.  Thus I keep current on high energy physics and, to a lessor
degree, nuclear physics.  If y'all have any questions in this area I'll
be glad to try to answer them.

[email removed] (Jack Dietz) writes:

>As far as I can remember, current theories call for a charge carrier
>particle for all four forces.  The carriers of electromagnetism (the
>photon), the strong nuclear force (the pion) and the weak nuclear
>force (the intermediate weak boson) have been found.  All have zero
>rest mass and all have a set velocity of c, 2.998x10^8 m/s.  The
>reason that they are affected by gravity is that they have a mass
>due to their speed -- zero mass and infinite (relativistic) speed
>yields any number for relativistic mass you want... ^_^  Since they
>have mass, black holes can haul them back, and they end up orbiting
>inside the event horizon if they enter it.

This description is out of date in some ways and wrong in others.
Current thinking goes like this.  Electromagnetism (the unification of
the 1800's) and the weak nuclear force have been unified by Glashow,
Weinberg, and Salam into the Standard Model of the Electroweak
Interaction, or for short, the Standard Model.  These fellows won a
Nobel Prize for this a few years ago.  This model describes the
common things from electromagnetism and reactions covered by the weak
force, such as beta decay of nuclei or neutrino interactions.  The
"carriers" of electroweak are the photon, W, and Z bosons.  Some properties
are:

    mediator   charge   rest mass         possible velocities
    ----------------------------------------------------------------
    photon     none     none              only at the speed of light (c)
    W          + or -   80.22 GeV/c**2    anything < c
    Z          none     91.17 GeV/c**2    anything < c

Note that the electroweak interaction affects both hadrons (mesons and
baryons) and leptons (electron, muon, tau, and their neutrinos).

Now the strong interaction.  This covers the interactions between quarks
but does not have anything to do with leptons.  It is mediated by the
gluon, which carries the **color** charge and has zero rest mass.  Color
charge comes in three varieties, red, green, and blue.  Hadrons must be
color neutral or "white".  Thus mesons are color-anticolor doublets of
quarks and baryons are RGB triplets of quarks.  By the way, remember
this is NOT color in the visual sense.  The mathematics describing this
(Quantum Chromodynamics or QCD) works out in a certain way having nothing
to do with visual color.  The use of the word "color" for the strong charge
and that the color charge is red, green, or blue is just a convenient
analogy.

A little bit about the pion.  It is made up of a quark doublet (actually a
superposition of doublet states) and hence is classified as a meson.  It
comes in both electrically charged and neutral forms and has a rest mass.
The mass of the neutral pion is 134.97 MeV/c**2 and the charged pion mass
is 139.57 MeV/c**2.  It can move at any velocity below the speed of light.
In nuclear physics pion exchange between nucleons was given as the
mechanism for some reactions, but this is not a fundamental interaction.

As to the graviton, I don't have any information off the top of my head.
If anyone is interested, I can pursue this with my theory colleagues
down the hall and then post an overview of the situation.

Regards,
Richard

P.S.  Is anyone else as completely disgusted with the description of how
meson weapons work as I am?

******************************************************************************
*  Richard Fernholz               Internet:  [email removed]                 *
*  Dept. of Physics               HEPnet:  muhep::fernholz (19191::fernholz) *
*  McGill University                                                         *
[address removed]                 voice:  [phone removed]                       *
*  Canada                                                                    *
******************************************************************************

Just a few corrections to Jack Dietz's comments on the exchange
particles for physical forces.

> All have zero rest mass and all have a set velocity of c,
> 2.998x10^8 m/s

Best estimates of particle masses, from the Particle Data Group
handbook are:
	PARTICLE	FORCE			MASS-ENERGY
	photon		Electromagnetic		< 3.0e-33 MeV
	W		Weak Nuclear		81 GeV
	pion		"Strong Nuclear"*	135 MeV (uncharged)

According to Yukawa, the strength of a force's potential as a function
of range is

	V = - K * exp(-u * r) / r

where K is a constant, r is the distance from the "charge", and u
is directly proportional to the exchange particle's mass (it's
mass times the speed of light divided by Plank's constant). For
zero mass, V = - K / r, which is the potential for the electric
field; so we can conclude that the Electric Force's exchange particle,
the photon, has zero mass (or close to it) and therefor travels at
the speed of light.

Since the gravitational force also has a potential proportional
to 1 / r, we can conclude that the gravitaional exchange particle,
dubbed the graviton, will also have a mass of zero, and will also
propagate at the speed of light.

				Rob Flammang

* The strong nuclear force is actually intermediated by the gluon,
but a long ranges (more than a few femtometers) the pion (which has
gluons in it) does most of the exchanging.

In TML-nightly [email removed] (Jack Dietz) writes:

>As far as I can remember, current theories call for a charge carrier
>particle for all four forces.  The carriers of electromagnetism (the
>photon), the strong nuclear force (the pion) and the weak nuclear
>force (the intermediate weak boson) have been found.  All have zero
>rest mass and all have a set velocity of c, 2.998x10^8 m/s.

Whoa! Neither the pion (aka pi-meson) nor the intermediate vector
boson (W-particle) have a zero rest meas. Quite the contrary. The
pion is massive and the W is *extremely* massive. In fact, this is
*why* these are "short range" forces.

The range of a force depends on how long the virtual particles that
carry it can last. This is given by a function that relates their
energy to the time they can exist using h (planck's constant). The
higher the energy, the shorter the time.

Since photons and gravitons have a rest mass of 0, they don't have to
"waste" any of their energy on mass. But the W and the pion *do*.
This sets an upper limit on the time they can exist, even with *zero*
energy devoted to carrying the force. That time gives the maximum
range they can reach.

Leonard Erickson            [email removed]
CIS: [70465,203]           [email removed]
FIDO:   [email removed] [email removed] (preferred)
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uucp: [email removed]
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