Gravity and lightspeed question

A star is a billion light years from earth. Today, we see the light
it emitted a billion years ago.

A billion years ago, it was more massive than it is today. That
star's gravity is acting on the earth today, ever so slightly.

Question: Is the gravity we "feel" from that star a result of its
billion-year-ago mass, or its mass today?

[Not a quiz; I don't know the answer, and have been intending to ask
this for some time.]

On a sunny day (Wed, 9 Jan 2008 17:57:14 -0800 (PST)) it happened Steve2
wrote in
:

A star is a billion light years from earth. Today, we see the light
it emitted a billion years ago.

A billion years ago, it was more massive than it is today. That
star's gravity is acting on the earth today, ever so slightly.

Question: Is the gravity we "feel" from that star a result of its
billion-year-ago mass, or its mass today?

[Not a quiz; I don't know the answer, and have been intending to ask
this for some time.]

Concensus in physics 'today', or what most of those guys keep telling each other,
is, that graffity moves at the speed of light.
One experiment was done that seemed to confirm that, but it is challenged by
some as simply measuring light speed.

So.. maybe the official answer is:
Yes it its the result of its mass long time ago.

But, officially earth was flat once, officially epicycles once explained the
movement of the planets, etc etc.
http://en.wikipedia.org/wiki/Speed_of_gravity
will show you some more and some links too.

"Steve2" wrote in message
...
|A star is a billion light years from earth. Today, we see the light
| it emitted a billion years ago.
|
| A billion years ago, it was more massive than it is today. That
| star's gravity is acting on the earth today, ever so slightly.
|
| Question: Is the gravity we "feel" from that star a result of its
| billion-year-ago mass, or its mass today?
|
| [Not a quiz; I don't know the answer, and have been intending to ask
| this for some time.]

While the supercranks are looking for gravity waves with LIGO
and we ordinary cranks are scoffing at them, it is refreshing to
see such a pertinent question.

The point you are making is 'gravity is proportional to mass'.
If a star loses mass as a result of radiation then it loses gravity
also.

Let's look at the problem a little closer to home.
As the Earth turns in the Moon's gravitational field we see tides.
The height of water in New York is different to the height of
water in London, when one is high the other is low and vice versa.
That is how we "feel" the mass of the Moon (and the Sun).
If you imagine weakening the mass of the Moon or Sun then
you'll have a smaller tidal effect.
Let's call that a deltatide, a small change in the tide.
Now... the twice daily change in the tide is entirely local, it occurs
as the Earth turns and is independent of changes in the Moon or
Sun. The deltatide, however, is a change in the remote body.
Your question is "how long does it take for the deltatide to reach
us?"
Acceleration is instantaneous, velocity is not.
The back of the train accelerates when the front of the train
accelerates, instantaneously.
Even an elastic train where the front moves before the rear
still applies instantaneous acceleration to the rear, but the rear
has a lower initial acceleration.
A gradual change in the mass of a star, detected by the deltatide,
is also a gradual change in the deltatide here, and in that sense
is instantaneous.

In sci.astro message ,
Thu, 10 Jan 2008 10:41:12, Androcles
posted:

Acceleration is instantaneous, velocity is not.
The back of the train accelerates when the front of the train
accelerates, instantaneously.
Even an elastic train where the front moves before the rear
still applies instantaneous acceleration to the rear, but the rear
has a lower initial acceleration.

Nonsense. The effect of the engine propagates along the train at the
speed of sound in steel.

--
(c) John Stockton, Surrey, UK. /
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Correct = 4-line sig. separator as above, a line precisely "-- " (SoRFC1036)
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"Dr J R Stockton" wrote in message
nvalid...
| In sci.astro message ,
| Thu, 10 Jan 2008 10:41:12, Androcles
| posted:
|
| Acceleration is instantaneous, velocity is not.
| The back of the train accelerates when the front of the train
| accelerates, instantaneously.
| Even an elastic train where the front moves before the rear
| still applies instantaneous acceleration to the rear, but the rear
| has a lower initial acceleration.
|
| Nonsense. The effect of the engine propagates along the train at the
| speed of sound in steel.

The hypothetical elastic "train" is made of latex, not rigid steel.
It is elastic because it returns to its original shape when the force
is removed. By the principle of relativity the caboose is pulling the
locomotive backwards.
Incidentally, the speed of sound in a rigid body is infinite. No
body is truly rigid, all bodies are made up of particles with space
between them.
If you are going to scream "nonsense" first check your facts, idiot.

In sci.astro message ,
Fri, 11 Jan 2008 14:22:45, Androcles
posted:

"Dr J R Stockton" wrote in message
. invalid...
| In sci.astro message ,
| Thu, 10 Jan 2008 10:41:12, Androcles
| posted:
|
| Acceleration is instantaneous, velocity is not.
| The back of the train accelerates when the front of the train
| accelerates, instantaneously.
| Even an elastic train where the front moves before the rear
| still applies instantaneous acceleration to the rear, but the rear
| has a lower initial acceleration.
|
| Nonsense. The effect of the engine propagates along the train at the
| speed of sound in steel.

The hypothetical elastic "train" is made of latex, not rigid steel.

Scientifically, steel is an elastic substance, when the stresses of
normal engineering usage are applied. Indeed, all true solids are
elastic when the stress is not too high, because that is in essence the
definition of a solid.

For a train made of haberdasher's elastic, the effect of the engine
propagates along the train at the speed of sound in haberdasher's
elastic.

Relativity teaches that no effect can propagate instantaneously.

It is elastic because it returns to its original shape when the force
is removed. By the principle of relativity the caboose is pulling the
locomotive backwards.

No; by the principles of Sir Isaac Newton. You should read the
Principia.

Incidentally, the speed of sound in a rigid body is infinite. No
body is truly rigid,

True.


If you are going to scream "nonsense" first check your facts, idiot.

This is one of those rare occasions on which you should take your own
advice.

--
(c) John Stockton, Surrey, UK. /
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Correct = 4-line sig. separator as above, a line precisely "-- " (SoRFC1036)
Do not Mail News to me. Before a reply, quote with "" or " " (SoRFC1036)

"Dr J R Stockton" wrote in message
nvalid...
| In sci.astro message ,
| Fri, 11 Jan 2008 14:22:45, Androcles
| posted:
|
| "Dr J R Stockton" wrote in message
| . invalid...
| | In sci.astro message
,
| | Thu, 10 Jan 2008 10:41:12, Androcles
| | posted:
| |
| | Acceleration is instantaneous, velocity is not.
| | The back of the train accelerates when the front of the train
| | accelerates, instantaneously.
| | Even an elastic train where the front moves before the rear
| | still applies instantaneous acceleration to the rear, but the rear
| | has a lower initial acceleration.
| |
| | Nonsense. The effect of the engine propagates along the train at the
| | speed of sound in steel.
|
| The hypothetical elastic "train" is made of latex, not rigid steel.
|
| Scientifically, steel is an elastic substance, when the stresses of
| normal engineering usage are applied.

It doesn't exist in my hypothetical scenario. You don't get to change
the conditions I set up and then describe your model, that's nonsense.

| For a train made of haberdasher's elastic, the effect of the engine
| propagates along the train at the speed of sound in haberdasher's
| elastic.

That's fine. The velocity of sound in haberdasher's elastic is
the velocity of the train plus the velocity of sound in
haberdasher's elastic, relative to the track the train is moving on.



| Relativity teaches that no effect can propagate instantaneously.

Einstein's relativity is nonsense, that is the only lesson to be learnt
from it. The OP made no mention of relativity, his question concerned
gravity.


| It is elastic because it returns to its original shape when the force
| is removed. By the principle of relativity the caboose is pulling the
| locomotive backwards.
|
| No; by the principles of Sir Isaac Newton. You should read the
| Principia.

Yes; by the principles of Sir Isaac Newton.

To every action there is always opposed an equal reaction; or the mutual
actions of two bodies upon each other are always equal, and directed to
contrary parts.
Whatever draws or presses another is as much drawn or pressed by that other.
If you press a stone with your finger, the finger is also pressed by the
stone. If a horse draws a stone tied to a rope, the horse (if I may so say)
will be equally drawn back towards the stone: for the distended rope, by the
same endeavour to relax or unbend itself, will draw the horse as much
towards the stone as it does the stone towards the horse, and will obstruct
the progress of the one as much as it advances that of the other.

I have read the Principia, you should read and understand LAW III,
Dr. Nonsense.
Now why don't you be a good little boy, go away and stop arguing about
things you have no knowledge of, you pompous, arrogant and ignorant idiot?


| Incidentally, the speed of sound in a rigid body is infinite. No
| body is truly rigid,
|
| True.
|
|
| If you are going to scream "nonsense" first check your facts, idiot.
|
| This is one of those rare occasions on which you should take your own
| advice.

And you should shut your ****ing mouth, you have your foot in it.

Now be a good little boy and go and read Principia. Come back when

you have a question.

In article ,
Steve2 wrote:

A star is a billion light years from earth. Today, we see the light
it emitted a billion years ago.

A billion years ago, it was more massive than it is today. That
star's gravity is acting on the earth today, ever so slightly.

The star's mass wasn't destroyed; it was radiated away. So the change
is effectively a movement of mass and the question of how fast it
affects distant objects is the same as the case of gravity from, for
example, a moving moon.

Question: Is the gravity we "feel" from that star a result of its
billion-year-ago mass, or its mass today?

According to general relativity, the change propagates at the speed of
light. It's apparently hard to verify experimentally as the
interpretation of the data will itself depend on general relativity.

-- Richard
--
:wq

Dear Steve2:

On Jan 9, 6:57*pm, Steve2 wrote:
A star is a billion light years from earth. *Today, we see the
light it emitted a billion years ago.

A billion years ago, it was more massive than it is today.
*That star's gravity is acting on the earth today, ever so
slightly.

The star's mass is leaving by:
- radiation of light
- stellar wind

Question: Is the gravity we "feel" from that star a result of its
billion-year-ago mass, or its mass today?

[Not a quiz; I don't know the answer, and have been intending
to ask this for some time.]

The stellar light that you see is passing you, so the gravitational
effect is decreasing as you see it, but represents a source with the
total mass it was when the light left. The stellar wind probably did
not drift far, so that effect is still largely "over there".

You cannot really get to "the speed of gravity" this way either.

David A. Smith

In sci.astro message
oglegroups.com, Wed, 9 Jan 2008 17:57:14, Steve2
posted:
A star is a billion light years from earth. Today, we see the light
it emitted a billion years ago.

A billion years ago, it was more massive than it is today. That
star's gravity is acting on the earth today, ever so slightly.

Question: Is the gravity we "feel" from that star a result of its
billion-year-ago mass, or its mass today?

It is generally thought, but not necessarily proven, that gravity
travels at the speed of light.


But the situation must be considered with adequate care.

If, a billion years ago, mass was magically dematerialised from the
star, then the effect of that change would be arriving about now.

But mass is conserved.

Material emitted by the star omnidirectionally at speeds much less than
that of light can be considered as a nested set of spherical shells, and
a fairly simple geometrical argument (Newton, Principia; JRS,
gravity1.htm) shows that the field outside the shell is as if the
material were at the centre. Therefore, we will still be feeling the
same gravity from that material, and will feel each shell until we are
inside it.

For other material emitted at speeds much less than that of light, the
centre of mass of the system will be unaffected, and the gravity felt
here therefore will not be much affected.

The spherical shell argument needs further consideration for photons and
fast particles, because of the finite speed of gravity; the question
needs a proper relativistic treatment.


As you acknowledge, the effect on us of a star a billion light years
away will be negligible - the effect on us of the star 1 A.U. away is
about (2 pi / (86400*365))^2 * 150e9 ~ 6 mm/s/s ~ 0.6 milligee, to be
reduced by a factor of (1e9 * 365 * 1440 / 8)^2 to get to a billion
light years.

--
(c) John Stockton, Surrey, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
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