1. I accept that I am under the control of a higher power (Muppet).

### Re: The speed of gravity

Gravity does not have a speed.  Gravity has an accelleration, a force.  Gravity does not have a velocity.

Please fix the header, it makes us sound more like morons than we normally do.
SaveTheHubble
March 5th, 2007 11:37am
Allan, when an object moves, what is the time lag between that movement and another object being affected by the new location?  If the sun were to suddenly vanish, how long before the Earth was no longer in a nearly circular trajectory?
Aaron F Stanton
March 5th, 2007 11:39am
The discussion was about the speed of gravity. How quickly does gravity affect change? Not how quickly does it move two objects together.
strawberry snowflake
March 5th, 2007 11:42am
Velocity is accelleration over some unit of time.  Gravity does not HAVE a unit of time.

I'm not saying gravity won't accellerate things over time, nor that accellerated things don't acquire a velocity.

I AM saying that gravity is a Force, not a result of Force applied over Time.  "Speed" is a quantity resulting from Force applied over Time.

You might just as well claim that gasoline has a 'speed' associated with it, since gasoline is used to accellerate automobiles.
SaveTheHubble
March 5th, 2007 11:58am
Oh, wait, you're talking about the "speed of the affect of gravity"?  The delay in space-time between when a mass comes into existence, and when that mass begins affecting neighboring masses?

Ah.  English Verb Ambiguity.  Sorry about that.
SaveTheHubble
March 5th, 2007 12:00pm
I suppose if you wanted to be really precise, it could be "the rate of propagation of change in local curvature of spacetime".
Aaron F Stanton
March 5th, 2007 12:01pm
Isn't that the speed of light?
JoC
March 5th, 2007 12:04pm
i know fuck-all, but when you are talking conceptually about "time difference" between events in a framework where space and time are somehow functions of each other - does the concept of "the time taken for something to go from x to y" actually have a meaning, and if so what?
$-- March 5th, 2007 12:05pm Yes, you can talk about a spacelike separation of events, as well as a timelike separation of events. Aaron F Stanton March 5th, 2007 12:07pm Apparently, all this comes from the 'graviton' thread below. And yes, that thread arrived at "the speed of light" as the answer. But then the question is, can't the propagation of the gravity affect travel slower than the speed of light? And how do we know? I'm a little curious at this point, if E=MC^2, does Energy curve space the way mass does? I would suspect not. If it does not, then the amount of mass transformed into energy in a supernova would 'remove' mass from the curved space-time, and that mass removal should generate 'gravity waves', which would propagate. Also, the reason Light travels slower in some materials I thought was because of interactions between the Light and the material. Gravitic interactions seem to be much less than Light interactions, so I would assume it's affects would travel at the speed of light in a vacuum. Nothing to slow it down, you see. SaveTheHubble March 5th, 2007 12:12pm > I AM saying that gravity is a Force How can it be force when it is simply a warping of space-time? son of parnas March 5th, 2007 12:12pm Not real sure why I thought that. I suppose because it is the theoretical limit. But it is a limit on the maximal velocity of matter, right? I did have an initial inclination to go with instantaneous. Isn't gravity sort of the monkey wrench in a unifying theory? Only to say, we don't really seem to understand/know that much about what gravity actually is. JoC March 5th, 2007 12:13pm Oh, and the "simultaneous event times requiring some independent observer, which doesn't really exist" idea bothered Einstein too, I understand. SaveTheHubble March 5th, 2007 12:13pm right, but you are talking about the distance between two point in space-time, not in space, right? for instance, that could be the distance between two points that are identical in space, but distant from each other in time. The time interval between two points in space-time must be the difference in their positions along the time access, no? So if we say that somthing happens, "instantaneously" (like the effect of a gravitational field) that means that it happens at the same point on the time access. so doesn't that mean that it's still instantaneous in space time?$--
March 5th, 2007 12:14pm
If we had some wormholes we could do some experiments. Like, hey, wow, this mass just appeared here in Lucerne. When will the moon start to fall faster toward the earth? Immediately or will it take a few seconds.
strawberry snowflake
March 5th, 2007 12:14pm
> Immediately or will it take a few seconds.

When an apple falls to the earth doesn't the earth instantaneously fall a little towards the apple? That's what Newton said anyway.
son of parnas
March 5th, 2007 12:19pm
So these huge colliders and whatnot that they are building will do some interesting things creating black holes and such.

They don't really know what all will happen in these experiments. I wouldn't go so far as to advocate against whatever floats their boat.

But do you think we'd notice if we got enveloped by a black hole? Because I'm thinking that's when my summers ended.
JoC
March 5th, 2007 12:19pm
Energy does curve space the same way mass does, but it is more commonly observed the other way round--for example light is affected by gravity and follows the curvature of space-time.

You don't need to create or destroy mass to make gravity waves. Unsymmetrical movement of mass, like rotating binary stars, can do it by "stirring" space.
bon vivant
March 5th, 2007 12:21pm
Well, SoP, gravity IS.  It's what makes the apple fall, instead of hovering in the air.

So, given that, several models have been developed to try to explain that.  One of the simpler models is the Force model, that there's some un-explained attraction between matter, depending on that matter's mass, which we call Gravity.

A much more sophisticated model explains that attraction as a curving of space-time, that the curved space-time IS how Gravity works.  And that curved space-time results in what LOOKS like a 'local force' that tends to draw two masses together.

And as Einstein argued, whether you're standing in an elevator on the surface of the earth, feeling the earth's gravity on you, or you're in that same elevator in the middle of "empty" space, being accellerated by a rocket at 1 Gravity accelleration, there is no perceptual difference you can use to tell the difference between the two situations.

So, "Force" or "Curved Space-Time" are just two ways to explain how and why Gravity works.  "Gravitons" are an attempt to have a mathematical "carrier of gravitic property".
SaveTheHubble
March 5th, 2007 12:22pm
The apple and the earth are already there. The question we're asking is ... if new matter just appears, how long will it take before it starts messing with the gravitational field? How long before the objects around it 'feel' its presence? Why should this effect take the speed of light to propagate? Why not sooner? Why not slower?
strawberry snowflake
March 5th, 2007 12:22pm
> So, "Force"

Force is wrong IMHO because it uses an incorrect metaphor. When I push something that is force. Gravity doesn't appear to be the same.
son of parnas
March 5th, 2007 12:26pm
Trying to resolve that issue is what forced Newton to become a mystic.
strawberry snowflake
March 5th, 2007 12:27pm

It should propagate at the speed of light because that's the fastest we've seen anything propagate in this universe.

The only time light does not propagate that fast ALL the time is because it is interacting with things in its path.  Implying light DOES propagate that fast ALL the time, it's just when there's stuff in its path it takes a longer path throught the stuff.

Since we've seen NOTHING to 'slow gravity down' (anti-gravity would be SO useful for that perpetual motion machine) it should propagate as fast as any phenomenon in our space-time, which would be light-speed.

If the affect of gravity COULD propagate faster than that, it would be nice.  We could use that for instantaneous communications.  I believe they're currently trying to make gravity wave detectors to time-correlate the arrival at the earth of gravity waves from a nova, with the time the light of the nova arrives.
SaveTheHubble
March 5th, 2007 12:30pm
> It should propagate at the speed of light because that's the fastest we've seen anything propagate in this universe.

We see quantum entanglement that is faster than light.
son of parnas
March 5th, 2007 12:32pm
See Einstein-Podolsky-Rosen paradox ... some things (information about a quantum state) can propagate faster than the speed of light.
strawberry snowflake
March 5th, 2007 12:33pm
I think Allan's use of force is correct here.

It's better to think of the apple and the earth as attracted towards each other by the force of gravity than to think that they are falling towards each other. The force follows an inverse square law.
Senthilnathan N.S.
March 5th, 2007 12:35pm
why can't information about change in mass also travel instantaneously?

"Yo, baby got a boob job, them bazookas got massive. You better up the attraction."
strawberry snowflake
March 5th, 2007 12:37pm
Newton's first law of motion --

I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

If Gravity were not a "Force" as called out in this law, then the apple would hang in the air until you reached out and touched it, applying what you DO recognize as a "Force".

Since, instead, the apple behaves as if you had reached out and pushed it toward the ground, therefore the apple is behaving AS IF Gravity was an equivalent "Force" to that you would apply with your hand.

Now, that "AS IF" could be one of those Engineering Simplifications, and Gravity could be ACTUALLY behaving quite differently from being pushed with your hand.  That's quite likely, in fact.  The EFFECT of the whole situation can be summarized by that "Force" equation.

Oh, and law II:
II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors. In this law the direction of the force vector is the same as the direction of the acceleration vector.

So, "by definition", if you have a mass, and it undergoes accelleration, there IS a "force" being applied.

And, just for completeness, Newton's third law:
III. For every action there is an equal and opposite reaction.
SaveTheHubble
March 5th, 2007 12:38pm
> Newton's first law of motion

Newton's laws are descriptive, they don't mean the world actually works that way.
son of parnas
March 5th, 2007 12:40pm
You all realize the question is about a change in force not the force itself? That's like so 10th grade.

dF/dt, baby, dF/dt.
strawberry snowflake
March 5th, 2007 12:41pm
...they gibbered...
zestyZuchini
March 5th, 2007 12:49pm
Isn't the 'default' for everything to be drawn together into a singularity?

The motion of objects towards one another that we see as gravity is present at all times. It takes force to defy the natural state.

The expansion of the universe is the only thing that keeps that in check, preventing an implosion of the entirety back into a singularity of all mass.
JoC
March 5th, 2007 12:57pm
I think some of you are making this up.  it _sounds_ impressive, but Im getting suspicious about the actual semantic content.
zestyZuchini
March 5th, 2007 1:00pm
Yes, SoP.  That's why I'm an Engineer, and not a Scientist, or a Magus.

Because I'm WAY more interested in doing useful things with our understanding of the universe, than I am in getting written down the exact mechanism of how these things work.

People have been using "Gravity as force" since Newton to make useful predictions, without complete understanding (even now) of how Gravity works.

Don't get me wrong, I have nothing against getting written down the exact mechanism of how stuff works.  It's just that I'd MUCH rather be sending missions to the Moon and Mars and collecting data, (and detecting asteroids that might end civilization) while others spend time in labs working out the minutia.

Give me a sufficient level of minutia to make useful predictions, and I'll build working solutions.
SaveTheHubble
March 5th, 2007 1:04pm
> Give me a sufficient level of minutia to make
> useful predictions, and I'll build working solutions.

That's why I have a hard time sometimes. It's difficult for me to accept useful abstractions as real enough to make a working model in my mind, so it just falls apart and I don't understand anything.
son of parnas
March 5th, 2007 1:08pm
http://www.cbc.ca/health/story/2000/07/20/speedlight000720.html

Kinda relevant I guess. Interesting at least.
Mikael Bergkvist
March 5th, 2007 1:22pm
Good instincts, zucchini. My undergraduate degree is in physics, and at first I thought they must have a deeper understanding of it than me, then I decided it was bs.
no label
March 5th, 2007 1:26pm

"The scientific statement "nothing with mass can travel faster than the speed of light" is an entirely different belief, one that has yet to be proven wrong. "

so does gravity have mass?  cause if not they appear to have proven it is possible for it to travel faster than the speed of light.
zestyZuchini
March 5th, 2007 1:31pm
> Isn't that the speed of light?

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html
Senthilnathan N.S.
March 5th, 2007 1:36pm
>> "nothing with mass can travel faster than the speed of light"

Where is this statement from? I only know that "nothing with mass can *accelerate* beyond the speed of light".
के. जे.
March 5th, 2007 1:56pm
Woah, Senthi, GREAT link, man!  Thanks!
SaveTheHubble
March 5th, 2007 2:34pm
This is the problem with degrees, it's more fun to argue the particulars than to answer the man's simple question:

http://en.wikipedia.org/wiki/Terminal_velocity
For example, the terminal velocity of a skydiver in a normal free-fall position with a closed parachute is about 195 km/h (120 mph or 54 m/s).

Since I basically knew this, it only took me a minute to find it, and I didn't have to read any of those long replies.
LinuxOrBust
March 5th, 2007 2:35pm
We're talking about jerk not velocity. That's two derivatives behind, bro.
strawberry snowflake
March 5th, 2007 2:38pm
Well, yes, LorB, but since you didn't read the replies, you never realized that the original issue WAS in fact "How fast does the effect of gravity propagate", NOT "what is terminal velocity in the earth's atmosphere".

In other words, if the Earth's Sun suddenly vanished, would that effect on the orbit of the Earth propagate instantaneously?  Or would that effect propagate at the speed of light?

Current conclusion, the Gravity effect propagates at the speed of light (certain binary stars are 'slowing down', which wouldn't happen if it was infinite), but we're working to verify that.
SaveTheHubble
March 5th, 2007 2:44pm
Strawberry, sing along time.

Momentum equals mass times velocity!
Force equals mass times acceleration!
Yank equals mass times jerk!
Tug equals mass times snap!
Snatch equals mass times crackle!
Shake equals mass times pop!
के. जे.
March 5th, 2007 2:47pm
I dont think gravity has speed, but objects reacting to it has, so a wave would mean a chainreaction of stellar objects shifting position, seeming like a wave moving with the speed of light.
This is assumable so because gravitation appears to have accumulative properties, meaning that a group, like the solar system can be seen as a single source of gravity.
So, gravity would include objects outwardly, affecting them in an accumulative order, moving forward with he speed of light in doing so, even though gravity in itself might have no speed and could theoretically affect distand objects instantly.
Mikael Bergkvist
March 5th, 2007 3:09pm
Einstein's equations form the fundamental law of general relativity. The curvature of spacetime can be expressed mathematically using the metric tensor — denoted gμν — and with respect to a covariant derivative, \nabla, in the form of the Einstein tensor — Gμν. This curvature is related to the stress-energy tensor — Tμν — by the key equation

G_{\mu \nu} = \frac{8\pi G_N}{c^4} T_{\mu \nu}\ ,

where GN is Newton's gravitational constant, and c is the speed of light. We assume geometrized units, so GN = 1 = c.

With some simple assumptions, Einstein's equations can be rewritten to show explicitly that they are just wave equations. To begin with, we adopt some coordinate system, like (t,r,θ,φ). We define the "flat-space metric" ημν to be the quantity which — in this coordinate system — has the components we would expect for the flat space metric. For example, in these spherical coordinates, we have

\eta_{\mu \nu} = \begin{bmatrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & r^2 & 0 \\ 0 & 0 & 0 & r^2 \sin^2\theta \end{bmatrix}\ .

This mathematical structure holds information regarding how distances are measured in the space we are dealing with. Because the propagation of gravitational waves through space and time change distances, we will need to use this to find the solution to the wave equation.

Now, we can also think of the physical metric gμν as a matrix, and find its determinant, \det\ g. Finally, we define a quantity

\bar{h}^{\alpha \beta} \equiv \eta^{\alpha \beta} - \sqrt{|\det g|} g^{\alpha \beta}\ .

This is the crucial field, which will represent the radiation. It is possible (at least in an asymptotically flat spacetime) to choose the coordinates in such a way that this quantity satisfies the "de Donder" gauge conditions (conditions on the coordinates):

\nabla_\beta\, \bar{h}^{\alpha \beta} = 0\ ,

where \nabla represents the flat-space derivative operator. These equations say that the divergence of the field is zero. The full, nonlinear Einstein equations can now be written[1] as

\Box \bar{h}^{\alpha \beta} = -16\pi \tau^{\alpha \beta}\ ,

where \Box = -\partial_t^2 + \Delta represents the flat-space d'Alembertian operator, and ταβ represents the stress-energy tensor plus quadratic terms involving \bar{h}^{\alpha \beta}. This is just a wave equation for the field with a source, despite the fact that the source involves terms quadratic in the field itself. That is, it can be shown that solutions to this equation are waves traveling with velocity 1 in these coordinates.
Ward
March 5th, 2007 3:22pm
See?  Simple...

(cut and pasted from Wikipedia, too much trouble to type in the same stuff from Landau and Lifshitz)
Ward
March 5th, 2007 3:23pm
If we allow our selves a little "flight of fantasy" and free speculation, then it becomes a problem that gravity is supposed to travel by the light of speed, since it holds together great masses across the universe that are *very big*, sometimes spanning several hundreds or more, lightyears.
With this *information* moving this slow, it's hard to understand how these structures keep together the way that they do.

It's like the signals of a humans nervous system would travel at the speed of a slow ant climbing up a (insanely high) tree.
Not very fast that is..
Mikael Bergkvist
March 5th, 2007 4:12pm
>>> With this *information* moving this slow, it's hard to understand how these structures keep together the way that they do.

What's the problem?  So what if it's slow, the structures you're talking about have been around for a loooooooooooooooong time.

A really long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long
time.
Ward
March 5th, 2007 4:26pm
> It's like the signals of a humans nervous system would travel at the speed of a slow ant climbing up

Perhaps gravity is intelligent so like ants it may be slow but it can do a tremendous amount in a parallel and autonomously?
son of parnas
March 5th, 2007 4:29pm
It's a known problem.
Mikael Bergkvist
March 5th, 2007 4:31pm
> does the concept of "the time taken for something to go from x to y" actually have a meaning,

Yes.

> and if so what?

Banana.

March 6th, 2007 9:14am