
What is the Speed of Gravity? - sdfx
http://scienceblogs.com/startswithabang/2010/08/what_is_the_speed_of_gravity.php
======
ewjordan
_The Sun, as it is right now, won't have its gravity affect Earth for another
8+ minutes, and the gravity that the Earth feels right now pulling it towards
the Sun is actually pulling it towards where the Sun was 8+ minutes ago!
(Weird, isn't it?)_

I'm almost positive this statement is incorrect.

Relativistic force laws tend to be forced to contain correction factors that
ensure that constant velocity motion is "predicted" and the direction of the
force is adjusted accordingly - as long as a body is not accelerating, a
purely attractive or repulsive force will be pointing at its current position,
_not_ its position 8 mins ago.

To see why this must be the case for a repulsive force, at least, imagine two
charges riding on frictionless rails that keep them at a constant, finite
distance. Suppose they're both moving with some constant velocity (to start,
at least) in the same direction - now the place that the force from the other
charge _appears_ to be coming from is behind the charge, so if there was no
correction factor, each charge would be getting an extra push forwards. This
would lead to a runaway "bootstrap" acceleration, and the particles would
accelerate to the speed of light. That's a pretty clear violation of the
conservation laws, so...

With attractive forces like gravity, there's still a conservation problem,
since the charges would slow down to zero speed eventually, but it's always
more convincing to cite the runaway solution as a violation of conservation
laws than the run-down one, because while the energy could possibly leak out
of the charges into the fields, there's nowhere to pull infinite energy from,
so it's pretty clear there's a problem if we'd need to.

And this is somewhat different from the runaway self-action solutions that we
grudgingly "accept" in E+M, because those tend to involve some limit to
infinitesimal size, whereas this is a completely finite situation that we
could theoretically set up in the real world with a couple of charged beads or
something like that.

~~~
achille
Wait, so you are saying we would feel the effects immediately, but the light
would not disappear for eight minutes?

So let's assume once can cause the sun to vanish instantly somehow. If that's
the case, would it then be possible to setup a 'gravitational wave'
communication system that travels faster than light?

Ie, I would put a whole bunch of stars together, and then have them disappear
at certain intervals. My buddy who sits 100 light-minutes away would then
detect the changes and decode the message. TADA: Faster-than-light
communication.

I feel safe to say that's not possible.

~~~
gnok
Out of curiosity, why do you think this is not possible?

~~~
achille
You can't transmit data faster than the speed of light.

~~~
benJIman
<http://cdsweb.cern.ch/record/1122131/?ln=cs>

~~~
sliverstorm
I love the names they give to things nowadays. In this case, specifically
"Spooky action"

~~~
pavel_lishin
I think that when it comes to the term "spooky action", "nowadays" would be
around the time Einstein was working on all of this.

------
acqq
The point of the article is in

Clifford M. Will

"The Confrontation between General Relativity and Experiment" (2001)

<http://relativity.livingreviews.org/Articles/lrr-2001-4/>

"Gravitational wave damping has been detected in an amount that agrees with
general relativity to half a percent using the Hulse-Taylor binary pulsar,"

I also have something to say to the first question in comments there, to which
the article author answered that he can't answer. The question was "Is the
speed of gravity reduced by the medium through which it travels in a analogous
manner to the slowing of the speed of light through various media?"

I was lucky enough to read Feynman Lectures recently, so as far as I
understood him, the photons themselves don't really slow down in various
media. That is, what we see as the result is the slowdown, but only because
it's a new light (new photons) on the other side of the media. The photons on
the other side are photons which were pushed out of the atoms of the media,
that's why it appears that they come out slower. Inside, between the atoms of
the media, photons still move at the speed of light.

~~~
randallsquared
Something that puzzles me about "the speed of light in X" discussions is
exactly the confusion over whether the speed of light is "different" in a
medium. On the one hand, it seems like it isn't, since no actual photons are
moving more slowly. On the other, things like Cherenkov radiation seem to
indicate that the apparent speed of light matters for things that you'd expect
to be based on the constant speed of light in a vacuum. There's a physicist,
Ronald Mallett, who has suggested an experiment to see if the constant speed
of light or the apparent speed of light is what matters for frame-dragging
(!).

~~~
acqq
But AFAIK Cherenkov radiation occurs in medium, so there isn't any
contradiction to the generally accepted limits, and Ronald Mallet is highly
unlikely to be trusted, see the wikipedia entry:

<http://en.wikipedia.org/wiki/Ronald_Mallett>

~~~
randallsquared
I was under the impression that tachyons (should they exist) would be expected
to give off Cherenkov radiation in vacuum, but I'm not sure where I got that
impression.

I hadn't actually heard anything about Mallett for a few years, and I guess
this is why. :)

~~~
ars
That would imply that tachyons interact with the electromagnetic force.

But if they do we would have detected them long ago.

But it's wrong anyway, it's not the moving particle that gives off the
Cherenkov radiation, it's the matter that they move near. And there is no
matter in a vacuum.

~~~
eru
Only virtual matter.

~~~
ars
Virtual matter can not give off photons - it would violate conservation of
momentum (unless it was in pairs, but that would not work with Cherenkov
radiation).

~~~
eru
There seems to be one paper about Cherenkov radiation in a vacuum [1]. But I
have only read the abstract. So you are probably right, that there's no
Cherenkov radiation in a vacuum. Or does anyone has any better knowledge?

[1] "Cherenkov radiation in vacuum and plasma-filled microwave sources in the
absence of guiding magnetic fields" by GS Nusinovich

------
ccarpenterg
I've always thought what we call 'speed of light' is the indirect observation
of a structural property of (this) the universe. A property that has something
to do with movement in general and not only the movement of light (the
movement of space itself?).

Another interisting property is the 0º Kelvin (absolute zero)
<http://en.wikipedia.org/wiki/Absolute_zero>

~~~
gigafemtonano
I'm pretty sure that Einstein intuitively "guessed" that the speed of light
would be the fastest possible speed and used that fact to work out the rest of
the equations. E = mc^2 was the bizarre discovery that those equations
yielded, and it was so weird that he felt like he had made a mistake at first.

~~~
splat
That's not quite right. Einstein (and several other physicists of the time)
inferred that the speed of light might be invariant in all reference frames
from the structure of the Maxwell Equations where the speed of light enters as
a true, fundamental physical constant of the universe.

~~~
gigafemtonano
From the actual paper: <http://www.fourmilab.ch/etexts/einstein/specrel/www/>

_We will raise this conjecture (the purport of which will hereafter be called
the "Principle of Relativity") to the status of a postulate, and also
introduce another postulate, which is only apparently irreconcilable with the
former, namely, that light is always propagated in empty space with a definite
velocity c which is independent of the state of motion of the emitting body.
These two postulates suffice for the attainment of a simple and consistent
theory of the electrodynamics of moving bodies based on Maxwell's theory for
stationary bodies._

------
Tycho
Reminds me of a thought experiment that's always bugged me: if you had an
incredibly long see-saw or lever, like the length of the galaxy, would
movement at one end instantaneously be matched by movement at the other end?
(I think the reason it bugs me is because the object itself is preposterous,
but I still want to know)

~~~
Luc
If it were a physical object (say, a long steel bar), the atoms would
communicate at the speed of light, so your object would bend or squish, with a
wave travelling along it.

Now, let's say you move an ideal laser across an ideal screen some distance
away. The further the screen is, the faster the laser dot will move. At some
point the dot will move faster than c... But the dot is not an object. If you
think in terms of individual photons, no speed limit is being broken.

IANAPhysicist...

~~~
philh
>Now, let's say you move an ideal laser across an ideal screen some distance
away. The further the screen is, the faster the laser dot will move. At some
point the dot will move faster than c...

I'm pretty confident that you're wrong about this, but I don't really
understand the scenario you're describing. Could you clarify?

edit: I eat my words and stand corrected.

~~~
Luc
For fun: the moon has a radius of about 1730 km. At full moon we can see about
half of the moon, so the distance from one side to the other is about pi *
1730 km. So how fast do you need to move a laser pointer across the moon
surface to make the dot attain the speed of light?

[http://www.google.com/search?q=%28%281730000+*+pi%29+m%29+%2...](http://www.google.com/search?q=%28%281730000+*+pi%29+m%29+%2F+c+in+s)

A bit less than 2/100 of a second. That seems quite feasible really!

(of course the moon is not a flat screen, but hey, back of the envelope and
all that).

~~~
Tycho
So say the dot was fairly large and you had a vantage point on the moon near
where the dot sweeps by... What would you actually see??

~~~
Luc
I think philh already explained it pretty well. In your question you suppose
the laser dot to be 'fairly large', which I think would complicate things (the
shape of the dot 'smearing out' as it goes faster). For me, philh's
explanation works best if you imagine the laser pointer to be a cannon of
photons, emitting one photon after another at a very high rate and in a single
direction, with the electrons of the moon's surface always reflecting the
photons towards your eye (perhaps a bunch of strategically placed mirrors...).

~~~
jerf
Actually, the shape of the dot is unchanged by going faster. Since it isn't a
real object, it isn't affected by relativistic contraction. If you emit a
circle, it'll be a circle.

It is of course changed by what it gets reflected off of, but that's not
special to this situation at all. If you're at the center of an enormous
hollow sphere and shine an astronomically-bright laser circle around, it'll
always be a circle to you no matter how fast you move it.

~~~
Luc
If the laser emits all photons constituting the dot at the same time, in
pulses... I was thinking of the photons that make up the dot arriving at
random intervals, but I'm probably making things too complicated.

------
Andrew_Quentin
I am not a physicist nor an astronomer. I wanted to ask however, when they
explain gravity by the analogy of holding a blanket, first the blanket is on
earth thus any object that is put on it is being pulled down by earth and the
blanket is being held by two people, thus what is pulling the earth and sun
down and who is holding the space blanket? Also, seeing some of the images on
the site, it is clear that space is something and it is not nothing. Thus,
what is space?

The second thing is that if you carry out the above experiment, the ball would
eventually stop. That is because of friction. Why does the earth never stop?
What makes it overcome friction?

Someone is probably going to shoot me down for being a complete idiot and
knowing nothing, but I am genuinely interested to know the answer or to know
if there is no answer.

~~~
lutorm
It's an analogy, and as such it has limited applicability. Noone is "holding"
space, it's infinite. The blanket is two-dimensional and embedded in the three
dimensions of our world, hence it has boundaries. Space is, well, space. It's
not embedded in anything. This is quite nonintuitive and here's where the
blanket analogy hurts more than it helps.

The best way I know how to think about curved spacetime is by analogy to the
Earth. We are effectively two-dimensional on the Earth's surface, as the
radius of curvature is much larger than we are. Yet you can imagine drawing
straight lines (i.e., great circles) on the ground to make (large) triangles
and discovering that the angles don't add to 180 degrees and hence deduce that
the surface is curved.

It's sort of the same way with space. Since we are three-dimensional, we can't
directly perceive the curvature as we can an embedded two-dimensional object.
A straight line would always look straight to us, yet we could _measure_ the
curvature by observing that the fundamental theorems of Euclidean geometry
don't hold.

The Earth _does_ slowly spiral towards the Sun. There's no "friction" per se,
but it loses angular momentum to other stuff, like gravitational waves and the
dark matter hslo. It just operates _extremely_ slowly.

~~~
Andrew_Quentin
Do we know that space is infinite?

~~~
lutorm
That depends on what you mean by "know", I guess... According to the current
best estimates of the cosmological parameters, yes, it is infinite. The basic
theory could be wrong, of course, and outside the observable Universe the
properties could be different.

~~~
pavel_lishin
I thought it was finite but unbounded.

~~~
lutorm
That would have been the case if the universe was closed, i.e. if the matter
density was above the critical density. However, the matter density is only
about 20% of the critical, which means it's infinite. (The remaining 80% of
the critical density seem to be the cosmological constant, which means the
universe has flat geometry to within experimental error.)

------
sliverstorm
It always seems rather low-class to take shots at Newton as if he was a fool
for coming up with Newton's theory of gravity instead of Einstein's.

~~~
doki_pen
Where was there at shot taken a Newton? AFAICT, he explains how Newton's
theory addressed the problem, but there is no shot taken. I've never heard of
such a problem as people taking shots at Newton..

~~~
sliverstorm
There's no single remark, it just seemed somewhat condescending

------
jockc
I hate that curved space graphic. Now you have some mysterious gravity pulling
both the sun and earth down. I wish someone would come up with a better
example to show gravity as curved space.

~~~
Eliezer
But gravity isn't curved space, it's curved spacetime. Think of a graph that
shows both space and time, so that it shows constant motion as a straight line
(four units of space per one unit of time with the slope constant). Then
acceleration is a curved line. Now curve the graph paper - not through some
higher dimension, but by stretching it and compressing it in ways that change
the shortest distances between two points - so that the Earth orbits the Sun
in a straight line. Not a straight line in space, mind you, but a straight
line in spacetime: no acceleration. After all, since no one is pushing on the
Earth, it _should_ move in a straight line, right? Got it?

~~~
ars
Why should it move in a straight line? If no one is pushing it, it shouldn't
move at all.

That graphic is just wrong, and I also hate it.

And gravity isn't the cause of the curvature! It's the effect. The cause is
energy (often in the form of mass).

~~~
Eliezer
> If no one is pushing it, it shouldn't move at all.

Thank you, Aristotle. Objects not being pushed on continue with constant
velocity, they do not stay motionless.

~~~
ars
So?

Electrostatic force also causes objects to move. Just making an object move
does not make gravity special.

------
1053r
This is all well and good, but what the article is talking about is the speed
of the propagation of gravity waves. I've never heard anyone who wasn't a
crank argue that gravity waves travel at anything other than the speed of
light. As the article confirms, observation of pulsars has confirmed the speed
of gravity waves to very close to the speed of light.

However, I think the larger objection goes something like this:

Gravitational attraction contains information about the location of an object
(say, for the purposes of argument, a singularity). Information may not travel
faster than the speed of light, by relativity. Information may not escape the
event horizon of a black hole, because to do so would require it to travel
faster than the speed of light (or to use some funny quantum teleportation
that Hawking describes as the mechanism behind Hawking radiation), but which
is not described by relativity.

Therefore it follows that the information about the location of a singularity
behind the event horizon of a black hole is somehow travelling "faster than
light". This is of course impossible if relativity is correct, which leads to
a big WTF?

My guess, and I am most emphatically not a physicist, is that you get some
funny macroscopic quantum effects near a black hole, which allows Hawking
radiation (and therefore also the encoded information about the objects that
fell into the black hole) out, and also lets out the information about the
location of the singularity itself, so that objects outside the event horizon
can be attracted to it.

Perhaps studying black holes in sufficient depth (pardon the pun) will allow
us to finally unify QM and relativity.

~~~
adamtj
What is the technical definition of "information"? I know what information is
in a colloquial sense, and I know something about it's technical definition in
the field of computer science, but I don't know what physicists mean when they
use the word. Is information the same thing as light? Is it analagous to
light? Is it a property of light? Matter?

~~~
JanezStupar
I guess that information is change of state that is clearly distinguishable
from some "random" change of state and caries a "meaning".

Eg. If you had an entangled pair of particles on this side of galaxy and
another on other side of galaxy. Now you "wiggle" (change its state) one then
the other one "wiggles" too - but you didn't send any information - since the
other observer cannot know if the particle changed state because of your
message - or it changed state "of its own accord". Thus you would still need
to notify him of you wiggling the first particle eg. via photon - thus
information only moves at the speed of light :)

Bear with my awful analogy - since I really don't know anything about physics
:)

~~~
eru
Meaning isn't necessary, and randomness is still information. The computer
science definition works well for physics, too.

------
kaffeinecoma
The article asks what would happen to gravity if you suddenly "removed" the
sun. My question is- is that even possible? How could you instantaneously
"disappear" a giant blob of mass/energy? Is this kind of like trying to divide
by zero? I'm not being facetious, just wondering if the whole idea is
predicated on an impossibility.

~~~
SteveC
You're thinking too deeply into that example. It's just a thought exercise to
better understand how gravity works and easier for people to grasp.

------
ck2
Personally I think it's easier to tell people to stop thinking of gravity as a
particle or wave - it's not like a beam of light or radio wave.

Think of the space/time as the blanket the universe is wrapped in. If that
blanket is stretched out like a trampoline, every object on it or moving
around on it is causing flexing and ripples. That's gravity.

When a girl bounces off a trampoline, the specific instance of her gravity is
gone but the ripples from that instance continue on for some time until
everything "settles".

But apparently the goo that is space/time really does not tolerate anything
going faster than the speed of light for some reason. Doesn't mean things
aren't trying to go faster than that, they just cannot achieve it.

~~~
doki_pen
Our reality was build on a 299 792 458 m / s processor. Newer realities are
much faster.

~~~
ck2
Ha! But what if it was the opposite and we are simply a cycle down the road of
big bang/big crunch/big bang/big crunch/big bang where the speed of light used
to be faster but is slowing each cycle because of lost mass/energy.

What if the speed of light was slightly faster or slightly slower, how would
it affect the development/entropy of the universe?

------
thailandstartup
Something I wonder about gravity - While it is often mentioned that objects
with a large gravitational force can 'bend' light (gravitational lens), can a
sufficient amount of light 'pull' objects towards itself?

So I wonder if a star's gravity is mostly the mass of the star itself, but
also, in small part, the huge amount of light that surrounds the star on all
sides, being densest nearest the star itself.

There's some evidence that gravity may change a little during a solar eclipse
<http://en.wikipedia.org/wiki/Allais_effect> and also the Pioneer anomaly,
<http://en.wikipedia.org/wiki/Pioneer_anomaly>

I am most definitely not a physicist!

~~~
sp332
> can a sufficient amount of light 'pull' objects towards itself?

Yup. As the article says, all forms of energy cause gravitational attraction.
Even light! But, as you can see from E=m * c^2, it takes an insane amount of
energy to make even a little mass-equivalent. C=3 * 10^8 m/s, so c^2 = 9 *
10^16 or 90000000000000000 m^2/s^2. So 1kg of mass at rest is equivalent to 9
* 10^16 joules of energy. It's about equal to a 4.5 megaton hydrogen bomb
blast.

~~~
thailandstartup
Mmmm. Okay, but a star puts out a lot of energy, over a long period of time.
Is it enough that we calculate the gravitational effect of all those photons,
rather than just the mass of star itself when calculating say the acceleration
of the pioneer crafts?

~~~
sp332
[http://www.wolframalpha.com/input/?i=3+cubic+au+times+682+ki...](http://www.wolframalpha.com/input/?i=3+cubic+au+times+682+kilojoules+per+meter+squared+per+light-
second&a=UnitClash_*au.*AstronomicalUnits--)

so 2.5 * 10^14 kg inside the earth's orbit, or equivalent to 254 cubic
kilometers of water - on an astronomical scale, insignificant. Still, bigger
than I expected.

------
wccrawford
My first thought was 'Find a way to create some matter and we'll measure it',
but apparently they have a way. ;)

~~~
Locke1689
Your trouble is on focusing on the analogy too much. Instead of, "how can we
measure what the effects of gravity would be if matter or energy were created
or destroyed?" ask "how can we measure the movement of the curvature of
spacetime?"

------
maeon3
The speed of gravity is either infinity or millions of times faster than the
speed of light. If the speed of gravity was the speed of light, then heavy
objects passing near the earth would have more gravity than objects passing
away. Less of the gravity beans would be arriving to be measured. Like the red
shift in light or the doppler effect with sound.

I see gravity as more of a property of the universe, every atom in the
universe is attached to each other atom in some way.

~~~
Locke1689
Wait, what? Did you read the article? The propagation of the curvature of
spacetime happens at the speed of light. If it didn't you could use large
amounts of energy to propagate information faster than the speed of light,
which screws the hell out of causality (using the light cones). If you have a
good math background you can solve for it using the equation.

 _I see gravity as more of a property of the universe, every atom in the
universe is attached to each other atom in some way._

Please don't do science like this. You should use experimentation to
differentiate between theories.

~~~
maeon3
Maybe data could travel faster than light if we find a property of the
universe which goes faster than light.

I imagine a bowling ball rolling down a blanket stretched tight at half light
speed. The bowl shape of the blanket in front of the bowling ball would be
very different from the rear. I'm just pointing out that if gravity goes light
speed, then something coming toward the earth at half light speed will have a
longer delay for us to feel the gravity, and as the object leaves the earth,
its gravity will linger longer. Maybe it's true, it boggles my mind.

If gravity is light speed, then an object travelling toward you at light speed
will have no gravity until it has touched you. We should be able to detect
shock waves like a jet breaking the sound barrier breaking all the windows in
a community because the sound builds up. Gravity would build up too.

~~~
ars
That's not how you add speeds in a relativistic world.

If something is moving at the speed of light, then light (or gravity) is
emitted from it also at the speed of light.

But if you add the two together, it doesn't equal twice the speed of light.
Yes, I know it's weird, but that's where the whole time dilation comes in.

