"The only problem with the speed of light, is it gets here too early in the morning." -Danny NevrathOne of the most common questions I get asked is whether gravity is instantaneous, or whether there's a speed limit to how fast the force of gravity can travel.
It's a good question! After all, we know how fast light travels, and if the Sun were to suddenly wink out of existence, we'd still receive light from it for just over 8 minutes after it disappeared! But what about gravity, and the Earth's orbit? Would the Earth simply fly off in a straight line, like a twirled poi ball the instant a string broke?
Or would it continue to move in its planetary for some time, and perhaps suffer some more interesting effects? Believe it or not, this is one of the most severe differences between Newton's old school theory of gravity and Einstein's General Relativity. According to Newton, you have two masses separated by a distance, and that determines the force. You take one of those masses away, and the force instantly goes away. End of story.
But in general relativity, things are much more intricate, and incredibly interesting. First off, it isn't mass, per se, that causes gravity. Rather, all forms of energy (including mass) affect the curvature of space. So for the Sun and the Earth, the incredibly large mass of the Sun dominates the curvature of space, and the Earth travels in an orbit along that curved space.
If you simply took the Sun away, space would go back to being flat, but it wouldn't do so right away at every point. In fact, just like the surface of a pond when you drop something into it, it snaps back to being flat, and the disturbances send ripples outward!
In Einstein's theory of gravity, these ripples move at the speed of light, not instantaneously.
This is a really amazing idea, and leads me to ask another question. Think about it; if the Earth was stationary, it would feel the ripples in one way, but if the Earth were moving over the surface of space, wouldn't it feel the ripples differently?
It turns out, that while Newton doesn't care what your velocity is, Einstein does. 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?)
The Earth, of course, since it's also moving, kind of "rides" over such a ripple, so that it comes down in a different spot from where it was lifted up. It looks like we have two effects going on: velocity affects gravity, and so do changing gravitational fields.
So, in theory, we know that the speed of gravity should be the same as the speed of light. But the Sun's force of gravity out here, by us, is far too weak to measure this effect. In fact, it gets really hard to measure, because if something moves at a constant velocity in a constant gravitational field, there's no observable affect at all. What we'd want, ideally, is a system that has an object moving with a changing velocity through a changing gravitational field. What would that take?
Something intense, like two neutron stars orbiting each other extremely close together! Occasionally, we get very lucky, and a neutron star emits very regular blips of light, pulsing with incredible precision: this makes it a pulsar! If one of these neutron stars is a pulsar aimed at us, we can test whether gravity moves at the speed of light or not!
Incredibly enough, we've discovered multiple independent binary pulsars with this exact configuration!
Not only is the gravitational source (star #1) moving, but the other object (star #2) is changing its velocity, as it changes its direction in orbit around the gravitational source! Remarkably, this effect causes the orbit to ever-so-slowly decay, which leads to time changes in the pulses!
The predictions from Einstein's theory of gravity are incredibly sensitive to the speed of light, so much so that even from the first binary pulsar system, PSR 1913+16 (or the Hulse-Taylor binary), we have constrained the speed of gravity to be equal to the speed of light with an error of less than 1%!
While we'd love to be able to detect these gravitational waves directly, rather than make an indirect measurement, we're likely going to have to wait until close to 2030. Why? We'll need to have LISA up and running, where it's capable of detecting a system like this and directly measuring the speed of gravitational radiation.
But until then, indirect measurements of very rare pulsar systems like this give us the tightest constraints, and tell us that the speed of gravity is between 2.993 x 108 and 3.003 x 108 meters per second, which is an amazing confirmation of general relativity and a terrible difficulty for alternative theories of gravity! (Sorry, Newton!) And now you know not only what the speed of gravity is, but where to look to figure it out!
Physical Science
Comments
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?
Posted by: Richard | August 25, 2010 8:45 PM
A paper by Van Flandern in Physics Letters A disagrees with you, Ethan. (Warning: craziness level high.)
Posted by: Physicalist | August 25, 2010 9:25 PM
It has long bugged me that this difference between Newtonian and Einsteinian gravitation wasn't detectable in solar system motions, but apparently the near-circular orbits make it hard to measure. Why can't we detect the effect in comet orbits?
By the way, I'm hoping to read some extended commentary on variable radionuclide decay rates, now that lots of people have had the chance to do repeated measurements.
Posted by: Nathan Myers | August 25, 2010 9:27 PM
To quote an observation by Arthur Eddington:
“If the Sun attracts Jupiter towards its present position S, and Jupiter attracts the Sun towards its present position J, the two forces are in the same line and balance. But if the Sun attracts Jupiter toward its previous position S’, and Jupiter attracts the Sun towards its previous position J’, when the force of attraction started out to cross the gulf, then the two forces give a couple. This couple will tend to increase the angular momentum of the system, and, acting cumulatively, will soon cause an appreciable change of period, disagreeing with observations if the speed is at all comparable with that of light.”
So, it appears that Eddington was wrong to believe that there was such a thing as a gravitational “force” acting on the planets.
Although the Sun moves at 250 kps in its galactic orbit, the geometry of its gravitational influence remains in a constantly updated and unchanged state around it, thereby providing all planets with stable and totally predictable orbits.
If it’s all a matter of geometry, then there’s no burning need to assume any kind of speed for gravity. The calculations of GR don’t really appear to need it any more than they need an ether. After all, the answers are indisputably correct and, in the final analysis, that’s what really counts.
Posted by: Alan L | August 25, 2010 10:11 PM
Richard, I don't actually know for certain! I don't know of anything that acts that way on gravity in theory, and I don't know of any observations that indicate that such an effect exists in practice.
Physicalist, that website is the notorious Meta Research by Tom Van Flandern, who, to be polite, is wrong. Check him out if you like: http://en.wikipedia.org/wiki/Tom_Van_Flandern
Nathan, the post-Newtonian effect is simply too small. Not because of the circularity of their orbits, but because of the incredible weakness of the Sun's gravity at the distance of Solar System objects.
Alan, the thing that Eddington likely goes on to say is that the effect of Jupiter having a non-zero velocity cancels out that effect almost exactly if the speed of gravity is c.
However, if you only do the calculation accounting for the retarded position alone, without accounting for the velocity of Jupiter, that would reasonably cause the effect that your quote talks about.
Posted by: Ethan Siegel | August 25, 2010 11:53 PM
You're starting from the wrong premise. Gravity is not a force like electro-magnetism, gravity is the name we give to the phenomenon that results from the bending of space-time by mass. When an object responds to this curvature it is responding to local conditions, information is not being transmitted at any time.
Now what if the mass in question were to be suddenly removed? Then the curvature would go away, because there would be nothing to curve space-time. Would this violate the cosmic speed limit? No, because the change would be local and not transmitted.
The error you make arises from our identification of gravity as a classical force —electromagnetism for example — and not a different phenomenon, what I like to call a first order force since it curves space-time in a different way than what I like to call derived, or secondary forces. (It's a long story and involves a bit of thinking I've done on the subject.)
By point is, the speed of light plays no role in the phenomenon because it does not involve anything like mediating particles and the transmission of information. The phenomenon we call gravity is entirely local because it is a response to local conditions. No need to transmit data, so no speed limit.
Posted by: mythusmage
| August 26, 2010 12:42 AM
This may be the most interesting post of yours I’ve read yet (and I wouldn’t have kept reading for very long if the mean interesting level was pretty high to begin with). Flattery thus achieved, is there any way you could elaborate a bit on how binary pulsars are used to measure the speed of gravity, and perhaps elaborate a bit on gravitational waves, as well?
Posted by: Petter Häggholm | August 26, 2010 12:43 AM
So, my only question is;
Does this mean that nothing can go faster than the speed of light, or that nothing can go faster than the speed of gravity?
Posted by: Mane | August 26, 2010 1:39 AM
Haven't several Threadizens on Pharyngula (myself included) explained that to you already?
Of course we have. Gravity is a force like electromagnetism, and it results from the bending of spacetime by mass. Hey, waves and particles are the same thing, too. Gravity waves are gravitons, the same way that light waves and electromagnetic waves in general are photons.
But not instantly, because spacetime curves at the speed of light. Changes in spacetime curvature propagate at the speed of light (± < 1 %). Did you read the post? Really?
It would be transmitted. It couldn't help being transmitted -- as gravity waves.
Without math and without observations of reality, that's just philosophy and not science. Show your work if you have any.
Either local means "within the same tetrahedron with a side length of 1 Planck length", or you're wrong. Or both.
Posted by: David Marjanović
| August 26, 2010 6:32 AM
Don't bring your blog down with all this gravity. Let's raise the tone and talk about the global helium crisis:
www.npr.org/templates/story/story.php?storyId=15231209
Posted by: IanW | August 26, 2010 7:01 AM
Hey, pretty much off topic, but the actual topic raised a question to me -
What would the ripples in spacetime represent? Specifically, If the curved space below the plane is causing gravity, what can be said about the area of curved space above the plane?
The implication is that it would be a force in the opposite direction, but would it still be considered gravity?
Posted by: William | August 26, 2010 7:35 AM
Interesting topic, noob question here. Why is it said that the 'plane' of space is curved by gravity? A plane is a wo dimensional object, but space is three-dimesional. Is this just a way to explain this 'curvature' thing, or is there a 'plane' out there somewhere?
Posted by: TEO | August 26, 2010 8:04 AM
Is this just a way to explain this 'curvature' thing
Yes. The two-dimensional plane is an analogy which is useful because absent an out-of-plane force an object orbiting a second object will remain in a plane. Also, it's easier to visualize a plane being distorted in the perpendicular direction of space than to visualize the equivalent of a space being distorted in the perpendicular direction of hyperspace.
What happens mathematically is that the mass distorts what is called the metric, which among other things allows you to compute distance in space-time. The resulting distortions in distance are equivalent to the "indented sheet" picture. Phrased another way, the geodesic (shortest path from A to B), which would be a straight line in space-time if there were no gravity, effectively changes to the path the orbiting object actually follows.
Posted by: Eric Lund | August 26, 2010 9:09 AM
Darn... all those science fiction books are wrong. Poodle.
So, my question is with these waves of gravity... the assumption being that they act like EM waves and other waves we're familiar with. Can there be interference between waves of gravity? Or because everything with gravity is already in place and not bouncing around like a kid on a trampoline, are there no ripples in spacetime caused by gravity? Is the pulsar above more of an instance of frame dragging rather than gravitational effects?
Posted by: OgreMkV | August 26, 2010 9:16 AM
So, if the speed of light and the speed of gravity are (nearly) identical, does this infer that light and gravity are manifestations of the same property or fundamental force?
Posted by: bomoore | August 26, 2010 10:54 AM
Also, there is a semantics problem: "speed of gravity" implies that speed is an intrinsic quality of gravity, whereas "the speed at which gravitational effects can be felt" (measured, observed) implies that factors other than gravity itself are involved. There is a HUGE difference.
Posted by: bomoore | August 26, 2010 11:11 AM
"[...] 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!"
Actually, to first order it's pulling the Earth toward where the Sun is now, not where it was 8+ minutes ago. This is actually part of van Flandern's confusion, and is explained in this Carlip paper debunking van Flandern.
(To pick nits, the direction of the "force" is directed neither toward the retarded position or the instantaneous position of the Sun, but to the "linearly extrapolated instantaneous position". That is, "where the Sun `ought to be' now if I linearly extrapolate from its position 8+ minutes ago".)
Posted by: Ambitwistor | August 26, 2010 12:19 PM
mythusmage,
It is correct that GR is not a "force field" or "mediating particle" based theory.
It is not correct to say that there is, therefore, no speed-of-light limit in GR to the propagation of gravitational effects. There is. Changes in spacetime curvature at one location affect spacetime curvature at nearby locations, and these changes propagate throughout spacetime at the speed of light. The speed of light is built into the structure of spacetime in a very fundamental way.
Posted by: Ambitwistor | August 26, 2010 12:23 PM
William,
As Eric mentioned, the "ripples in spacetime" are partly an analogy. There is no "above" or "below" the plane of spacetime. The ripples are ripples in the spacetime metric which governs how distances are measured.
Gravitational waves don't manifest themselves as "sticking up out of spacetime". Rather, they manifest themselves in changes in the distances between objects: two freely falling bodies will suddenly become closer or farther from each other as a gravitational wave passes by. (This is how the LIGO observatory tries to measure them, using hanging mirrors as approximations to "freely falling bodies".)
Posted by: Ambitwistor | August 26, 2010 12:27 PM
OgreMkV,
Yes, there can be interference between gravitational waves, and orbiting bodies do generate gravitational waves.
Posted by: Ambitwistor | August 26, 2010 12:28 PM
bomoore,
In relativistic theories all interactions propagate at the same speed (the speed of light) -- be it electromagnetism, gravity, a nuclear force, etc. This does not itself imply that those interactions are unified. Rather, it's a reflection of the fact that all of them take place in spacetime. Relativity imposes a particular geometry on spacetime, and that geometry always has a special speed built into it which is always the same. We call it the "speed of light" because this speed was first measured for electromagnetism, but that same speed shows up in all relativistic theories.
Posted by: Ambitwistor | August 26, 2010 12:31 PM
Interesting. The reason the speed of gravity seems instantaneous is because gravity (IMHO) is cause by a gravitational field, which emanates at the speed of light, but affects all matter within the field, so it seems instantaneous. See my alternative theory of gravity..
http://www.theoryofgravity.com
Posted by: Bill | August 26, 2010 12:42 PM
are telling me warp drive is impossible?
I always thought the speed of light was the maximum speed allowed in traveling within/in the fabric of space-time but nothing prevented the fabric of space-time itself from traveling faster.
Posted by: luke | August 26, 2010 4:23 PM
Forgot to add-
So why cant the ripples of space-time travel faster than light?
Posted by: luke | August 26, 2010 4:25 PM
Thanks Ambitwister! I'm curious though about information: does gravity "carry" information, or does light carry information about gravity?
Posted by: bomoore | August 26, 2010 5:16 PM
4 Why can't we see it in commetary orbits?
Comets are notoriously bad at following ballistic trajectories. The problem is that when they are in the vicinity of the sun, they are losing mass (and creating a tail). The hard to predict forces from this mass-loss process produce unpredictable accelerations that are too large to factor out.
If gravity worked like say EM waves, then you'd would have gravity not being able to escape from a black hole (i.e. the BH would completely vanish). So I assume treating it like a classical force only gives good results in the nonrelatavistic case. The bottom line (I would think) is you gotta do general relativity to understand it, and intution based upon classical physics will lead you astray.
Posted by: Omega Centauri | August 26, 2010 5:59 PM
So, Eric and Ambitwistor, let me see if I got this right, as this is something I have wondered about for a while. Are you saying that the trampoline-like analogy for space-time deformation is really for a deformation of distances in that space-time rather than a deformation into a fifth dimension? I always thought that it involved a fifth dimension and that this was one of the eleven dimensions, with the other dimensions being similarly involved in the three other forces.
Posted by: Brad | August 26, 2010 7:16 PM
One more question: Do all physicists agree that the universe has a speed limit and that both light and gravity "obey" this limit?
Posted by: bomoore | August 26, 2010 9:20 PM
Gravity is the end result of mass separation, given that distance, is time of the speed of light in space.
If light cannot escape a black hole then the curvature of the black hole would be immense, the curvature due to density and the density due to the lack of distance between particles. The closer mass is to mass the greater the effect and the more immediate the effect ,due to close proximity.
If you have a single body then the gravity or space-time curvature does not have the same effect, Space/time gravity are flat, any gravity wave produced would be meaningless, there is no counter effect from a second body, hence meaningful /useful space/time /gravity are all artifacts of mass separation.
It is my contention that Gravity, space and time cannot exist separately, they are closely interwoven, one single entity alone cannot posses any of these artifacts. One single quark in a abyss of nothing cannot know the artifacts.
Distance between mass is the most fundamental aspect of all nature, it is the lack of distance that creates the monstrosity of a black hole, it is the gain of distance that accelerates the universe.
The speed of gravity may never change, yet the speed in which the gain of distance between mass does increase at universal scale and decrease at galaxy cluster scale and it is this increase that will someday rob us of the pleasure to see the other astrological bodies…
Gravity from an entity at such great distances is almost by itself meaningless, it is the accumulation of all mass gravity and their specific distances that dictate the shape of our universe and it’s components.
I do agree with some relativist who state that gravity is a manifestation of the curvature of space time and I would take that further to state that gravity is the curvature of space/time and the distance of time/space.
So due to this effect the time of gravity does change to any body not in the same acceleration. The speed remains the same.
Posted by: Sphere Coupler | August 26, 2010 9:38 PM
The LISA hardware sounds very similar to an old proposal for a super-duper-giant optical space interferometer. I guess with radio waves the alignment requirements will be much easier to meet. I hope I get to see all these toys in their operational phase.
Posted by: MadScientist | August 27, 2010 2:08 AM
William, you are thinking of transverse waves due to the two dimensional nature of the rubber sheet visualisation, i.e. waves perpendicular to the direction of travel. Instead, think of longitudinal waves where the waves or oscillations are in the direction of travel. I.E. the difference between AM and FM radio or between simplified surface waves on water and sound waves through a medium.
Think of how sound waves oscillate as they radiate outward three dimensionally from an omnidirectional source. You have areas of rarefaction (the equivalent of the trough of a transverse wave) where air (space time) is less dense and areas of compression (the equivalent of the peak of a transverse wave) where the air (space time) is denser. At any point, the direction of oscillation is along (longitudinal) the same axis as the direction of travel of the gravity wave at that point.
I.E. instead of the up down effect of a transverse wave, it would be a push pull effect on anything the gravity wave passes through. It is the action due to this push pull effect that they hope to measure one day using very very large interferometers.
Posted by: John Phillips, FCD
| August 27, 2010 5:56 AM
luke,
Warp drive is impossible in general relativity unless you postulate the existence of "exotic matter" (with negative mass-energy, or the like). See this paper for a proof.
Posted by: Ambitwistor | August 27, 2010 8:13 AM
luke,
The geometry of spacetime defines a causal structure, in which the speed of light plays a privileged role. If you perturb a gravitational source, the change in its local gravity will itself induce changes in gravity at nearby locations, and so on, with the disturbance propagating outward as a wave. Like all waves, this has a speed, and the geometry of space requires that this speed always equal the speed of light in vacuum.
Posted by: Ambitwistor | August 27, 2010 8:28 AM
bomoore,
Yes, gravity does carry information. That's why we're building gravitational wave observatories: to see what we can learn from a gravitational, rather than optical or radio, approach to astronomy.
Light can carry indirect information about gravitational fields (e.g., gravitational light bending), but elecromagnetism is not fundamental to the operation of gravity. Gravity works the same regardless of whether there is any light in the universe.
With respect to the speed of light, it's generally accepted that you can't propagate information faster than light in any relativistic theory (electromagnetism, GR, QFT, or whatever), since you always get a relativistic wave equation for disturbance propagation. But there are loopholes, as in my response to luke: if you're willing to mess around with the source of gravity in sufficiently weird ways, you can alter the causal structure of spacetime geometry enough to violate this constraint. It is highly debatable whether this is physically possible.
(Messing around with gravitational sources is the only way you can do this in relativity, because gravity determines the structure of spacetime, and the structure of spacetime determines the speed of light limit for all other relativistic theories such as electromagnetism.)
Posted by: Ambitwistor | August 27, 2010 8:38 AM
Brad,
"Are you saying that the trampoline-like analogy for space-time deformation is really for a deformation of distances in that space-time rather than a deformation into a fifth dimension?"
Yes, exactly.
"I always thought that it involved a fifth dimension and that this was one of the eleven dimensions, with the other dimensions being similarly involved in the three other forces."
No. For the purposes of visualization it's often possible to embed a 4-dimensional geometry into an "extrinsic" higher dimensional space (though perhaps not 5 dimensional). But this is purely for the purposes of visualization: it's hard to imagine what a curved 2D surface looks like without picturing it inside of a 3D embedding space.
The theory of general relativity itself does not refer to or require the existence of any higher dimensional space. All of its dimensions are "intrinsic", and it is possible to speak of the curvature of spacetime with respect to measurements made purely internal to the four dimensions of spacetime itself.
The eleven dimensions of string theory are similarly intrinsic and have nothing to do with "embedding" spacetime in some higher dimensional space in which to curve.
Posted by: Ambitwistor | August 27, 2010 8:44 AM
This just gets more fascinating! It's clear that science education misses the boat entirely, and that the "U" is more delicate than we non math-speaking types can imagine. I find myself reacting in perhaps a stereotypical way: it seems that when a theory encounters a hiccup, the solution is to say, Well, we NEED a "thingy" to fix this, so let's say that this "thingy" exists.
It seems that gravity is so fundamental to the "U" that without understanding gravity, the foundation on which theories rest is a bit inadequate to support exotic notions. Aren't physicists unconfortable with this?
Posted by: bomoore | August 27, 2010 9:55 AM
Thanks for the answers to my question. This is a very interesting topic. Ethan has a knack for that.
Posted by: William | August 27, 2010 11:47 AM
this may seem far fetched and ignorant, but its simply an outsiders observation... could the speed of gravity perhaps be explained by how much depth we allocate in the fabric of spacetime, the sun with more mass, creates a larger tube if you will, in the fabric of spacetime thus creating stronger gravity? wouldn't gravity have a variable speed depending on the mass of the object obstructing the spacetime fabric? again this may seem silly but its all in theory :]
Posted by: matt | August 27, 2010 12:24 PM
Omega Centauri,
"If gravity worked like say EM waves, then you'd would have gravity not being able to escape from a black hole (i.e. the BH would completely vanish)."
Not quite. Gravitational waves and EM waves are fairly analogous, at least in the perturbative limit.
EM waves cannot escape a black hole, but a black hole can have a charge and therefore an electromagnetic field. Likewise, gravitational waves cannot escape a black hole, but a black hole can have mass and therefore a gravitational field. Waves propagate changes in a field (EM or gravity), but that doesn't mean the field itself can't exist. It just means that whatever goes on inside the black hole can't communicate information outside the hole via a field.
See this FAQ for more.
Posted by: Ambitwistor | August 27, 2010 1:18 PM
bomoore,
Physicists generally believe that for most everyday phenomena, our current understanding of gravity (at the level of general relativity) is adequate.
This may not prove true for some extreme phenomena at very short distance or time scales (i.e., black hole or Big Bang singularities), or very high energies (typically far beyond anything we can access in a lab, although some extra-dimensional theories permit lower-energy consequences).
Posted by: Ambitwistor | August 27, 2010 1:21 PM
"If you simply took the Sun away"...
In my humble opinion:
Maybe this question just has no sense unless we define what "take away" means in this case.
Why are we assuming that the mass can be simply "took away" but not the gravity?
If gravity is an effect of the mass it should change at the speed the mass changes otherwise the gravity should present a kind of Doppler effect with velocity (maybe it can be tested).
Well, the main thing I wanted to say is that if you start your assumption with a fact that breaks the physical laws then nothing guarantees that you are going to end with something physically correct or possible.
Posted by: Victor | August 27, 2010 1:23 PM
matt,
It is conceivable to have a theory in which the speed of light depends on mass or gravitational field strength. But general relativity is not such a theory. The speed of light (in vacuum, in a locally inertial reference frame) is constant in GR. (In an arbitrary coordinate system it can equal anything, but that's an artifact of the coordinate system, not some observer-independent physically measurable quantity.)
Posted by: Ambitwistor | August 27, 2010 1:24 PM
Victor,
That's right. In general relativity it's not possible to conduct a thought experiment of "taking the Sun away", even in principle, because that would be inconsistent with that theory's laws (namely, conservation of mass-energy). It's a question with no answer since it contradicts the premises of the problem.
However, among physicists it's understood that this impossible thought experiment is shorthand for something that is physically permissible (if implausible), such as the Sun exploding outward at near-light speed or whatever. And in that case, the answer is "the sudden absence of matter in the vicinity of the Sun's previous location would make itself felt throughout space, gravitationally, at a rate equal to the speed of light".
Posted by: Ambitwistor | August 27, 2010 1:28 PM
Then a star moving at near-light speed will leave a trail of gravity like a boat in the sea?
Posted by: Victor | August 27, 2010 1:58 PM
Strictly speaking, it's accelerated motion that creates gravitational waves. Orbiting stars do produce gravitational waves, as Ethan mentioned.
Posted by: Ambitwistor | August 27, 2010 5:37 PM
(Technically, what you need for gravitational waves is a time-varying quadrupole moment.)
Posted by: Ambitwistor | August 27, 2010 5:40 PM
So the string theory guys agree with Einstein's relativity theory?
Posted by: bomoore | August 27, 2010 7:42 PM
We call it the "speed of light" because this speed was first measured for electromagnetism, but that same speed shows up in all relativistic theories.
Posted by: Ambitwistor | August 26, 2010 12:31 PM
---
Thank you.
Posted by: Douglas Watts | August 27, 2010 9:12 PM
String theory is fundamentally a relativistic theory: it can only be perturbatively formulated in spacetimes which obey the field equations of general relativity (generalized to higher spacetime dimensions). In fact, the gravitational dynamics of general relativity can be derived, as a low order approximation, from the dynamics of string theory: strings imply gravity. (This is in contrast to quantum field theory, which not only does not imply gravity, but doesn't even appear to be consistent with it.) That's one reason why string theorists like strings. At higher energies, however, stringy gravity stops acting exactly like general relativity.
Posted by: Ambitwistor | August 28, 2010 7:31 AM
When an object responds to this curvature it is responding to local conditions, information is not being transmitted at any time.
Posted by: Hamza | August 28, 2010 8:34 AM
David,
comment #9
The Graviton is a hypothetical virtual particle that is an attempt to “concrete” =( mass of coalesced particles) a mass formed when particles coalesce the actions of gravity), however it is nonrenormalsable and that is a definite problem for a real particle.
Gravity is not a force like electromagnetism,
No real graviton has been detected, while there is a real photon.
Dirac's second-order perturbation theory can involve virtual photons. The magnetic field between magnetic dipoles is caused by the exchange of virtual photons.
Virtual photons may have three or four polarization states, instead of the two states of real photons
Virtual particles are an artifact of perturbation theory , and do not appear in a non-perturbative treatment.
These virtual particles are indirectly detected as interactions between particles.
Real photons are directly detectable as individual particles in particle detectors
Quantized general relativity is not renormalizable , unlike quantum electrodynamics.
General relativity is said to be background independent. In contrast, the Standard Model is not background independent, while gravitons are not considered to be a part of the Standard Model there has been no good experimental evidence that they exist. The Standard Model of particle physics contains only renormalizable operators.
Waves and particles are not the same (thing)phenomena, they are however two separate descriptions of RQM relational quantum mechanics
http://en.wikipedia.org/wiki/Relational_quantum_mechanics
Posted by: Sphere Coupler | August 28, 2010 12:51 PM
Hmmm... it would seem that most of the universe exists as extreme conditions, while our human / animal / planet scale, although the product of extreme conditions, is actually the oddball state. ???
Posted by: bomoore | August 28, 2010 5:26 PM
4% normal matter = oddball state? Hmmm...
On a lighter note I Personally feel stuck in the middle between (clowns and jokers)extreme states.
What is odd, is that "It will make sense".
Posted by: Sphere Coupler | August 28, 2010 7:00 PM
Perhaps oddball is the wrong word, but abnormal sounds weird: still, 4% normal matter wouldn't be normal, would it? Minority matter? Familiar matter? Friendly matter? Home space matter? We are stuck: we really can't exist anywhere but "here" because "here" made us. Personally, I'm a child of earth, specifically Wyoming, and I wouldn't want to live anywhere but the home planet.
Posted by: bomoore | August 28, 2010 8:05 PM
Indeed, I agree,Yet traditionaly it is "normal" to us,It is our first experience.
Do you think their was more or less "of the 4%" in the past?, that is "While gravity works to change the Universe".
Posted by: Sphere Coupler | August 28, 2010 8:20 PM
There is no past; there is information about the past, and the human brain is probably the least accurate recorder of that. I suppose physicists are lucky in that they access the past as data. As a geologist, teasing out what happened is forever out of reach; experience is the greater part of our understanding of existence and we can only imagine what earth's history actually looked, smelled and felt like. Oh my! No wonder physicists and engineers complain that geology isn't a science. But geologists are so very NORMAL....
Posted by: bomoore | August 28, 2010 9:31 PM
u huh,
ha,ha,ha,ha, don't get me started.As far as normal matter, .4% luminous and 3.6 nonluminous, seems their may have been a paradigm shift to call normal matter-ordinary matter.
OK then, Just a point to ponder, will the 4% ordinary matter increase or decrease in the future, given the data from the past and what we now know.
Posted by: Sphere Coupler | August 28, 2010 10:00 PM
One thing that puzzles me is we attribute a % Percentage to Dark matter, a % to Ordinary matter, yet their is a void of knowledge of % when it comes to Black hole matter. Sure Black hole matter is in a category separate from ordinary matter...right?, we do know that Black hole matter consumes Ordinary matter and IF we find that Dark matter is consumed by Ordinary matter then we would need to know which matter consumes the most aggressively, I've got a pretty good guess. We speak of matter in the context of force carriers, phases, states, yet we rarely speak of the percentage matter in transition from one type to another (due to gravity of course).
Does the consumption of Ordinary matter cause Dark matter to maintain 4% Ordinary matter?
Or will they both conspire to eliminate Ordinary matter?
Posted by: Sphere Coupler | August 28, 2010 10:53 PM
I never got Einstein's physics. I find it impossible to visualize the curvature of space, or how time can be altered by velocity.
I just can't visualize it. Personally, I'm incredulous.
Posted by: GuerillaOntology | August 29, 2010 12:15 AM
I just can't visualize ti too. can you show any video ?
Posted by: antalya tasarım | August 29, 2010 3:53 AM
I think that warp drive is impossible in general relativity unless you postulate the existence of "exotic matter".
Posted by: Oknast | August 29, 2010 5:33 AM
@Sphere Coupler: We know the abundance of baryonic matter ("ordinary matter", as you call it) from primordial nucleosynthesis, which happened in the first minutes after the Big Bang. Since in all likelihood no black hole were around there, there was no "black hole matter" at that time. So we can conclude that today there is probably *less* baryonic matter than the percentage we know from primordial nucleosynthesis - if we count "black hole matter" separately.
Posted by: Bjoern | August 29, 2010 7:28 AM
Personally I believe it is instantaneous, there's actually no speed or a way to measure it. A simple but yet effective example would be that of putting your hand over fire, you unconsciously remove your hand because you get burned, the signals are sent to the brain and the brain makes the reaction to remove the hand so we cant actually give it a speed or measure it
Posted by: Pushime ne Turqi | August 29, 2010 7:46 AM
@GuerillaOntology: Huh? You are skeptical of relativity because you can't visualize it?!? Since when does the validity of laws of nature depend on if one can visualize them or not?
Posted by: Bjoern | August 29, 2010 8:03 AM
Thanks Bjoen,
Yet I still don't understand why BHM does not have it's own slice of the pie today.
http://en.wikipedia.org/wiki/File:Matter_Distribution.JPG
Posted by: Sphere Coupler | August 29, 2010 8:37 AM
Never mind, I guess it is lumped together (intergalactic gas and about 0.1% neutrinos and 0.04% supermassive black holes)in the non luminous matter catagory.
Posted by: Sphere Coupler | August 29, 2010 9:05 AM
Can I have your permission on using this article for my science shcool papers ?
Posted by: odienk | August 29, 2010 9:05 AM
So, if the speed of light and the speed of gravity are (nearly) identical, does this infer that light and gravity are manifestations of the same property or fundamental force?
Posted by: Adim | August 29, 2010 9:21 AM
This is a wonderful post indeed. When I read this I remember why I didn't become a scientist. You explained very well but I still couldn't understand :(
Th speed of gravity is... well, sorry I can't, but thanks for the post anyway! :)
Posted by: I am a self help junkie!!! | August 29, 2010 9:22 AM
The theory that the speed of gravity=speed of light has come under question by numerous sources, most recently by the AAS. Of course physicist will continue to argue both sides of the story. At this point it's about which side do you believe.
Posted by: Dallas movers | August 29, 2010 9:29 AM
So, if the speed of light and the speed of gravity are (nearly) identical, does this infer that light and gravity are manifestations of the same property or fundamental force?
Posted by: Adim | August 29, 2010 9:21 AM
No, it means that light moves in the environment of gravity and the environment of gravity dictates the speed of light.
Right Ethan?
Posted by: Sphere Coupler | August 29, 2010 9:34 AM
When an object responds to this curvature it is responding to local conditions, information is not being transmitted at any time.
Posted by: Hamza | August 28, 2010 8:34 AM
Hamza,
The curvature found in the local environment IS the result of mass information...All mass, it's just that local mass has a much greater gravitational effect due to the closer proximity.The strength of this field at any given point is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.
Posted by: Sphere Coupler | August 29, 2010 10:33 AM
Sphere Coupler,
The way I would phrase it is that space and time are connected, and they are related, proportionally, by the speed of light. The speed of light, as far as we can tell, is a constant, and is always the same constant, and is the only speed that ever shows up in general relativity.
Light travels at the speed of light (by definition), and gravitational information (gravitational waves, for instance) also travels at that same speed.
Posted by: Ethan Siegel | August 29, 2010 11:08 AM
Yes, your phrasing is clearer and more concise, thank you.
Yet, if light can not escape a black hole and presumably gravitational waves also can not escape ( the standing field generated by Black Hole Mass is not effected) then the "time in flight" of the speed of light and gravitational waves is affected within the dense environment of Black Holes but not the speed...right?
Posted by: Sphere Coupler | August 29, 2010 11:44 AM
I am not from this professional background and am here out of sheer simplicity of your content. I want to ask a layman question : How do you distinguish between speed of gravity and escape velocity?
Posted by: Casey Pearson | August 29, 2010 12:42 PM
I always wondered whether the speed of gravity would be affected by black holes. If a planet existed closer to a black hole, would the force of the pull within the black hole affect gravity?
Posted by: Ted Jervis | August 29, 2010 12:57 PM
Can't agree with mage more!
Gravity is not a force like electro-magnetism, gravity is the name we give to the phenomenon that results from the bending of space-time by mass. When an object responds to this curvature it is responding to local conditions, information is not being transmitted at any time.
Posted by: iPod Tricks | August 29, 2010 2:46 PM
I have not this professional background but at least i can read and understand the theme.
I think in our dimension the speed of light and gravity is limeted to the known values. In the inner of the Black Holes
there is another room time travel possible at least in theory which are faster then light and gravity speed.
Posted by: Strom Wechseln | August 29, 2010 5:03 PM
Right so what is effectively being said is that the gravity we feel at current is actually caused by the sun's actions from 8 minutes ago? I'm struggling to understand how the speed of light can be any indication of how long it would take for gravity's effect to be known? It says 8 minutes difference for light but if the sun was to vanish would we feel the force immediately while still seeing the light? I know what I mean but can't explain it lol
Posted by: national lottery numbers | August 29, 2010 5:11 PM
Next question: Can gravity waves be used to send out information from within a black hole's event horizon.
Posted by: Ferrous Patella | August 29, 2010 5:52 PM
I'm not a physicist, but my theory is that the effects of gravity would be instantly lost if the 'mass' were to suddenly vanish. Of course, mass can't just vanish. Everything has a cause and effect, without one, you can not have the other. Again, I'm not a physicist, but I think photon particles and graviton particles act vastly different, and are not interconnected in the way that's being explained here.
Posted by: Martin | August 29, 2010 6:09 PM
For those having trouble visualizing relativity: these are objects and relationships between matter, energy, space, and time that the human brain is not equipped to visualize. Mathmatics is the language that describes things outside our "normal" experience. Our brain evolved to operate in physical conditions on earth; it has no ability to understand the universe directly: scientific instruments expand our senses and create conditions (atom smashers, etc.) that exist elsewhere in the universe.
The drawings, graphs, and illustrations that scientists use to demonstrate possibilities are not the actual "things" - they are crude visual tools geared to our vision-dominated minds.
Posted by: bomoore | August 29, 2010 6:15 PM
This is a really fantastic article. I'm going to forward this on to my husband. He is a big science guy and hes been researching and discussing gravity quite a bit lately.
Posted by: julie | August 29, 2010 10:32 PM
Amazing article, but I don't think scientists have even touched on what gravity truly is and how it might benefit is.
Posted by: Joye | August 29, 2010 10:48 PM
Wow, fascinating stuff, if I was an NBA Player, I'd definitely be using this kind of science in alternating my shot. Gravity is on your side, lol.
Posted by: NODJZONE | August 30, 2010 12:04 AM
wow.. I wish I understood this in highschool.. I would have been able ot alter my shot and get better a balance
Posted by: clutch | August 30, 2010 12:09 AM
It says something when some of the real commentators posts are indistinguishable from the spam rehashes. Re: 75-79,81
Posted by: Robert S. | August 30, 2010 12:10 AM
The 2D representation of a 3D phenomenon is of course misleading. Space isn't so much "curved" as "puckered". :-)
Posted by: fnxtr | August 30, 2010 3:01 AM
Robert S. - how true. but you missed a few. Have a look at numbers 83, 84,85.
Posted by: Rick | August 30, 2010 7:14 AM
Very nice post and comments. May I am ask 2 questions.
1) Gravitational waves are ripples in spacetime. As photons move along such a rippled path from A to B, do photons lose energy as they ride up and down the ripples of spacetime?
I assume that gravitons would not lose such energy because there is no shielding of the gravitational force by mass (or energy, i.e. gravitational waves) and hence gravitons would not feel (so to speak) the ripples of gravity; rather they would tunnel right through. But it would seem that photons would lose energy riding the ripples of spacetime.
2) Why will LISA succeed; where LIGO and others have failed to detect gravitational wave?
Posted by: OKThen | September 2, 2010 12:49 PM
If Super-massive Black hole matter is approx. 0.04% of indirect observable matter and ordinary baryonic matter is under 4%, then I think that the Schwarzschild radius is insufficient to describe the amount of matter contained within the radius. Their is no evidence to indicate that BHM is in the form of Ordinary baryonic matter and many clues to indicate that it is not. Due to the extreme curvature of density and the Coupled Angular Momentum of Black Hole (baryonic?) Matter , the condition of this matter could be quite different from Ordinary baryonic matter and if this extreme condition of black hole matter produces a diverging gravitational effect due to this extreme curvature-due to extreme density-due to extreme lack of distance then the gravitational effect could be dampened so that it would take more Ordinary baryonic matter to see he effect that we see. One way we could work this into the mathematics is including the time and consumption rate from the beginning, the amount of converted Ordinary baryonic matter to Black hole matter could be grossly under-determined and could constitute a larger portion of the missing mass.There may even be a dynamical relationship between Dark, Ordinary, and Black matter (and most likely is)where condensation of Dark matter slows the consumption of Ordinary baryonic by converted Black Matter.Sure the Schwarzschild radius works if we assume that black hole matter is baryonic but why would ordinary baryonic matter not be extremely converted by extreme curvature/density/LOD?
I think that we must count Black Hole Matter separately, and I think that in the future it will require a higher portion of the pie chart in comment 65.
As it stands as of 9/4/10
73% Dark energy
23% Dark Matter
4% Baryonic Ordinary Matter divided as 0.04 luminous and 3.6 non luminous.
It has to change, because if it don't then the song I posted in comment 53 won't make sense and we just can't have that!
Stuck in the middle with you, cover by Steelers wheel
Posted by: Sphere Coupler | September 5, 2010 11:11 AM
Sorry, should be
dampened so that it would take more (consumption of)Ordinary baryonic matter to see he effect that we see.
then the song (and video) I posted in
I'm done.
Happy Labour Day!
Posted by: Sphere Coupler | September 5, 2010 11:45 AM
Too funny. I got here through random surfing and saw the quote a the top.
It's Danny NEVERATH (it's misspelled a lot on the net). He was a radio (and sometimes TV) host in Buffalo from the 1960's through the early 1990s. I didn't care for him much, but someone must have.
Posted by: Dave Tufte | September 7, 2010 1:11 PM
What would be the fastest way to remove the mass of the sun? Spreading out the mass in all directions at the speed of light would still mean that the center of mass would act in a similar way during the first 8 minutes - sending the mass in the opposite direction of us would give us a gradual change in gravity during those 8 minutes. Could we turn all that mass into energy? And would that be closer to taking the sun out of the time-space, or how would the curvature of space react then?
Posted by: Joffemannen | September 8, 2010 4:36 AM
@Joffemannen: Instantaneously teleport every particle of the sun into another universe.
Posted by: crd2 | September 8, 2010 5:30 AM
Casey Pearson asked, "How do you distinguish between speed of gravity and escape velocity?"
No one else answered, so I'll take a stab at it.
The faster you throw a rock into the air, the farther it will arc before it lands, yes? If you throw it fast enough (and are above the atmosphere) then the rock will orbit around the earth instead of falling back down to the surface. If you throw it even faster, it will have an orbit with peak farther away.
How fast do you have to throw the rock so it will escape the Earth's gravity and not orbit it? That is escape velocity. Escape velocity depends on the mass of the object. The moon's escape velocity is slower than Earth's, but Jupiter's is faster.
And these speeds are all within speeds that we can make rockets and satellites go: not very fast compared to the speed of light. It takes years for a space ship to get from Earth to Jupiter, but light just takes a few hours.
Escape velocity is not how fast gravity "travels". The speed of gravity was described in the article, but let me try a different analogy. Let's say you had a telescope and an imaging gravity detector. (There isn't really such a thing, but let's just pretend you could "seen" objects by their gravity.) Since it takes hours for Jupiter's light to reach us, when you point your telescope at Jupiter, you're really looking at where it was hours ago. So where do you point your imaging gravity detector? Same place, or where you think Jupiter is now?
Physicists are really really sure you point the gravity detector at exactly the same location as your telescope: the speed of gravity is the same as the speed of light. And that is very different from escape velocity.
Finally, let me add that one of the definitions of a black hole is an object whose escape velocity is equal to or greater than the speed of light. Which means, of course, that you can't get out.
Posted by: Timberwoof | September 9, 2010 11:04 AM
OKThen;
I’ll address/Part2) of comment 90
LIGO is a dual ground based system incorporating laser interferometry 1,865 miles apart, thus triangulation is possible.
LISA is a triad space based system incorporating laser interferometry approx.5-million-kilometer apart, thus equilateral triangulation is possible.
Further more LISA will have the capacity to systematically rotate at a annual rate.
So you can see that LISA will be more sensitive to curvature fluctuations (gravitational waves)... than LIGO and the combined viewing of LISA and LIGO may enable a better understanding, since the two systems deal with differing difficulties to overcome, the two systems may in conjunction enhance the total overall gain than one system alone.
In the future it may be possible and fruitful to synchronize three consecutively linear orientated satellites with even greater distance between them to lend us a view from a different perspective.
As with any phenomena, the more views you obtain the clearer the picture will be, The faint elusive structure dynamics in this context is a challenge.
Check out the link provided above, by Ethan http://lisa.nasa.gov/
Posted by: Sphere Coupler | September 9, 2010 9:00 PM
Speed of gravity 'c' is a wrong interpretation.
1. Aberration of gravity is for real. If there is a propagation delay there will be aberration.
2. For the sake of argument let me assume the speed of light and therefore speed
of gravity is a mere 10 miles per hour, do you think the orbits of planets relative to the motion of the solar system will hold up and be stable under that kind of aberration?
3. Space time curvature may be a local effect but the local curvature which directs the motion of planets is dependent on the actions of the far away sun. The local curvature must be updated in real time instantaneously in order to have stable orbits. In that respect gravity is not a locally motivated source.
4. The so called gravitational waves are so flimsy(We don't feel them and our sensitive detectors cannot find them) and therefore cannot be the cause of real gravity even if they exist.
5. Just because speed of light appears in GR equations, it should not lead to speed of gravity conclusions. It is merely for coupling mass and energy.
Real Gravity
1. Space is the carrier of gravity(and matter). Properties of space will determine the speed of gravity.
2. The ZPE energy of space is 120 orders as currently estimated. That kind of energy would easily
endow space with extreme energy densities and create conditions of space with extreme superluminal wave propagation properties.
3. Gravity is basically transmitted as a bulk tidal force wave as opposed to a kind of surface wave.
4. Accordingly the gravity speed is of about 26 orders of magnitude over speed of light.
5. This kind of speed cannot be even detected by any experiment on earth, therefore gravity waves will not be found in the near future.
Posted by: Tissa Perera | September 15, 2010 11:34 AM
I would post re comments but am lost.
Question re statement from above: But in general relativity, things are much more intricate, and incredibly interesting. First off, it isn't mass, per se, that causes gravity. Rather, all forms of energy (including mass) affect the curvature of space. So for the Sun and the Earth, the incredibly large mass of the Sun dominates the curvature of space, and the Earth travels in an orbit along that curved space.
My question is: First you back off the mass statement causing gravity then go forward acting on it alone. Confusing. The problem is that the greatest by far mass of the universe is dark matter & dark energy (having mass)so wouldn't it logically follow that we have no clue? Since we can't even id DE or DM? Recent studies trying to correlate visible matter with unided matter/energy have worked from about a 17 to 1 ration & discovered that even our supposedly visible matter can't be all ided. So, doesn't that sound like our understanding/concept of gravity is so far off base that it can't be considered workable? See Velikovsky's work referencing why gravity is wrong. Over 60 years ago before I was born.
Posted by: katesisco | September 21, 2010 11:57 AM
bomoore - I agree that our brains evolved specifically for the conditions and laws on Earth and that is what is easiest for us to understand. But nontheless, our brains are incredibly powerful and it is quite possible with the correct amount of understanding and practise, to visualise even quite alien concepts (and many scientific laws are alien or counterintuitive to what we may actually perceive in nature). I don't think that the human race has yet to come across something that we can't 'visualise', provided that we fully understand how it works via mathematics and such.
As for relativity - it's also quite possible to see how it works with your mind's eye and why it works the way that it does, thereby gaining an actual 'understanding' of it rather than just memorising the rules and assuming its true. I have managed to do so at least - although I inevitably end up forgetting half of it after the explanation or text is over. In my mind the problem is two-fold:
1. It is indeed conceptually hard to understand - although certainly far from impossible.
2. But more than anything - it's just that the vast majority of explanations of relativity tend to be either flawed or poor at relating the mechanics behind it - instead just using a jumble of big words and implying concepts that the author just takes for granted that his readers would be immediately familar with.
I for one, was never able to grasp time-dilation at speeds approaching c until I read of the analogy, where it was explained what would happen if 2 spaceships both travelling at a good fraction of the speed of light attempted to bounce a beam of light between each other - or something to that effect. Until I read that example - I simply never really grasped the concept, yet afterwards it became a lot easier.
Posted by: Max Kalininskij | October 10, 2010 9:20 PM
@Richard
Hi, The very first post here, probably asked the most interesting question...
"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?"
If we imagine a ripple passing through a large (and dense) object such as a star, and if we assume that a medium has an affect on gravity, then we should expect "ripple" of the wake of the star to fall behind.
I'm no scientist (I have more heart than raw brainpower unfortunately), but I would love to find out how I can answer my own (and in this case Richards) question(s). What resources of astronomical data are publicly open for us lesser mortals to make our own "scientific" inquiry?
Posted by: Konrad | October 11, 2010 9:29 AM
@Richard
Hi, The very first post here, probably asked the most interesting question...
"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?"
If we imagine a ripple passing through a large (and dense) object such as a star, and if we assume that a medium has an affect on gravity, then we should expect "ripple" of the wake of the star to fall behind.
I'm no scientist (I have more heart than raw brainpower unfortunately), but I would love to find out how I can answer my own (and in this case Richards) question(s). What resources of astronomical data are publicly open for us lesser mortals to make our own "scientific" inquiry?
Posted by: Konrad | October 11, 2010 9:31 AM
it seems to me that Einstein's proposition that gravity would travel at the speed of light would disprove his theory that gravity consists of the bending of space. space is either bent, or it is not, and it seems if you remove the object it is not bent, and the gravity disappears. On the other hand perhaps discovering the speed of gravity is crucial to understanding what it actually is. Even so,, what do we mean by speed of gravity anyhow? Can gravity even travel? Or is it in fact only other types of matter, objects that can travel? By the way if we assume that gravity can travel that would have very strange implications for gravity itself. It really can't travel, if you think about it.
Posted by: jeff | October 16, 2010 3:31 AM
the answer is: Since the mass that causes gravity cannot be moved away faster than the speed of light, then gravity will never be changed faster than the speed of light. If that mass could be removed instantaneously, then gravity would be instantaneously changed for any object in the area
Posted by: jeff | October 16, 2010 3:45 AM
We know that even the light can not escape black holes. Why? There is only one possibility that gravitational force pulls light with greater speed than the speed of light. The speed of gravitational wave is many times higher than the speed of light at black holes.
Posted by: Anirudh Kumar Satsangi | December 4, 2010 4:07 AM
Ethan, I appreciate the article. I think you are one of those who "gets it" while also being able to explain to those who are not physicists themselves in relatively graspable terms (Though i'm sure my engineering background helps me along a bit too). I didn't get to read all the comments, but you answered something i've wondered about: Does the fact that the speeds of gravity and light are identical suggest that gravity and electromagnetism are unified. You say not necessarily. It is more like a speed limit of spacetime that both light and gravity and other forces are hitting. I have to point out though that the expansion of space is actually faster than the speed of light, that is why we are out here being able to observe the big bang! Do you agree?
Posted by: dave | December 30, 2010 10:16 PM
The formation of the universe was virtually instantaneous even as the gravitational fields were forming to establish the current uniformity that we see today. That being given, gravity is instantaneous.
Posted by: Michael Opitz | January 17, 2011 3:23 PM
According to tests done by Harvard since the 1950's the visual affects of an eclipse do not align with the gravitational alignment, in fact the tests show that the force of gravity arrives "before" the visual event. Light has aberration, it appears gravity may be faster, some theories may need revised.
http://home.netcom.com/~sbyers11/grav11d.htm
I do not necessarily agree with the authors interpretation of emission and blockage, but the data shows that the gravitational and visual events do not align.
Quote "Conclusion (from Saxl and Allen via OCR program) "Quantitative observations made with a precise torsion pendulum show, in agreement with many earlier less precise recordings made at Harvard since 1953, that the times required to traverse a fixed fraction of its total angular path vary markedly during the hours before the eclipse and during its first half, i.e., up to its midpoint. Also the significant changes in these times do not coincide exactly with the astronomically determined onset, midpoint, and endpoint of the eclipse", (Note: marked a..b..c respectively on the above graph.)
Posted by: Steven White | April 11, 2011 1:10 AM
"We know that even the light can not escape black holes. Why? There is only one possibility that gravitational force pulls light with greater speed than the speed of light."
Nope. If, for example, the emitting surface were retreating at the speed of light, there would be no light leaving where that surface was.
Just one example.
Alternatively, gravity could move at zero speed. It is, however, everywhere. And it subtracts energy from the photons until there is no more energy left.
That one isn't serious, but it's as physically sensible as your "only one possibility" that nonetheless causes light to extinguish and doesn't travel faster than light.
Posted by: Wow | April 11, 2011 10:22 AM
"If that mass could be removed instantaneously, then gravity would be instantaneously changed for any object in the area"
Unless (and this is where my maths doesn't get anywhere near where it needs to be to answer this), the energy that that mass contained itself gravitates. If photons self-gravitate, then there may be no different from a mass that blows itself apart and matter that annihilates to nothing.
Also, and this goes back to my reflective box idea that the panicky one avoided answering: when you accellerate the box with just photons inside, the energy of the photons hitting the side you're pushing increase in energy and therefore resist the motion.
Is that inertia?
Isn't that reflective box containing solely photons now a model of matter?
I suspect that a model of matter based on a reflection of photons would require creation of photons to make inertia match the inertia of the mass and that the mechanism for this creation would break many things so is not useful as a model, but it's an interesting realisation, isn't it.
Posted by: Wow | April 11, 2011 10:27 AM
Gravity is not instantaneous. It travels at the speed of light. The fact that gravity has no observable aberration does not contradict this. Rather, it highlights a misunderstanding.
Many of you are assuming that when gravity reaches an object, the object is pulled in the same direction that the gravity came from. But this is wrong.
When the source of the gravity is moving (relative to the object) the force acting on the object is skewed. Think of the force as the combined effect of the gravity and the movement. So, gravity arrives at the object from a particular direction but the force pulls the object in a different direction. The angle between the two directions depends on how fast the source is moving.
When light and gravity from the sun reach the earth, they are both coming from where the sun was about 8 minutes ago. But the gravitational force is skewed at a slight angle, because the sun is moving relative to the earth. This angle mostly cancels out the aberration caused by the movement, i.e. the earth is pulled towards a point which is indistinguishable from where the sun is now.
If (hypothetically) the sun's movement suddenly slowed, the earth would be pulled towards a point away from the sun. This would continue for over 8 minutes, until the information about the sun's change in velocity propagated out as far as the earth.
Since the force of gravity appears to be directed towards the sun, it is a common mistake to believe that the gravity comes from that direction. This then leads to the incorrect conclusion that gravity must travel faster than light. But, realise that the direction of force and the direction of travel are not the same, and everything makes a lot more sense.
There is no reason to abandon General Relativity. Or even to revise it.
Posted by: cSense | April 14, 2011 4:09 PM
Ok... 1% in the scheme of things that margin is huge... to presume that gravity moves at the speed of light give or take 1% is simply grasping at a straw. Einstien said alot of very inteligant things but he was not caperble of calculating the speed of gravity as he still belivied in gravitons.
Come on I dont have a degree in physics and this si simple to me to understand.
The speed of light is taken so often as a constantbut yet we know that the speed of light is affected by gravity (Eienstien) so how can we measure somthing by the the fact that effectes the actual item we sre trying to measure it by?
Posted by: Christopher | September 9, 2011 7:50 PM
Why gravity itself can escape a black hole?
Why gravitational waves do not get weaker as they escape the black hole?
Since gravity escapes the black hole, it should provide information about what is going on into the black hole (like different densities).
Posted by: me | September 14, 2011 12:43 PM
"Why gravity itself can escape a black hole?"
Because gravity in particles is the exchange of gravitons between two bodies.
The gravitons from a black hole can't catch up to each other to exchange, but they CAN catch up to, for example, a photon, and exchange gravitons that will pull that photon down.
"it should provide information about what is going on into the black hole (like different densities)."
There's no reason why it should.
And indeed it doesn't.
You can get some topology changes from rotating black holes, but whether that is the mass itself rotating (at what speed?) or (more consistently plausible), merely the rotation of the field, is rather deeper than I can answer.
Posted by: Wow | September 14, 2011 1:04 PM
"The speed of light is taken so often as a constantbut yet we know that the speed of light is affected by gravity (Eienstien)"
No, light still moves at lightspeed.
It just doesn't go in a straight line and is redshifted.
If that light is redshifted to a zero frequency, then there is no light.
In fact a redshift would be expected even if you didn't know any general relativity since the frequency and energy of a photon are related and it takes energy to move out of a gravitational well.
Posted by: Wow | September 14, 2011 1:07 PM
Reference Mr. Jug Suraiya’s very thought provoking article ‘Einstein Won’t Mind’ (Speaking Tree, Oct. 9, 2011, pg. 7). Einstein is great, but Newton is all time great. I would like to quote his few excellent lines from this article before I come to main theme of this article: “Faith and religious beliefs are destinations reached; science and skepticism are journeys without end”. But it’s to be modified. In Veda it is written, ‘Neti, Neti’, ‘not the end, not the end’. So in Vedic religion journeys never end. I consider Vedas the most honest scriptures which do not limit the scope of further exploration of truth. Now I come to the speed of light. This has long been proved since the time of discovery of black holes that the speed of light is not the fastest. Black holes do not allow even light to escape. It means the escape velocity at the black holes is much higher than the speed of light. Black holes are the infinitely dense ball of gravitation force. All creational forces of the universe have originated from the gravitational force field and will end up in it. The speed of light is no doubt fastest in our solar system. The source of light is Sun in our solar system. But how this light is originated? We should study the various stages involved in the formation of a star. Our Sun is also a star.
The starting material for the formation of a star is mainly hydrogen gas and helium gas. If the hydrogen cloud contains a very large number of atoms, each atom feels the gravitational pull of all the atoms in the hydrogen cloud. (Here is NO LIGHT)
The gas cloud becomes a permanent entity, held together by the mutual attraction of all the atoms present in it. The cloud then begins to contract under its own gravity setting off the process which will convert this huge condensed gas cloud into a star. Such a tight contracting cluster of atoms held in the grip of its own gravity, is called a protostar. The protostar is not yet a star and does NOT emit LIGHT. The temperature of this star is as low as -173 degree C.
The force of gravity acting on different atoms in the protostar draws every atom towards centre. As a result, the protostar shrinks in size and its density increases. As the atoms in the protostar fall towards the centre, they pick up speed. Because of the high speed and greater density of atoms, the atoms in the gas cloud collide with one another more frequently, thereby raising its temperature from -173 degree C to about 10 ^7 degree C. At these extremely high temperatures the proton (hydrogen nuclei) at the centre of the protostar collide together and undergo a nuclear fusion to form helium nuclei. In this reaction a tremendous amount of energy is released. This further raises the temperature and pressure. The release of nuclear energy marks the birth of the star. The protostar now beings to GLOW and becomes a STAR. Here at this stage LIGHT is ORIGINATED. Thus light is NOT ETERNAL. It has a beginning and an end. So LIGHT cannot be claimed as Cosmic Constant. However, Gravitation Force is eternal.
It is evident from the above description that light is latent before the birth of star. Light originates and become kinetic only after the action of gravitation force. So the speed of light can never exceed the speed of gravitation force. It cannot be ruled out that the speed of gravitation force is infinitely greater than the speed of light at black holes.
Posted by: Anirudh Kumar Satsangi | October 17, 2011 12:03 PM
Wolverine can't be wrong!
Posted by: malnati | November 17, 2011 1:03 PM