The speed of Gravity

[Event Horizon]

I was wondering how fast does gravity propagate through spacetime. I assume it propagates at lightspeed, but assumptions are dangerous things sometimes. Presumably we can calculate this from great celestial events like binary stars or black holes colliding. And I guess a follow-up question would be, does gravity always propagate at the same speed? Through matter vs through the vacuum of space for instance. Do we know this yet?

[Event Horizon]

Ok, I found an article on this that I can understand:

https://www.forbes.com/sites/startswith ... f838666211

Although Einstein wins out over Newtons ideas, it does not seek to explain if gravity travels through dense objects at the same speed or not. The range, or reach of gravity may or may not be infinite too, and I dug this up..

https://www.reddit.com/r/askscience/com ... _infinite/

So it seems we are getting to grips much more with gravity than I thought.

So, do we have a unified gravity theory that works the same throughout the universe? It seems Einstein would say we can have some confidence that it does, but it's worth remembering we only detected gravitational waves a few years ago. It could be one day we may need "Gravitologists" to navigate space if we ever take to the stars.

[Lincoln]

https://www.livescience.com/60695-why-g ... tters.html

[zetreque]

https://www.livescience.com/60695-why-gravitational-wave-discovery-matters.html

Very interesting.

One fundamental question that can be answered is the speed of gravitational waves. Long thought to be the speed of light, an initial measurement in 2003 made as Jupiter passed in front of a distant quasar confirmed that light and gravity traveled at the same speed — although the measurement was not very precise. However, today's measurement is stunning. After traveling for 130 million light-years, gamma-rays traveling at the speed of light and gravitational waves traveling at the speed of gravity arrived at Earth within 1.7 seconds of each other, with the gravitational waves arriving first. This is a conclusive measurement of the speed of gravity.

While today's measurement is an exciting one, it's important to remember that it's but a single observation. Even more thrilling is the fact that astronomers have just begun to exploit this capability. Gravitational-wave detectors are just beginning to tell us about some of the most violent events in the universe. I don't yet know what story they will tell us, but it's going to be fascinating.

[Watson]

Seems to suggest there is a new upper limit on the speed within the UNIVERSE. Spooky Action at a Distance happens instantaneously, or measured at the speed of light, I think.

I think I read that this was problematic, conflicting with the speed of light as the upper limit. If the instantaneous could happen just faster than the speed of light then the conflict would disappear?

[mitchellmckain]

Seems to suggest there is a new upper limit on the speed within the UNIVERSE. Spooky Action at a Distance happens instantaneously, or measured at the speed of light, I think.

I think I read that this was problematic, conflicting with the speed of light as the upper limit. If the instantaneous could happen just faster than the speed of light then the conflict would disappear?

Incorrect. It is already well known that the velocity with which light actually travels is affected by materials it travels through (in this case low density interstellar gas). The index of fraction of a material divides this speed, so for example, light travels slower by a factor of 1.33 in pure water at 20 degrees Celsius. The constant used in relativity is the speed of light in a perfect vacuum.

Thus the upshot is that the measurements here have absolutely NOTHING to do with any increased upper limit but only that the speed of gravity waves is a bit more reliable measure of that constant over great distances.

[zetreque]

If the speed of light in a vacuum is x
and if the speed of gravity in a vacuum is x+1
How does that not increase an upper limit to which I think Watson was thinking.

[DJ_Juggernaut]

I was wondering how fast does gravity propagate through spacetime. I assume it propagates at lightspeed, but assumptions are dangerous things sometimes.

The speed of gravity is instantaneous. This is true in any mathematical model of gravity. [GR and Newton's theory of Gravity, for eg.]

Gravitational waves and the 'speed of gravity' are the not same. Below is an attempt at calculating the 'speed of gravity'.

https://en.wikipedia.org/wiki/Speed_of_gravity#Laplace

The first attempt to combine a finite gravitational speed with Newton's theory was made by Laplace in 1805. Based on Newton's force law he considered a model in which the gravitational field is defined as a radiation field or fluid. Changes in the motion of the attracting body are transmitted by some sort of waves.[5] Therefore, the movements of the celestial bodies should be modified in the order v/c, where v is the relative speed between the bodies and c is the speed of gravity. The effect of a finite speed of gravity goes to zero as c goes to infinity, but not as 1/c2 as it does in modern theories. This led Laplace to conclude that the speed of gravitational interactions is at least 7×106 times the speed of light.

[Lincoln]

DJ_Juggernaut, your statement is simply incorrect. In GR, gravity is assumed to travel at the speed of light. In addition, the recent observation of the merging of two neutron stars with associated optical and gravitational wave detection proves without any doubt that the speed of gravity is equal to the speed of light. The differences are under 1.7 seconds out of a few hundred million years in travel time. Very compelling.

[mitchellmckain]

If the speed of light in a vacuum is x
and if the speed of gravity in a vacuum is x+1
How does that not increase an upper limit to which I think Watson was thinking.

Because the measured speed of speed of light x in this case is NOT the speed of light in a perfect vacuum c, which is the upper limit due to relativity. So instead we have the speed of gravity = c = x + a very small amount to make arrive 1.7 seconds earlier out of a few hundred million years. It just means that the index of refraction of the space the light traveled through was slightly higher than 1 due to interstellar gas.

[zetreque]

If the speed of light in a vacuum is x
and if the speed of gravity in a vacuum is x+1
How does that not increase an upper limit to which I think Watson was thinking.

Because the measured speed of speed of light x in this case is NOT the speed of light in a perfect vacuum c, which is the upper limit due to relativity. So instead we have the speed of gravity = c = x + a very small amount to make arrive 1.7 seconds earlier out of a few hundred million years. It just means that the index of refraction of the space the light traveled through was slightly higher than 1 due to interstellar gas.

You are hypothesizing here.

[Watson]

No Z, 'twas I that was hypothesizing, or at very least, surmising.

[zetreque]

until we have more data, you are all hypothesizing.

[DJ_Juggernaut]

DJ_Juggernaut, your statement is simply incorrect. In GR, gravity is assumed to travel at the speed of light.

At the risk of agreeing to disagree with you, I will retort that the distance between most celestial objects (sun and earth for eg) is too large for it not to be instantaneous. If the earth suddenly changed its distance relative to the sun, then the new resultant gravity force would have to travel or take effect instantaneously in order for it to maintain a stable orbit. If speed of gravity travels at the speed of light, it would take 8 mins for the new resultant gravity force to take effect. By then the earth's orbit would travel in a tangent for 8 mins. And the earth is constantly changing its distance relative to the sun. So...

Note, that I agree with you that a 'gravitational wave' can travel at a finite velocity. But not the "speed of gravity". They are not the same entities. Apples and oranges. The evidence you speak of is highly questionable if you say that it addresses the "speed of gravity". They are not the same beasts, so to speak.

[mitchellmckain]

You are hypothesizing here.

Nope. No hypothesis anywhere in sight. I was simply explaining the scientific facts.

[Event Horizon]

In the OP I was probing to find how gravity propagates, and if it is slowed by planets or suchlike. I think knowing it's equivalent of terminal velocity, and ways it might be manipulated could be critical to our survival as a race that will eventually have to abandon earth.
I also want to know why it's such damn awkward thing. Are clumps of dark matter real, or some kinda wrinkle in the Higgs field perhaps?
Is it better to know the right questions, or know the right answers? That's tough.

[Event Horizon]

https://www.space.com/41077-einstein-ge ... tification

Interesting related article if the link works...

[mitchellmckain]

In the OP I was probing to find how gravity propagates, and if it is slowed by planets or suchlike. I think knowing it's equivalent of terminal velocity, and ways it might be manipulated could be critical to our survival as a race that will eventually have to abandon earth.

Seems to me, it is more critical that we learn to behave, treating the earth and each other better. That is the more immediate requirements for survival, and if we cannot do that then I hope we do not spread the disease to the rest of the universe. I guess that means I am not a humanist, putting more value in the species than anything else.

The sun has billions of years left. If you are going to talk of eventualities then as far as we know, everything ends in the heat death of the universe.

I also want to know why it's such damn awkward thing. Are clumps of dark matter real, or some kinda wrinkle in the Higgs field perhaps?
Is it better to know the right questions, or know the right answers? That's tough.

What is so awkward?

Dark matter? Now THAT is an hypothesis.

The right questions usually lead to the right answers which in turn leads to the asking of a lot more questions. At least, that is what science has shown to be right sort of answers -- answers which shed light so we can see more to ask questions about. Not at all like the "Goddidit" kind of answer.

[Event Horizon]

Even if we turned right around and protected our planet it should last a few billion years for sure. But the Sun will eventually become a red giant with Earth on the perimeter of its atmosphere. We can't stay here and we would have to launch many many craft over a long time to promote success. We gotta go then, or before then. Either way, we gotta go.

[Event Horizon]

Anyway. If dark matter contains so much gravity, shouldn't it also be interacting with the Higgs field, or perhaps a manifestation whereby the Higgs field is distorting spacetime more than in other areas perhaps? A lot of gravity would indicate high levels of interaction with the Higgs field wouldn't it?
No Higgs, no gravity(Via mass), I'd like to know how this interaction works but I don't think the instruments have been invented as yet.
Apparently the standard model can probably said to be compatible, but we need metrics to be developed. Nailing gravity would be simply epic.
Gravity is also marginally faster than light in experiments, but I think photons have a quanta of mass albeit small which may account for the really very marginal difference in recorded speed.
And somehow I got back to where I started.
And is gravity super-relativistic in some way? How do we test for that?

[Watson]

The other thing I was pondering the about the neutron is, given the two options, forming a BH or coalescing into a larger star. In this case it form a BH, but how can it be otherwise?
How can the two rather large massively dense objects just coalesce into a single larger, more massive object? How can this happen without a massive display of energy and debris? Seems to me, the weakness of gravity would not be able to contain the energies and material of such a collision, despite the suggestion in the article. Obviously I'm missing a piece of the puzzle?

[Event Horizon]

A kinda unified gravity theory should be a common goal I think, or its equivalent. Ever since Newton we have pondered this. It looks like we're gonna have to ponder it some more.

[mitchellmckain]

The other thing I was pondering the about the neutron is, given the two options, forming a BH or coalescing into a larger star. In this case it form a BH, but how can it be otherwise?
How can the two rather large massively dense objects just coalesce into a single larger, more massive object? How can this happen without a massive display of energy and debris? Seems to me, the weakness of gravity would not be able to contain the energies and material of such a collision, despite the suggestion in the article. Obviously I'm missing a piece of the puzzle?

A low velocity collision is certain to produce a black hole. Neutron stars have a pretty narrow mass range (between 1.4 and 2.16 solar masses) and so just doubling the minimum will put you over the limit where the mass will collapse into a black hole. But with a high enough velocity collision enough matter will blow off to bring the total mass in under the limit and you get a neutron star. More velocity might blow off enough mass to leave you with one or more white dwarfs. Of course, with a high enough velocity collision you could end up with just a large cloud of hot matter (with all kinds of heavy elements) -- great stuff for making rich planets..

[mitchellmckain]

I was just thinking...

You know a neutron star is like a gigantic atomic nucleus and so one of these collisions would be like watching a nuclear reaction writ very large. This is very very interesting stuff for the scientist... BUT not something you can watch close up and expect to survive (huge understatement)!

[mitchellmckain]

On further reflection my suggestion of white dwarf star(s) as possible remnants would be highly unlikely. Even if you still had a mass of neutronium under the lower limit after blowing off enough mass due to the high speed collision, the destabilization of the mass of neutronium would give you an explosion too violent to leave a white dwarf remnant.

[Watson]

When two neutron stars collide, one of two things can happen. If the two stars are small enough, the outcome will be a single, larger, neutron star. However, if the combined mass of the two stars is above a threshold, the neutron stars will disappear into a single black hole. The data reported today cannot yet determine which happened in this instance. The remnant is either one of the heaviest neutron stars ever observed or one of the lightest black holes.

This is the part of Lincoln's article I'm referring to. If it is a gentle coming together of a pair of neutron stars, seems to me they would become a pair of hard rock rolling around each other in some gravitational dance. What energies would cause them to become one single, larger neutron star?

And if they collide with some force, wouldn't the collision + gravitational forces combined be enough to create a super nova, or at least appear as one, to us?

[mitchellmckain]

When two neutron stars collide, one of two things can happen. If the two stars are small enough, the outcome will be a single, larger, neutron star. However, if the combined mass of the two stars is above a threshold, the neutron stars will disappear into a single black hole. The data reported today cannot yet determine which happened in this instance. The remnant is either one of the heaviest neutron stars ever observed or one of the lightest black holes.

This is the part of Lincoln's article I'm referring to. If it is a gentle coming together of a pair of neutron stars, seems to me they would become a pair of hard rock rolling around each other in some gravitational dance. What energies would cause them to become one single, larger neutron star?

And if they collide with some force, wouldn't the collision + gravitational forces combined be enough to create a super nova, or at least appear as one, to us?

If they do not touch then it is not a collision. Though a near miss should cause severe disruption and friction due to tidal forces. Thus a gentle collision will reform into a larger mass with blow off to get rid of any kinetic energy. What you would do is compare that kinetic energy to the potential energy required to remove the mass from the gravity well and thus you would know how much mass is lost.

Neutron stars have such a narrow mass range because it is a delicate balance between the gravity and the forces needed to compact it down to neutronium. Too little mass and the gravity is not enough and the neutronium expand explosively. Too much mass and the gravity overcomes the force due to Pauli exclusion and it collapses into a black hole.

[Dave_Oblad]

Hi All,

The following is largely personal opinion, so take such with a grain of salt..lol.

I believe it is a mistake to group the wave propagation speed of a Gravity Wave with the speed of Gravity Collapse.

In the first case of a Gravity Wave, we are looking at the propagation of a Gravity Wave in the medium of Space-Time. It should be at the Light Speed (given my personal Model of Space-Time).

However, for Gravitational Collapse, we are talking about a density differential of a Gravitational Field or the Curvature in Space-Time (as more commonly used). As such, the structure of Space-Time is supported by the local surrounding structure of Space-Time. This means that if one could instantly remove.. say.. a Star like our Sun.. then the structure of Space-Time must relax (density decrease) as a function of its local area spread over distance.

This would mean that the instantaneous removal of our Sun would NOT result in releasing our Earth from its Solar Orbit 8 (plus) minutes later as per the propagation of Light Speed. What seems more probable is the relaxation of Space-Time Curvature spread over some Period of Time and not.. an instantaneous shift in Curvature. Or in other words.. the Earth would eventually break Solar orbit but it would be a gradual outwards spiral and not a sudden drop causing the Earth to shoot off in a straight line 8+ minutes after the Sun has been teleported elsewhere.

The Sun does NOT pull on the Earth. The Sun Warps Space-Time and that warping (Gravitational Density Gradient) causes the Earth to always be in an Acceleration Mode towards the Sun.. which also includes Clock Dilation on Matter that is subjected to said same Space-Time distortion. (Clocks run slower on the Sun then they would on the Earth)

This was a topic explored several years ago by myself and several Experts (I am not an expert..lol). It required a Model of a field of clocks surrounding a Black Hole and what would happen to the clocks should the Black Hole suddenly disappear. The answer demonstrated that the Gravitational Field Collapse cannot be propagated at the Speed of Light because it includes the element of Time (Temporal Adjustment) for one thing.

In other words: Given the Space-Time stress/tension created by a Star, the direction of Stress Dissipation must be from the outside in and not the inside out. A crude example is imagine a group of people crowded inside an enclosed area. Now instantly remove the enclosure. Notice the people near the center are the last to know the enclosure has been removed and they may now move further apart or dissipate (reduce density). As far as I know, this would seem true of all Gradient Density Effects which become suddenly unbounded. They dissipate outside-in as in released tension.

Anyway, the results of that discussion ended without a solid resolution. For the time being, Science holds that the speed of Gravitational Collapse is the same as Gravity Wave propagation (unless something has changed since that time). My objection was that they are two completely different things from a mechanical point of view (Stress Release in a Medium <vs> Wave Propagation in a Medium).

Best Regards,
Dave :^)

[Event Horizon]

Hi Dave. That makes quite a bit of sense to me.

I hadn't considered the possible effects of gravitational collapse. If you can find a pulsar imploding or something, and measure gravitational variations, it should be interesting. Unfortunately I don't think Cern are gonna find any gravitons for a while. I'm still trying to figure what kind of energy is the darn Higgs field made of. This on top of zero-point energy, contiguously? Separately? What stops all bosons gaining all their mass, and when this mass is given off as energy can the Higgs field get its energy back?

I tend to think too much sometimes. Heh!

[Dave_Oblad]

Hi Event Horizon,

Imagine the collision two equal Black Holes where one is composed of Anti-Matter. Sure, they would annihilate each other and supposedly produce a tremendous amount of energy.. inside of a Black Hole.

The results would be the conversion of both Matter types to Energy.. But would the Energy trapped within still maintain the Gravitational Field Space-Time distortion? Can that Energy escape the Gravity Well? Is Time itself actually distorted within the Black-Hole.. or is Matter (ie: clocks) the only real (temporal) distortion affected by a Gravity Well?

Fun to think about..

Best wishes,
Dave :^)