# The Speed of Gravity:

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Nov 4, 2017.

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Physicists make rapid progress in bounding the speed of gravity:

(Phys.org)—Recent gravitational wave detections have allowed physicists to confirm with greater and greater precision what Einstein predicted over 100 years ago in the theory of general relativity: that gravity does not act instantaneously as Newton thought, but instead propagates at the speed of light.

"The speed of gravity, like the speed of light, is one of the fundamental constants in the Universe," Neil Cornish, a physicist at Montana State University, told Phys.org. "Until the advent of gravitational wave astronomy, we had no way to directly measure the speed of gravity."

Over the past few months, physicists have made very rapid progress in bounding the speed of gravity using gravitational wave observations.

Initially, the first LIGO detections of gravitational waves constrained the speed of gravity to within 50% of the speed of light.

In a paper published last week in Physical Review Letters, Cornish and his coauthors Diego Blas at CERN and Germano Nardini at the University of Bern have combined the first three gravitational wave events reported by the LIGO and Virgo collaborations, allowing them to improve the original bounds to within roughly 45% of the speed of light.

Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light to between -3 x 10-15 and 7 x 10-16 times the speed of light.

The reason for the huge leap in precision is that the neutron star event did not emit only gravitational waves, but also electromagnetic radiation in the form of gamma rays. The simultaneous emission of both gravitational waves and light from the same source allowed the scientists to set bounds on the speed of gravity that is many orders of magnitude more stringent that what could be set using gravitational wave signals alone.

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https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.161102

Bounding the Speed of Gravity with Gravitational Wave Observations

ABSTRACT
The time delay between gravitational wave signals arriving at widely separated detectors can be used to place upper and lower bounds on the speed of gravitational wave propagation. Using a Bayesian approach that combines the first three gravitational wave detections reported by the LIGO Scientific and Virgo Collaborations we constrain the gravitational waves propagation speed cgw to the 90% credible interval 0.55c<cgw<1.42c, where c is the speed of light in vacuum. These bounds will improve as more detections are made and as more detectors join the worldwide network. Of order 20 detections by the two LIGO detectors will constrain the speed of gravity to within 20% of the speed of light, while just five detections by the LIGO-Virgo-Kagra network will constrain the speed of gravity to within 1% of the speed of light.

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http://iopscience.iop.org/article/10.3847/2041-8213/aa920c/pdf

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

Abstract:

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is 5.0 10 ´ -8. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of (+  1.74 0.05 s ) between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between 3 10 - ´ -15 and 7 10 + ´ -16 times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and sub threshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity

7. ### geordiefRegistered Senior Member

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Is it correct to equate the "speed of gravity" with the speed of propagation of gravitational waves?

Is "speed of gravity" a well defined term or can it mean different things ?

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8. ### Michael 34530th March coming up - Well behaved FridayValued Senior Member

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I thought of gravity as a "field" ie as being attached to, say the Sun

Take away the Sun and you instantly take away the field and the planets head off in the direction heading at the time

If gravity travels at light speed Mercury would head off first with Earth off 8 minutes after the vanishing act

To me it seems odd with a vanished Sun gravity still present

Huey Dewey and Louie are shaking their heads

My current uptake is consciousness

When I solve that next week I will do gravity in January after holiday and Christmas

9. ### geordiefRegistered Senior Member

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I feel very confident that the field would not vanish as soon as the Sun disappeared. It would take 8 minutes for the Earth to change its direction of motion wrt,say another Star in the Galaxy.

To inhabitants on the Earth it might seem like to change was instant but it would only take effect t when the huge change in the gravitational field had reached them (after 8 minutes, those 8 minutes measured from ,perhaps Jupiter as it might not be easy to do those measurements in the vicinity of the Earth)

10. ### Michael 34530th March coming up - Well behaved FridayValued Senior Member

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I'm sure you are correct but my gut (not a very scientific gut

) looks at gravity, as I said, as a field attached to the Sun
Somewhat like atmosphere attached to Earth
Remove Earth atmosphere goes with it
If gravity detaches from the Sun to travel to Earth how does it attach to Earth and let the Sun know "I got it"?

At the meeting of the gravity field from the Sun and Earth's gravity field my non scientific gut imagines a depression in the Sun field. The motion of the Earth forms a temporary trench (if gravity was solid the trench would remain)

As soon as the Earth has pushed a way forward the field behind fills in

Still I am still waiting on alternative ideas from Huey Dewey and Louie

11. ### geordiefRegistered Senior Member

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I am starting to see this field as not (statically**) attached to the Sun but emanating dynamically from the Sun ,being continuously replenished by the Sun's presence (visually like a continuous tsunami).

If the Sun disappears the "back end " of this tsunami goes quiet and this "lack of tsunami£ reaches us after a 8 min delay.

The Earth is also creating its own smaller tsunami and I guess that these two "tsunamis of gravitation" meet and interact in ways I do not understand (can there interfere destructively ,I wonder idly?)

** confused here as I have heard that gravity fields are static and I am very much out of my depth here.

12. ### sculptorValued Senior Member

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Last edited: Nov 4, 2017
13. ### Janus58Valued Senior Member

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Gravity is a field much in the way magnetism is a field. Place a hunk of Iron in the vicinity of an electromagnet and it will be attracted to the magnet. Turn off the magnet, and the magnetic field goes away and it is no longer attracted. But if the vanishing of the field(or even its weakening) happens across the whole field at once, then you've found a way of transmitting information faster than light.
All you have to do is rig up a away to a way to modulate the electromagnet to encode information. So let's use an alternating current to power the electromagnet, then vary the strength amplitude of the current. We can them remotely measure the variations in the magnetic field an decode the message... Oh wait, I just described the basic idea behind AM radio. Radio is transmitted by electromagnetic waves. Electromagnetic waves are essentially "ripples" in an electromagnetic field. They carry information about changes in the field from one point of the field to another, and do so at the speed of light.

This is no different for the gravitational field and gravitational waves. The gravitational field is there, but the only way to transmit info about changes in it is by gravitational waves, which travel at c. So, if the Sun were to suddenly disappear, then that information would move as a gravitational wave "ripple" through the gravitational field at c. The Earth would react to this change at the same time as we saw the Sun disappear in the sky.

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14. ### exchemistValued Senior Member

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I presume this means that for a massive object moving at a significant fraction of c relative to another, we might be able to see the "lag" in response to its moving gravitational field as it passes. Do we have any recorded instances of this?

15. ### Michael 34530th March coming up - Well behaved FridayValued Senior Member

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Sounds very reasonable and correct (here comes the but) but if you take my example of Earth and the atmosphere. You can transmit sound through the atmosphere but remove the Earth along with the atmosphere at the same time and you remove the sound waves at the same time. Now faster than light speed required

Sounds scientificity and I hope Janus58 has a scientificity answer but in visual English

My understanding comes more from being able to visualise answers

16. ### DaveC426913Valued Senior Member

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12, 463.2 years ago, I answered a similar question about the propagation of the start and stop moments of EM waves from a generator.

This diagram applies to gravity as well as it applies to EM waves.

Last edited: Nov 4, 2017
17. ### DaveC426913Valued Senior Member

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It is helpful to remember that gravity is a field. That means it pervades the universe, and always has a value at every point in space. There is no such thing as a space without a value for gravity.

Gravity does not propagate; it is already there.

What propagates is changes in its value at given points across space. It is these changes that move at the speed of light.

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18. ### DaveC426913Valued Senior Member

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Wow. I guess this big question is pretty much definitively answered once and for all.

What speed does gravity (gravitational changes) travel at?
The speed of light. Full stop.

19. ### geordiefRegistered Senior Member

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A bit off topic ,but could you perhaps answer me this?
Do these 3 kinds of field exist? Are they different and distinct?
1:An electromagnetic field
2: A magnetic field
3:An electric field

I ask because I (i think in this thread**) came across the term "electromagnetic field" but had till then only really thought the other two kinds of field existed.

**yes it was Janus58 in post#9 I see.

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Bingo! well put. Gravity is simply after all just spacetime geometry.

21. ### SchmelzerValued Senior Member

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While I fully agree with this point of view, it is, unfortunately, not really the mainstream position.

There is the concept of GR as a field theory. This was advocated by Feynman. But this would require that there is some notion of a spacetime point - where the field has some value - as some entity which exists independent of the field. From the point of view of the spacetime interpretation, there exists no such notion of points.

This becomes problematic in quantum gravity. In a field theory, you have a fixed point, and the values of the fields in this point may be uncertain, with the quantum FT telling you what are the probabilities for various values of the field in that point. A superposition of, say, $\alpha_1|f_1(x)> + \alpha_2|f_2(x)>$ will have in point $x$ the field value $f_1$ with probability $|\alpha_1|^2$ and $f_2$ with probability $|\alpha_2|^2$. But if you have two metrics $\alpha_1|g_{mn}^1(x)> + \alpha_2|g_{mn}^2(x)>$, it makes no sense to talk about the value of the field $g_{mn}(x)$ in $x$ being $g_{mn}^1$ with probability $|\alpha_1|^2$ and $g_{mn}^2$ with probability $|\alpha_2|^2$.

22. ### RajeshTrivediValued Senior Member

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His statement is unwittingly true in reality, even though he is making this as the mainstream point of view, which is not so.
In simple words for this to be true, it will call for presence of non empty space either ether comes in or some other form.

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https://arxiv.org/pdf/1702.00319.pdf

EPFL Lectures on General Relativity as a Quantum Field Theory

Abstract

These notes are an introduction to General Relativity as a Quantum Effective Field Theory, following the material given in a short course on the subject at EPFL. The intent is to develop General Relativity starting from a quantum field theoretic viewpoint, and to introduce some of the techniques needed to understand the subject.

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