<p>To answer the title question &quot;How can we be sure...&quot;:  We can't.</p>
<p>Instead, this is a testable prediction of the model of mechanics proposed by Relativity. Relativity predicts that gravitational effects propagate at <em>c</em>. This (combined with other relativistic effects) predicts that two body orbits should have a particular shape: almost but not quite an ellipse. When we carefully observe the shape of orbits they are almost but not quite ellipses,
exactly as predicted (to our current ability to measure them).  This gives us  confidence.</p>
<p>Relativity predicts that gravitational waves should also travel at <em>c</em>.  When we observe gravitational waves we find that they have travelled very great distances in the same time that light has taken.  This gives us more confidence that the theory is correct, or close to correct.</p>
<p>If the &quot;speed of gravity&quot; was faster or slower than <em>c</em>, then orbits and gravitational waves would be different. This allows us to test the theory. So far, General Relativity has passed every test we have devised. This gives us very great confidence in the theory.</p>
<p>This doesn't mean that we are sure that gravity travels at the speed of light, but we have a pretty good model in which it does.</p>

CC BY-SA 4.0
2024-12-10T21:34:54.490Z
This is essentially the same standard we use to be "sure" of anything else in science. Every test or observation that's consistent with the theory makes us more confident, until and unless we observe something that contradicts it.
Barmar
CC BY-SA 4.0
2024-12-10T22:03:02.130Z
Indeed, my point is that we are never "sure" in science.  But the model predicts that the speed of gravity is the same as light, and this can be tested.  So far the model has passed every test.
James K
CC BY-SA 4.0
2024-12-09T22:22:05.727Z
2024-12-13T02:25:01.243Z
2024-12-13T02:25:01.243Z
James K
<p>One way to check is to observe an object that emits both gravitational waves <em>and</em> electromagnetic radiation. If such an object emitted a pulse of both gravitational and electromagnetic waves at almost the same time and these travelled a great distance before being detected, then any time lag between the detection of the different wave types would give an indication of any wave speed difference.</p>
<p>Such a test was provided by the kilonova and gravitational wave source GW170817. The merging neutron stars causing this explosion emitted both a chirp of gravitational waves as they merged <em>and</em> a pulse of gamma ray radiation. Both were detected within about 2 seconds of each other, even though the source was estimated to be at a distance of about 80 million light years from Earth. According to <a href="https://iopscience.iop.org/article/10.3847/2041-8213/aa920c" rel="noreferrer">Abbott et al. (2017)</a>, and considering other sources of delay between the two signals, this constrains the speed of gravitational waves to be the same as the the speed of light to within about 3 parts in <span class="math-container">$10^{15}$</span>.</p>

CC BY-SA 4.0
2024-12-10T22:25:09.833Z
Which of the two was faster in this example?
Paŭlo Ebermann
CC BY-SA 4.0
2024-12-10T22:59:23.233Z
@PaŭloEbermann the gamma rays arrived 1.74s later, but this small difference is predicted because of a small delay in the gamma ray generation after the merger. So there is no significant speed difference.
ProfRob
CC BY-SA 4.0
2024-12-11T23:52:14.750Z
I believe the first check involved Jupiter passing in front of a star: gravitational effects on the star's apparent position matched Jupiter's visual position well enough to give a speed of gravity of 90% the speed of light (plus or minus 30%).
Mark
CC BY-SA 4.0
2024-12-10T06:37:36.677Z
2024-12-10T06:37:36.677Z
ProfRob
<p>As gravity is the only force that affects spacetime, how can we be sure that effects of gravity travel only at the speed of light? Wouldn’t our observables be constrained by the speed of light and affected by gravity itself adding uncertainty to our observations?</p>

CC BY-SA 4.0
2024-12-09T21:25:05.640Z
Gravity isn't a force in General Relativity.
ProfRob
<gravity>
How can we be sure that the effects of gravity travel at most at the speed of light
Solo Man
# How can we be sure that the effects of gravity travel at most at the speed of light

(Asked by: username_0 on 2024-12-09)

As gravity is the only force that affects spacetime, how can we be sure that effects of gravity travel only at the speed of light? Wouldn’t our observables be constrained by the speed of light and affected by gravity itself adding uncertainty to our observations?

**Tags:** gravity

> **Comments:**
> **username_1**: Gravity isn't a force in General Relativity.  

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## 2 answers

### Answer by username_1 on 2024-12-10. Score: 47

One way to check is to observe an object that emits both gravitational waves *and* electromagnetic radiation. If such an object emitted a pulse of both gravitational and electromagnetic waves at almost the same time and these travelled a great distance before being detected, then any time lag between the detection of the different wave types would give an indication of any wave speed difference.

Such a test was provided by the kilonova and gravitational wave source GW170817. The merging neutron stars causing this explosion emitted both a chirp of gravitational waves as they merged *and* a pulse of gamma ray radiation. Both were detected within about 2 seconds of each other, even though the source was estimated to be at a distance of about 80 million light years from Earth. According to [Abbott et al. (2017)](https://iopscience.iop.org/article/10.3847/2041-8213/aa920c), and considering other sources of delay between the two signals, this constrains the speed of gravitational waves to be the same as the the speed of light to within about 3 parts in $10^{15}$.

> **Comments:**
> **username_4**: Which of the two was faster in this example?  
> **username_1**: @PaŭloEbermann the gamma rays arrived 1.74s later, but this small difference is predicted because of a small delay in the gamma ray generation after the merger. So there is no significant speed difference.  
> **username_5**: I believe the first check involved Jupiter passing in front of a star: gravitational effects on the star's apparent position matched Jupiter's visual position well enough to give a speed of gravity of 90% the speed of light (plus or minus 30%).  

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### Answer by username_2 on 2024-12-09. Score: 27

To answer the title question "How can we be sure...": We can't.

Instead, this is a testable prediction of the model of mechanics proposed by Relativity. Relativity predicts that gravitational effects propagate at *c*. This (combined with other relativistic effects) predicts that two body orbits should have a particular shape: almost but not quite an ellipse. When we carefully observe the shape of orbits they are almost but not quite ellipses,
exactly as predicted (to our current ability to measure them). This gives us confidence.

Relativity predicts that gravitational waves should also travel at *c*. When we observe gravitational waves we find that they have travelled very great distances in the same time that light has taken. This gives us more confidence that the theory is correct, or close to correct.

If the "speed of gravity" was faster or slower than *c*, then orbits and gravitational waves would be different. This allows us to test the theory. So far, General Relativity has passed every test we have devised. This gives us very great confidence in the theory.

This doesn't mean that we are sure that gravity travels at the speed of light, but we have a pretty good model in which it does.

> **Comments:**
> **username_3**: This is essentially the same standard we use to be "sure" of anything else in science. Every test or observation that's consistent with the theory makes us more confident, until and unless we observe something that contradicts it.  
> **username_2**: Indeed, my point is that we are never "sure" in science.  But the model predicts that the speed of gravity is the same as light, and this can be tested.  So far the model has passed every test.  

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