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Topic: How “Fast” is Gravity? Post ReplyPost New Topic
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John Byrne
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Posted: 29 November 2021 at 9:56am | IP Logged | 1 post reply

If the Sun suddenly disappeared, vanished (not exploded) it would be about nine minutes before we’d see it happen from Earth.

But how soon would we feel it? Gravity is a bending of spacetime, and it seems like the Sun being gone would have an immediate effect. The Earth would start at once to drift out into space, no longer having anything to orbit.

But is that right? I googled various versions of “how fast is gravity” and couldn’t get any kind of answer.

Hmm…….

(No, this is not for a story. Just curious.)

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Michael Penn
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Posted: 29 November 2021 at 10:15am | IP Logged | 2 post reply

https://www.sciencealert.com/speed-of-gravitational-waves-an d-light-same
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Michael Roberts
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Posted: 29 November 2021 at 10:17am | IP Logged | 3 post reply

General relativity predicts that gravity moves at the speed of light, and observational research has found that gravity moves somewhere around the speed of light.
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Peter Martin
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Posted: 29 November 2021 at 10:20am | IP Logged | 4 post reply

I have wondered about this before and it is fascinating. I arrived at the same assumption as you, that it is somehow immediate.

Looking into it more, it seems this assumption is incorrect. According to general relativity, gravity propagates at the speed of light.

The best measurements we have, suggest it is slightly slower though:

[LINK to a paper on this. From the paper: "

We use the observed time delay of (+1.74 + or - 0.05)s between GRB 170817A and GW170817 to constrain the difference between the speed of gravity and the speed of light to be between -3 x (10^-15) and 7 x (10^-16) times the speed of light]
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Michael Roberts
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Posted: 29 November 2021 at 10:37am | IP Logged | 5 post reply

The best measurements we have, suggest it is slightly slower though

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 QUOTE:
the difference between the speed of gravity and the speed of light to be between -3 x (10^-15) and 7 x (10^-16) times the speed of light

Slightly slower or slightly faster. Basically, the speed of light is within the range of observations that have been made, and it is likely the speed of light, as General Relativity predicts.
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Peter Martin
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Posted: 29 November 2021 at 10:45am | IP Logged | 6 post reply

Yes, fair point.
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Vinny Valenti
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Posted: 29 November 2021 at 11:10am | IP Logged | 7 post reply

I've spent way too much time thinking about this very topic. In my mind it messes with the idea of what "now" really means. If the Sun was snuffed out "now", and there' no measure or effect that we would not perceive for minutes later, then "now" is also a function of the distance between objects. At Earth-scale, that's small enough to not factor in, but at inter-planetary scale (and beyond!)......
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John Byrne
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Posted: 29 November 2021 at 11:26am | IP Logged | 8 post reply

I’ve noticed astronomers tend to talk as if the universe is happening “now”. A while back there was some thought that Betelguese might be about to explode—ignoring the fact that if we witnessed this event today it would actually have happened more than 300 years ago.

“You may think it’s a long way down the street to the chemists….”

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Bob Brown
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Posted: 29 November 2021 at 1:29pm | IP Logged | 9 post reply

I'm far from an expert on this, but my interpretation of what I've read is that our "common sense notion" of two events (a significant distance apart) happening "at the same time" is fundamentally flawed...
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Eric Sofer
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Posted: 29 November 2021 at 1:37pm | IP Logged | 10 post reply

We're limited by the speed of light,
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James Woodcock
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Posted: 29 November 2021 at 1:40pm | IP Logged | 11 post reply

My go to scene whenever I talk about space.
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Mark Haslett
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Posted: 29 November 2021 at 3:38pm | IP Logged | 12 post reply

If "near the speed of light" is the correct answer, then the original post would have a sequence of events like this, right?:

The sun disappears...

8 minutes later, we SEE it disappear and we FEEL it disappear.

Until then, we feel its gravity and see its light.

We would also experience its heat and all other functions, no?

I wonder if there could possibly be any expected subtle warning before the 8 minute mark.
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