<p>it's an interesting question. It could ask about any violent transient effect that produces light and gravitational waves, though "popping out of existence" is the ultimate step function. </p>

<p>electromagnetic (EM) radiation is slowed very slightly by the presence of interstellar medium (especially by free electrons) and it's wavelength dependent. The image below from <a href="https://astronomy.stackexchange.com/a/29549/7982">this answer</a> shows the lower frequency radio waves arriving later than the higher frequency ones due to such an effect.</p>

<p>The farther away an object is, the larger the time difference, so this technique can be used to determine a likely distance for a given object.</p>

<p>So if you relax the term "light" to include a wide range of wavelengths which many astronomers now do, you can see a quite a big delay.</p>

<p>Answers to <a href="https://astronomy.stackexchange.com/q/13813/7982">How fast can and do gravitational waves travel?</a> say that gravitational waves generally travel at the speed of light, and answers to <a href="https://astronomy.stackexchange.com/q/33009/7982">Have more recent LIGO/VIRGO gravitational wave measurements narrowed down the speed of gravity further?</a> addresses experimental confirmation. I've never heard of an "index of refraction" for gravity that deviates from unity and could slow it with respect to the speed that an EM wave would travel between the same two points, so I think it's probably safe to assume that it will always come before the EM waves.</p>

<p><a href="https://i.sstatic.net/Zz4IS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Zz4IS.png" alt="enter image description here"></a></p>

CC BY-SA 4.0
2019-12-26T14:37:39.027Z
So, if the EM waves travel in vaccum then both arrive at the same time otherwise gravity hits first. But I read somewhere that whatever travels across space is limited at the speed of light but space itself is not limited by that speed. So if gravity is the curvature of space-time, then sudden absence of something should remove that curvature and it should be felt instantly. Does it work that way?
raptorAcrylyc
CC BY-SA 4.0
2019-12-26T14:44:04.490Z
@quantumbiker I've answered the question as asked. If you have a new question, ask it separately as a new question post. But "I read somewhere that..." type questions will usually be immediately challenged with "What exactly did you read, and where exactly did you read it?"
uhoh
CC BY-SA 4.0
2019-12-26T15:12:55.877Z
Alright, Thanks for the answer.
raptorAcrylyc
CC BY-SA 4.0
2019-12-27T13:24:56.680Z
@quantumbiker Also, we don't normally answer impossible hypothetical questions on this site. It gets tricky giving scientifically precise answers when the scenario itself contradicts the laws of science. ;) We need to use GR (General Relativity) to precisely answer your question about gravitational waves, but GR says that mass-energy cannot pop into or out existence.
PM 2Ring
CC BY-SA 4.0
2019-12-27T18:00:48.653Z
Yeah, I understand, I was just trying to find a way to create a situation where an event might have an instant effect. I just wanted to get the idea whether gravity is a property of space or time, but then you used the term general relativity and then I realised it's a property of spacetime. Thanks anyways :)( please excuse the terminology if it's not technically correct, I'm just a hobbyist who's trying to grasp these concepts...)
raptorAcrylyc
CC BY-SA 4.0
2019-12-27T23:55:13.570Z
@quantumbiker In addition to Astronomy SE there is [Physics SE](https://physics.stackexchange.com/) at one end of the spectrum and [Worldbuilding SE](https://worldbuilding.stackexchange.com/) at the other end. It's great to be curious about these things; you may find several *existing* and answered questions here and in those sites that address your interests, and if one leaves something unanswered for you you can always ask a new question and link back to an old answer.
uhoh
CC BY-SA 4.0
2019-12-27T23:58:50.357Z
@quantumbiker [Gedankenexperiments](https://en.wikipedia.org/wiki/Thought_experiment) are obviously a recognized way to address problems in physics for sure and that's what you were doing here, introducing a step-function and asking about how it might propagate. But in Stack Exchange there is some resistance to hypothetical/impossible scenarios because sometimes people just ask random impossible questions without thinking things through, and it's hard to tell the difference from a two-sentence question that doesn't show evidence any prior reading.
uhoh
CC BY-SA 4.0
2019-12-27T23:59:58.730Z
@quantumbiker including  links to something (anything!) you've read before asking really help, and they can be links to other Stack Exchange questions and/or answers in the same or another SE site.
uhoh
CC BY-SA 4.0
2019-12-28T03:22:21.910Z
I've asked another question with references [https://astronomy.stackexchange.com/questions/34513/can-gravity-be-faster-than-light-in-some-specific-cases]
raptorAcrylyc
CC BY-SA 4.0
2019-12-28T09:01:12.140Z
You may find these questions interesting: https://physics.stackexchange.com/q/27845/123208 and https://space.stackexchange.com/q/21467
PM 2Ring
CC BY-SA 4.0
2019-12-26T13:53:46.947Z
2019-12-27T12:12:45.740Z
2019-12-27T12:12:45.740Z
uhoh
<p>Let's assume a person is close enough to an object in space time to be able to observe it's gravitational effects and the light coming from it.
If that object suddenly pops out of existence (hypothetical situation), what will the person observe first,</p>

<ol>
<li>The gravitational field's absence</li>
<li>The object disappearing (light stops coming)</li>
</ol>

<p>P.S.- pardon me for the noobness of this question, physics isn't one of my strongest subjects.</p>

<gravity><light>
If an event occurs at a point in space time what will an observer experience first, it's gravity or the light from it?
raptorAcrylyc
# If an event occurs at a point in space time what will an observer experience first, it's gravity or the light from it?

(Asked by: username_0 on 2019-12-26)

Let's assume a person is close enough to an object in space time to be able to observe it's gravitational effects and the light coming from it.
If that object suddenly pops out of existence (hypothetical situation), what will the person observe first,

1. The gravitational field's absence
2. The object disappearing (light stops coming)

P.S.- pardon me for the noobness of this question, physics isn't one of my strongest subjects.

**Tags:** gravity, light


---

## 1 answers

### Answer by username_1 on 2019-12-26. Score: 4 | Accepted answer

it's an interesting question. It could ask about any violent transient effect that produces light and gravitational waves, though "popping out of existence" is the ultimate step function.

electromagnetic (EM) radiation is slowed very slightly by the presence of interstellar medium (especially by free electrons) and it's wavelength dependent. The image below from [this answer](https://astronomy.stackexchange.com/a/29549/7982) shows the lower frequency radio waves arriving later than the higher frequency ones due to such an effect.

The farther away an object is, the larger the time difference, so this technique can be used to determine a likely distance for a given object.

So if you relax the term "light" to include a wide range of wavelengths which many astronomers now do, you can see a quite a big delay.

Answers to [How fast can and do gravitational waves travel?](https://astronomy.stackexchange.com/q/13813/7982) say that gravitational waves generally travel at the speed of light, and answers to [Have more recent LIGO/VIRGO gravitational wave measurements narrowed down the speed of gravity further?](https://astronomy.stackexchange.com/q/33009/7982) addresses experimental confirmation. I've never heard of an "index of refraction" for gravity that deviates from unity and could slow it with respect to the speed that an EM wave would travel between the same two points, so I think it's probably safe to assume that it will always come before the EM waves.

[![enter image description here](https://i.sstatic.net/Zz4IS.png)](https://i.sstatic.net/Zz4IS.png)

> **Comments:**
> **username_0**: So, if the EM waves travel in vaccum then both arrive at the same time otherwise gravity hits first. But I read somewhere that whatever travels across space is limited at the speed of light but space itself is not limited by that speed. So if gravity is the curvature of space-time, then sudden absence of something should remove that curvature and it should be felt instantly. Does it work that way?  
> **username_1**: @quantumbiker I've answered the question as asked. If you have a new question, ask it separately as a new question post. But "I read somewhere that..." type questions will usually be immediately challenged with "What exactly did you read, and where exactly did you read it?"  
> **username_0**: Alright, Thanks for the answer.  
> **username_2**: @quantumbiker Also, we don't normally answer impossible hypothetical questions on this site. It gets tricky giving scientifically precise answers when the scenario itself contradicts the laws of science. ;) We need to use GR (General Relativity) to precisely answer your question about gravitational waves, but GR says that mass-energy cannot pop into or out existence.  
> **username_0**: Yeah, I understand, I was just trying to find a way to create a situation where an event might have an instant effect. I just wanted to get the idea whether gravity is a property of space or time, but then you used the term general relativity and then I realised it's a property of spacetime. Thanks anyways :)( please excuse the terminology if it's not technically correct, I'm just a hobbyist who's trying to grasp these concepts...)  
> **username_1**: @quantumbiker In addition to Astronomy SE there is [Physics SE](https://physics.stackexchange.com/) at one end of the spectrum and [Worldbuilding SE](https://worldbuilding.stackexchange.com/) at the other end. It's great to be curious about these things; you may find several *existing* and answered questions here and in those sites that address your interests, and if one leaves something unanswered for you you can always ask a new question and link back to an old answer.  
> **username_1**: @quantumbiker [Gedankenexperiments](https://en.wikipedia.org/wiki/Thought_experiment) are obviously a recognized way to address problems in physics for sure and that's what you were doing here, introducing a step-function and asking about how it might propagate. But in Stack Exchange there is some resistance to hypothetical/impossible scenarios because sometimes people just ask random impossible questions without thinking things through, and it's hard to tell the difference from a two-sentence question that doesn't show evidence any prior reading.  
> **username_1**: @quantumbiker including  links to something (anything!) you've read before asking really help, and they can be links to other Stack Exchange questions and/or answers in the same or another SE site.  
> **username_0**: I've asked another question with references [https://astronomy.stackexchange.com/questions/34513/can-gravity-be-faster-than-light-in-some-specific-cases]  
> **username_2**: You may find these questions interesting: https://physics.stackexchange.com/q/27845/123208 and https://space.stackexchange.com/q/21467  

---

