Echoes Beyond the Horizon: Gravitational Secrets?

[Richard Townsend]

Could gravitational waves from colliding black holes carry echoes of their internal structure, revealing what's inside without ever looking?

 

[No Ghost In This Shell]

No. The source term for a gravitational wave is the quadrupole (and higher multipole) moment of the system, so the wave form can depend only on masses, angular momenta, and (if present) electric charges, plus boundary conditions that give the initial orbital parameters. It does not depend on anything to do with their internal structure.

 

[Richard Townsend]

No. The source term for a gravitational wave is the quadrupole (and higher multipole) moment of the system, so the wave form can depend only on masses, angular momenta, and (if present) electric charges, plus boundary conditions that give the initial orbital parameters. It does not depend on anything to do with their internal structure.

I get what you’re trying to say, but it’s not quite right as stated. The quadrupole moment is the source of gravitational radiation, yes, but for anything other than black holes the quadrupole does depend on internal structure.

Black holes are the special case where the exterior field is fully fixed by mass, spin, and charge, so the waveform really doesn’t care about anything else. That’s basically the no‑hair theorem.

But if you’re talking about neutron stars, white dwarfs, or any object made of matter, the waveform absolutely carries information about internal physics. Tidal deformability, the equation of state, resonant modes, finite‑size effects - these all show up in the phase evolution. That’s exactly how LIGO/Virgo extracted constraints on the neutron‑star EoS from GW170817.

So, the statement is fine for binary black holes, but it’s not true in general. For material bodies, the internal structure matters.

 

[exchemist]

I get what you’re trying to say, but it’s not quite right as stated. The quadrupole moment is the source of gravitational radiation, yes, but for anything other than black holes the quadrupole does depend on internal structure.

Black holes are the special case where the exterior field is fully fixed by mass, spin, and charge, so the waveform really doesn’t care about anything else. That’s basically the no‑hair theorem.

But if you’re talking about neutron stars, white dwarfs, or any object made of matter, the waveform absolutely carries information about internal physics. Tidal deformability, the equation of state, resonant modes, finite‑size effects - these all show up in the phase evolution. That’s exactly how LIGO/Virgo extracted constraints on the neutron‑star EoS from GW170817.

So, the statement is fine for binary black holes, but it’s not true in general. For material bodies, the internal structure matters.

Botshit.

 

[Pinball1970]

Botshit.

I've reported him

 

[exchemist]

I've reported him

You can easily tell when it's him and when it's his AI chatbot. The "What if the sky was made of concrete?" questions are him, and the wordy replies full of science terminology that he wouldn't understand are his bot.

 

[Richard Townsend]

You can easily tell when it's him and when it's his AI chatbot. The "What if the sky was made of concrete?" questions are him, and the wordy replies full of science terminology that he wouldn't understand are his bot.

You’re reading way too much into this. I’m just asking questions and taking part in the discussion. Sometimes I write short posts, sometimes longer ones - it depends on the topic and how much I feel like saying. There’s no conspiracy behind it.

If you want to talk about the actual subject, I’m here for that. If not, that’s fine too.

 

[exchemist]

You’re reading way too much into this. I’m just asking questions and taking part in the discussion. Sometimes I write short posts, sometimes longer ones - it depends on the topic and how much I feel like saying. There’s no conspiracy behind it.

If you want to talk about the actual subject, I’m here for that. If not, that’s fine too.

If post was really your own words, you would never have asked the question you asked in post 1.

 

[Richard Townsend]

If post was really your own words, you would never have asked the question you asked in post 1.

I’m not sure why you think you can infer that. I asked a question, then followed up with a longer post after reading more about the topic. That’s all. People learn as they go - nothing unusual about that.

If you want to discuss the subject itself, I’m here for that.

 

[Pinball1970]

You can easily tell when it's him and when it's his AI chatbot. The "What if the sky was made of concrete?" questions are him, and the wordy replies full of science terminology that he wouldn't understand are his bot.

I've had to click him.

Nine threads on everything from Martian genes to Doughnuts, usually opening with a silly question.

 

[Richard Townsend]

I've had to click him.

Nine threads on everything from Martian genes to Doughnuts, usually opening with a silly question.

I’m just posting questions about things I’m curious about. They’re all different topics because I read widely, not because there’s anything odd going on. If a thread doesn’t interest you, feel free to skip it. BTW, I never asked why the sky was made of concrete - that's ridiculous.

 

[DaveC426913]

I’m just posting questions about things I’m curious about. They’re all different topics because I read widely,

OK, why don't you start with the things you've read about the subject? For example, if you want to know about atmospheric rentention of planets, why not start with what you already know, so we don't just go over the same ground?

BTW, I never asked why the sky was made of concrete - that's ridiculous.

It's no more or less ridiculous than any other form of suspending the laws of physics and then asking what would happen - for example: two coins at opposite ends of the universe being magically correlated.

You see the point?

 

[Richard Townsend]

OK, why don't you start with the things you've read about the subject? For example, if you want to know about atmospheric rentention of planets, why not start with what you already know, so we don't just go over the same ground?




It's no more or less ridiculous than any other form of suspending the laws of physics and then asking what would happen - for example: two coins at opposite ends of the universe being magically correlated.

You see the point?

I wasn’t trying to re‑cover basics, just asking the next question that came to mind. What I already know is the broad picture: a planet keeps an atmosphere if its gravity is strong enough and the gas molecules aren’t moving fast enough to escape, with temperature, composition, and solar radiation all playing a role. Beyond that, I’m trying to understand the finer points, not repeat the intro material.

It wasn’t meant as a literal proposal, just a way of asking whether perfect correlations at a distance would tell us something about how randomness works. Physics already deals with that kind of question in things like entanglement and hidden‑variable ideas. I was just putting it in everyday terms.

 

[DaveC426913]

It wasn’t meant as a literal proposal, just a way of asking whether perfect correlations at a distance would tell us something about how randomness works.

If such perfect correlations at a distance were actually observed, they might tell us that something is happening, yes.
But again, what can be gleaned by speculating about what would happen if something that doesn't happen were to magically happen?

Start with what we observe. Build on that.

 

[Richard Townsend]

If such perfect correlations at a distance were actually observed, they might tell us that something is happening, yes.
But again, what can be gleaned by speculating about what would happen if something that doesn't happen were to magically happen?

Start with what we observe. Build on that.

I’m not treating it as something that “magically happens.” Thought experiments are a normal way to probe what our theories would imply if certain patterns were observed. We already start from what’s actually observed - this is just a way of testing how the concepts behave at the edges; the same way physicists use idealised setups to clarify what a theory really says.

 

[DaveC426913]

I’m not treating it as something that “magically happens.” Thought experiments are a normal way to probe what our theories would imply if certain patterns were observed.

It is not a valid thought experiment. That's not how they work.

this is just a way of testing how the concepts behave at the edges; the same way physicists use idealised setups to clarify what a theory really says.

You can't test how a concept behaves at the edge - if it doesn't behave that way at the edge - and wouldn't have any reason to behave that way.

You might as well ask 'what if one of the coins always turned blue?' and then start trying to figure out the secrets of the universe from that.

 

[Richard Townsend]

It is not a valid thought experiment. That's not how they work.


You can't test how a concept behaves at the edge - if it doesn't behave that way at the edge - and wouldn't have any reason to behave that way.

You might as well ask 'what if one of the coins always turned blue?' and then start trying to figure out the secrets of the universe from that.

A hypothetical scenario used to test what a theory would imply is a thought experiment. It doesn’t need to be physically realised or currently observed; it’s just a way of checking the logic. Physicists do this all the time with idealised setups like frictionless surfaces or perfect detectors. They’re not claims about reality, just tools for clarifying ideas.

 

[exchemist]

I’m not treating it as something that “magically happens.” Thought experiments are a normal way to probe what our theories would imply if certain patterns were observed. We already start from what’s actually observed - this is just a way of testing how the concepts behave at the edges; the same way physicists use idealised setups to clarify what a theory really says.

It's not a thought experiment. It's a silly scenario - like asking what if the sky was made of concrete.

A sensible scenario might be what would we expect if people at opposite ends of the universe each toss a coin repeatedly. The answer is we would expect no statistical correlation between the outcomes. But by adding in the stipulation that they are correlated, you are stipulating that the laws of physics are broken - and then asking what that would mean for the laws of physics.

Which is silly.

 

[DaveC426913]

A hypothetical scenario used to test what a theory would imply is a thought experiment.

A thought experiment can ignore things that are not relevant to the thing being tested.

You are trying to ignore the very thing being tested - i.e. a correlation of two objects on opposite sides the universe

You can do that if you want, it's just that you get answers that are meaningless.

It doesn’t need to be physically realised or currently observed; it’s just a way of checking the logic.

What logic is there in two coins 97Gly apart always correlating their flips?

Yes - if that happened - it would be a crazy world indeed. We might get unicorns.

Physicists do this all the time with idealised setups like frictionless surfaces or perfect detectors.

Yes - ignore elements that are not relevant to the outcome of the very thing being tested.

(I really should write up an article about this; it is such a common misunderstanding of how thought experiments work.)

 

[Richard Townsend]

A thought experiment can ignore things that are not relevant to the thing being tested.

You are trying to ignore the very thing being tested - i.e. a correlation of two objects on opposite sides the universe

You can do that if you want, it's just that you get answers that are meaningless.



What logic is there in two coins 97Gly apart always correlating their flips?

Yes - if that happened - it would be a crazy world indeed. We might get unicorns.


Yes - ignore elements that are not relevant to the outcome of the very thing being tested.

(I really should write up an article about this; it is such a common misunderstanding of how thought experiments work.)

There’s is no logic in two coins 97 billion light‑years apart matching flips - that’s exactly why the scenario is useful. It strips the idea down to its essentials. A thought experiment doesn’t need to describe something that happens; it just tests what a theory would imply if it did.