Hasn't Gravitational Lensing Already Proved Einstein? (LIGO)

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Discussion Overview

The discussion centers around the implications of recent LIGO findings for the General Theory of Relativity, particularly in relation to gravitational lensing and other experimental evidence supporting the theory. Participants explore the significance of gravitational waves, their detection, and the potential for new astronomical insights.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants note that LIGO's detection of gravitational waves is a significant technological achievement that provides direct evidence for general relativity.
  • Others argue that gravitational lensing and observations of the Hulse–Taylor binary pulsar already offer strong evidence for general relativity, likening LIGO's findings to the first detection of a radio signal.
  • A participant highlights that the observed waveform from LIGO aligns well with general relativity predictions for black hole mergers, suggesting a unique test in strong gravitational fields.
  • There is curiosity about whether gravitational waves from supermassive black hole collisions would be more energetic than those from smaller black holes, with some questioning the relationship between mass and gravitational radiation power.
  • One participant expresses interest in the potential for gravitational waves from neutron-star mergers to provide insights into the equation of state of neutron-rich matter.
  • Concerns are raised about the nature of black holes, questioning whether they could be composed of unknown forms of matter rather than being true black holes, and whether gravitational wave observations could help clarify this.
  • Another participant mentions that observations suggest black hole candidates have no surface and are consistent with a horizon, rather than being exotic objects.

Areas of Agreement / Disagreement

Participants generally agree on the significance of LIGO's findings and the existing evidence for general relativity, but there are multiple competing views regarding the implications of gravitational waves, the nature of black holes, and the potential for new discoveries in astrophysics.

Contextual Notes

Some discussions involve assumptions about the relationship between mass and gravitational radiation, as well as the definitions of black holes versus other potential forms of matter, which remain unresolved.

dlivingston
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The news out of LIGO is being heralded as one of the most important experimental verifications of physics in decades, as it provides experimental support to the General Theory.

The news makes it seem as though it were like the Higgs Boson was; theoretically concrete, but up in the air until this research confirmed it.

But hasn't the well known gravitational lensing effect already provided nearly as strong experimental evidence?
 
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You are right. We have much experimental evidence in favour of general relativity, and the binary pulsar observations of Taylor and Hulse showed that gravitational waves exist.

LIGOs achievement is
1) technological, allowing an amazing direct detection of a gravitational wave;
2) hopefully will allow a new type of astronomy, just like radiowaves and x-rays allow us to see more of what is out there than visible light allows.
 
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dlivingston said:
But hasn't the well known gravitational lensing effect already provided nearly as strong experimental evidence?

Sure, as has the observations on the Hulse–Taylor binary pulsar along with plenty of other tests. However this is akin to detecting the oscillation of a radio signal for the first time ever. It's a very, very strong piece of evidence in favor of general relativity, and it potentially puts great constraints on any present or future competing theories.
 
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In addition to the points that others have made above, the waveform that was observed agrees well with GR-based calculations for the merger of black holes. That is a unique test of general relativity in the regime of very strong gravitational fields.
 
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bcrowell said:
That is a unique test of general relativity in the regime of very strong gravitational fields.

Do they get any stronger than that?!
 
Drakkith said:
Do they get any stronger than that?!

I do still want to see the gravitational wave signal from two supermassive black holes colliding :-)
 
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Are you sure it's more energetic? The masses are larger, but they are moving slower and are farther away from each other. Does the gravitational radiation power increase monotonically with mass? There's also this issue that the wavelength will be much longer - ten million miles.
 
Oohh, great insights folks. I've learned something and am 200 feet deep in a Wikipedia rabbit hole that started with the Hulse–Taylor binary pulsar. Appreciate it. :)
 
bcrowell said:
I do still want to see the gravitational wave signal from two supermassive black holes colliding :-)

As a nuclear physicist I'd be even more eager to see (transient) gravitational waves from neutron-star mergers, because this could help to restrict the equation of state of nuclear (neutron-rich) matter.

Black holes are quite boring due to the no-hair theorem: It's just some object with a mass, a spin, and an electric charge. This brings up another question, I always ask myself, when I listen to talks about black holes (including Sagittarius in our own galaxy): The only argument we have that these very massive objects are truly black holes is that they have very large masses, ruling out any so far thinkable kind of matter composition (like e.g., neutron or quark stars), but maybe they could be just "stars" of a today unknown kind of matter. So is it possible with a very accurate observation of gravitational waves to falsify the assumption that these objects are just black holes or can one perhaps learn something about possible yet unknown forms of matter?
 
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vanhees71 said:
So is it possible with a very accurate observation of gravitational waves to falsify the assumption that these objects are just black holes or can one perhaps learn something about possible yet unknown forms of matter?

A good question. I'd also like to find a new kind of matter that can withstand the force of 4 million solar masses compressed into an area of space less than 50 million km across.
 
  • #11
vanhees71 said:
As a nuclear physicist I'd be even more eager to see (transient) gravitational waves from neutron-star mergers, because this could help to restrict the equation of state of nuclear (neutron-rich) matter.

Black holes are quite boring due to the no-hair theorem: It's just some object with a mass, a spin, and an electric charge. This brings up another question, I always ask myself, when I listen to talks about black holes (including Sagittarius in our own galaxy): The only argument we have that these very massive objects are truly black holes is that they have very large masses, ruling out any so far thinkable kind of matter composition (like e.g., neutron or quark stars), but maybe they could be just "stars" of a today unknown kind of matter. So is it possible with a very accurate observation of gravitational waves to falsify the assumption that these objects are just black holes or can one perhaps learn something about possible yet unknown forms of matter?
There are a growing set of observations to the effect that BH candidates have no surface, consistent horizon rather than exotic body.
 

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