Gravitational Waves Finally Prove Stephen Hawking's Black Hole Theorem (newscientist.com)
(Friday September 12, 2025 @03:00AM (BeauHD)
from the he-was-right-again dept.)
- Reference: 0179170762
- News link: https://science.slashdot.org/story/25/09/11/2133252/gravitational-waves-finally-prove-stephen-hawkings-black-hole-theorem
- Source link: https://www.newscientist.com/article/2495377-gravitational-waves-finally-prove-stephen-hawkings-black-hole-theorem/
Physicists have [1]confirmed Stephen Hawking's 1971 black hole area theorem with near-absolute certainty, thanks to gravitational waves from an exceptionally loud black hole collision detected by upgraded LIGO instruments. New Scientist reports:
> Hawking proposed his black hole area theorem in 1971, which states that when two black holes merge, the resulting black hole's event horizon -- the boundary beyond which not even light can escape the clutches of a black hole -- cannot have an area smaller than the sum of the two original black holes. The theorem echoes the second law of thermodynamics, which states that the entropy, or disorder within an object, never decreases.
>
> Black hole mergers warp the fabric of the universe, producing tiny fluctuations in space-time known as gravitational waves, which cross the universe at the speed of light. Five gravitational wave observatories on Earth hunt for waves [2]10,000 times smaller than the nucleus of an atom. They include the two US-based detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) plus the Virgo detector in Italy, KAGRA in Japan and GEO600 in Germany, operated by an international collaboration known as LIGO-Virgo-KAGRA (LVK).
>
> The recent collision, named GW250114, was almost identical to the one that created the [3]first gravitational waves ever observed in 2015 . Both involved black holes with masses between 30 and 40 times the mass of our sun and took place about 1.3 billion light years away. This time, the upgraded LIGO detectors had three times the sensitivity they had in 2015, so they were able to capture waves emanating from the collision in unprecedented detail. This allowed researchers to verify Hawking's theorem by calculating that the area of the event horizon was indeed larger after the merger.
The findings have been [4]published in the journal Physical Review Letters .
[1] https://www.newscientist.com/article/2495377-gravitational-waves-finally-prove-stephen-hawkings-black-hole-theorem/
[2] https://www.ligo.caltech.edu/page/what-are-gw
[3] https://science.slashdot.org/story/16/02/11/1558252/its-official-ligo-scientists-make-first-ever-observation-of-gravity-waves
[4] https://journals.aps.org/prl/abstract/10.1103/kw5g-d732
> Hawking proposed his black hole area theorem in 1971, which states that when two black holes merge, the resulting black hole's event horizon -- the boundary beyond which not even light can escape the clutches of a black hole -- cannot have an area smaller than the sum of the two original black holes. The theorem echoes the second law of thermodynamics, which states that the entropy, or disorder within an object, never decreases.
>
> Black hole mergers warp the fabric of the universe, producing tiny fluctuations in space-time known as gravitational waves, which cross the universe at the speed of light. Five gravitational wave observatories on Earth hunt for waves [2]10,000 times smaller than the nucleus of an atom. They include the two US-based detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) plus the Virgo detector in Italy, KAGRA in Japan and GEO600 in Germany, operated by an international collaboration known as LIGO-Virgo-KAGRA (LVK).
>
> The recent collision, named GW250114, was almost identical to the one that created the [3]first gravitational waves ever observed in 2015 . Both involved black holes with masses between 30 and 40 times the mass of our sun and took place about 1.3 billion light years away. This time, the upgraded LIGO detectors had three times the sensitivity they had in 2015, so they were able to capture waves emanating from the collision in unprecedented detail. This allowed researchers to verify Hawking's theorem by calculating that the area of the event horizon was indeed larger after the merger.
The findings have been [4]published in the journal Physical Review Letters .
[1] https://www.newscientist.com/article/2495377-gravitational-waves-finally-prove-stephen-hawkings-black-hole-theorem/
[2] https://www.ligo.caltech.edu/page/what-are-gw
[3] https://science.slashdot.org/story/16/02/11/1558252/its-official-ligo-scientists-make-first-ever-observation-of-gravity-waves
[4] https://journals.aps.org/prl/abstract/10.1103/kw5g-d732
One non-inconsistent observation != PROOF (Score:2)
by greytree ( 7124971 )
1. This is one observation that shows two merged black holes have an event horizon with an area not smaller that of the sum of the event horizons of the merging black holes.
How is that a PROOF of a theorem stating that "when two black holes merge, the resulting black hole's event horizon -- the boundary beyond which not even light can escape the clutches of a black hole -- *cannot* have an area smaller than the sum of the two original black holes" ?!
It shows that in this case it is not smaller than the sum,
I don't understand how this isn't obvious (Score:2)
I'm honestly confused. Wouldn't a higher mass black hole have a bigger event horizon?
Can someone give a slightly more indepth description of what interesting fact was shown here?
Re: I don't understand how this isn't obvious (Score:3)
You wouldn't expect the relationship to be linear for area. You would naively expect the merging to increase volume linearly, but area more slowly. When you put two spheres of clay together, their surface areas don't sum linearly, but their volumes do.
Re: I don't understand how this isn't obvious (Score:3)
It is not a volume, it is the relationship between escape velocity, distance and mass of the central object. In classical Newtonian physics it is v = sqrt(2GM/R) from setting the needed kinetic energy equal to the needed potential energy. Now setting v to the speed of light, you get R=2GM/c^2, which is the radius of a black hole.
Re: (Score:3)
A black hole compresses matter into a smaller space. Would two black holes compress even further, i.e. does more mass inside a black hole cause more compression? Hawking felt that a black hole was 'maximum compression' and thus this theory was stated, and now is has finally proven.
Just because something is obvious, doesn't mean its reality until proven. If you don't know better, its obvious the sun moves around the earth. A different perspective and more data was needed to prove otherwise.
Re: (Score:2)
Correct. A higher mass means a bigger event horizon.
The formula for this is the Schwarzschild radius, r = 2GM/C^2 where G is the gravitational constant, M is mass and C is the speed of light. So yes, the higher the mass, the bigger the event horizon.
Which maps neatly onto this observation that when the blackholes merged the resultant blackhole was not smaller than the radius predicted by adding the two masses together.
Though its more a vindication of Schwarzschild since the conclusion flows naturally from
Re: (Score:2)
> I'm honestly confused. Wouldn't a higher mass black hole have a bigger event horizon?
Yes, intuitively you are right. The question is "how much bigger".
> Can someone give a slightly more indepth description of what interesting fact was shown here?
Let me try.
Hawking said "bigger enough, that the area of the event horizon of the new black hole will have an area at least as large as the area of the two previous black holes combined". That's interesting because (at this point my understanding is based on normal Euclidean math - I don't think it can be wrong, but maybe someone will correct me - is there a special case?) surface area increases as radius^2 whilst volume increases as radius^3