On January 14, 2025, the LIGO-Virgo-KAGRA collaboration observed an exceptionally loud gravitational-wave signal, named GW250114. The signal was recorded by the LIGO detectors in Hanford, Washington, and Livingston, Louisiana, during the second segment of the fourth LIGO-Virgo-KAGRA observing run (O4b). The black holes were strikingly similar (in their masses and distance) to those observed in LIGO’s first detection, in 2015: GW150914. But, after 10 years of improvements to both the global network of gravitational-wave detectors and the methods used to analyze their data, LVK researchers were able to “hear” GW250114 three times as clearly as that pioneering first detection. With this unprecedented observation, LVK researchers have confirmed that black holes only ever grow in size and that, when disturbed, they ring like a bell with the “sound” predicted by Einstein.
GW250114 – The Clearest of Chirps
Hawking's Area Theorem: Was he right ?
By analyzing the frequencies of gravitational waves emitted by the merger, the LVK team was able to provide the best observational evidence captured to date for what is known as the black hole area theorem, an idea put forth by Stephen Hawking in 1971 that says the total surface areas of black holes cannot decrease. When black holes merge, their masses combine, increasing the surface area. But they also lose energy in the form of gravitational waves. Additionally, the merger can cause the combined black hole to increase its spin, which leads to it having a smaller area. The black hole area theorem states that despite these competing factors, the total surface area must grow in size.
Later, Hawking and physicist Jacob Bekenstein concluded that a black hole’s area is proportional to its entropy, or degree of disorder. The findings paved the way for later groundbreaking work in the field of quantum gravity, which attempts to unite two pillars of modern physics: general relativity and quantum physics.
In essence, the LIGO detection allowed the team to “hear” two black holes growing as they merged into one, verifying Hawking’s theorem. (Virgo and KAGRA were offline during this particular observation.) The initial black holes had a total surface area of 240,000 square kilometers (roughly the size of Oregon), while the final area was about 400,000 square kilometers (roughly the size of California)—a clear increase. This is the second test of the black hole area theorem; an initial test was performed in 2021 using data from the first GW150914 signal, but because that data was not as clean, the results had a confidence level of 95 percent as compared to 99.999 percent for the new data.
Data from LIGO Hanford (left) and LIGO Livingston (right) and GW250114 signal reconstruction. Times are relative to January 14, 2025, 08∶22∶03UTC. The top panels show whitened data versus time and signal reconstructions (90% credible regions), either with a waveform model for black hole binaries in general relativity [19] or via a model-agnostic wavelet-based approach [20, 21]. Data and models have been downsampled to 2048 Hz, whitened (effectively, divided) by the detector noise amplitude spectral density, and finally bandpassed to [20, 896] Hz. The bottom panels show a time-frequency spectrogram of the data. The signal reaches > 10 σ above the noise.
Thorne recalls Hawking phoning him to ask whether LIGO might be able to test his theorem immediately after he learned of the 2015 gravitational-wave detection. Hawking died in 2018 and sadly did not live to see his theory observationally verified. “If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase,” Thorne says.
The trickiest part of this type of analysis had to do with determining the final surface area of the merged black hole. The surface areas of pre-merger black holes can be more readily gleaned as the pair spiral together, roiling space-time and producing gravitational waves. But after the black holes coalesce, the signal is not as clearcut. During this so-called ringdown phase, the final black hole vibrates like a struck bell.
In the new study, the researchers were able to precisely measure the details of the ringdown phase, which allowed them to calculate the mass and spin of the black hole, and subsequently determine its surface area. More precisely, they were able, for the first time, to confidently pick out two distinct gravitational-wave modes in the ringdown phase.
Another study from the LVK, submitted to Physical Review Letters today, places limits on a predicted third, higher-pitch tone in the GW250114 signal, and performs some of the most stringent tests yet of general relativity’s accuracy in describing merging black holes.
Ringing Black Hole Animation (GW250114)
Listening to Black holes ring:
Black hole mergers goes through three phases, the Inspiral, the Merger and the Ringdown phase.
Using ringdown-only analyses, which focus solely on the part of the signal after the merger, we look for these tones without using any information from the earlier inspiral. We find that the data favor a description with the main fundamental tone and its first overtone, which is a faster-fading version of the same vibration. The first overtone can be tracked for about three milliseconds after the merger, after which it can no longer be resolved above detector noise. Its measured amplitude and phase agree with detailed numerical-relativity simulations of black-hole mergers in general relativity. The tones’ frequencies and decay times are found to be consistent with theoretical predictions within measurement uncertainties.
We also analyze the full signal – including the inspiral, merger, and ringdown – with models calibrated to numerical simulations, but allowing the tones’ frequencies and decay times to vary freely, rather than fixing them to the Kerr prediction. By incorporating information from the entire signal, we can measure the ringdown’s properties even more precisely. With this method, the fundamental tone’s frequency is measured to about two-percent precision, and its decay time to about nine percent, both in excellent agreement with general relativity. This analysis also provides the first constraints on a higher-frequency secondary tone, which rings at roughly twice the frequency of the fundamental, once again matching theoretical predictions. Together, the ringdown-only and full-signal analyses offer complementary tests, confirming that the remnant’s properties are consistent with those expected for a Kerr black hole when measured in different ways.
Consistency Across the Inspiral-Merger-Ringdown Signal:
We also test general relativity’s predictions for other parts of the signal, beyond the ringdown. In the early inspiral phase, where the black holes orbit each other more slowly than in the final merger, we find no evidence for departures from Einstein's predictions. We also compare the mass and spin of the final black hole estimated from the early part of the signal with the values inferred from the later merger and ringdown, again finding agreement.
Remarkably, the strength of GW250114 allows us to place constraints on possible deviations from general relativity that are not only comparable to, but in some cases two to three times more stringent than those obtained by combining information from dozens of signals in the latest fourth Gravitational-Wave Transient Catalog (GWTC-4.0), which includes signals up to the first segment of the fourth LIGO-Virgo-KAGRA observing run (O4a).
Finally, we check whether subtracting the best possible general-relativity waveform from the data leaves behind any unexplained pattern. It does not, and the leftover signal behaves like ordinary detector noise.
The gravitational waves are separated into the two modes of the ringing remnant black hole that were identified in the GW250114 observation: the quadrupolar fundamental mode (labeled "First tone") and its first overtone ("Second tone ). It also shows a predicted third tone that the data place limits on. The detected tones have a quadrupole pattern, while the third tone has a hexadecapolar pattern.
Einstein holds firm
In every test, GW250114 matches the expectations of general relativity, within the precision of current observations. The strength and clarity of this signal make it the most precise confirmation yet of Einstein’s theory for black hole mergers, significantly tightening the limits on how much any alternative theory of gravity could differ from it. Together with the detection paper, which tests Hawking’s area law and provides complementary results on the Kerr nature of the remnant, this study illustrates the breadth of science unlocked by GW250114.
To read more:
GW250114: COSMIC CARILLON OF CHAOS (Science Summary)
Black Hole Spectroscopy and Tests of General Relativity with GW250114