Frequently one sees science press headlines describing observations of black holes: the discovery of a black hole at the galactic center or the discovery of a pair of orbiting super massive black holes in merging galaxies or the aLIGO detection of gravitational waves created in the spin-down merger of a pair of binary black holes. These days, there are so many astrophysical observations on Earth and in space that are attributed to black holes that questioning their existence seems rather absurd.
However, it is important to point out that a black hole's event horizon, the region where time comes to a complete halt, has never been observed. Further, Hawking radiation, the predicted emission of thermal photons arising from quantum effects at the event horizon, has never been detected. The fact is that the term "black hole" is commonly used to indicate any collapsed stellar object that is more massive than a neutron star. Most of the "black hole" observations that we hear about come from emissions from the accretion discs of such massive compact objects, which may or may not actually be black holes.
What is the difference between black holes and other massive compact objects? Black holes are a major prediction of Einstein's general theory of relativity (GR), currently our standard model for gravity. GR predicts that when a massive fuel-exhausted star can no longer be prevented from final collapse by the repulsion of nuclear forces, its core falls inward in a supernova explosion and a black hole forms, complete with an event horizon at the Schwarzschild radius rg given by rg=2GM/c2, where G is Newton's gravitational constant, c is the speed of light and M is the mass of the star. Half a radius outside the event horizon, black holes have a region called the photosphere. In other words, the photosphere has a radius of 3/2 times rg. In the surface of the photosphere light is bent into a circular orbit by gravity and can orbit the compact object.
The presence of an event horizon is the essential defining characteristic of a black hole. It is the surface within which gravity is so large that the time-slowing effect of the gravitational red shift causes time to stop altogether. To an external observer, any infalling object would appear to stop and freeze on the event horizon and would never go any farther in. On the other hand, according to GR an unfortunate observer who was falling into the black hole would notice no particular effect when crossing the event horizon.
The interior of a black hole inside the event horizon is causally disconnected from the outside, and nothing that enters the event horizon can escape (except for advanced waves; see my book The Quantum Handshake, Sec. 6.21). Like Las Vegas, whatever happens inside the event horizon stays inside the event horizon. Furthermore, GR predicts the eventual formation of a pathological singularity within the event horizon. The singularity is completely isolated from the outside world, shielded from it by the event horizon. However, within the singularity's domain of influence the familiar laws of physics must break down, with unknown consequences.
There are a number of reasons why many physicists distrust the above GR description of black hole formation and behavior. The singularity, whether hidden or not, is considered by some to be an unphysical deal-breaker as a prediction of a physics theory. Further, the GR description ignores the effects of quantum mechanics. There is an intrinsic incompatibility between our present standard theories of general relativity and quantum mechanics, and we do not have a theory that correctly includes the effects of both. It is speculated the if we did have a more comprehensive theory of quantum gravity, it would significantly alter the scenario of black hole formation, particularly in the regions of the event horizon and the singularity.
One critic of GR is George Chapline of the Lawrence Livermore National Laboratory (see AV 129 in the September-2005 issue of Analog). Chapline has argued that, if properly applied, quantum mechanics will not permit gravitational time dilation to stop time altogether. He cites a similar situation that occurs within a cylinder of super-fluid helium, in which at a certain vertical location the speed of sound attempts to go to zero. He points out that a phase transition in super-fluid helium prevents this from happening, and he argues that a similar phase transition must occur in a collapsing star, which will prevent the formation of an event horizon. Protons and neutrons must transition to leptons and mesons, which must in turn transition to dark energy, and the repulsion generated by the dark energy halts the collapse before an event horizon can form. Chapline's work is but one example of the many GR critics who have proposed alternatives or modifications of GR that would qualitatively change the stellar collapse scenario and would prevent the formation of black holes with event horizons. This raises the question of whether these ideas and theories, questioning the very existence of black holes, can be tested by observation.
Recently, V. Cardoso and P. Pani of the University of Lisbon have proposed such an observational test. They first survey the various non-GR models of star collapse and divide the resulting collapsed stars into: (1) exotic compact objects (ECOs) with radii smaller than a neutron star but larger than the photosphere, (2) ultra-compact objects (UCOs) with radii near the photosphere radius, and (3) clean photosphere objects (ClePhOs) with radii only about 1.65% larger that the event horizon radius rg. All of these objects would have an accretion disc and, based strictly on observation of their emissions of light and radio waves, would be indistinguishable from a black hole. However, they are predicted to show distinguishable differences when gravitational waves are generated by a merger.
The aLIGO Detector, with sites in Hanford, Washington and Livingston, Louisiana, has now detected gravitational waves three times: on September 14, 2015, on December 26, 2015, and on January 4, 2017, with a fourth lower confidence event also detected on October 12, 2015. The analysis of these events indicates that all were produced by mergers of pairs of massive compact objects with masses ranging between 6 and 37 times the mass of our Sun.
The gravitational wave signature of such mergers is a "ringdown", a set of wiggles in the signal that rise in frequency during the transition from two massive compact objects to a single one. The ringdown is predicted to be slightly different for ECOs and UCOs than for black holes, and more precise observations of gravitational waves from mergers could reveal these differences. For ClePhOs Cardoso and Pani predict that the ringdown itself is the same as for black holes, but that a fraction of the gravity waves generated should be briefly trapped between the object's surface and its photosphere. In this case, the characteristic ringdown signal would be followed by a sequence of "echo" wiggles that would persist for a few hundred milliseconds after the primary signal. The magnitude of these echoes and how fast they die out would depend on the details of the ClePhO formation, permitting some discrimination between different models.
One group of physicists from Waterloo, Canada and Tehran, Iran has analyzed the three aLIGO events that were detected in 2015 in search of echo signals. Interestingly, they obtained a positive indication of the presence of echoes. However, because of the noise present in the data their result had a statistical significance of less than three standard deviations. A result must have six or more standard deviations to be considered a definite observation. Thus, it is inconclusive but tantalizing. Improved analysis of this type will probably have to wait for more events and for more new gravitational wave detectors to come online.
If such echo signals were actually observed with convincing statistical significance, the observation would have major consequences for gravitational physics. It would be interpreted as falsifying GR and supporting rival non-GR theories that predict compact objects with no event horizon. However, while the detection of gravitational waves by aLIGO has been a major triumph, the aLIGO system has been fighting noise since its original construction, and the present noise level only makes possible the unambiguous detection of the ringdown signal. The two aLIGO stations currently in operation are simply too noisy to observe the predicted ringdown differences or echoes.
However, this may change soon. The Virgo gravitational wave detector in Pisa, Italy is scheduled to join the aLIGO configuration next year, and there are also gravitational wave detector stations under development or construction in India, Germany, and Japan. Reduced noise from design improvements and signal averaging among more detector stations observing the same gravitational wave event may reduce the noise level significantly. The addition of more interconnected detectors positioned around the planet will also greatly improve that sensitivity of the system to the polarization of the gravitational waves. This is a crucial observable in distinguishing between GR and many of its alternatives.
On a longer timescale, the 2017 LISA space-based gravitational wave detector, re-planned several times, is now scheduled to be launched in the 2030s. This project of the European Space Agency is a gravitational wave detector in the form of an equilateral triangle 2.5 million kilometers on a side with a laser interferometer unit at each vertex. The 2017 LISA gravitational wave detector is presently planned to orbit the Sun at the Lagrange L3 point on the side of the Sun away from Earth, but in the same orbit as the Earth.
Earth-based gravitational wave detectors like aLIGO have sensitivity in the frequency range from 1 Hz to 30 kHz, which is the region where signals from massive black hole mergers are strongest. The 2017 LISA gravitational wave detector will have sensitivity in the 10-5 Hz to 1 Hz region, where many binary star systems have their peak emission of gravitational waves. For technical reasons 2017 LISA cannot use the high-finesse Fabry-Pérot resonant-arm cavities and signal recycling systems used by aLIGO. For this reason, its absolute sensitivity to changes in arm length will be an order of magnitude poorer than aLIGO. However, since its arm lengths are a million times greater, this should not be a problem, and excellent low noise detections are expected from the 2017 LISA system.
In any case, in 2015 the aLIGO detection of gravitational waves opened a new window on the universe. The developing detector technology promises to make it possible to answer the question of whether the black holes, as predicted by Einstein's general theory of relativity, actually exist, or whether quantum effects and other considerations rule out black holes and indicate that collapsed stars have a different form. As more and better detectors come online, we can expect an answer to this important question.
Black Hole Existence Tests:
"Tests for the existence of horizons through gravitational wave echoes", Vitor Cardoso and Paolo Pani, Nature Astronomy 1, 586-591 (2017); https://arxiv.org/abs/1709.01525.
"Echoes from the Abyss: Evidence for Planck-scale structure at black hole horizons", J. Abedi, H. Dykaar, and N Afshordi, (December-2016); arXiv:1612.00266 [gr-qc]; and
"Echoes from the Abyss: The Holiday Edition!", J. Abedi, H. Dykaar, and N Afshordi, (January-2017); arXiv:1701.03485 [gr-qc].
John Cramer's new book: a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406 .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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