Cosmologists, astrophysicists and other ‘space scientists’ have a good idea how massive objects such as stars, dwarf stars, neutrino stars, magnetars, galaxies and quasars function within our Universe today. Theoretically the mechanics of what forms a “black-hole” are understood by many physicists. But what is a black-hole?
The conventional explanation is that a black-hole is a region of space-time from which nothing, not even light can escape. A black-hole is formed by a stellar object that has collapsed in on itself due to the star’s own gravitational forces. Surrounding a black-hole is a mathematically defined region of space called an event horizon; this is a boundary where not only nothing can escape but also the current laws of physics break down. Where boundaries exist within scientific knowledge, usually a theory is incomplete.
Missing knowledge with black-hole theory is a problem, not least because the two main descriptions of the Universe are currently incompatible: relativity (general) and quantum theory. Black-holes were first postulated by Karl Schwarzschild. Many other physicists have subsequently analysed Albert Einstein‘s theory of general relativity and produced new insights into the theoretical behaviour of gravitationally large objects.
John Archibald Wheeler who collaborated with Einstein, also worked with Niels Bohr to explain the basic principles behind nuclear fission (how stars make heat and light); he also coined the term “black-hole”. Wheeler tried to unite the fundamental forces (electromagnetic, strong, weak and gravitation), carrying on Einstein’s search for a unified field theory. To this day, there remains gaps in our knowledge between the fundamental forces. While three of the forces can be tangled together in various theories, the weakest force gravity, remains for the most part left out in the cold.
Quantum electrodynamics (QED) describes how light and matter interact unifying the electromagnetic force , quantum mechanics (very small-scale) and special relativity (the larger scale). However no current theory currently unites the forces of gravity as described within Einstein’s general theory of relativity to quantum mechanics without having to invent extra dimensions or describe the structure of matter using strings; none of these higher-dimensional theories are testable with our current level of knowledge. This once again demonstrates a boundary to these theories, boundaries in knowledge usually exemplify a lack of understanding or missing information; the jigsaw is still incomplete.
During the 1970s a young British physicist called Stephen Hawking postulated a theory about black holes. Professor Hawking demonstrated theoretically that black-holes radiate energy (Hawking radiation) and thus evaporate away as heat energy (entropy) through quantum effects.
With Professor Hawking’s theory, what happens if a complicated but highly ordered entropy object such as a space rocket were to fall onto a black-hole? In theory, all the information that made up the space rocket should still be retained by the black-hole; according to the laws of entropy. However if black-holes can evaporate, what happens to that information? I would conjecture a simple answer in that, as the object falls onto the black-hole, its entropy state moves from a highly ordered state to a highly disordered state, possibly through ‘spaghettification’ process, while still remaining complex; however this is pure conjecture on my part.
Although we have scientific and mathematical explanations of what a black-hole could be, no one has directly observed a black-hole; however with newly available telescopes, observation of a black-hole’s effect on neighbouring stellar matter may be possible. In 1972 a bright X-ray source was detected somewhere in the region of the constellation of Cygnus. A young blue star in this region was producing X-rays, however according to our current physics theories, blue stars don’t emit X-rays. Discovered close to this blue star was a ‘dark’ companion object, measured to have at least 6 times the mass of our Sun, with a physical size smaller than the Earth. This object has been named Cygnus X-1 and was the first astronomically detected candidate black-hole object.
Black-holes represent not just a boundary in our knowledge of physics, but also a real descriptive boundary with an event horizon. Although we can’t physically look at a black-hole, we can see its effect on other close-by objects, such as stars. Black-holes are often described using Karl Schwarzschild’s theory of non-rotating black-holes with a ‘dot’ singularity at their centre. While this is a nice curiosity, in reality, stars tend to rotate.
This is where Kerr black-holes come-in. A rotating black-hole as described by Roy Kerr has two main properties: its gravitational strength and the amount of rotation. With the Kerr solution of a rotating black-hole, there is a region of space-time called the ergosphere that surrounds the black-hole’s event horizon. The ergosphere in effect acts similar to a secondary event horizon. The rotational speed of a Kerr black-hole may also allow odd effects such as time-travel or passing safely through the inside of a black-hole. However this is dependent upon: how your space rocket approaches the ergosphere, the rotational spin speed of the black-hole, the black-hole’s size and its electrical properties if it has any.
We can indirectly ‘see’ a black-hole by looking at the electromagnetic radiation being emitted when fuelled by a companion star. Although Hawking radiation is being looked for in black-hole candidate objects, this process has not yet been observed. Quantum effects may still be occurring, they may not be detectable with current technology as other electromagnetic radiation could be swamping out the Hawking radiation process.
Another very troubling idea follows from the very nature of a black-hole: black-holes ‘forget’ what they consume, from the point of view of an observer who resides outside a black-hole; they can’t see inside it. The aforementioned complex but highly ordered entropy (the information of what makes an object an object) of say a space rocket is lost from the Universe as it falls into a black-hole’s event horizon; like socks in a wash never to be found again. One might speculate that the total information of an object that falls into a black-hole is moved from a highly ordered state to a highly un-ordered state to an external observer. If however information on the space rocket does goes missing, where does it go?
Physicists have worked out the current set of limits for stars. Black-hole candidate objects such as Cygnus X-1 are small in measurable size, have very strong gravitational fields (they appear to be massive objects) and emit no visible light (indication of nuclear fission within a star) but emit X-rays or radio-waves. How other types of stellar objects function, such as dwarf stars, neutron stars and their more bizarre cousins magnetars is known. Cygnus X-1’s behaviour does not match any of the directly observable stellar objects.
Before I go further dear reader, I would like to explain a little background information on the scientific approach. The scientific method looks for answers by testing hypothesis against observation and accordingly updates knowledge with newly learnt information. Following on from new innovations and inventions, scientific theories are updated if new information is learnt from new observations; this process is iterative. Science does not set knowledge in unchangeable boundaries, which consequently results in existing scientific theories being updated.
Although current theoretical descriptions for black-holes appear to match indirect observation, there are other theories which also should be considered. George Chaplin discusses in Scientific American’s blogs that maybe black-holes don’t really exist. Chaplin’s argument centres around the yet unexplained phenomena of a jet of charged particles moving at nearly the speed of light being ejected: “but there is no obvious reason why the tidal disruption of star by a black hole should give rise to such a jet“. Chaplin also states: “Pawel Mazur and I realized some time ago that quantum gravitational effects modify the collapse process”. These jets are produced in “crystal stars” or “dark energy stars”. There is merit in Chaplin’s theory and although many astrophysicists may easily dismiss Chaplin’s ideas, they should take a closer look. George Chaplin’s “crystal stars” are testable in a way that black-holes may not be.
The very description of a black-hole with a singularity, whether Schwarzschild or Kerr is troubling. It’s troubling because singularities also infer another area of physics where there is a knowledge boundary. However there have been lots of knowledge boundaries in past human history, as we innovate and invent, often knowledge boundaries can be overcome. With better telescopes and future experimentation, the stellar dust can be laid-to-rest.
Adding to the black-hole theoretical crucible is another idea which suggests “black holes turn into fuzzballs“. Professor Jon Butterworth discusses a talk given by Samir Mathur of Ohio State University at the Tata Institute in Mumbai on the Lepton Photon. Entropy is key to this debate. As previously mentioned, the troubling idea that black-holes forget the information they consume goes against thermodynamics which states that entropy cannot disappear from a system. Mathur has applied string theory to a black-hole and the entropy problem becomes resolved by removing the singularity and applying quantum gravity to the event horizon; thus producing “fuzzballs”.
From observational evidence, physicists know that an object which rotates very fast is going to exert a greater gravitational force than an object with little or no rotation. How an object moves in space-time can also effect its mass; special relativity confirms this. Professor Frank Tipler in 1974 produced a paper entitled “Rotating Cylinders and the Possibility of Global Causality Violation” where he proposes a solution for creating a time machine; Tipler’s solution focuses on the use of the frame-dragging effect. If dark-star object measurements are accurate, such as with the March 28 Swift event, where a “dark-star” rotates at 98% – 99% the speed of light, this would create massive gravitational forces and consequently frame-dragging effects that would in-turn generate weird quantum effects around the surrounding dark-star’s space-time. If the dark-star also is partly composed of neutrons or quarks in addition to dark-energy, it could create various density pressures, moving elliptically inside its internal structure producing possible time-dilation effects. Other theoretical explanations can be thought up in place for black-holes.
Another theory put forward by physics professor Martin Bojowald suggests the Universe could become forgetful, due to a combination of quantum theory and space-time physics. This theorizing could lead one to ask the question is “fundamental physics too theory-led?” Professor Jon Butterworth suggests three counter points to this question: thought experiments, electroweak symmetry breaking and gloating, i.e., experimental evidence within this context.
To expand knowledge, we need to do experimental science to confirm theories, such as the LHC, space telescope astronomy, etc. Many past scientific theories have been swept away with newer and better versions. Bad science, misrepresentation, often in the media while reporting results or exaggeration of scientific claims can damage science in the public’s perception. Niels Bohr talked about the importance of language and the words we use to describe something; choosing an incorrect or misleading word can easily misinform or misdirect a debate.
There are examples in science of exaggerated claims, such as the human genome project’s original claim that following 10 years of decoding, which is now complete in 2011, we would fully understand the human genome and be able to find cures to illness. These exaggerated claims in the media have not held up, but this does not mean that a very important milestone has not been reached. We now have a new vast tome of information added to the human library of knowledge. New information is constantly being added as more is learnt, including how much more complex understanding the human genome is. A whole new branch of scientific research has grown following on from the human genome project: bio-hacking.
There were claims over 30 years ago in the media of a personal transportation system in the form of a jet-pack being available for all by 2000; personally I am still waiting for mine. Some technologies move faster forward than others, such as computing. Without the scientific research, observation and understanding of what is about us in-conjunction with innovation and invention, we may stagnate or even go backwards. Many of the inventions and technologies we enjoy today, from books, medicine, heating and cooling systems, transportation, computing, drawing with a pencil, to name a few have a basis on scientific thought.
Theoretical science should be explored, including esoteric ideas such as warp engines, time travel and black-hole physics. After all people taking to the skies has been postulated by many. We know of manuscripts that remain today from Leonardo da Vinci, however human flight in the 14th century would have seem fantasy to many; as science fiction in today’s world with an idea on deep space travel does. The aviation pioneers of the late 19th and early 20th centuries turned an idea into a technology in the early 20th century, based upon the scientific knowledge of the age. As we moved into the jet age we take aircraft for granted, but the basis is scientific theory with its application in technology.
So what do we understand about black-holes? When a black-hole is formed, in theory nuclear forces have ceased to function in the star’s core. However there maybe other elements at play, such as compact neutrons, electrical charge, neutrinos, quantum chromodynamics. Other quantum effects may also be at play; research will continue and new theories may arise, some may even be testable in the future. Those who follow scientific principles will continue to seek new knowledge, that we can be assured of.
So do space-time black-holes really exist? Well, the existing evidence would suggest that they do. This does not rule out other exotic stellar objects such as “dark-stars” or “fuzzballs”. I think it is fair to say that astrophysicists have a good idea of the sort of stellar object a black-hole candidate is not, but also good theories of what it might be.