Black holes: what we think we know, we don’t
Our understanding of black holes has changed over time
What is a black hole?
The notion of black hole reflected in popular science hinges on the idea of event horizon — this is when the velocity needed to escape the gravitational pull of the black hole exceeds the speed of light. Whatever falls into the event horizon is lost forever.
The Schwarzschild radius is the radius of the event horizon,and is proportional to the mass of the black hole. But Schwarzschild’s definition has a pitfall: it requires us to know that nothing will emerge from the black hole. This means that the black hole must be monitored forever to know that nothing exits. In practice, this is impossible.
Another mathematical solution to Einstein’s equationsdescribes the formation of a black hole through the collapse of a spherical shell of light. An event horizon forms at its centre, expands outwards,meets the infalling shell of light at the Schwarzschild radius where it stops—et voilà!— a black hole is formed.
New black holes
Perfectly isolated or unchanging black holes do not exist. Real-world black holes are surrounded by disks orbiting them, stellar winds and dark matter, all of which produce infalling matter that increases their masses.
Black holes often exist in pairs, spiraling closer and closer to each other and emitting gravitational waves until they merge into a larger black hole, the horizon changing in time, dramatically so at the merger.
The2016 LIGO/Virgo gravitational wavesoriginated in the spectacular merger of two black holes. By the time these waves reached Earth, they were weaker than the ambient noise and could only be identified by matching theoretical templates of the expected signal to the data.
Large banks of templates are generated in computer simulations that obviously cannot run forever, as would be necessary if the black hole was characterized by the eternal event horizon. Instead, simulations use theapparent horizon, characterized by the property that nothing can escape from itnow.
Apparent horizons have played a crucial role in the newly borngravitational wave astronomy, but are surprisingly little known.
Black holes change because they live in anexpanding universe. Theoretical physicist Stephen Hawking predicted that all black holesradiate energy due to quantum mechanics, which makes them shrink. Although negligible for practical purposes, this radiation is unavoidable in principle.
New understandings
Our understanding of black holes is based on the mathematical definition of horizon. The apparent horizonis defined by the behavior of light rays in its vicinity: rays cannot escape (and since nothing moves faster than light, nothing escapes)at the present moment.
But how light rays behave depends on the observer describing them using mathematical simulations. Since, in relativity, time and space depend on the observer, the location where rays stop and the present time are different for different observers. So, the apparent horizon itself depends on the observer.
The very existence of the black hole has come to depend on the observer, while the old event horizon was universal.
The mathematics expressing the new black hole concept tells us that, even in Schwarzschild’s case,certain observers existaccording to whom there is no apparent horizon and, therefore, no black hole! Admittedly, these mathematical observers are very artificial. All natural observations (those that occur through observing the actual behavior of a black hole) that perceive the Schwarzschild geometry as spherical, agree onthe existence and location of the apparent horizon.
Scientists have finally detected gravitational waves from black holes but had to change the way they understand them. The essence of black hole theory is now different.
This article byValerio Faraoni, Professor, Physics & Astronomy,Bishop’s University, is republished fromThe Conversationunder a Creative Commons license. Read theoriginal article.
Story byThe Conversation
An independent news and commentary website produced by academics and journalists.An independent news and commentary website produced by academics and journalists.
Get the TNW newsletter
Get the most important tech news in your inbox each week.
