Theoretical physicists have recently been frustrated by a bold hypothesis concerning black holes—specifically, that they don’t exist.
In March, at the 22nd Pacific Coast Gravity Meeting in Santa Barbara, Calif., George Chapline, an applied physicist at Lawrence Livermore National Laboratory, gave a talk based on ideas he’s been incubating for several years. His goal: to amend astrophysics by applying theories of dark energy and condensed matter physics.
His work reinvents black holes as so-called “dark energy stars,” which are what is left over when matter transitions to dark energy as it passes a point of no return similar to a black hole’s event horizon. That redefinition, if correct, would invalidate much of the intellectual framework of traditional black holes.
Chapline’s ideas take inspiration from his colleague Robert Laughlin, a condensed matter physicist at Stanford University who won a Nobel for his work on quantum fluids.
Laughlin is quick to point out that the hubbub he and Chapline’s ideas have caused “is a battle of words rather than a battle of science.
“In science, you decide whose theory is right (or wrong) by means of an experiment,” he said, “not by polling experts.”
“Who wouldn’t want to be the researcher who dismantles Einstein and Hawking?”
Unfortunately for theoretical physicists, experimenting on the nature of the universe is not an easy undertaking. Revisionism of one sort or another is constantly occurring, due to the field’s heavier-than-normal reliance on theories based on observation, extrapolation and imagination.
“In some ways our playground is too big,” said Leonard Susskind, a theoretical particle physicist at Stanford and an outspoken critic of the Chapline-Laughlin theory.
“Practically speaking, much of our subject matter is inaccessible to direct experimentation,” he continued. “It doesn’t make the science any less valid—we didn’t need to go to the Moon to know that it wasn’t made of cheese.”
But indirection, inference and, ultimately, guesswork all chafe against some of science’s core values. Understandably, some researchers inevitably suggest less-fuzzy alternatives, which is how Chapline and Laughlin see their work.
“George and I made a very plausible case that general relativity, as we have observed it experimentally, could be perfectly true, and yet fail to describe a black hole event horizon properly,” said Laughlin. “What would allow this to happen is failure of the relativity principle on very short-length scales.”
His and Chapline’s model, he argues, fixes violations of quantum mechanics—such as information loss and the freezing of time at a black hole’s event horizon— in traditional black hole models. Laughlin notes that the argument may offend his peers, but that they have no valid criticism of his and his partner’s arguments. He insists their redefinition is correct.
“The point is that there is no way to tell one way or the other right now,” he said. “If there were, there would be no controversy.”
The Chapline-Laughlin hypothesis will linger like most cosmological theories, which are only partially or indirectly testable as well as often incomplete and replete with corrections needed to describe the universe we actually observe. The process of pinning on these amendments can get messy.
“This is starting to bug a lot of people,” said Geoff Marcy, an astronomer at the University of California at Berkeley. “You can end up with a patchwork that’s so ad hoc, with so many after-the-fact add-ons and addenda and caveats, that you might as well throw the whole thing out.”
Chapline and Laughlin face an uphill battle among the many theoretical physicists who have already devised their own fixes for the quantum violations of black holes either via string theory or a concept called “black hole evaporation,” wherein two particles fluctuate at the event horizon of a black hole so that one is sucked in while the other is shot out, making it seem as though the black hole is emitting the particle, or “evaporating.”
Samir Mathur, a physicist at the Ohio State University who has his own theories of black holes, which he calls “fuzzballs,” has no use for the Chapline-Laughlin theory.
“I feel comfortable dismissing it,” he said. “Their model does not account for the entropy of black holes, or for Hawking radiation. These are basic signatures of what black holes are. It appears that what is most appealing to them about their theory is that they are the ones who thought of it.”
For his part, Chapline suggests his critics are predictably lashing out at him using what he calls “the first law of physics,” where an idea is immediately derided if it questions well-ingrained notions.
“Experts don’t like it when you tell them they are not experts anymore, that books they have written are obsolete,” he said. “They don’t like to have to learn new things.”
Luboš Motl, a theoretical physicist at Harvard University, doesn’t buy the idea that black holes don’t exist. In fact, at Harvard, a NASA/Smithsonian partnership using the Chandra X-Ray Observatory has produced swarms of black hole data.
“Who wouldn’t want to be the researcher who dismantles Einstein and Hawking?” Motl said. “That is seductive. But this is a matter of ego, not science.”
Looking for Hawking Radiation in space is likely impossible with our current technology. But scientists here on Earth recently used flowing water to simulate a black hole and create event horizons, testing Stephen Hawking’s famous prediction that the event horizon creates particles and anti-particles.
Black holes resemble cosmic drains where space disappears like water draining out of a sink. Space seems to flow, and the closer one gets to the black hole, the faster it flows. At the event horizon, space appears to reach the speed of light, so nothing, not even light, can escape beyond this point of no return.
Researchers from the University of St. Andrews and the University of Nice used a water channel to create analogues of black holes, simulating event horizons.
The scientists sent waves against the current, varied the water speed and the wavelength, and filmed the waves with video
They used a 30-meter-long water channel with a powerful pump on one end and a wave machine on the other, which is normally used to test the environmental impact of currents and waves on coasts or the hulls of submarines.
While the water didn’t create anti-particles, the researchers may have seen “anti-waves.” Normal waves heave up and down in the direction they move, whereas anti-waves do the opposite.
One of the researchers, Professor Ulf Leonhardt said, “It is probably impossible to observe the Hawking radiation of black holes in space, but something like the radiation of black holes can be seen on Earth, even in something as simple as flowing water.”
“We definitely have observed these negative-frequency waves. These waves were tiny, but they were still significantly stronger than expected. However, our experiment does not completely agree with theory and so much work remains to be done to understand exactly what happens at the event horizon for water waves.”
Their research will be published in the New Journal of Physics.
Technically we're all travelling in time just by existing. But we can't seem to control our motion through time in the same way we can control our motion through space. What most people usually mean by time travel is the ability to drive around in time they same way they'd drive around in a city. 
Let's look at a spacetime diagram that shows a few examples of different ways observers or objects can travel in time and space. 
