Life on the Holodeck
When Albert Einstein published his general theory of relativity 100 years ago, he revolutionized the way scientists perceived the physical world. Redefining the force of gravity as a curvature in the combined fabric of space and time, the theory presented myriad previously unimagined possibilities.
The concept also led to new ways of contemplating the fundamental properties of the universe and what it is all made of. Seeking answers, theoretical physicists have turned to one of the more bizarre phenomena in the cosmos — black holes.
Breaking free
Formed when a massive star, in its last death throes, collapses in on itself, a black hole creates a gravity well — a warp in space-time — so strong that even light cannot escape. (Hence, the name.)
A defining feature of a black hole is its event horizon, the point at which gravity becomes inescapable. Once across a black hole’s event horizon, nothing — no atom, no planet, not even a starship traveling at light speed — can return from the super-dense interior.
Or can it?
“According to general relativity, stuff falling into a black hole never comes out,” explained Nicholas Warner, professor of physics and astronomy and mathematics at USC Dornsife, “but Hawking showed it can.”
Warner, who studies black holes using string theory, was referring to noted theoretical physicist and cosmologist Stephen Hawking. In the 1970s, Hawking showed that black holes can, in fact, emit radiation, at least on paper. Now termed “Hawking radiation,” the process is akin to evaporation. So, what goes into making a black hole will come back out, albeit in a much different form and after many eons.
A black hole with the mass of Earth’s sun “will take 1067 years — that’s a ‘1’ with 67 zeroes after it — to decay with Hawking radiation. It’s an incredibly, insanely long time,” Warner said.
Skimming the surface
Hawking’s work, along with that of theoretical physicist Jacob Bekenstein, led to another surprising revelation: a black hole’s entropy — a measure of all of the substance that formed it — is proportional not to the volume of the black hole, as expected, but rather to its surface area.
“For most things if you ask, ‘How much can I put in a box?’ it depends on the volume of the box,” Warner explained. “The bigger the box, the more I can put in — the more books, or rocks or information.
“So, usually information in the box is proportional to the volume of the box. Not true for a black hole! It has information in it that’s proportional to the surface area, not the volume.”
That insight — that the surface area provides a window into the inside of a black hole — gave a vital hint at a different way to view not just black holes, but the universe altogether.
“That was the hint that for theories of gravity, if you dig deep enough, the most information you need may actually be just three-dimensional,” said Clifford Johnson, professor of physics and astronomy.
In other words, the universe may be a hologram.
One less dimension
Most people recognize a hologram as a two-dimensional image that has three-dimensional information. A common example is the small security feature on many credit cards.
In the broader sense, however, a hologram is an image of an object in one less dimension that preserves its higher dimensional information.
For physicists, the insight provided by Hawking and Bekenstein into the nature of black holes led to the realization that the four-dimensional universe, comprising three dimensions of space and one of time, might also be described mathematically using just three dimensions, two of space and one of time. One spatial dimension might be unnecessary, perhaps even an illusion, Johnson explained.
Because gravity, as Einstein showed, is tied up in those four dimensions that constitute space-time, this can simplify how scientists model the universe and how gravity fits within it.
“The idea essentially is that theories involving gravity, or dynamical space-time, are such that when you try to characterize gravity … you don’t need as many dimensions as you thought,” Johnson said. “The theory of gravity in four dimensions may be more efficiently described as a theory that is not gravitational in one dimension fewer.”
That means that physicists can more simply describe everything in the four-dimensional world that includes gravity by using three dimensions without gravity, he explained.
This also works in reverse; physicists can describe what takes place on a three-dimensional surface by addressing its effect on the gravitational field in the fourth dimension, according to Warner, who provided some of the first conclusive evidence that supported this notion.
“If you want to understand something happening on a surface … you can do it by studying gravity in one extra dimension because the things moving on that surface source the gravitational field out in the extra dimension,” he said. “I did some of the earliest, most nontrivial tests of this hypothesis, and it worked!”
Living in the matrix
This idea of the world as a hologram has further led to conjecture of a more fanciful nature — that the universe, rather than being a four-dimensional reality, might instead be a vast computer simulation using a hologram.
If an intelligence were to construct a “virtual reality,” holography would simplify the process, Johnson said. “Your simulation wouldn’t need to have gravity in it; gravity would emerge because of holography.”
Ultimately, he dismisses the concept as extremely speculative. But, Johnson does acknowledge its allure. “It’s a fun idea. It’s appeared in a million and one pieces of fiction,” he said. “If we were going to make sense of it in physics, it probably would be through this holographic idea.”
Read more stories from USC Dornsife Magazine‘s Fall 2015-Winter 2016 issue