What Happens if You Pour a Cup of Tea Into a Black Hole?

Israeli physicist Jacob Bekenstein changed the way science thinks about space: even Stephen Hawking came around to his theories.

 

At the dawn of the space age, the sky over Mexico City seemed limitless to Jacob Bekenstein and his classmates at the city's Jewish elementary school. Inspired by the rockets the United States and Soviet Union were hurling into space, in 1957 the boys began meeting after school to launch their own homemade missiles.

Bekenstein sees this youthful frisking as the beginning of his ground-breaking career in theoretical physics.

It all began during a casual conversation with his doctoral advisor at Princeton, John Archibald Wheeler.

"Wheeler asked me, 'What happens when you pour a cup of tea into a black hole?'” Bekenstein recounted.

Less prosaically, his adviser had presented the young Bekenstein with a contradiction that stood at the heart of the way black holes were understood. Mission: Resolve the paradox, which centered on the concept of entropy.
Entropy is a measure of disorder. The higher the entropy, the greater the disorder.
The second law of thermodynamics holds that the entropy of a closed system cannot decrease, only stay the same or increase.

A teabag placed in hot water illustrates this principle nicely.

The tea concentrated in the bag will diffuse in the cup, increasing the entropy of the system. But the reverse process is extremely unlikely – the tea particles won't return to cluster in the teabag.

Black holes – massive stars that collapse into infinitely dense points in space – seemed at the time to pose a serious conundrum for the second law of thermodynamics.

“If the black hole swallows the tea, the tea's entropy disappears,” Bekenstein explains. Ergo, the entropy of the entire universe becomes smaller. "So either the second law of thermodynamics is irrelevant or it isn't working. This is a serious consequence for such an established law of physics. Because this law served us well for 150 years, I was concerned by this contradiction, and I tried to find a way to save it and fix things up.”

Bekenstein's solution was to attribute the property of entropy to black holes, holding these mysterious celestial bodies to the old-fashioned rules of thermodynamics.
"My idea was that when you throw into a black hole some entropy, from a cup of tea for example, the black hole's surface area grows a bit, so the entropy created in the black hole offsets the entropy of the tea and anything else that was thrown into it," he said.

Beaker go boom!

Bekenstein, 65, received the prestigious Wolff Prize last month along with five other scientists from different fields. The award is the latest in a long list of honors for the Israeli physicist, including the 2005 award of the prestigious Israel Prize, which is given to Israelis who made exceptional contributions to their fields or to the country.
His academic career actually started in chemistry, at the Polytechnic Institute of Brooklyn. Kids adore bangs, booms and color changes in chemical reactions, he says. "But when I got to the university I realized that the more essential questions pertain to physics."

After his doctorate at Princeton University and post-doctoral work at the University of Texas at Austin, he moved to Israel on his own – “Zionism,” he explains in a word. His next years were spent at the Ben-Gurion University in the Negev Desert. Today, he researches and teaches at the Hebrew University in Jerusalem.

His thesis that began with the question of the tea, a groundbreaking theoretical work that eventually thrust him onto the international stage. But when it was just published, in 1972, many thought it "a silly idea" Bekenstein says.

Silly it wasn't.

"Bekenstein brought about a great revolution,” said Eliezer Rabinovici, a theoretical physicist at Hebrew University. “We're used to thinking about thermodynamic systems in which some important quantities, such as the energy of a system and its entropy grow in proportion to the volume. This was the paradigm for years."

Technically, Bekenstein postulated that when a system reaches the tremendous level of energy in which a black hole can form, physical quantities stop growing in proportion to the volume, and instead are proportional only to the surface area.

"This way, he showed that the growth in a black hole's surface area offsets any loss in entropy," says Rabinovici. “This challenges our usual concept of space in ways which we are only now beginning to grasp”.

The scientific community did not rush to embrace Bekenstein's radical proposal. After all, he was an obscure 25-year-old scientist who had just completed his doctorate.
Among the idea's early detractors was British physics celebrity Stephen Hawking.
“When I came out with my idea in 1972, Hawking and his two partners [Brandon Carter and James Bardeen] argued that my equations weren't part of thermodynamics but just describing mechanical properties of black holes,” Bekenstein says.

But just two years later, Hawking affirmed his theory, confirming the opinion that this is how things work, Bekenstein says.

In fact, Bekenstein's idea was the basis for Hawking's theoretical predication of the existence of Hawking radiation, emitted by black holes.

Bekenstein has continued to make major scientific contributions, for which he has received heaps of accolades.

But in some ways, he remains the product of his upbringing in Mexico City, where he first launched rockets with friends from the local Jewish school. The yarmulke he wears on his head reflects his unshaken faith.

“I look at the world as a product of God,” he said. “He set very specific laws and we delight in discovering them through scientific work, finding out how everything fits together.”

But he emphasizes that his faith does not influence his actual scientific work.
“Religious scientists calculate like everyone else and lecture like everyone else." And while his personal life may not affect his work, the reverse is certainly not the case, he says.

“I feel much more comfortable in the world because I understand how simple things work. I get a sense of security that not everything is random and that I can actually understand and not be surprised by things," he said. "I enjoy seeing a phenomenon, like the flickering of a light bulb, and understanding what's actually going on. Another person might not have a clue, and might not be interested. But when I see this phenomenon, I see the expressions of the laws and order of physics.”
 

Jacob Bekenstein, at his home in Jerusalem, in a photograph from 2002.
Emil Salman