A Light Take On The Gravity-Time Relationship
Let's talk about tiptoeing right next to a black hole. Not that you could do that, because the nearest one we know of, near the center of our galaxy, is impossible to get to. Even if you could travel at the speed of light, you wouldn't be able to reach it in your lifetime.
But let's pretend.
If you traveled the distance and then hopped into a little one-person cruiser and zoomed very close to the lip of a black hole, says professor Brian Greene, a theoretical physicist at Columbia University, the hole would not suck you into its vortex. Black holes, contrary to popular belief, are not giant sucking machines.
They only pull in whatever happens to pass directly in front of the hole. If you slip over the border, then you're in trouble. But if you hang just off to the side, if you cruise around the edge, being careful not to get into the danger zone, you will not get sucked in.
That doesn't mean you won't be affected. Albert Einstein's theories say the closer you get to a black hole, the more it will affect you. And thereby hangs a tale.
Greene has written a short (less than 40 cardboard pages) new picture book called Icarus at the Edge of Time. It tells the story of a young boy who slips off in a space ship and cruises over to a black hole, only to discover that he's made a terrible mistake: He forgot one of Einstein's fundamental observations, which is that time is not the same for everybody everywhere. But before I tell you what happened to the boy, let me pause and tell a version of this same story in New York City, which is nowhere near a black hole. But the same logic applies.
An 'Empirical' Physics Test
If you and I go to the Empire State Building and I go up to the 102nd floor while you stay down on the street, Einstein teaches that time for me will be different than time for you.
You, down there on 34th Street with the cabs and the pedestrians and the hot dogs, are a wee bit closer to the center of the Earth, while I, upstairs with the wind, the clouds and the occasional pigeon, obviously am a little farther from it.
The center of the Earth, I should remind you, is a center of gravity. That center pulls at you, which is why rain and bricks and pennies and even softballs eventually land on the ground. The Earth pulls them "home."
Einstein's theories posit that as one gets closer to a center of gravity, time will "slow down." So if you spend the rest of your life closer to the Earth's center of gravity on 34th Street while I spend the rest of my life at the top of the Empire State Building, time for you will tick a teeny, teeny bit more slowly than time for me.
Einstein meant this not poetically, but literally. If you and I each had a watch, ticking off hundred-billionths of seconds, the watch on your wrist down below on the street would tick fewer times than the watch I was wearing up in the sky. It wouldn't be a big difference -- a few billionths of a second over 20 years -- but it would be a real difference. If we decided after several decades to meet and compare watches, we'd see that they would literally differ, that time for the two of us had indeed ticked differently.
From Skyscraper To Black Hole
In his new book, Greene has come back to this theme, this time using the gravitational pull of black holes, not the Earth.
Black holes have enormous (and I mean enormous) gravitational pull. If you stand next to a black hole and I stand off at a distance, not only will time tick more slowly near the black hole, it will tick so much more slowly that the difference will be obvious.
From where I watch, as you approach the lip of the black hole you will appear to slow your movements, slow even the blinking of your eyes. You will look like a person in slow motion. You will feel perfectly normal. Time for you will seem as tick-tock regular as ever. But should you glance back at me -- I will look frantic and agitated.
What's happening here is we are experiencing very different time sequences. We are also modeling one of Einstein's great insights: that time is not the same for everybody everywhere.
Getting A Feel For The Problem
Experiments have proved that Einstein was right, that this is literally true. But because we don't visit black holes, none of us -- not even Einstein -- ever gets to feel what fractured time is like. And here is Greene's motivation for storytelling.
Greene says he feels strongly that "if people recognize that science is something that can be taken in emotionally, viscerally -- something that can really help frame your whole way of interacting with the world -- that emotional pull of science, I think, would change the way people interact with it."
As for Icarus, the boy in Greene’s story, he zings around the lip of the black hole for what he thinks has been an hour or two, until, triumphant, he turns his spaceship around. But he discovers that the reality he knew has been gone for millennia.
"Then, in a flash, it hit him. Icarus understood what had happened," Greene says. He gives voice to the character: " 'Gravity and time. Gravity and time. … I didn't take account of gravity and time.' "
In our broadcast, Brian Greene performs a very abridged version of this Icarus tale. Greene, who has acted in movies, plays most of the parts; Robert Krulwich briefly plays the captain of a space ship.
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STEVE INSKEEP, host:
If back in high school you got a C or a C minus, or maybe even flunked physics, do not despair. Our science correspondent, Robert Krulwich, knows a physics teacher who thinks he can help.
ROBERT KRULWICH: Physics is hard, says physics Professor Brian Greene. Because if you try to understand, say, Einstein's theories, what Einstein describes is not a world you can see or touch or taste or feel. It is so strange, you can't feel it in your bones. But, says Professor Greene, there is a way to aim science not only at your head, but also at your heart.
Dr. BRIAN GREENE (Professor of Physics, Columbia University): That's the point. And I think that if people recognize that science is something that can be taken in emotionally, viscerally - something that really can help frame your whole way of interacting with the world, with the universe - that emotional pull of science, I think, would change the way people interact with it, if they could take in science in that way.
KRULWICH: But how do you make abstract science feel emotionally true? Well, Brian has created, of all things, a picture book with thick cardboard pages, spectacular images, mostly from the Hubble Space Telescope, that spin a simple tale about one of Einstein's trickier ideas, the relationship between time and gravity because everybody knows, or thinks they know, about time.
Dr. GREENE: We're constantly guided by time. We're always thinking about time. We are living our lives within this constant tick, tick, tick dragging us into the future.
KRULWICH: But time can tick in surprising ways, says Einstein.
Dr. GREENE: I mean, most people, they hear relativity, they know it's something important. But they don't really take it in. And the goal here in this little story is to have them take it in.
KRULWICH: OK, so who's the main character?
Dr. GREENE: Well, he's a young boy on a ship. He's a very precocious kid...
KRULWICH: And he's traveling through space on a voyage that will take 150 years. His great-grandfather planned the mission, and his grandpa set off, and then his dad was born onboard...
Dr. GREENE: But in the story, he's not only recognized that he's born on the ship, he's recently come to realize that he will die on the ship. So he realizes that his whole life is really going to be encased within this ship, within this journey. And when the ship unexpectedly encounters a black hole, for him that's the moment.
KRULWICH: Because our boy, whose name is Icarus, has been given his own little spaceship. It's a kind of runabout that he has secretly reengineered so it can do what no ship has ever done.
Dr. GREENE: To go out to the black hole, to get close to it, careful not to step over the edge, the event horizon, because he knows that if you do that, he'll never get back. But he wants to go around this black hole and explore it close up.
KRULWICH: So when the adults are busy, Icarus hops into his ship. He casts off. He heads straight for the black hole...
Dr. GREENE: And the story goes...
(Soundbite of story "Icarus at the Edge of Time)
Dr. GREENE: (Reading) Moments later, the runabout's radio crackled.
Unidentified Man: (As the captain) This is an unauthorized journey. Return at once.
Dr. GREENE: (Reading) The captain urgently commanded. Icarus didn't reply.
Unidentified Man: (As the captain) Return.
Dr. GREENE: (Reading) The captain repeated his order, but Icarus stayed on course for the black hole. The next voice was his father's. Icarus, turn back. Don't go near the black hole. You won't survive. Icarus answered, I'm sorry, Dad, I must go. It's my one chance. But don't worry. I built the micro-warp drive engine. I've done the calculations. And I'm the best pilot. I will make it. Icarus shut off his radio. Oh, Icarus, his father pleaded across the icy stillness of space. Even if you can do what no pilot has ever done, in your calculations you didn't take account of time, the slowing of time near the black hole's edge. But Icarus couldn't hear his father's words and headed onward.
KRULWICH: Einstein teaches that time does not tick at the same rate for everyone, everywhere. That's one of his big lessons.
Dr. GREENE: And moreover - and this is the one that's maybe even a little less familiar - if you go into a strong gravitational field, the strong gravity has the effect of slowing your passage through time. So time goes slower if you are experiencing strong gravity.
KRULWICH: And a black hole has very strong gravity.
Dr. GREENE: If you go near the edge of a black hole, time for you will slow down dramatically, relative to someone who's far away.
KRULWICH: So as Icarus approached the black hole, his father, watching with powerful binoculars, could literally see his son slow down.
Dr. GREENE: As if he is in a movie that's being run in slow motion, as Icarus approaches the black hole, his arms start to move slowly, his eyes start to blink slowly. Everything about him slows down because time is running slower near the black hole.
KRULWICH: From his father's point of view.
Dr. GREENE: From his father's point of view.
KRULWICH: But inside the capsule for Icarus, time is very normal. His clock ticks just as usual. His body moves just as usual. He zings around the lip of the black hole, for how long?
Dr. GREENE: Oh, he could've just been there for in the order of an hour or two.
KRULWICH: Until triumphant, he turns around, heads back, flips on his communicator.
Dr. GREENE: Dad, what do you think now, he asked, trying his best not to sound too self-congratulatory. I'm the first person to journey to the edge of a black hole. There is no response.
KRULWICH: Because what for Icarus had been an hour or two, away from the black hole time was ticking at a much faster rate. So from his dad's point of view, how long did Icarus' trip last?
Dr. GREENE: Depending on how close he got to the edge, it could be many years. In this story I took it to be 10,000 years.
KRULWICH: Ten thousand years. Two hours for Icarus is 10,000 years for his dad. So, dad, I guess, is long gone. And Icarus?
Dr. GREENE: Exactly. Everything that he knew about - his father, his family, his friends, the reality that he knew about - has been gone for millennia.
KRULWICH: Everybody he knew is dead.
Dr. GREENE: Yes. They are profoundly gone. They are gone for thousands of years.
KRULWICH: And staring at the empty space where his father used to be, Icarus remembers his Einstein.
Dr. GREENE: Gravity and time, gravity and time, he said over and over, almost buckling under the weight of what had suddenly become clear. I didn't take account of gravity and time.
KRULWICH: So now Icarus knows. But more importantly, says Brian Greene, so do the readers of this tale.
Dr. GREENE: The idea is that it's a journey where, by the end, you've taken in one of Einstein's most profound insights, not by studying it or really cogitating over it, but by allowing a story to wash over you.
KRULWICH: And especially when the subject is very abstract. And you wonder, why should I care about this? Stories can remind us, says Brian...
Dr. GREENE: That science is something that can make your heart pound and can make you really care about something, care about a character, and how it can really entwine with life in an organic way. It's not something that you just think about.
KRULWICH: Brian Greene's book, which we abridged, is called "Icarus At The Edge of Time." I'm Robert Krulwich, NPR News, in New York. Transcript provided by NPR, Copyright National Public Radio.
