Black Holes: The Weird Science of the Most Mysterious Objects in the Universe
All the way back in fact to the late seventeenth century That’s when an apple falling from a tree is said to have led English physicist and mathematician Sir Isaac Newton to formulate his law of universal gravitation GRAVITY IT’S NOT JUST A GOOD IDEA IT’S THE LAW By the mid-sixteenth century scientists had discovered that the planets in our universe travel around the sun in an elliptical oval pattern Nobody knew why the planets moved that way or what kept them in orbit Newton OH THAT’S JUST A THEORY In everyday conversation people tend to use the word theory to describe a hunch or guess Evolution say creationists people who believe in the biblical explanation for the creation of the universe That’s just a theory In fact evolution is a theory To scientists a theory is not a half-baked idea A theory explains a collection of facts about some aspect of the natural world It can be tested and used to make predictions Peter Godfrey-Smith a philosopher of science at the City University of New York and at the University of Sydney in Australia compares theories to maps Just as a map represents an area of land a theory represents a territory of science A road map shows highways rivers and cities that we know exist In the same way a theory is made up of observable facts A good theory like a good map is an accurate depiction of the physical world It’s important to understand the difference between facts laws hypotheses and theories When an apple drops from a tree it falls down not up That’s a fact Newton’s law of universal gravitation describes the observation that the apple falls down
tackled that problem in his 1687 book Philosophiæ Naturalis Principia Mathematica Mathematical Principals of Natural Philosophy known as the Principia for short In it he described three laws of motion Although it took Newton more than five hundred pages to explain his laws they boil down to three sentences Every object stays in its state of rest or uniform motion in a straight line unless some kind of force acts upon it It allows us to calculate all sorts of things about a falling apple the strength of the gravitational pull between it and Earth its acceleration as it falls how long it will take to hit a daydreaming Newton on the head and so on THE HOW AND WHY OF GRAVITY What Newton’s law doesn’t tell us is why gravity exists or how it works Scientists develop testable hypotheses or smart guesses to explain observations Until the late nineteenth century scientists believed that gravity was due to what scientists call ether tiny invisible particles all around us that pushed things down But the evidence didn’t support that hypothesis so scientists rejected it Einstein proposed that what we perceive as gravity is actually the result of a distortion in the fabric of space and time Einstein’s hypothesis held up through repeated observation and experimentation So it became a scientific theory the general theory of relativity Not only did Einstein’s theory explain gravitational attraction but it also predicted gravitational waves and black holes Theories may change as scientists make new discoveries They can be adapted to include that new knowledge 7
The force acting on an object is equal to the mass of that object times its acceleration In mathematical form it is expressed like this F ma where F is force m is mass and a is acceleration For every action there is an equal and opposite reaction Newton used those laws in a thought experiment to try to understand the motion of the planets It went something like this What if he were to shoot a cannonball from the top of a very high mountain where just as in outer space there is no air resistance Without a force to act on it the cannonball would continue in a straight line forever But in reality Newton knew that the cannonball would move forward and eventually drop back to Earth Newton came up with the idea of gravity to name the downward pulling force acting upon the cannonball Isaac Newton 1642 1727 was a physicist and mathematician He laid out what he understood about laws of motion the behavior of fluids and gravitation in his book Philosophiae Naturalis Principia Mathematica The book established Newton’s reputation as one of the greatest minds of science 8 BLACK HOLES
Newton reasoned that the faster the cannonball was moving the farther forward it would travel before dropping to Earth If the cannonball were moving fast enough it would fall into an orbit around Earth instead of into it much as the moon orbits Earth If it moved really fast it would reach escape velocity leaving Earth and its orbit altogether He realized that the same force gravity that pulls an apple to Earth when it falls from a tree could also explain the orbits of the moon around Earth and the planets around the sun Newton’s law of universal gravitation published as part of the Principia states that the gravitational force between any two objects in the universe is proportional to the product of their masses The gravitational force is inversely proportional to the square of the distance between the center of each mass So more massive objects have greater gravitational pull than less massive objects And that gravitational pull gets weaker with distance Fast-forward to 1783 in a village in West Yorkshire England The head of the parish in the village a brilliant clergyman named John Michell had an unusually keen interest in science He met with some of the leading scientists of the time including Benjamin Franklin and published influential papers about the nature of magnetism astronomy and earthquakes He was trying to figure out a way to measure the mass of distant stars He concluded that the gravity of some really massive stars might be so great that not even light could escape them Michell published his calculations in a paper that described a universe with many invisible dark stars The only way to detect them he said would be to observe their gravitational effects on nearby objects 9 WHAT IS A BLACK HOLE
