Today’s post subject is gravity. It all begun yesterday morning, when I wanted to see a pictue of David Hockney’s, and run by accident into Garrowby Hill, the picture you see below.
I immediately felt the instinctive need to jump on the winding road and let gravity do the rest of the work, propelling me down the twisted path without regard for time and speed. It was after this delirium that lasted for a few seconds that I started thinking about gravity.
This took me back to Newton’s theory of gravity: gravity is a force, that is directly proportional to the mass of the objects involved.
Newton’s law of universal gravitation states that every massive particle in the universe attracts every other massive particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The second theory is Einstein’s General Theory of Relativity. Einstein realized that Newton’s theory of gravity had problems. He knew, for example, that Mercury’s orbit showed unexplained deviations from that predicted by Newton’s laws. However, he was worried about a much more serious problem. As the force between two objects depends on the distance between them, if one object moves closer, the other object will feel a change in the gravitational force. According to Newton, this change would be immediate, or instantaneous, even if the objects were millions of miles apart. Einstein saw this as a serious flaw in Newtonian gravity. Einstein assumed that nothing could travel instantaneously, not even a change in force. Specifically, nothing can travel faster than light in a vacuum, which has a speed of approximately 186,000 mi/s (300,000 km/s). In order to fix this problem, Einstein had not only to revise Newtonian gravity, but to change the way we think about space, time, and the structure of the Universe. He stated this new way of thinking mathematically in his general theory of relativity.
In a nutshell, Einstein set forward the hypothesis that “Gravity” is a curve in space-time.
Many of the predictions of general relativity, such as the bending of starlight by gravity and a tiny shift in the orbit of the planet Mercury, have been quantitatively confirmed by experiment.
Einstein said that a mass bends space, like a heavy ball making a dent on a rubber sheet. Further, Einstein contended that space and time are intimately related to each other, and that we do not live in three spatial dimensions and time (all four quite independent of each other), but rather in a four-dimensional space-time continuum, a seamless blending of the four. It is thus not “space,” naively conceived, but space-time that warps in reaction to a mass. This, in turn, explains why objects attract each other. Consider the Sun sitting in space-time, imagined as a ball sitting on a rubber sheet. It curves the spacetime around it into a bowl shape. The planets orbit around the Sun because they are rolling across through this distorted space-time, which curves their motions like those of a ball rolling around inside a shallow bowl. (These images are intended as analogies, not as precise explanations.) Gravity, from this point of view, is the way objects affect the motions of other objects by affecting the shape of space-time.
Einstein’s general relativity makes predictions that Newton’s theory of gravitation does not. Since particles of light (photons) have no mass, Newtonian theory predicts that they will not be affected by gravity. However, if gravity is due to the curvature of space-time, then light should be affected in the same way as matter. This proposition was tested as follows: During the day, the Sun is too bright to see any stars. However, during a total solar eclipse the Sun’s disk is blocked by the Moon, and it is possible to see stars that appear in the sky near to the Sun. During the total solar eclipse of 1919, astronomers measured the positions of several stars that were close to the Sun in the sky. It was determined that the measured positions were altered as predicted by general relativity; the Sun’s gravity bent the starlight so that the stars appeared to shift their locations when they were near the Sun in the sky. The detection of the bending of starlight by the Sun was one of the great early experimental verifications of general relativity; many others have been conducted since.
One can imagine how many measurements have ben carried out in order to validate and/or refute einstein’s theory and its components, one of which is “frame-dragging”.
I quote one of the reports on such measurements (BBC NEWS):
“Frame-dragging” is the effect wherein a massive body like Earth drags space-time around with it as it spins.”Frame-dragging” is the effect wherein a massive body like Earth drags space-time around with it as it spins.
Ignazio Ciufolini and Erricos Pavlis measured frame-dragging by studying the movements of two satellites in Earth orbit over a period of 11 years.
Ciufolini, from the University of Lecce, Italy, and Erricos Pavlis from the Joint Center for Earth Systems Technology in Baltimore, US, analysed millions of laser range-finding signals that are reflected by the satellites Lageos and Lageos 2.
These reflected signals are normally used to map variations in the Earth’s gravitational field.
But the researchers analysed them for evidence that the satellites’ orbits were altered by frame-dragging, also known as the Lense-Thirring effect after the Austrian physicists who predicted it in 1918.
Ciufolini and Pavlis say their result is 99% of the value predicted by Einstein’s theory, plus or minus 5%. This result has an uncertainty of about 10% say the scientists.
Commenting on the research, Neil Ashby of the University of Colorado, US, said the result was “the first reasonably accurate measurement of frame dragging.”
He added: “Further analysis is anticipated as additional geodesy missions are undertaken to improve our knowledge of Earth’s gravity field.”
The same researchers reported preliminary findings in 1998, which were roundly criticised. But reaction to the latest measurements has been broadly positive. The new work is based on a new gravity map released last year.
And so the story goes. the good news for Newton’s theory is that it is still good for most cases, as it starts having problems when the mass involved is huge. So we can continue having Newton in our everyday lifes, while Einstein is waving at us from a distant star, riding a light beam.
P.S. I am not going to join Albert this time. I return to Garrowby Hill to visit David Hockney and have a beer at the local inn. And now that we talk about it, I have a small contribution to make to the theories of gravity. I have noticed that after the third pint of ale, my feet are heavier. I have therefore prepared an amendment to the Newtonian theory: the force of gravity is also directly proportional ot the alcohol content of the masses involved. Cheers!!!