Einstein’s Warped Universe
University of Maryland
About the Lecture
Einstein's Relativity Theory revealed that time depends on the motion of the clock measuring it, and that gravity is nothing but the warping of time (and space), so that time depends also on the location of the clock. Such effects are both a practical matter, for example in the global positioning system, and a window to the cosmos. Bending of light rays, black holes, expansion of the universe and gravitational waves (ripples of time and space) are some striking examples. These are well understood, but the theory also presents yet deeper mysteries which it is unable to address.
About the Speaker
THEODORE A. JACOBSON is a Professor of Physics at the University of Maryland. He earned a bachelor's degree in physics and mathematics at Reed College and a Ph.D. in physics at the University of Texas at Austin. He is a fellow of the American Physical Society, and presently serves on the editorial board of Physical Review Letters. His current research includes probing the validity of relativity theory, black hole thermodynamics, and the microstructure of spacetime.
President Robert Hershey called the 2,198th meeting to order at 8:20 pm December 10, 2005. The minutes of the 2,197th meeting were read and approved. Mr. Hershey then introduced the speaker of the evening, Mr. Ted Jacobson of the University of Maryland. Mr. Jacobson spoke on “Einstein's Warped Universe.” “I'm glad to have the opportunity to tell you some things about Einstein's warped universe,” Mr. Jacobson began. He noted that we were in the anniversary of Einstein's “miraculous year,” 1905, in which he published three seminal papers that spanned all of the revolutions of modern physics of that time. One introduced the idea of photons and the particle nature of light. Another gave calculations of Brownian motion, which established the relative nature of matter. Third, he introduced the idea of mass as not independent of energy, but as an aspect of energy. He was 26 at the time and a clerk in a patent office. He could not get an academic job, Mr. Jacobson joked. Einstein did say that “a practical profession is a salvation for a man of my type. An academic career compels a young man to scientific production, and only strong characters can resist the temptation of superficial analysis.” The 1905 work is called special relativity. In it, time, space, and mass are taken as not absolute. It omits the whole matter of gravity. It wasn't until 1915 that he incorporated gravity and inertia. He conceived gravity as a warping of time and space. What does it mean to say that gravity is the warping of space and time? In 1905, Einstein had inherited Maxwell's theory of electrodynamics and Newton's theory of mechanics. Neither of them implied a preferred state of rest. Maxwell's theory did predict that light and electromagnetic waves propagate at a definite speed. Light from a flashlight always travels at the same speed, regardless of whether the flashlight moves. Therefore we cannot attach any absolute signification to the concept of simultaneity. Consider a flash of light in a box. If the flash originates in the middle of the box, it reaches both ends at the same time. If an observer is running toward the middle, he sees it before it reaches the other end of the box. Thus, a time interval between two events depends on who is observing. This is the time dilation effect. One aspect of this relativity is that time elapsed between two events depends on the path in spacetime. One who travels a straight line from one point to another might age, say, fifty years. One who visits a distant intermediate point on the interim ages less, and the difference is relative to the additional distance traveled. The terms time and space are not used. Instead, they use timelike and spacelike. Newton conceived gravity as a universal force, an attraction of masses to each other. It explained both the falling of apple and the orbit of the planets around the sun. Einstein said he was sitting in the patent office in Bern when all of a sudden a thought occurred to him: If a person falls freely, he won't feel his own weight. He was startled. This simple thought made a deep impression on him. Then he had the happiest thought of his life, that a gravitational field has only a relative existence. To illustrate Einstein's happy thought, he showed a picture of a dancer in an airplane, dancing in the air in a space that was static relative to the confines of the airplane and feeling none of the weight of Newton's force. She seemed to be having a good time. Gravity is the curvature of spacetime. When an apple appears to fall, what really happens is that the apple and the earth approach each other. When an apple appears static relative to the earth, they are actually both traveling through space. When intervening forces are removed, they curve toward each other. What true gravity is can only be understood by considering all the movement of both. A freely falling object is following straightest line it can in spacetime. He described a little of how the global positioning system works. The earth devices have clocks on them. Locations are determined by comparing the times of the origin of the different signals. Gravity bends light. Mr. Jacobson showed some pictures of a galaxy viewed through the gravitational lens of another galaxy. Rings of light from the distant object that appear around the nearer object are called Einstein rings. Actually, you usually see only arcs, not complete rings. In some cases, the time difference between two paths of light from the same source is over a year. To bring the matter closer, he showed a picture of the Smithsonian castle and another picture of it as it might appear through a gravitational lens, as if a black hole were between the camera and the castle. Parts of the building seemed to bend around and enclose parts of the clouds behind it. He showed a picture of an antenna in Puerto Rico. This instrument showed in 1974 that gravitational waves exist by finding a binary pulsar. It has an orbit period of eight hours and a pulse period of 59 ms. Such systems emit gravitational waves. Gravitational waves carry energy, the orbits are reduced, and the orbit period decreases at the predicted rate. As another approach, there is an attempt underway to measure gravitational waves directly using very-long-baseline radio interferometry. Instruments will be placed far apart on the earth, one in Oregon, one in Louisiana. Other interferometers elsewhere on earth may also be used, and eventually interferometers in space. Then he took up some other questions about the universe. Is it closed or open? Is it curved or flat? How did it begin? Does it expand forever? What about the beginning of time? What is time like inside a black hole? He offered some speculations on these questions. Mr. Jacobson offered to answer questions from the audience. In response to questions, he noted that Einstein made gravity equal to the curvature of spacetime and therefore gravity and inertia the same thing. omeone complained that he would never understand the spacetime thing. Where does it get started? How do you measure time without an instrument? You can't, Jacobson said. Time is what a clock measures. Is the cosmos as a whole warped? Yes, it is. Does the 3-dimensional universe have an analog in a 2 dimensional universe? Yes, this is an outgrowth of string theory. It is a theory in two dimensions that has no gravity. Electromagnetic waves are harnessed for useful purposes, is anything like that possible for gravitational waves? Yes, for astronomy. Could we generate them? Anything we can generate would be extremely weak. He guessed it won't happen in the next 50 years. Then we had the annual business meeting. I won't recount it because the business records are kept by the corresponding secretary. Mr. Hershey announced the next lecture and invited guests to join the Society. Finally, he made the parking announcement, invited everyone to enjoy the social hour, and, at 9:50 pm, adjourned the 2,197th meeting. Weather: Unremarkable Temperature: 6°C Attendance: 71 Respectfully submitted, Ronald O. Hietala, Recording secretary