The ultimate test of our current theory of gravity and general relativity will come from LIGO and other gravity wave detectors. Gravitational waves present a rich phenomenology that can be studied, involving all kinds of objects in the universe. That's still a couple of years away. Meanwhile, an experiment recently completed in Earth orbit has tested general relativity and gravity theory in a new regime. The Gravity Probe B experiment was looking for distortions of space and time that uniquely occur in general relativity and are not predictions of Newtonian theory. It was not a completely successful mission. It never produced results of the eras that had been planned. But it improved on current tests of general relativity by over an order of magnitude and affirmed several exotic phenomena that are result of space-time distortions caused by mass energy. Gravity Probe B had four exquisitely accurate gyroscopes, the most accurate made at the time; although they've since been exceeded, and a telescope locked on a star. To understand the phenomena, the Gravity Probe B was designed to detect. Let's look at the conceptual differences between Newton's theory and Einstein's theory. Although Gravity Probe B is an esoteric experiment to detect general relativity effects that are extremely small, it made its way into the popular culture. Welcome to our program. The space time continuum. Is it distorted by the gravitational fields of massive objects like the Earth as Einstein postulated or is it simply lazy? Last week, NASA set out to answer that question by launching its latest insult to the nation's homeless. Gravity Probe B, a $750 million satellite designed to test several key components of the theory of relativity. Mostly the equals part. They got the edmc square that they know equals, we don't know what's going on with that. GPB blasted off into space aboard a Boeing Delta Two rocket. Let's listen to the historic count down. Five, four, three, two, made ignition start, one, ignition, and liftoff of the Delta rocket carrying Gravity Probe B, testing for [inaudible]. Why was he standing so close? That seems falsely comic. GPB will precisely measure the gravity of Earth's orbit using four gyroscopes designed around golf ball-sized courts spheres, said to be the most perfect ever made by man. Believe me I've seen better spheres. The measurements will then be used to test Einstein's theory. Now, a lot of you are in no doubt confused by the geodetic effect, the way it alters Earth's mass that is suddenly warping the space time's continuum. So if you would be so kind as to indulge me, here's how it works. I don't actually know how it works. Did you really think I knew how it works? I got nothing. I don't even know how the damn binocular`s works quite frankly. Jesus. I think I poked my eye. The two distinct effects the Gravity Probe B was designed to see occurred because of the Earth itself, not an extremely massive nor an extremely dense object. So it's distortions of space are very subtle indeed. One distortion is the curvature of space itself, which should have manifested in a slight bending of the axis of the gyroscopes in the satellite as it orbited the Earth. The second effect was 20 times smaller still, called frame dragging. It's the subtle distortion of the contours of space-time caused by a spinning object, essentially twisting the space time by a tiny amount. Gravity Probe B detected both effects by their deviations imposed on the gyroscopes while the telescope was locked in a very distant object at a fixed location in space. The extreme distortions of space in time expected near a black hole remain to be tested. The distortion of space as you approach the black hole produces a tunneling of what can be seen looking outwards, should someone venture close to the black hole. At the event horizon of course, all view of the external world is lost forever. The distortions of time are equally extreme. In principle, if two astronauts had synchronized timekeeping pieces and one stayed in orbit of a black hole at a safe distance while the other spiraled into the black hole, according to general relativity as seen from the astronaut at a safe distance, the clock of the astronaut falling in would run slower and slower, and would asymptotically slow as they approach the event horizon. In principle, at that point they reached the event horizon, their clock and their image as seen from the outside, would be frozen forever in time. Is it possible to ever test this in real life? Unfortunately, black holes are rare. The likely nearest example would be hundreds of light years away, too far for us to venture in the near future, perhaps even in the next few centuries. Also, falling into a solar mass black hole or the remnant of a massive star would be an extremely unwise thing to do. The differential gravity or stretching force between your head and your feet if you fell in or one side of your body and the other, would leave sufficient to stretch you apart and kill you. This is not a mild form of stretching like pizza dough. This is called spaghettification, where every aspect of your body from muscles down to nerve fibers down to molecules are stretched as falling into the black hole, the gravity field increases. So falling into a black hole is not recommended. How big would a black hole have to be for this effect not to be disruptive? Probably about 3,000 times the mass of the sun. At that point, the stretching force or the tidal force on a human-sized object would not be so extreme as to kill that person or disrupt the object. The second problem of an experiment of falling into the event horizon of a black hole, is that no information could be transmitted out on the conditions inside? They will remain according to general relativity forever sealed off from us. There are many Hollywood type simulations of black holes, and black holes have featured in TV shows and movies for decades. But the real astrophysics of a black hole general relativity can be used to predict what it would look like in terms of space and time as you fall in. The following animation showing how the clock is affected and how the contours of light are affected, shows a spiraling passage in towards the event horizon. How can we summarize the complex states and stages of stellar evolution, the cycle of star birth and death that includes our story to? Perhaps this is best done in the words of a popular song, from Pink Floyd, from the mid 1970s Shine On You Crazy Diamond. Remember when you were young, you shone like the sun. Shine on you crazy diamond. Now there's a look in your eyes, like black hole in the sky. Shine on you crazy diamond. You were caught in the cross fire of childhood and stardom, blown on the steel breeze. Come on you targets of faraway laughter come on you stranger, you legend, you martyr, and shine. Nobody knows where you are, how near or how far. Shine on you crazy diamond. Pile on many more layers, and I'll be joining you there. Shine on you crazy diamond and we'll bask in the shadow of yesterday's triumph and sail the steel breeze. Come on you boy child, you winner and loser, come on you miner for truth and delusion, and shine. The song is a double metaphor. It was written in honor Sid Barrett, founder member of the group, present on their first two albums who left, and eventually went into dissolution to mental illness and drug use. He died a few years ago. They wanted to honor Sid Barrett as the creative spark, the young flame who lived hard, died young, and left a bright legacy. But also in this song, they pay an honor to most of the stages of stellar evolution. There's an awful lot of astrophysics in this tune. In just two verses, we see references to the main sequence, to the strange carbon-like diamond form in a white dwarf, to black holes, to the end states of massive stars in their onion skin layering, to the difficulty of measuring stellar distances, and in a wonderful metaphor, sailing on the steel breeze. The nucleus synthesis of heavy elements and then blast wave of the supernova. General relativity has been tested in a new regime by Gravity Probe B, which sought the distortion of space time and the twisting of space time itself caused by the spinning earth, both effects were detected. The ultimate test of general relativity however is the study of black holes themselves. The nearest example is likely to be dozens or hundreds of light years away, so too far for us to venture with space probes or astronauts. We will have to do clever astrophysical experiments to decide whether event horizons are real, and whether perhaps a singularity lurks at the center of every black hole.
