Albert Einstein’s theory of general relativity, completed 90 years ago this year, stands as one of the greatest achievements of modern physics. With it, Einstein created an entirely new way of looking at the universe.
General relativity was an extension of Einstein’s special relativity of 1905. Special relativity showed that the only measurement two moving observers could agree on was the speed of light. Their measurements of time, space and momentum would disagree.
Special relativity dealt with observers moving at constant velocities (hence the term, special), and Einstein realized he had to generalize the theory to include accelerated movement (changing velocities). The mathematics involved was quite difficult, taking him another decade to master and then apply.
General relativity had some startling predictions, which I summarize below. Most of GR’s predictions have been verified, but one — the existence of gravitational waves — has not, as yet. Gravitational waves are ripples in the fabric of the universe created by rotating, supermassive objects, colliding galaxies and exploding stars.
Gravity is a comparatively weak force and interstellar distances are vast, so detecting gravitational waves is quite a feat. There are several earth-bound gravitational wave detectors, and there will soon be an orbiting one. They all churn out reams of data, which project supervisors need to analyze to catch a wave.
Enter the WWW and all those personal computers attached to the internet. Bruce Allen of the University of Wisconsin – Milwaukee has developed einstein@home, a program that runs on your computer and churns a tiny batch of wave detector data. The site also has tons of information about relativity, the sources of gravitational waves, and the wave detectors.
Though its predictions seem quite bizarre, GR has been on target so far, so we expect to find gravitational waves. Here are some of the predictions that have been verified.
- Matter warps space, thereby bending light beams. GR proposes that matter affects the shape of space. Light follows the shape of that space as an airplane follows the curvature of Earth’s surface. Einstein predicted and calculated how much the Sun’s gravity would bend distant starlight during a solar eclipse. This prediction was first verified in 1919, catapulting Einstein to international fame.
- Space tells matter how to move. Rather than conceiving gravitation as a force traversing the space between objects, Einstein proposed that the shape of space determines the inertial motion of objects moving in that space. Using his equations, he was able to explain a longstanding puzzle about the orbit of Mercury, which Newton’s law of universal gravitation had failed to solve.
- The warping of space coincides with a stretching out of time. If matter bends space, it would imply a change in the speed of light. For relativity to be self-consistent, matter must also slow down time proportionately to keep the speed of light invariant. Scientists have verified this prediction with high altitude atomic clocks, astronomical observations and gravitational redshift experiments. Satellites in the global positioning system (GPS) have to take time dilation into account to stay synchronized in time.
- Gravitation will redshift light waves. Light is a wave in space and time. When we say light is redshifted, we mean its wave frequency has increased, or moved in the red direction of ROYGBIV. This shift reduces the energy of light moving away from a massive body, just as an object loses speed (kinetic energy)when it is thrown upward.The gravitational redshift has also been detected many times since the 1960s.
- The universe is expanding. Einstein at first rejected this implication of his equations, then regretted his error when Edwin Hubble in 1927 concluded that the galaxies he was observing were moving away from us and from each other. The space between the galaxies is increasing in size, suggesting that at one time the universe was much smaller. This evidence is a cornerstone of the theory of the Big Bang.
- A massive star can collapse in on itself, folding space and time around it so much that it “disappears” from the universe. John Archibald Wheeler in 1967 coined the phrase, “black hole,” for these objects, a number of which have since been detected.
Wikipedia has a concise summary of general relativity.