Einstein predicted that space is curved. Now it will be possible to measure it
Uriel Brizon
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American scientists are about to launch into space a research satellite that should confirm Einstein's theory of general relativity. The equations of general relativity are the basis of the scientific explanation for the movement of the stars and galaxies and the formation of the universe.
According to Einstein's theory, space itself curves in proximity to large bodies (like the Earth). The satellite - Gravity Probe B - will try to measure the curvature of space, which is manifested in the existence of a kind of spatial "pit" around the Earth. To measure the spatial changes, unique sensors were installed on the satellite, capable of making measurements with unprecedented precision. The launch of the satellite is the culmination of a huge scientific and technological project based on four decades of research and development.
A new explanation for gravity
In 1905, Albert Einstein, then a clerk in the patent office in Bern, published an article that revolutionized the world of physics. On this day, what later became known as special relativity was born. Einstein delved into the problems that arose from previous experiments in which the speed of light was tested. These experiments showed that, contrary to what happens in normal movement, the speed of light does not change in relation to the movement of the observer. If a person is driving a car at a speed of 20 km/h and another person is driving in front of him in a car at a speed of 10 km/h, they are moving towards each other at a relative speed of 30 km/h. Light rays do not behave like this. Light, moving at Einstein's speed.
About a billion km/h, not relative to the speed of the viewer. Various attempts to explain the phenomenon failed until Einstein proposed to change the fundamental assumptions of physics. He claimed that the speed of light is constant and that it is impossible to move faster. On the other hand, other quantities such as length, mass and even time itself, which were considered constant, are the ones that change during movement.
Speed is constant, time is variable
At speeds approaching the speed of light, strange phenomena occur. Energy invested in acceleration will cause an increase in the mass of the moving body. You can think of a car moving at a speed close to the speed of light. Under such conditions pressing the gas pedal in the car will not cause an increase in speed but an increase in the mass of the car. To the viewer from the side of the road, the car will look more Asian, and the movements inside it will seem slow to him - time in the car will move at a slower pace than on the watch of the viewer.
Although Einstein's conclusions seem improbable, the phenomena predicted by special relativity have been measured in many experiments. In particle accelerators, for example, an increase in the mass of particles as a result of their rapid movement was measured as well as a change in the time rate (by detecting a deviation in the half-life time of the particles). Einstein's famous equation E=mc2 derives directly from special relativity and underlies the operation of nuclear power reactors and atomic bombs.
In 1915, after spending about a decade expanding the special theory of relativity, Einstein published the foundational article of the general theory of relativity. In general relativity, Einstein applies the new insights about the nature of space and time (which were united into a factor known as space-time) in a broader framework. Einstein proposed a new explanation for gravity, the central force in describing the behavior of the universe.
According to the theory of general relativity, bodies are not attracted to each other due to a force arising from each body with mass (as Newton believed), but move in a curved space. The idea can be clarified by the following image: Suppose a heavy metal ball rests on a soft mattress. The ball creates a kind of hole in the mattress. The pit simulates the depression created by massive bodies in space. A small marble that moves on the mattress in a straight line towards the ball will "fall" to it. She will accelerate her fall the closer she gets to the ball since the sides of the pit will be steeper. To someone who cannot see the mat (let's say it is transparent) it will appear as if the small marble is being pulled towards the ball by some force accelerating its motion. Similarly, the bodies falling to the earth actually move in a kind of spatial pit; Their movement is dictated by the curvature of space which is not "flat" but curves around massive bodies.
Over the years, many proofs of the special theory of relativity have been found. The equations of general relativity, on the other hand, are very difficult to confirm. Important experiments, including the measurement of the change in the position of stars during a solar eclipse and the measurement of the slight change in the orbit of the planet Mercury, which is closest to the Sun and its gravity well, examined limited aspects of general relativity, and therefore did not satisfy the scientists.
A deviation of 1.8 thousandths of a degree
In 1959, Leonard Schiff, a physicist from Stanford University, proposed an experiment that would make it possible to directly measure the curvature of the space around the Earth. Schiff suggested using a gyroscope - a mechanical component found in various navigation devices that has a central mass that rotates quickly and maintains the direction of the axis of rotation with great precision. The idea was to measure the curvature of space-time by measuring the deflection in the movement axis of the gyroscope with great precision. The direction of the axis of movement of the gyroscope, Schiff claimed, would change according to the curvature of the space around it, and the deviation could be measured by comparing it to a distant point that is not affected by the curvature of the local space.
Schiff proposed sending a satellite with a gyroscope into orbit around the Earth and measuring the angular deviation that would occur in its axis of motion after a year. He calculated and found that in an orbit at an altitude of 640 km above Israel, a deviation of approximately 1.8 thousandths of a degree should be created over the course of a year. In addition, Schiff calculated that another deviation should be created, a hundred times smaller, on the axis perpendicular to the axis of the first deviation. The second deviation will be caused by a phenomenon known as "frame dragging". Spatial drag is caused by the movement of the Earth around its axis. The movement "drags" the space near the earth and creates a kind of spatial vortex. The measurement of space drag will be considered a particularly significant boost to general relativity because it is a very unique phenomenon to the theory.
After proposing the experiment, Schiff realized that the technology in 1959 did not allow it to be performed. Later, in 1976, scientists from the American Space Agency launched the research satellite Gravity Probe A. Its purpose was to measure the change in the rate of time at different points in the Earth's gravitational field (the change in time, accompanying the curvature of space, is an effect predicted by general relativity). The satellite, in which a precise atomic clock was installed, stayed in space for only 115 minutes and during that time reached an altitude of about ten thousand kilometers. When moving away from the center of the Earth's gravity field, the changes in the time rate were measured and found to be consistent with the predictions. But the scientists were not satisfied with that and aimed to make a direct measurement of the curvature of space.
In 1980, NASA conducted a renewed review of the technological readiness to carry out the Schiff experiment, and it was decided to start the project. After years of examining and rejecting various alternatives, the satellite structure was decided upon. Its important components are four sensitive gyroscopes and a telescope with high resolution. It was decided that the satellite would indeed orbit the Earth at an altitude of 640 km, and the duration of the mission was set for two years.
The satellite's telescope will aim at a guide star - IM Pegasi in the Pegasus constellation. The star will serve as an external reference point, and thus the satellite will maintain its exact direction for the duration of the mission. At the beginning of the experiment the axis of rotation of the gyroscopes will be parallel to the axis of the telescope and the satellite body. After a year, the angle difference between the axes of motion of the telescope and the satellite body and the axis of motion of the gyroscope will be measured. The deviation to be measured should indicate the curvature of the space.
Gravity Probe B resembles a metal urn and is 2.74 meters tall. The body of the satellite will be filled with about 2,500 liters of liquid helium, which is kept at a constant temperature of 1.8 degrees above absolute zero (minus 273.15 degrees Celsius). Keeping the temperature low is essential for the operation of the sensors located in the center of the satellite, and a complex system of insulators and filters should prevent a change in temperature when the satellite is exposed to sunlight during its movement around the Earth.
To ensure accurate measurements, the surfaces inside the telescope and the spinning balls in the gyroscopes are made of pure quartz. The scientists chose to use material that was mined in Brazil, in a mine known to have particularly high quality quartz. After refining the material in laboratories in Germany, quartz was obtained so pure that the International Organization for Standardization (ISO) decided to change the definitions relating to the purity and uniformity of materials. The angular deviation inside the gyroscopes will be measured by highly sensitive sensors, and the low temperature in the body of the satellite will allow these sensors to report angular changes of up to one ten millionth of a single degree.
Gravity Probe B, which costs about 650 million dollars, is a joint project of NASA, Stanford University and Lockheed Martin. The original launch date was set for these days, however a malfunction in one of the voltage converters on the satellite caused a delay, and according to estimates the launch will take place in the coming weeks. If the satellite measures the expected deviations, it will be direct observational evidence that will confirm the theory of general relativity and scientists all over the world will receive reinforcement for the equations that guide their research. If the expected deviations are not measured, great embarrassment will be felt. Many scientists will be forced to re-evaluate their work and new theories may rush to fill the void that will be created in the research field.
News on Space.com about the delay in the launch of the spacecraft
