Subtle twists
There is a certain degree of understanding of relativity
Even if people who are professional in physics know a lot, they can't say "real understanding", and non-professionals of my generation can only understand it through metaphor and imagination. In 1915, Einstein combined space and time into a mathematical object.
This thing, called space-time, has similar properties to rubber film: it deforms. Placing a ball (such as the sun) on top of it will press the membrane down to create a pit. Objects passing near the sun will fall into the pit. Of course, we usually call this phenomenon that objects are gravitated by the sun.
According to our daily experience, the path taken by light is the straightest and shortest. However, even light without mass will bend the path as it travels through curved space, but this path is still the shortest path in this curved space, called a "geodesic". When light passes near a massive celestial body, its gravitational route will be deflected, called the "geodesic effect".
The degree of deflection can be calculated based on the speed of light and the mass of the celestial body. Einstein calculated that the deflection angle of light near the sun was 1.7′. In 1919, the British astronomer Eddington (who, according to legend, once claimed to be the only person besides Einstein who understood the theory of relativity) led an observation team to confirm that the starlight did indeed have the expected deflection as it passed near the sun. General relativity, supported by experimental evidence, was an important turning point in the history of science.
Another famous test of relativity was on June 18, 1976. NASA's Gravity Detection A satellite was launched into a 10,000-kilometer orbit and carried an ultra-precise atomic clock over the Atlantic Ocean for 116 minutes.
Meanwhile, another identical, calibrated atomic clock was running on the ground. As predicted by the theory of relativity, the rate at which the atomic clock carried by the satellite is different from that of the atomic clock on the ground.
That is to say, gravity affects the speed of time, which is the effect of gravitational redshift.
General relativity predicts that there is another, weaker distortion of space-time, proposed in 1918 by Austrian physicists Joseph Lance and Hans Tieling. They say that a rotating object, especially massive matter, in addition to the space bending caused by the "geodesic effect", will also produce another space distortion effect called "inertial drag" due to rotation, such as a spoon stirring in the syrup to form a vortex. This effect is much weaker than the geodesic effect, so it has not been tested for more than 80 years after its proposal. In the late 1990s, some X-ray astronomers believed that they had observed this effect indirectly. Around neutron stars or black holes with enormous masses,
There are rotating disks of gas and dust that emit intense streams of X-rays. If a spinning neutron star or black hole distorts the surrounding space-time, it will cause the dust disk to shake, causing the X-ray flow to change. The scientists say they observed such changes. However, some theories unrelated to the "inertial drag" effect also seem to explain this change, so this is not a conclusive evidence.
The mission of "gravitational detection B" is to observe the "geodesic effect" with unprecedented precision, and then deduct its effect and directly look for signs of the "inertial system dragging" effect from the remaining data.
Precision, more precision
The core component of "Gravity Detection B" is 4 gyroscopes. This element, also known as a gyroscope, is commonly used to position and stabilize the aircraft. Theoretically, the "gravitational detection B" thing to do is very simple: fix the gyroscope to the telescope, and during the flight, the telescope always faces a star, so that the rotation axis of the gyroscope coincides with the straight line between the telescope and the star. When the Earth rotates, a space-time vortex is formed around it, and the rotation axis of the gyroscope will also be slightly deflected. Carefully measuring the degree of deflection, deducting the effect of the earth's mass itself leading to the curvature of space, can observe the existence of a space-time vortex.
Just as hanging a bell around a cat's neck sounds easy, but there are huge technical difficulties in actual operation, so does Gravity Detection B. The problem is that the "inertial system drag" effect is too weak. Calculations show that this effect of the Earth will deflect the axis of rotation of these gyroscopes by 41% o arcseconds. This angle is roughly equivalent to watching a coin placed in Los Angeles from Washington.
The 4 gyroscopes of gravity detection B are the most sophisticated gyroscopes in the world, and their main components are made of quartz. It is a very stable mineral that is largely unaffected by temperature changes. The rotor of each gyroscope is a ping-pong ball with an extremely thin layer of metallic niobium coated on the surface, which can rotate at a speed of 10,000 revolutions per minute. These quartz balls are the closest thing humans have ever made to perfect spheres. Even if it is magnified to the size of the Earth, the height difference between the apex of the highest mountain on its surface and the bottom of the deepest trench is less than 5 meters.
The gyroscope is encapsulated in a 2.74-meter-long structure shaped like a cigarette. Inside this structure, it is ten times more empty than outer space, close to absolute vacuum. On the outer layer, there is a jar containing 2441 liters of liquid helium. The temperature of these liquid helium is only 1.8°C higher than absolute zero, which is used to cool the gyroscope and avoid heat affecting the experimental results. Such a low temperature is enough to make the metal niobium on the surface of the quartz ball enter a superconducting state, and the direction of the rotation axis of the quartz ball is determined by the magnetic field of this superconductor.
In addition to the rotor of the core, other components must also do everything possible to eliminate the effects of any electronic or mechanical defects. In addition, people also need to observe the movement of a pair of binary stars in the constellation Pegasus, which is used to locate the pointed star, and take this factor into account to ensure that there is no misalignment and abandonment of the previous achievements due to the slight drift of the star's position. As for the orbit of the satellite itself, it is also almost perfectly circular.
Designed by Stanford University and produced by Lockheed Martin, the "Gravitational Detection B" satellite weighs 3100 kg, is 6.43 meters long and 2.64 meters in diameter. If you remove the solar panel, it looks a bit like a cement mixer. This huge, heavy and unusually precise thing is naturally extremely expensive. It was one of the first projects NASA decided to launch, but the high price almost ruined the project. Especially since it is testing a theory that few people doubt. Although the scientists in charge of the project have a very high profile and say that they are "open-minded" about all outcomes, to be honest, there is little chance that general relativity will actually be overturned by this experiment.
The history of "gravitational detection B" dates back nearly half a century. The program was first proposed in 1959 and received the first grant from NASA in 1963. However, the technical conditions at that time were not sufficient to implement the plan. During this period, NASA implemented the "Gravitational Detection A" program in 1976, confirming the gravitational redshift effect. But in the second year of the experiment's success, NASA's grant expired and the Gravitational Probe B program stalled.
The project was revived in the early 1980s. Scientists had intended to use the space shuttle to put the satellite into orbit, but the explosion of the Challenger in 1986 canceled the plan, and the Delta 2 rocket became the new launch vehicle of gravity detection B. In the years that followed, the progress of the project was slower than expected. It was in danger of being terminated by Congress several times, but on each occasion scientists succeeded in convincing lawmakers to keep the project.
The last life-or-death decision occurred in 2003. When the satellite was assembled and the thermal vacuum experiment was performed, it had some failures. NASA nearly ended the lengthy project, but considering that the problems with the thermal vacuum experiment were only minor technical problems, it was eventually allowed to survive. On April 20, 2004, "Gravity Detection B," finally lifted off at Vandenberg Air Force Base in California.
Rocket liftoff
Not every scientist involved was lucky enough to see it fly into space. The initiator of the programme, Professor Leonard Schiff, died in 1971. One of the main scientists in charge, Stanford Professor Francis Evert, left his native Britain in 1960 to spend two or three years in the United States, joining the program in 1962, and 40 years later, he is still here. The cross-century program has produced 94 PhDs, 15 masters in engineering and more than 300 research positions.
All of these efforts are for one purpose: to confirm general relativity prophecies. Butchman, the program's scientific director, said the results of the experiment were not simply a right or wrong sign for general relativity, but rather helped scientists discover whether the fainter effects predicted by Einstein's theory could be detected. "It will help us better understand the theory of relativity."