When Albert Einstein proposed his theory of general relativity in 1916, one of his predictions was that light could be deflected by the mass of a nearby object. In 1919 Arthur Eddington took a trip to Principe and photographed stars during a total eclipse. The results confirmed Einstein’s theory.
Ten Billionths of a Gee
Pioneer 10 was launched in 1972. The next year Pioneer 11 was launched. Their mission was to fly past Jupiter and then Saturn, making the first detailed studies of the planets. Afterwards, they continued their journey to the outer reaches of the solar system. At this point they entered a second phase of their mission, to study the diffuse gases in the solar system on their way to interstellar space. Their observations indicated that something rather strange was going on.
Behind the Veil
Last September a planet-massed gas cloud known as G2 will made a close approach to the supermassive black hole at the center of our galaxy. At minimum distance it will pass within about 260 AU of the black hole, which is about a third as close as any other object so observed. It will be close enough that it will enter the hot accretion region of the black hole, and may provide the first observation of matter as it is absorbed by the black hole.
The Planet That Never Was
While there were speculations about a planet closer than Mercury going back to at least the 1600s, it wasn’t until the 1850s that Urbain Le Verrier discovered the first indirect evidence for such a planet. Le Verrier carefully calculated the orbit of Mercury, and determined that the orientation of its orbit rotated slowly over time. This is known as a perihelion advance, and it is due to the small gravitational pulls from other planets.
Dance Magic Dance
Janus is a small moon of Saturn. It is somewhat oval in shape and has a diameter of about 180 kilometers. Epimetheus is another moon of Saturn, with a diameter of about 120 kilometers. The two moons are very similar, even down to their orbits. They share the same orbital plane, and at the moment the orbit of Janus is only about 50 kilometers closer to Saturn than that of Epimetheus. In other words the gap between the orbits is less than the size of the moons.
Newton’s Apple
You probably know the story of Isaac Newton. He was sitting under an apple tree when he saw an apple fall to the ground. This inspired his idea of universal gravity. There’s long been some debate as to the truth behind this tale. The story comes most famously from “Memoirs of Sir Isaac Newton’s Life” by William Stukeley in 1752. Earlier mentions appear in works of Voltaire and Robert Greene. Whether true or not, it is a story we love to tell. It portrays Newton as a genius with a revolutionary insight.
Eddies in the Space-Time Continuum
We often refer to gravity as a gravitational field because it can be described mathematically as a field. That is, each each point in space and time has certain characteristics, and they are related to each other through field equations. For gravity, the field equations are the equations of general relativity. Lots of physical quantities can be described as fields, including electromagnetism and quantum objects. Fluids are also described mathematically as fields. With a flow of water or air, for example, every point has properties such as pressure and speed, and they are related by equations of fluid dynamics.
There And Back Again
When someone mentions time machines, you might think of fantastical machines such as Dr. Who’s TARDIS or the DeLorean in Back to the Future, but several physicists have made a serious study of time machines. Most of this work focuses on “what if” scenarios, which are really about testing the limits of a particular theoretical model, rather than actually engineering a device that can travel to the past.
Battle of the Planets
The 55 Cancri system is in some ways very similar to our own. The star is about the same mass and size as our Sun, and it has a Jupiter-sized planet at a distance of 5.7 AU, which is just slightly more distant than Jupiter’s 5.2 AU distance. It has three other gas planets, as well as a super-earth. One would think that the system might be similar in other ways, such as having rocky planets close to the star and gas planets more distant. But this is not the case.