First look
The idea, in plain words
Newton’s law of gravity says how strongly two things pull on each other, and it works beautifully. It never said how the pull gets across empty space. In 1915 Einstein gave an answer that did away with the pull altogether. A heavy thing like the Sun bends the space and time round it. A planet is not being tugged. It is going as straight as it can through space that is curved.
His way in was a simple thought. Somebody shut in a falling lift feels no weight at all. Let go of an apple and it hangs in the air beside them, because the apple and the lift and the person are all falling together. From inside, falling freely cannot be told from floating far out in space. Einstein took that to mean gravity is not a force you feel. It is the shape of the space and time you are moving through.
The new theory had to explain something the old one could not. Mercury’s orbit is a stretched circle, and the whole orbit slowly turns, so that its closest point to the Sun creeps round. The pull of the other planets accounts for nearly all of that turning, but a small part was left over that nobody could explain. Einstein’s sums gave exactly the missing amount.
Then came a prediction that could be tested. If space is bent near the Sun, starlight passing close to the Sun must bend too, twice as much as Newton’s gravity would allow. The only time you can see stars beside the Sun is during a total eclipse. This is a real photograph from the eclipse of 29 May 1919. The stars near the Sun had shifted, by about the amount Einstein said.
Time is bent as well as space. A clock low down, where gravity is stronger, runs slower than one higher up. This has been measured between two clocks only a third of a metre apart in height. It matters every day: the clocks on navigation satellites, high above the Earth, run fast by 45 millionths of a second a day, and the system has to allow for it.
The same theory said that enough mass in a small enough space would bend things so far that not even light could get out, and that space itself could ripple. Both took a very long time to find, and both have now been found. This is a black hole, 55 million light years away, pictured in 2019.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Gravity
- The pull that every mass has on every other mass.
- Perihelion
- The point of an orbit nearest the Sun.
- Ellipse
- A stretched circle, the shape of every closed orbit.
- Solar eclipse
- When the Moon passes between the Earth and the Sun and its shadow falls on the Earth.
- Corona
- The Sun's thin outer atmosphere, which at over a million kelvin is far hotter than the surface beneath it.
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- Gravitational wave
- A ripple in space itself, sent out when very heavy things move violently.
- Speed of light
- 299,792,458 metres per second in empty space, the fastest that anything can carry news from one place to another.
Where this comes from
- ESO, Highest resolution image of the 1919 solar eclipse
- Lawrence Berkeley National Laboratory, Precession of the perihelion of Mercury
- NIST, Pair of aluminium atomic clocks reveal Einstein’s relativity at a personal scale (2010)
- Richard Pogge, Ohio State University, Real-world relativity: the GPS navigation system
- ESO, First image of a black hole (2019). Picture: EHT Collaboration, CC BY 4.0
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.