In November 1915, Albert Einstein wrote the four papers that formed his general theory of relativity in just one week, changing our understanding of physics for the next century. This work took his special theory of relativity, which applied only to systems moving at constant speed, and expanded it to include objects under acceleration, in essence describing the entire universe.

The general theory of relativity described gravity in a new way. Isaac Newton had thought of gravity as of masses pulling on one another. By contrast, Einstein thought of gravity as a push.

Think of a bowling ball sinking into a flexible rubber sheet, and marbles all around being pushed towards it, responding to the distortion of the rubber being stretched. Like the bowling ball, all objects with mass create curves in the fabric of space-time – a four-dimensional description of the universe that includes height, length, width, and time – and these curves tell objects how to move.

Gravity Probe B orbits the Earth to take space-time measurements
Credit: NASA 

This distinction may seem small, but it has opened up all sorts of predictions about the universe, from black holes to the passage of time. Here are five fun facts and thought experiments based on the general theory of relativity.

Everything in the universe is connected to everything else. Yes, everything.

One of the mind bending aspects of relativity is that everything is relative to everything else. No matter how much distance separates you from another object in the universe, you are linked. To prove this, let’s consider a thought experiment.

Imagine that everything in the universe has vanished, except for you and your best friend, and one of you is spinning and the other is still. You start to wonder which of the two of you is spinning, but without any external environment for reference, it’s impossible to know for sure. From both perspectives, it appears that the other person is spinning. Your experiences are linked. And even when the rest of the universe is reintroduced, there is no reason why one of you should feel an effect different from what the other person is experiencing.

Now let’s consider that you and your best friend are free falling through empty space. With no external reference, it appears as though you are floating together. Regardless of your mass, you are falling at the same rate. If you were to drop your phone into the air beside you, it would also appear to float. Gravity behaves this way because it is the warping of space-time that drives your motion, not the pull caused by mass.

What makes this case study particularly fascinating, is that this is exactly what is happening on board the International Space Station. The gravitational pull of the Earth is nearly the same there as it is on the Earth’s surface, but astronauts and the space station are both falling towards the Earth together, giving the appearance of floating.

While perspective can change, every perspective is equally valid, so you can think of yourself as the literal centre of the universe – and be right!

The general theory of relativity predicted black holes, and we’re about to get a clear look at one.

Einstein’s new theory on gravity allowed us to predict black holes, and research now shows that there is a supermassive black hole at the centre of every galaxy – including our own. Prof. Sarah Gallagher, an astronomer at the University of Western Ontario, studies the winds from these black holes.

Typically we think of darkness when we think of black holes, but in her Orange Chair Interview, Gallagher explains that “the light that is generated in the gas falling into the black hole is so amazingly bright that it can outshine the trillions of stars in the host galaxy by a thousand times.”

Exactly what black holes look like is a bit of a mystery, but soon we’re going to get a very close look, as the globe-spanning Event Horizon Telescope, including nine radio observatories located around the world, is set to take the first direct images of the Milky Way’s supermassive black hole in the next decade.

General relativity and quantum mechanics are at odds with one another. And no one knows which one will win.

The battle between relativity and quantum has, ironically, been brewing since Einstein wrote a pair of papers in 1905 introducing both. While the special and general theories of relativity are robust descriptions of how objects behave on large scales, they fall apart when we approach very small systems, like atoms and molecules. At the same time, quantum mechanics is great on molecular scales, but breaks down when trying to describe behaviour on large scales, like planets and galaxies.

Prof. Lee Smolin, a theoretical physicist at the University of Waterloo’s Perimeter Institute, is a supporter of relativity. In an interview with the Guardian, he defends his choice saying that unlike quantum mechanics, “general relativity is not a description of subsystems. It is a description of the whole universe as a closed system.”

GPS needs the general theory of relativity to work.

According to special relativity, as objects speed up, time slows down. At speeds we’re used to here on Earth, this doesn’t normally make a practical difference, but it does make a difference for satellites, which zip around the Earth at a speed of 14,000 km/h.

But gravity also alters time. According to general relativity, clocks on satellites farther away from Earth run faster than they do on the Earth’s surface.

GPS devices use signals from a network of satellites to pinpoint locations on Earth. The signals from the satellites are transmitted continuously, containing information on the position of the satellite, and when each signal was sent. From this, the time it takes for the signals to reach the receiver gives the distance to each of four satellites, and this information is used to zero in on a location. However, for this to work, the clocks in space must synchronize to clocks on Earth.

Without compensating for relativity, these microsecond differences would reduce GPS accuracy by 11 km every day.

Everyday Einstein: GPS and Relativity
Credit: Perimeter Institute

About one fifth of Einstein’s brain is at McMaster University.