People love to drop Albert Einstein‘s theory of relativity into conversation, whether they’re discussing the latest Hollywood blockbuster or just trying to look smart. But what is the theory of relativity? You may know that it involves gravity pulling objects and some possible time dilation, but probably don’t know that it is a whole new way of describing reality itself. You may take for granted that you have to meet a friend for breakfast at your favorite place at ten in the morning, but without the forces outlined in the general theory of relativity you would meet at a completely different place at a different time.

First published in 1915, Einstein’s theory of relativity managed to solve problems physicists and astronomers had been grappling with for centuries. It’s revealed secrets about black holes, planetary orbits, and space travel. And as you learn more facts about the theory of relativity and spacetime, you’ll discover just how mind-blowing it is.

## It Was Born From Einstein’s Theory Of Special Relativity

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In 1905, Einstein published his theory of special relativity, which proved to be a pretty big deal in the mathematics and physics communities. The basic theory states that the laws of physics are exactly the same between objects moving at the same speed. The key is that those objects can’t be accelerating – they have to be at a constant speed. It also states that despite your acceleration or speed, the speed of light in a vacuum is the same for everyone.

## The General Theory Of Relativity Adds In Acceleration

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For ten years after creating the theory of special relativity, Einstein worked to add acceleration into the mix. He had determined that the laws of physics (as in, space and time) were the same for two people moving at the same constant speed. The minute one starts accelerating, however, then you run into problems. The faster you accelerate, the more of a time difference you’ll have when observing occurrences with others at different speeds.

## Gravity And Acceleration Produce The Same Effect

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In 1911, Einstein created the “equivalence principle,” which essentially states that gravity from the mass of an object or from the acceleration of a vessel are the same effect.

If you’ve ever wondered why a space shuttle launch is so hard on the astronauts, it’s because the constant acceleration needed to break the Earth’s gravity actually imposes gravitational forces on the people inside the craft. G-Force Centrifuge training prepares astronauts for the effects of that force on their bodies.

## Spacetime Is The Fourth Dimension

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In order for all these theories to work, the Universe has to be measured in a different way. Standard X and Y coordinates describe a position in a two-dimensional plane, but because the Universe is a three-dimensional space, a Z coordinate is needed as well. But there’s actually a fourth dimension at play in reality: time. “Spacetime” refers to this fusing of time and three dimensional space, and it gave physicists a whole new perspective on understanding the Universe.

## Gravity Affects Space-Time

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When it became apparent to Einstein that gravity could disrupt spacetime, it changed the constants that he was working with. When acceleration (and thus gravity) is constant or non-existent, spacetime is also constant. But when gravity changes, then you need to be prepared to change all the variables.

## Space And Time Work Together

Albert Einstein realized things tend to travel in straight lines. However, he also learned gravity could alter how a straight line might appear. In truth, the space of the line doesn’t curve, spacetime curves. The two separate concepts work together.

## Yes, You Have An Effect On Spacetime

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Just because you have a small gravitational pull doesn’t mean you don’t have an impact on the Universe. Everything with mass has a gravitational pull, which means that while the Earth is pulling you, you’re pulling it right back. But because the Earth has much more mass, it tends to win those tugs of war and keep you firmly planted on the ground.

However, everything with mass in the Universe has an impact on the other objects in it. The Moon’s pull creates the tides, for instance, and the Sun’s gravity pulls all the planets into elliptical orbits.

## Gravitational Lensing Shows That Spacetime Is Curved

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So, how do experts know for sure that spacetime is curved by gravity? Its effects can be seen through gravitational lensing. This process involves a piece of matter with gravity so massive that it bends spacetime around it to a huge degree. Usually, light isn’t as effected by gravitational bends because it moves so quickly, but when a large object – say, a black hole or quasar – creates a disruption that’s big enough, even light starts to curve. Astronomers can use this bent light to peek around massive object and see what lies behind it. It’s a perfect example of gravity disrupting and curving spacetime.

## The Theory Of Relativity Explains Mercury’s Weird Orbit

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Mercury’s orbit stumped astronomers for centuries. Sir Isaac Newton was able to accurately predict the orbits of every other planet, but Mercury proved difficult – its orbit was faster than expected. Some experts even theorized that an entirely new planet called Vulcan had an impact on Mercury’s orbit.

Einstein’s theory of relativity finally uncovered the reason behind that strange behavior. The Sun’s mass was warping space itself, causing Mercury, the closest planet to the Sun, to take an unusual path through the Solar System.

## Gravity Affects Light And Sound

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The theory of relativity also explains how gravity affects light and sound. When inside a gravitational field, mechanical waves (like sound waves) and electromagnetic waves (like light waves) tend to stretch out as they move farther away from the center of mass. This is known as the Doppler Effect.

## Gravitational Waves Are The Key To The Origins Of The Universe

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When a massive stellar event occurs, it creates noticeable ripples in spacetime called gravitational waves. This phenomenon might also help theorists discover how the Universe was created. If scientists can detect waves from the Big Bang – the most violent event in the known Universe – that discovery would go a long way to proving that it happened and pinpointing when the event actually occurred.

While only one confirmed gravitational wave has been spotted, space agencies all over the world are keeping an eye out for more.

## Quantum Mechanics And The General Theory Of Relativity Don’t Like Each Other

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Einstein’s general theory of relativity explains a whole lot of what goes on in the Universe, but it doesn’t always mesh with quantum mechanics. Quantum theories state that particles interact by sending other particles between them – consider electricity, which works by sending a photon between two charged particles. The problem is, the route that particle travels is pretty random. Tracking gravity means you should be able to tell how things travel through spacetime, but quantum mechanics doesn’t like to play by those rules.

A unified theory of quantum gravity is being sought, but no one knows when scientists will be able to reconcile these two concepts.

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