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Asked by to Aimee, Chris, Dave, Greig, Laurence on 13 Jun 2014. This question was also asked by .
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Dave Jones answered on 13 Jun 2014:
Gravity wasn’t ever really discovered, it was always there and people had always understood that things fall. Just it wasn’t always understood. Three main people contributed massively to understanding gravity: Galileo Galilei, Sir Isaac Newton and Albert Einstein.
Galileo was the first to show that the rate objects fall doesn’t depend on how heavy they are. He did lots of experiments rolling cannon balls down inclined tracks and even throwing them off the tower of Pisa! They repeated one of his experiments on the Moon during the Apollo 15 mission and they recorded it! https://www.youtube.com/watch?v=KDp1tiUsZw8
Sir Isaac Newton was maybe even more important. He found a mathematical way to explain how gravity works and that is fundamental in science (or physics at least).
Finally, Einstein showed how gravity works in very extreme cases, like things moving really quickly or with extremely massive objects, and how gravity affects beams of light.
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Laurence Perreault Levasseur answered on 14 Jun 2014:
Like Dave said, gravity has been around for much longer than humans have, and I don’t know about you, but for me it took only a couple of attempts at walking and falling face ahead when I was few months old to discover that gravity exist. So I think your question is not really about discovering that if I drop a pen it’ll fall on the ground or if I try to walk on a wire there’s a good chance I’ll get hurt (well, I’m not very good at that stuff), but more about discovering WHY things fall to the ground.
Like Dave said, this happened in many different steps. First a lot of people (like Galileo, Keppler, Newton…) tried to understand HOW things fall to Earth, or HOW planets turn around the sun. That means finding things like heavy and light things fall to the ground as quickly as each other, but also (and more importantly) the mathematical equations they obey as they fall. So, for example, Newton discovered that planets turn around the sun following a curve called an ellipse.
That worked great. People used these equations for more than 200 years. But people still didn’t understand WHY gravity works like this, WHY objects obey those mathematical equations. That’s where Einstein come in. His theory of general relativity proposed an explanation that answers this WHY question. And the amazing thing is that his theory makes predictions very similar to Newton’s for things on the Earth, or the orbits of the planets, but made slightly different predictions in very specific cases where gravity is VERY strong.
The thing is that people didn’t believe Einstein’s theory straight from the start. It had to make predictions and be tested for people to buy that Einstein’s ideas were correct. There are few important such tests, but even today some scientists spend their lives designing more such tests. (The thing is, actually, you can never completely prove a theory right, you can only prove it wrong…)
Here are some of those tests that made scientists over time think Einstein was correct:
1) Mercury’s orbit doesn’t quite follow an ellipse, and at the time no one knew why, people even thought there might me a new planet very close to the sun (which we know now isn’t correct). General relativity explains why.
2) A clock runs slightly faster when it’s on a plane or in orbit than when it’s on the ground. GPS have to take this into account to work properly!! (I find this absolutely mind-blowing!!!)
3) VERY strong gravity is strong enough to bend a ray of light (!!). That gives rise to something called ‘gravitational lensing’, which looks like: http://apod.nasa.gov/apod/ap111221.html
The blue arc is a normal background galaxy, it’s just been completely deformed by the gravity of the central yellow galaxy.
4) General relativity also predicts that the Universe is expanding, but I’m not sure I’d call this a test.
5) It predicts that two extremely massive stars or black holes orbiting close to each other will emit something called gravitational radiation, and this will cause them to inspiral toward each other with time. That has been tested to insane accuracy with more than 30 years of observations!
6) a lot more… -
Aimee Hopper answered on 15 Jun 2014:
Gravity exist if there is mass. As far as we know it always has, and always will, exist in our universe as we know it.
I think the real question here is why did it take so long for people to try and understand gravity…?
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Greig Cowan answered on 16 Jun 2014:
In some sense, we have always known about gravity: if you drop something it will fall to the floor. The link between the mass of the planets and the strength of the gravitational field was first described by great scientists such as Newton and Kepler as they tried to understand the interactions that they could observe in the solar system. Einstein took this knowledge to a new level when he introduced the concept of “relativity”, that linked how objects move and accelerate with the idea of gravitational attraction. These concepts are really important today in science since we using the fact that gravity can bend the path of light to map out the distribution of matter in the galaxy. It’s also important for navigation as the equations that Einstein wrote down are basically used in the GPS system to correct for the effect that the time measured by the satellites orbiting the Earth is a tiny bit different to the time measured on Earth.
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