I learned this today. Nobody knows why gravity exists. They can explain what happens, but not really why. Gravity can be observed, but not completely explained.
Gravity is the force whereby objects with mass are attracted to each other. This force is related to the mass of the objects and the distance between them. The sun attracts Earth, but Earth also attracts the sun. The force of Earth on the sun is far less because it is so much smaller, but there is an effect. All bodies in the universe attract each other, even bodies like galaxies that are light years apart.
Gravity is quite often explained by dropping a heavy ball onto a sheet. The heavy ball pulls down the sheet and any other bodies roll towards it. This represents space, which can be “pulled down” by heavy bodies, and other bodies roll towards them. Unlike with the demonstration of the balls on the sheet, the bodies in space that roll towards the larger objects have far more speed and roll around the object without contacting it.
Gravity also bends light, which is how black holes and dark matter can be detected. The same curvature of space pulls light photons towards heavy bodies. The light photons are travelling too fast to do anything more than bend. This is called gravitational lensing. It is a way of detecting gravity.
We can use gravity to detect objects that we can’t see. Watching how light bends is a way of detecting black holes and Neptune was discovered after astronomers noticed a gravitational pull on Uranus and theorized that there must be another large planet there.
Despite being able to detect gravity, nobody can explain why it happens. It is obviously a force and it is the weakest of the four fundamental forces. The others are electromagnetic, strong, and weak. It might not seem it if you have just jumped off something and are plummeting towards the ground, but gravity is very weak. It is 1029 times weaker than the weak force, 1036 times weaker than the electromagnetic force and 1038 times weaker than the strong force. It is so weak that when we get down to the level of atoms and molecules, there is no gravity.
A research team in China measured the speed of the force of gravity and it is the speed of light. That means, if the sun suddenly disappeared, the Earth would keep orbiting for 8 mins 20 seconds, which is the length of time it takes light to reach Earth from the sun.
Scientists cannot completely explain why gravity exists, but there are some theories. The two main theories seem to be gravitational waves, or particles called gravitons.
Gravitational waves are basically ripples in space time. If you think about the sheet and the heavy ball experiment again, when you drop the ball onto the sheet, it makes ripples. A body in space does the same thing. The sun is pulling down space time and making ripples. These ripples travel out from the sun at the speed of light and pull other bodies in, in the same way that ripples on a pond pull in floating things. In 2015, a team of astronomers detected gravitational waves. 2 black holes of 29 and 36 solar masses merged about 1.3 billion light years away. The new black hole was 62 solar masses, which meant that 3 solar masses of energy had been released as gravitational waves. The astronomers were able to detect this for the first time and in 2017, they won the Nobel Prize for the discovery.
The graviton is a theoretical particle that would have zero mass and would be responsible for the gravitational force. It must have zero mass because gravity travels at the speed of light. Some astrophysicists believe that gravitons are released with gravitational waves. The problem with this theory is that it is impossible to detect or observe gravitons. The only way to test for them would be at the Large Hadron Collider at CERN and they would be looking for missing energy after a test. Energy cannot be destroyed, so, if some energy was missing, that might mean it was contained in an undetectable graviton. However, it could also be contained in several other undetectable theoretical particles. We may never know if gravitons are real or not.
So, gravity is the pull of one body on another. It is affected by the mass of both bodies and the distance between them. Gravity can be easily detected, but nobody really knows why gravity exists. And this is what I learned today.
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Photo by Tom Fisk: https://www.pexels.com/photo/duo-of-sky-divers-falling-down-11124980/
Sources:
https://en.wikipedia.org/wiki/Gravitational_wave
https://en.wikipedia.org/wiki/Graviton
https://en.wikipedia.org/wiki/Gravity
https://en.wikipedia.org/wiki/Speed_of_gravity
https://spaceplace.nasa.gov/what-is-gravity/en/
https://www.sciencefocus.com/space/gravity/
https://www.livescience.com/1770-greatest-mysteries-gravity.html