Gravity;
It pulls you to the ground and makes sure you don't fly off
It is the reason why things fall down
There is no gravity in space
Issac Newton came up with gravity
Easy right? What else is there to know?
Many people (including me) sometimes forget to ask themselves why when looking at this seeming simple principle force in the universe. WHY does gravity exist ??
Issac Newton
The idea of gravity was first proposed by Sir Isaac Newton in 1687 although Galileo Galilei had experimented with the idea 100 years before. Newton realized that there must be a force pulling things down and that this force is universal, meaning that the same force that kept us from flying off, kept planets rotating in their orbits.
Newtons Universal Law of Gravitation:
The gravitational force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between their centers
Equation:
G --> Gravitational Constant (6.67408 x 10 ^-11)
m and M --> 2 masses
D (sometimes referred to as r) --> distance between centers
There are 2 key factors in the Universal law of Gravitation; the first is distance.
The relationship between the gravitational force of 2 objects and the distance in between them is inversely proportional meaning that as the distance increases the force decrease and vice versa.
The next key factor is mass; this relationship is just proportional meaning that if the mass increases the force will increase as well. This law also states that gravity is universal, meaning that the same force keeping us on the ground also keeps planets in orbit around the sun and stars in orbit around the next biggest thing.
And one more thing, gravity is also a mutual force meaning that when the Earth pulls on an apple making it drop in the ground, the apple is pulling on the earth as well, just with not as much force because it does not have enough mass.
Before Newtons breakthrough, people knew little about this mysterious force. They knew that objects fall and they knew, thanks to Kepler's Laws of Planetary Motion, that planets orbited in certain ways but no one put the two together, that is what made Newton's discovery all the more revolutionary. It opened up thousands of new pathways for us to explore and discover. And together with Galilee's Law of of falling bodies (all bodies fall at the same speed in conditions with no air resistance), and laws of motion it formed the basis for classical mechanics.
Classical mechanics is the study of the motion of bodies that stemmed from Sir Issac Newton and his book Philosophiae Naturalis Principia Mathematica (1687). It has had a humungous impact on today's world including many inventions and it was a key factor in getting people on the moon and in the future, Mars. Yet the classical version of gravity has one big problem that needs a whole new branch of physics to fix
Albert Einstein
Ever heard of the german patent clerk, Albert Einstein? He (being the rebel he was) took an entirely different take on gravity and he was (surprise, surprise) entirely correct. Ladies and gentlemen, I present to you, Space-Time!
Space-time... even the name sound like it just walked out of a sci-fi movie. But the fabric o space-time is not fictional, it is very very real and the reason behind gravity. You see Einstein was not satisfied with Newtons magical force of gravity that seemingly acted without an explanation and so he used his general theory of relativity to explain it.
In his theory of Special and General Relativity, Einstein fused the 3 dimensions of space with the one dimension of time creating the mathematical model of the universe creatively titled space-time. He believed that gravity is not a mysterious force, but instead a warp or distortion in this fabric.
Imagine the sun as a bowling ball on a trampoline, the material will curve and bend towards it, and the heavier the ball the larger the curve, right? It's the same principle with space-time.
Now imagine that you roll 2 small marbles, near the dent in the fabric, they would curve inwards toward the bowling ball right? This force is what Einstein defined as gravity; a warp in the fabric of spacetime.
But let's take a step back, in his theory of Special Relativity, Einstein stated that the laws of physics remain constant for all non-accelerating observers (inertial reference frames) and that the speed of light remains constant in a vacuum, no matter the position of the observer or the light. This has been tested and proven to be true many times.
The Speed Of Light: 299, 792, 458 m / s
You know that to calculate speed you multiply time and distance but when dealing with light in a vacuum, the speed of light fixed, which means that the other variables have to change. When time changes thats called Time Dilation (see What Is Time post) and when distance changes its Length Contraction . Time and space both compensate to account for the speed of light showing that they are directly connected to each other. Time & Space --> Space-Time.
In his Theory of General Relativity, Einstein declares that gravity is the result of the fabric of spacetime being curved by the objects with mass. The more mass the larger the dent.
Over the years there has been a lot of evidence proving his theory to be true, let's look at some examples.
Gravitational Lensing
Light travels parallel to the fabric of spacetime, so when that fabric curves, it does too, bending around the object to continue its path. This effect is called Gravitational Lensing since the curve of spacetime is caused by gravity. This effect is most prominent when dealing with larger objects such as galaxies and stars. An excellent example is Einstein's Cross, a quasar (a very very bright nucleus of a star that releases massive amount of energy) in the Pegasus constellation. This quasar, as you can see in the diagram above, lies 8 billion light years away from earth, with a galaxy positioned in front of it, 400 million light years away from Earth.
The light coming from the Quasar, bends around the galaxy to continue its path making it seem like there are 4 quasars instead of one. Here is a real picture of what Astronomers observed.
The discovery of gravitational lensing was one of the first pieces of evidence behind Einstein's theory of spacetime
Frame Dragging
Imagine the Earth was submerged in a pot of honey. As it spins the honey will twist as well and so would anything in the honey (like a satellite, perhaps). This same effect is seen with all bodies and spacetime and it is called frame dragging. Austrian physicists Joseph Lense and Hans Thirring predicted frame dragging in 1918 working from Einstein's theories, and it was first observed by NASA in 1990's. After observing 2 earth orbiting satellites for11 years they discovered that their orbits had moved in (towards the earth) by about 6 feet every year because of the twisting of spacetime. But the credit of this discovery goes to the Gravity Probe B which confirmed the findings with newer technology.
The confirmation of this theory is further evidence for Einsteins General Relativity.
Gravitational Redshift
General Relativity predicts that light (electromagnetic energy) should lengthen when it leaves a gravitational hole (dent made in space-time by mass) because it needs to expend more energy to escape. But since the speed of light can't change, the frequency must.
To those who are not familiar with frequency and the electromagnetic spectrum, click here, but for now, this is the EM Spectrum.
When escaping a gravitational hole, light starts off on the right side of the spectrum, with a high frequency, but as the energy of the photon decreases, the frequency also decreases. This causes an increase in the wavelength of the photon, or a shift to the red end of the electromagnetic spectrum, hence the name gravitational redshift.
Sometimes the light will stretch out so much that it passes the visible spectrum and extends into the invisible microwave and radio wave bands.
But gravitational redshift applies to time as well. If you have 2 very precise clocks and you hold them at the same altitude, they will run the same, but if you put one much higher than the other, it will appear to go faster than the one lower down. This is because the force of gravity is weaker higher up. This also explains why a day on the moon is equal to 29.5 earth days. See my TIME blog post for more information. This 3rd crucial piece of evidence was yet another piece of proof for General Relativity.
Gravitational Waves
And last but not least, gravitational waves. These are probably the most recent and influential of the pieces of evidence mentioned here.
Gravitational waves are... well, waves, like ripples on a pond, caused my the most violent and energetic precesses in the universe, this black holes or neutron stars colliding. The waves travel through space time until they hit something... like the Earth perhaps. This effect was first proposed by Henri Poincaré in 1905 and then predicted by Albert Einstein in his theory of General Relativity then observed by LIGO (Laser Interferometer Gravitational-wave Observatory) on Dec 14 2015.
When gravitational waves pass through something, everything stretches and contracts, so without the use of LIGO you and I would not be able to notice that anything has changed, so how does LIGO work??
<--- A ring of Particles affected by gravitational waves
LIGO is an interferometer, meaning that is it 2 light beams traveling between mirrors through pipes traveling in 2 different directions so that they form a right angle (like North and East). The effect of a passing gravitational wave should stretch space in one direction and shrink it in the other. On Earth, that would cause the mirrors to move (as they respond to the stretching and compression of spacetime) by minuscule amounts, causing the distance between one pair of mirrors gets smaller, while the other gets larger. These subtle changes are detected by the photo detector
The problem is that there is a lot of interference from cities and humans that mess up these detectors so they have to be in remote locations to avoid this. There are 2 LIGO's, at opposite ends of the US to make sure that the waves observed are not flukes.
Here is a video from Piled Higher and Deeper for more information
For some more information on our favorite genius and his theories of special and general relativity, watch this
PBS Space-Time is an amazing channel with lots of in depth complicated concepts explained clearly. These 2 videos are about space-time but they have a lot more information that could be confusing. It is recommended that you watch it a few times to fully grasp these mind-boggling concepts!
2
Now, before we close off, I want to quickly introduce you to the newest theory of gravity in town; Emergent Gravity.
In a nutshell, Erik Verlinde published a paper explaining why he believed that gravity was not a fundemental force of the universe, but instead an effect of the interactions between certain types of particles. This is an over simplified version but if you are interested in the subject you can click on the links below.
Emergent gravity: New Study Reshaped our View on the Theory of Emergent Gravity
Erik Verlinde Emergent Gravity: Video
This theory is very controversial between scientists and there is evidence for both sides of the argument. If it proves to be true, many of the laws that we know today may have to be rewritten.
I hope you enjoyed reading this article and that you learned something from it. If you have any questions don't hesitate to ask in the comments below :)
Comments