The Implications of Gravity in Spacetime

The theories in which modern science rest heavily upon were presented hundreds of years ago by the scientists we have grown to learn about and love. Their theories have yet to be disproved, and that is why those theories are the platform on which modern science now rests. It is a strong platform, but as it ages, we must fill in the cracks. That’s where we are at in this day and age, filling in the cracks of an old foundation. Gravity is the most fundamental force in the universe, yet it is a very weak force. The series of posts I am about to share rely heavily on our concept of Gravity, so it’s important to get the basics down first.

We will start with Galileo. In a uniform gravitational field, Galileo believed that all Galileoobjects fall identically-irrespective to their mass. To prove his theory, he climbed up the Leaning Tower of Pisa where he dropped various masses. In doing so, he proved that when an object is in free fall, it will not experience a force in relation to it’s mass and that all objects, regardless of their mass will move in the same direction, at the same time. Think of an elevator- your head and shoes will “fall” at the same time, even though your head is heavier than your shoes. Galileo gave us the understanding of inertia; where an object that is set into motion stays in motion until it is acted upon by some external force.

Aristotle believed that the Earth did not move because if you threw a ball straight up in the air it would come straight back down, instead of going to the left/right etc. Galileo argued this idea giving an example of the cabin of a ship. Inside the cabin, if there are no windows, there is no way to tell if the ship is moving or not. Galileo concluded that the laws of physics are identical in all Galilean (intertial) reference frames, providing us with our first encounter with relativity.

newton

Next, Sir Isaac Newton comes along to explain the force that acts upon all objects. Newton’s first law of motion is essentially Galileo’s concept of intertia. The second law of motion tells us that the force needed to act upon an object depends on it’s mass and acceleration. If you have a large mass with a large acceleration, you will need a large force to act upon it and so on. Finally, Newton’s third law of motion state’s that for every action, there is an equal but opposite reaction.

Newton’s laws gave us a better understanding of the physical world around us. Einstein decided to apply his idea’s to the entire universe. On November 25th, 1915 Einstein
published his Theory of Special Relativity. This theory has Einsteinprovided us a profound understanding of our universe, and much of what we know has been found using Einstein’s theories. Special Relativity tells us that the speed of light is the same in all constantly moving frames and that Time slows down the faster you travel and vice versa.

To better understand the idea of special relativity, I will provide an analogy from the book “Hyperspace” by Michio Kaku:

Think of a car chasing a train that is traveling at 100 mph. Now imagine that train is a beam of light. Even when the car reaches 99 mph, according to relativity the train still speeds ahead of us at 100 mph. No matter how much we try, we can approach, but never exceed 100 mph in our car. This is the ultimate speed limit of the universe (the speed of light). Also, no matter how close we come to reaching the speed of light.. the train still seems to be moving ahead of us at 100 mph as if we were not moving at all. Say that we in the moving car measure the same velocity of the train (light beam) as a person standing on the ground. How is this possible? The only way this is possible is if time slows down for us in the car. If the stationary person looks into our car with a telescope, he will see that everything is moving exceptionally slow. However, to us, everything seems normal because our brains have slowed down too. He also sees that the car has shrunk, but we do not notice this because our bodies have shrunk too. Thus, the speed of light will always be measured the same because the faster we travel, the slower, and smaller our measuring devices become, p. 82-83.

Because all things are always moving relative to each other, space and time are relative. As a result, we have the idea that space and time are a continuum known as space-time, and we know that gravity is thought of not as a force that is pushing and pulling on things, but as the curvature of spacetime. How do we get this curvature in space? What does curving have to do with gravity?

Space curves where a mass is present. Larger masses have a larger curve- or more intense gravity. Smaller masses = less curve = less gravity. This creates an illusion in spacetime as masses distort the spacetime around it, causing time warps etc.

To imagine the curvature of spacetime think of a sheet that is stretched out by all four

OLYMPUS DIGITAL CAMERAcorners. Wherever you place a rock on the sheet, it will create a dip. Picture a large rock in the center of the sheet which creates a large dip. You then place a penny at the edge of the surface (where the sheet begins to dip) and watch it go around in a circular motion, speeding up as it gets closer to the rock. At the outer edge, where spacetime begins to curve there is less of a gravitational pull. The penny is able to leave that orbit around the rock because gravity is weak. Once it gets closer to the rock the gravity gets stronger. Outside of the dip, the sheet remains flat, allowing objects to move in a straight line, unaffected by “gravity” because there is no curvature. This is where Newton’s third law can be applied. An object in motion stays in motion until acted upon by some external force- like the curvature of spacetime, for example!

Evidence for the curvature of spacetime is the fact that light bends around a mass, acting as a lens for what lie behind that mass, which is how we can study stars and galaxies that sit behind a massive object!

light

What is most fascinating about Einstein’s theory is that it predicts that violent, energetic episodes in the universe such as the collision of two black holes, or a supernova can cause “ripples” in spacetime known as gravitational waves (GW). The public was recently informed on February 11th, 2016 by Gaby Gonzalez (who was part of the ‘hangout’ that my online physics course organized to discuss GW) that LIGO- the Laser Interferometer Gravitational-Wave Observatory in Lousianna, and Washington detected GW on September 14, 2015. The historic event was announced to the public February 11th, 2016. This will be the topic of our next discussion.
Hope you enjoyed this intro to the topics that follow explaining gravitational waves, black holes, dark matter & dark energy, and the big bang!
This post is part of a series, to find the links to other posts click here
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