Student blog: Quantum Physics I – Relativity

Quantum Physics is the study of matter and energy at the most fundamental level, i.e. atoms and particles. It focuses on how the particles, that everything in the Universe are made up of, work when it comes to physics and on a much larger scale than the tiny particles they are. Quantum physics has been studied by scientists for decades and yet only a small scale has been unearthed so far. In this essay, I have researched and summarised the basics of these discoveries. 

A Brief History of Relativity

In his quantum theory, which came out in the 1900s, Max Plank (a German theoretical physicist) proposed that energy was absorbed or emitted in small bursts, called quanta. In 1905, Plank’s led Albert Einstein to independently produce a similar theory about light: the special theory of relativity

The Twins Paradox

In the special theory of relativity, each observer has their own measure of time

In the twins paradox, one twin travels closer to the speed of light on a space journey while the other remains on Earth. 

The faster an object moves through space, the slower it moves through time compared to an unmoving observer.

Because of the twin’s motion, time will pass slower on the spaceship however the twin onboard would not feel time pass slower. Only the twin on Earth would notice time passing slower for the other twin. Therefore, the twin who stayed on Earth would have aged more and thus, be older on their return.  

General relativity

It could be said that if the Earth were flat, there are two explanations, the apple fell on Newton’s head because of gravity or because Newton and the surface of the Earth were accelerating upwards. This equivalence between acceleration and gravity does not work for a round Earth – the people on opposite sides of the world would have to be accelerating in opposite directions but stay at a constant distance from each other. However, Einstein proposed that spacetime was curved and not flat, his idea was that mass and energy would warp spacetime. Objects would try to move in straight lines through spacetime but since spacetime is curved, their paths would appear to be bent by gravity. The new theory of curved spacetime was called ‘general relativity’ – to separate it from the original theory without gravity.

One big problem Einstein realized was that gravity would be pulling everything in the universe together. And so, he introduced a new term called the cosmological constant to counter the force of gravity and create a static (non-moving or changing – still) universe. However, later observations by Edwin Hubble (an American astronomer) proved that galaxies were moving away from each other due to red shift. Red shift is when light emitted from distant galaxies are stretched and appear shifted towards the red end of the spectrum. This implies that the universe is expanding, as the distance between us and the galaxies would need to increase for the light to be shifted more and more towards the red end. This shut down Einstein’s theory of a ‘cosmological constant’ as well as supported the Big Bang theory.  

The Shape of Time

General relativity combines the time dimensions with the three dimensions of space to form what is called spacetime

The theory incorporates the effect of gravity by saying that the distribution of matter and energy in the universe warps and distorts spacetime so that it is not flat. For example, imagine a sheet of rubber. As you place a ball on it (that represents the sun), the weight of the ball will depress the sheet and cause it to be curved near the sun. If one now rolls little ball bearings on the sheet, they will not roll straight across to the other side but instead will go around the heavyweight, like planets orbiting the sun. 

Image source: https://www.secretsofuniverse.in/wp-content/uploads/2020/03/Spacetime_curvature_pillars.jpg

However, this analogy is incomplete because in it only a two-dimensional section of space (the surface of the rubber sheet) is curved, and time is left undisturbed. Nevertheless, in the theory of general relativity, time and space are tangled up. One cannot curve space without curving time as well. 

Written by Dinuli, a Year 10 Student at Harris Academy Bromley

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