Time, Space and the Universe as we know it – Part 2 Relative Motion

With all it’s vastness and complexity it’s taken a long time for humans to reach the understanding that we have of the universe that we have. This is the story of evolving understanding and how simplicity gives way to incredible truth’s that seems to sit at odds with how we think life should be… and we’ve only just begun our journey of understanding.

Galileo, Telescopes and Uniform Motion

In the last post on the ancient understanding of the universe the stage was set for the next big step in understanding the cosmos as mankind’s thinking moved away from the geocentric model of the cosmos through to the sun centered cosmos where the planets moved in ellipses.

Galileo developed the fist telescope and one of the things he observed was Jupiter and what he was 4 little dots or moons around Jupiter and moved about the planet. This was a hammer blow in science at the time becasue it was another example of a cosmology like the Earth where these moons moved about Jupiter not the Earth and in confirmed the Copernican model of the cosmos becasue not all things moved around Earth.

Jupiter and the 4 ‘Galilean’ Moons

Galileo also observed the phases of Venus, the reason for these phases is similar to the reason for the moon’s phases – the position of the object in relation to the Earth and the Sun causes these phases. In the case of Venus, such phases also confirmed the Copernican notation that the Earth is not the centre of the universe.

Lastly Galileo’s observation of sunspots was direct evidence that the heavenly realm was not perfect. The notion of the perfect heavenly realm was no longer a reality.

The most relevant of Galileo’s discoveries though led to the laws of inertia. Up until that time the thinking was that the natural state of motion on the Earth was to get as close as possible to the centre of the Earth but what Galileo discovered was that it doesn’t take a push to move an object – say a metal ball. If we take the example of a trough and roll the ball inside it by tipping it then it will move. Galileo supposed that if air resistance and friction were to be taken away then that ball would keep rolling for as long as the trough is. Even if the trough was really, really long like miles long it would keep going.

From this Galileo stated that the natural state of motion was to move in a straight line at constant speed. If something is moving in a straight line and then changes direction, speeds up or slows down then a force has been acted upon it and has taken it out of it’s ‘state of uniform motion’. Perhaps the most well known example of this is the Voyage space probes – they’re moving through space in an almost straight line (depending on what gravitation forces are acting on it) and at constant speed and will do forever until something stops it.

For his discoveries Galileo was declared ‘heretical’ by the Catholic church as they were against the holy scriptures – he spent the later years of his life under house arrest and not allowed to teach or publish his work.

Newton’s Universal Gravitation Laws

The legendary story of Issac Newton is well known, an apple drops from the tree and he ‘discovers’ gravity. This story is however missing some context, gravity as a force that pulls all object towards the Earth had been known about by the earliest cavemen. What’s Issac Newton connected was how the force that pulled the apple to the ground is the same force that pulls the Moon from it’s uniform motion into a path that takes it around the Earth.

Photo by Zen Chung on Pexels.com

Because of this pull the Moon is essentially ‘falling’ towards the Earth in the same way that the apple was only becasue of the same force is pulling them both out of their natural state of motion from a straight line at constant speed. He supposed that if air drag was ignored if an object was to be ‘launched’ along the Earth fast enough it would come back around the Earth and keep circling. From these observations Newton came up with the universal laws of gravitation and from this he verified Keplers observations and the fact that orbits are not perfectly circular.

Newton’s universal law of gravitation was far more than just a law of gravity, it unified the movement of things on Earth and and things in the heavens. It meant that terrestrial and celestial motion followed the same laws, they were no longer separate and no longer divided. It meant that once again Earth was not a special place in the cosmos – it was merely one part of a whole cosmic clockwork.

Laws of Relative Motion

Because of this unifying law from Newton the Laws of Relative motion were born – combining Galileo’s laws of inertia and Newton’s laws of gravity it meant that these laws applied to all the cosmos. It simply states that the laws of motion (which at the time were the only laws of physics) are the same for all the universe as long it’s travelling in a straight line and at constant speed.

If a Mars alien had a Martian fruit drop on it’s head the same laws applied – provided they’re moving in a straight line and at constant speed.

In effect what his law means is that anyone in motion can use their ‘reference point’ or their perspective as the basis for saying something is in motion. So if a train is being driven at 50mph through a station, someone sitting on the station platform can say ‘That train is moving at 50mph relative to me‘ and the driver looks at that person sitting on the platform can say ‘that person is moving at 50mph relative to me‘.

What should be avoided is to make the statement ‘That train is moving‘ – this statement is now meaningless as it has no reference.

There is a slight issue with the statement of ‘In a straight line and at constant speed‘ when using Earth and indeed any other planet as the reference. Because they’re spinning and in orbit around the Sun they’re strictly not in uniform motion – but it’s close enough.

© Simon Farnell 2013 – 2023

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