Time, Space and the Universe as we know it – Part 5 Relative Time

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.

Relative Time

In the last post the principle that the laws of physics was the same for all observers in uniform motion – even if those observers are moving relative to us. Because of this simple principle of relativity, the notion that time moves differently for anyone moving relative to anyone else came about.

This must mean though that if I was to drive at an average speed of 60 miles per hour from Portsmouth to Glasgow that time would have moved differently for me relative to someone staying in Porstmouth.

Yes – but only a little. 60mph is a speed that’s insignificant compared to the speed of light and the time difference would be so small as to not be able to notice it.

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One of these tests of this was carried out by scientists using a number of atomic clocks that kept perfect time to each other. One clock stayed in the lab and the others were put onto jet planes and flown around the world. The clocks were brought back together and the times compared and there was a difference, only a few nanoseconds – but there was a difference.

This is called time dilation.

The Twin Paradox

A perfect example of this is the twin paradox. This is a thought experiment where the principle of time dilation can be demonstrated. Twins born on Earth at the same time grow up and one decides to go onto a spaceship that will travel to another star, the other will remain on Earth. With this I need to use the term light year, this is a measure not of time but space, it’s measuring how long it takes light to travel in a year.

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The twin boards the starship and zooms off at 80% of the speed of light to a star 10 light years away. On Earth the trip takes 12 years to get there. If we want to see what’s happened it’s going to be difficult as signals travelling between Earth and the spaceship will take 10 years to get there. The ship turns around and comes back to Earth again taking 12 years to get back by Earth time – the twin get’s off the spaceship and is 10 years younger.

Ship time has moved slower on the spaceship, 12 years at 80% of the speed of light means that 7.5 years have elapsed for the other twin.

What? How can this happen?

The passage of time has moved more slowly on the ship becasue it’s been moving relative to Earth. In order for the same physics to apply in any reference frame, time as an absolute has had the be given up.
The maths around this is based on the inverse square law, I’ve not included it in this but you can find it here.

Relative Ship or Earth

If motion is relative – the question could be asked “Why doesn’t Earth time move slower – Earth is moving from the ship’s reference frame surely?”

That is true, the clue comes from the principle that the laws of physics are the same for all observers – In uniform motion.

The ship has had to accelerate, to get to 0.8c from the Earth reference frame, it’s had to turn around and come back and then decelerate. These motions are not uniform, uniform motion is travelling in a straight line and constant speed. That’s why the ship is determined to be moving faster, relative to the Earth.

© Simon Farnell 2013 – 2023

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4 thoughts on “Time, Space and the Universe as we know it – Part 5 Relative Time

  1. Good explanation Simon.
    The maddening thing is that despite over the years having read, heard and seen explanations for this effect I still can’t absorb the principal into the Intuitive, it’s like constantly looking through frosted glass.
    It’s one of the things I am still ‘linear’ on. (if ‘linear’ is the right word)


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