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.
Events
Because of the different measures in time and space over different reference frames there can be a difference in perspective on the time between events and this can even lead to a conflict in causality.
Firstly it needs to be understood that an event is something that’s taken place in space and time. For example you’re reading this post at this time and in this place or it might be the time and place of your birth. In life there are many events. On Earth there’s no contradiction in the time of your birth and the time of you reading this. This is becasue the size of the Earth is small compared to the speed of light which is the maximum speed that information can travel.
If a spaceship travelling at 0.5c was to measure this time they would come up with a different answer between the time of these 2 events – as long as they weren’t too far away say less than 20 – 50 light years away. They would still know that the event of your birth came before the event of reading this post.
However in a galaxy over million light years away there could well be observers that if they could record these events would say that the time of you reading this came before the time of you being born. From their reference frame that wouldn’t be incorrect, the time it takes for that information to get to them is far longer than the years between your birth and now.
Let that sink in, it’s a hard concept to accept but relativity means that this must be true.
Simultaneity and Cause and Effect
To demonstrate this better let’s use the example of a Mars rover. Mars can be as little as 3 light minutes from Earth and as much as 22 light minutes. That’s the distance that it takes light to travel in one minute.
If Mars was 10 light minutes from Earth for example and NASA has a rover running around on Mars, collecting samples and taking pictures it will take signals from Earth 10 minutes to reach the Rover and equally 10 minutes for signals to get back. This is becasue radio waves and therefore the information travels at the speed of light.
If there was a danger and that danger was going to destroy the rover in five minutes there would be nothing we on Earth could do about it. Only if that danger was greater than 10 minutes away could we instruct the rover to get out of danger.
Equally if that rover was destroyed 5 minutes ago we wouldn’t know about it for five minutes.
Because of this – if on Earth we sent an instruction for the rover to drill a site or take a picture from our reference frame we sent the signal and 10 minutes after the rover completed the instruction. However if there were aliens on Jupiter another 20 light minutes further out they would see that the rover completed the instruction first and then the signal from Earth arrived. Or if a spaceship was sat exactly between Earth and Mars (provided they’re in uniform motion) these evens would be simultaneous.
This incredible nature of space and time comes about because relativity states that the laws of physics are the same for all observers in uniform motion.
© Simon Farnell 2013 – 2023
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Simon's Space 
More interesting than HS Science. 😁
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That’s what I like to hear… It’s all amazing 🙂
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🌠
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🙂
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It would be fun if there were some sort of interplanetary murder mystery story that hinged on determining the sequence of events as observed from differing relativistic frames of reference.
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That’s a good thought – the series of even would be right out.
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I LOVE your posts! They always make me think, lol
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I’m really pleased about that. Thank you 🙂
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Fascinating perspective Simon.
With this in mind it has been an incredible feat of engineering to construct the various space probes, monitor and thus guide their progress over even the comparatively small solar distances.
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