I saw this webpage on the internet yesterday about Einstein's Theory of relativity... I'd like to summarise what i learnt here...
Its says...
If u are travelling on a train that goes at 100km/h and u jump forward at 20km/h... How fast are u travelling to an observer beside the tracks? Logically... u'd be travelling at 120km/h, which is the total sum of the speed of the train + ur velocity... But to you, you are travelling at only 20km/h relative to the train.
Ok, so now einstein asks a question, If u are sitting on a train travelling at half the speed of light, 0.5c, and u shine a beam of light forward, what will be the speed of that beam of light observed by someone beside the tracks? Again logic tells you that it would be 1.5c, the sum of the speed of light and the speed of the train right? But appearantly, to an observer, the speed of light is still c.
Ok, then what would the speed of light be to u, who is sitting inside the train? In that case, it should be 0.5c right? Since the train is moving at halve the speed of light, so relative to the speed of the train, light should travel at half its speed right? Einstein tells us it is not the case. The speed of light observed by you would still be c.
Although this doesnt really make any sense to me... Einstein claims that this is because both you and the observer by the tracks perceive time differently...
So to illustrate this, he says that if u are travelling in a spaceship moving at the speed of light, and u bend down to tie your shoelace, to you, the time taken to tie your shoelace will still be 2 minutes, but to an observer on earth, it seems like you are taking hours and hours to tie your shoelace. So, if u travel outwards in space for 1 year at the speed of light and come back to Earth, to you, it only took 2 years. But to the people on Earth it seems like you were gone for hundreds of years. So you have actually travelled hundreds of years into the future...
Personally, i think there seems to be something wrong with the flow of logic:
1. I think time should be the same for both you and the people on Earth. So if you go into space and come back later, the time on Earth will still be 2 years later, it is just that you have travelled faster then the image of your spaceship... its the image of your spaceship that takes hundreds of year to come back to Earth. I dont understand why either since you are travelling at the same speed as your image, so logically you should return to Earth at the same time...
2. If you travel at the speed of light directly in front of the person on earth, he probably wont be able to see you, because light isnt able to catch up with u, thus ur space ship wont be able to reflect any light. So i think the spaceship can only be observed at an angle... and in that case... i dun think you would be able to see the image until some time later, because u will need to wait for light to catch up with the spaceship and reflect back into your eyes...
Check out this website if you think I've misinterpreted what I've read... http://www.pbs.org/wgbh/nova/time/think.html
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