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# why do light have the tendency to manage time? Answered

the speed of light is different in different mediums.when light enters in a more or less denser medium,it is deflected from its path,that is,it changers its path.it does so to maintain that time which would be reqired to cover the same distance in the absence of that medium.the question is why does light do so?,e.i.why light manages its time?

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It seems that English is probably not your first language. I think that you are trying to understand the "variational principle" (use that as the search term in Wikipedia). Light (classically) is a wave phenomenon, like sound or water ripples. Waves propagate in such a way that they follow the path of "minimum time" between their source and destination, which is just what you describe above. But how does light "know" which path to take? Seems kind of magical, doesn't it?

Feynman, back in the 1940s, found a great way to explain this. He was working in the context of quantum mechanics, but it applies to classical light waves, too. When light leaves a source, don't think of it as travelling in some special direction. It travels in all directions. Now, work out all of the possible paths that light could take (going in straight lines) to get to the destination. Next, keep in mind that light, being waves, can interfere -- a wave at the top of a cycle will combine with a wave at the bottom of a cycle to make zero.

You'll need to do some fairly complex math, but it turns out that for all of the paths very far away from the minimum-time path, interference makes them cancel out with one another. But the paths close to minimum time interfere constructively (all the top-of-cycle bits line up, and add together). The end result, when you sum up the effects of all the possible paths, is that the only path you actually see is the minimum time one.

Well, I just explained it to my 13 year old thanks to that. Cool.

Woo hoo! That's awesome, Steve. I wish I could take credit, but I've spent much of my career reading Feynman in order to learn how to explain this stuff. Thank you for the followup :-)

How well verses are you in General Relativity ?
Steve

It's not a relativistic issue. It's just the variational principle.

Well I suppose Fermat's principle states it, but that doesn't really explain WHY . Its underpinned by quantum Electrodynamics isn't it ? I seem to remember Richard Feynman made a lot about it in his book about QED.

Steve

Yes, exactly! The path integral formalism is what explains it, and makes it true for all particles. The cool bit is that you can use the same logic with purely classical EM waves to derive linear ray optics.

+1. By the way, Einstein's own book on relativity is surprisingly readable...

Because Chuck Norris wants it that way.

Cause time has places to go and people to see.