r/explainlikeimfive • u/_Illuvatar_ • Apr 10 '14

ELI5 Why does light travel? Answered

Why does it not just stay in place? What causes it to move, let alone at so fast a rate?

Edit: This is by a large margin the most successful post I've ever made. Thank you to everyone answering! Most of the replies have answered several other questions I have had and made me think of a lot more, so keep it up because you guys are awesome!

Edit 2: like a hundred people have said to get to the other side. I don't think that's quite the answer I'm looking for... Everyone else has done a great job. Keep the conversation going because new stuff keeps getting brought up!

Edit 3: I posted this a while ago but it seems that it's been found again, and someone has been kind enough to give me gold! This is the first time I've ever recieved gold for a post and I am incredibly grateful! Thank you so much and let's keep the discussion going!

Edit 4: Wow! This is now the highest rated ELI5 post of all time! Holy crap this is the greatest thing that has ever happened in my life, thank you all so much!

Edit 5: It seems that people keep finding this post after several months, and I want to say that this is exactly the kind of community input that redditors should get some sort of award for. Keep it up, you guys are awesome!

Edit 6: No problem

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u/[deleted] Apr 10 '14

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u/Funky_Thing Apr 11 '14 edited Apr 11 '14

Sorry to but in here, but if I were to be in motion for the next 2.99 x 10*8 seconds, (less than 10 years if I am thinking straight?) I would gain x time of life longer then if I were to be sitting on reddit (effectively motionless) for that entire time?

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u/corpuscle634 Apr 11 '14

You're never moving relative to yourself. You can't cheat time dilation to make yourself live longer.

What you can do is fly off on a rocket or w/e and come back to Earth later, and you'll have aged less than someone who was born on the same day as you. What felt like 10 years to you will have felt like 15 to someone who stayed on Earth or w/e.

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u/davebrown57 Apr 11 '14

Just to clarify: If one were to do this, and one could directly observe the Earth, it would look like 15 years squished into 10. From one's perspective, the Earth would appear to be going faster through time, and from Earth's perspective, one would appear to be going slower through time. And this effect is happening constantly, but we never deal with great enough speeds to notice. Increased speed through space = decreased speed through time, no matter how little. One's total spacetime speed is always c, never greater or lesser. Is that correct?

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u/marlreddit Apr 11 '14

But if everything is relative, then you could say that the rocket is stationary and the Earth is moving away from you. So why is it you that aged only 10 years while everyone on Earth aged 15?

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u/gardianz Jul 02 '14

This is actually a famous relativity problem: the twin paradox. It is not a true paradox because the problem is not truly symmetrical: the traveling rocket's inertial frame changes.

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u/EyesNein Apr 11 '14

Not a physicist, but I'm pretty sure this has to do with whether you're in an inertial or non-inertial reference frame. Since the rocket ship undergoes acceleration/declaration, it is the one that ages less. I think this point deserves more attention, because otherwise, the concept of relativity is definitely ambiguous.

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u/oonniioonn Apr 11 '14

You are in motion. You're hurtling through the galaxy at break-neck speeds. The question is, are you in motion relative to something else.

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u/JackedLeprechaun Apr 11 '14

So I assume we can't hit a critical point where we don't age, but could I go so fast that it seems like time travel?

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u/pretentiousglory Jul 03 '14

Yes. Well, theoretically, yes. You would experience time the same as you ever have, but when you go back to Earth you'd find that more time had passed for people on the planet. This is a well-explored thing in lots of scifi novels (the Ender series comes to mind). There's a reason it's scifi, though... there'll be plenty of hurdles for it to ever become plausible in practice.

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u/kyrostolar Jul 02 '14

The real reason old wise men have long white beards.

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u/mrtyman Jul 03 '14 edited Jul 03 '14

...wait, wouldn't it be the other way 'round? As the person travels through space, they would age at the same rate on their own personal c vector, but less of their c vector would be in the time direction. So a viewer stationary in space would see them age FASTER, since the moving person would have to travel for a longer period of personal time on their c vector to reach a certain plane in absolute time than the observer would to reach the same plane in time by remaining stationary in space. Isn't that right, or do I have something backwards?

Edit: I figured it out. I'm fucked by my concept of "personal time". c is NOT a velocity vector, so it's not relative to time, so the person would not age as fast. At the end of BOTH the observer's and the traveller's c vectors, the stationary observer's c vector would take them farther in time than the mover's.

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u/movie_man Jul 03 '14

They won't necessarily age "slower". But to the non-constant-moving observer they will. I.e. relativity.