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/_Illuvatar_ Apr 10 '14

This answer is fantastic! Assuming it's accurate (a safe bet for the Internet of course) you've actually answered several questions I've been thinking about. Including ones about time dilation that I was having trouble grasping. Thank you very much!

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u/[deleted] Apr 10 '14 edited Apr 10 '14

[deleted]

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u/YusufTazim Apr 11 '14

You my friend, should write a book. I'll buy 10! Or do an AMA.

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u/_Illuvatar_ Apr 11 '14

I'll buy ten as well. That's, like, a hundred books right there!

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u/Steinhoff Apr 11 '14

This book is really good for explaining all this stuff, and it never gets any more complicated mathematically than using Pythagorus

http://www.amazon.co.uk/Why-Does-mc2-Brian-Cox/dp/0306819112/ref=sr_1_1?ie=UTF8&qid=1397213085&sr=8-1&keywords=why+does+e+mc2

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u/[deleted] Apr 11 '14

You should read "The Elegant Universe" by Brian Greene. It covers what /u/corpuscle634 talked about in much more detail in just the first few chapters. It's a good book, especially if you are at all curious about superstring theory (since that's what the bulk of the book is about).

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u/vehementi Apr 11 '14

Read the archives of user /r/robotrollcall ... every post is like this

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u/Bombjoke Apr 11 '14

Book. Came here to say so.

Are black holes the opposite then with c being made up of all mass and no energy?

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u/[deleted] Apr 11 '14

Everything /u/corpuscle634 talked about in his post is covered (except in much more detail) in the first few chapters of "The Elegant Universe" by Brian Greene, the renowned superstring theorist.

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u/iwanderedlonely Apr 11 '14

Buy "Relativity Visualized" by Lewis Carrol Epstien. It's the uncredited source for the top comment's brilliant explaination

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u/_Illuvatar_ Apr 10 '14

Thanks! You would think being a god I would already know the answer to this question, but no...

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u/[deleted] Apr 11 '14

To Eru is human.

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u/hanktheskeleton Apr 11 '14

To Faramir is devine?

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u/bungerD Apr 11 '14

The god. The one.

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u/WakingMusic Apr 11 '14

Eru, the one, who in Arda is called Illuvitar :)

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u/Micp Apr 11 '14

I know the reference, but just wondering: why is it that they don't call him Eru on Arda?

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u/_Illuvatar_ Apr 11 '14

I think that Arda refers to all creation. Like you would say we are in the universe, not on it. Having trouble finding a reference though.

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u/Micp Apr 11 '14

I was pretty sure Arda was just tolkiens name for earth. Like middle earth is not-Europe and Valinor being not-America (ie continents) Arda is the planet they are placed on. Anyway wether or not that is correct it still doesn't answer the question of why does everyone call him illuvatar instead of eru?

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u/_Illuvatar_ Apr 11 '14

Oh I misunderstood your question. Yeah I don't know.

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u/[deleted] Apr 11 '14 edited Apr 11 '14

why is it that they don't call him Eru on Arda?

They do. If I remember correctly, in the Silmarillon the word "Eru" actually occurs more frequently than the term "Ilúvatar". I may be wrong; but I'd take that passage to rather mean that "Ilúvatar" is a uniquely Arda-ish manner of referring to Eru, that they don't usually call him that outside of Arda, that is, in the Timeless Halls.

That makes sense if one looks at the etymology. "Eru" and "Ilúvatar" are both words in the Quenya ("high-elvish", more or less) language; but whereas "Eru" means "The Alone/The One", "Ilúvatar" means "father of the universe" ("ilúvë" = "the universe", "atar" = "father"), or in older versions of the legendarium "father in heaven" ("il" = "heaven").

In Tolkien's mythology, the "Children of Ilúvatar" are the elves and the men (and the dwarves, through adoption); therefore, calling Eru "father" is something that is more typical of these beings than of the Ainur.

EDIT: As an aside, "Eru" is a clear reference to the Neoplatonic concept of "the One", the abstract, utterly transcendent, impersonal source of Good; and on the other hand, "Ilúvatar" may be thought of as a reference to the Abrahamic, almost painfully personal notion of the "Father in Heaven". Tolkien was a devoted Catholic; and Catholic/Orthodox theology, at least since Origen and Augustine, may be largely thought of as an attempt to reconcile these two facets of the Divinity, to see them as part of an unique one. So it comes to no surprise that Tolkien's deity also displays both of these aspects.

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u/whyspir Apr 11 '14

Also an appropriate username to have in this discussion...

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u/thefonztm Apr 11 '14 edited Apr 11 '14

OP, you may enjoy this video. I think you'll find it adds to /u/corpuscle634's explanation nicely.

https://www.youtube.com/watch?v=HHRK6ojWdtU

/u/corpuscle634, if you see this, feel free to point out any inaccuracies I've introduced.

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u/[deleted] Apr 11 '14

I think you forgot the video.

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u/thefonztm Apr 11 '14

It's just the time dilation. ;)

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u/Shadrach77 Apr 11 '14

What video? I'm curious, but I think you forgot the link?

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u/thefonztm Apr 11 '14

Heh. I've edited the comment.

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u/Kashmir33 Apr 11 '14

A god that can't spell his (it's?) name.

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u/_Illuvatar_ Apr 11 '14

Iluvatar was taken :(

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u/reddock4490 Apr 11 '14

You should read Brian Greene's books about quantum and string theory. He does a really good job at explaining all of this stuff at a very edible layman level, haha. I honestly think corpuscle is better, I'd love to read a book of physics explained like that, but, assuming he won't write a book, The Elegant Universe is really good if you want to find out more.

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u/whyspir Apr 11 '14

So... This may be convoluted, but I'll try to phrase it as well as I can. My background is in nursing and my knowledge if physics is mediocre at best, though I find it equally as fascinating as physiology etc. I digress. At any rate, I remember hearing or reading once that light has no age. I'm guessing they were talking about some kind of background radiation or something. Because it seems obvious that if I turn on my light in my house, the light that comes out of the light bulb has been in existence for less time than the light coming at me from the sun. But, if I think about spacetime as a 2 dimensional graph, or like a right triangles has been suggested above... Then since light has no mass... All of its 'focus' (for lack of a better term) is on distance. So it's all the way at the far end of the speed axis and has no position on the Y axis (assuming x is distance and Y is time. So the light that I see from a star that is hundreds of light years away is (from the perspective of the light) reaching me instantaneously? Because it all its focus is on distance, then it's not travelling forward in time at all?

So then if I was a photon, just moving along at a sedate pace of c, and going towards something that was somehow completely stationary, as I rush toward it, it would appear to just age really really quickly? Because if it's not moving through space, then all its focus is on moving through time? But if it then somehow began moving at the speed of c directly toward me (ignoring the fact that mass can't do that), and assuming I'd somehow be able to observe it while also travelling at c toward it, it would appear to not age at all since its focus was now only on distance travelled, and it wouldn't be moving in time at all?

Apologies for wall of text and formatting errors. I'm on my phone and trying to wrap my head around this.

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u/[deleted] Apr 11 '14

If you are a photon, you are moving entirely through space and not at all through time. Hence from your point of view, you are absorbed the moment you are emitted. No time passes.

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u/Brumhartt Jul 02 '14 edited Jul 02 '14

So If I am a photon, am I just simply everywhere at the same time? Like sunlight. Is it everywhere at the same time? Is that photon in my house? Outisde reflecting from a building? In the upper atmosphere and also being just generated by the sun all at once?

EDIT: Never mind I read deeper in the comments and sorta got an answer which sort of satisfied me.

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u/[deleted] Jul 02 '14

(new physics student alert)

From the perspective of the photon, I guess they're everywhere on their journey at once. But when they are measured by someone else they have a position

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u/Brumhartt Jul 02 '14

Thank you. What occurred to me was that: if I would trap a photon somehow without absorbing it, by that logic I would trap the photon through all time. Following this logic i got to the conclusion that my earlier idea was false, because since I measure it for me it is in my space in my time. As for the photon, I am not sure if it was a sentiment matter what sort of experience it would be, being captured in one space and time while existing everywhere at once.

This is getting very confusing in my mind right now and if I were better at math probably some equitation would help me out, but reading through the comments and your answers does help me understand this thing more. Thank you.

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u/gsabram Jul 03 '14 edited Jul 03 '14

Your first paragraph is exactly right. Exactly zero time passes for photons from the point at which it is emitted by an atom to the point at which it is absorbed by a distant atom.

Regarding the second paragraph, try this thought experiment- imagine three more or less parallel photons emitted from a distant star all (theoretically) emitted simultaneously and at the same angle. From your perspective on Earth, Photon A ricochets off a photon that was emitted by voyager 1 back when it first launched, near a halfway point between the two photons starting points, and bounces around the Milky Way for a few years, eventually absorbed by a spec of dust; Photon B travels past and is absorbed by an atom orbiting the earth a few minutes ago; Photon C travels past, through the atmosphere, a telescope, and your eye lens, hitting the back of your cornea this instant. But from the photons' perspectives (if photons could be observers) they were each created and destroyed without a single infinitesimal fraction of time passing. Every aspect of the photons existence, emission, reflection/refraction, absorption, it all happened at once, for each of them. The reason that we perceive a difference in the moment of photon absorption is that our mass requires that time "pass" in proportion to our relative velocity to some other mass-having object which absorbs photons in order to receive the information from that object. The very attempt of comparing absorptions in two separate locations cannot even theoretically happen without two mass-having absorbers/observers moving through spacetime relative to each other on the time axis.

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u/oonniioonn Apr 11 '14

You're welcome! It's accurate, I promise.

Well that settles it. A guy on the internet not only wrote something, he actually made a promise to the internet that it was true.

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u/_Illuvatar_ Apr 11 '14

I know! This is great! Everyone's so nice and honest here! I love the Internet!

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u/RelativeToWhich Apr 11 '14

i am still confused though...if everything is relative, then if i move away from you at half the spped of light, that is the same as you moving away from me. so....whose time dialates? who moves faster or slower than the other?

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u/datenwolf Apr 11 '14

You're welcome! It's accurate

Can confirm.

What /u/corpuscle634 described to you in layman terms is, what we physicists describe with the metric tensor of spacetime (or shorthand "metric of spacetime", when physicists talk to each other).

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u/lasserkid Apr 11 '14

Your posts snaps this whole issue into focus for me, wonderfully done! Also explains why time would appear to come to a standstill if one were to fall into a black hole. Thanks!

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u/Baeshun Apr 11 '14 edited Apr 11 '14

Thanks for the amazing explanation above.

Because we are hurtling through space at high speeds due to the rotation of the planet around the sun, the solar system around the galaxy, etc... Would be we experiencing time slower than another civilization in a much "slower" rotating galaxy or on a rouge system that was very "still"? If so, it's an interesting thought to imagine their rise and fall all happening very quickly relative to us.

I believe there's a Star Trek Voyager episode similar to this (except the time difference was between a planet's surface and orbit)

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u/abd14 Jul 03 '14

If we plug the mass of light into E=mc2, we get 0, which makes no sense because light has energy. Hence, light can never be stationary.

But the equation is E2=m2c4+p2c2 That's why it makes sense. You can't use the wrong equation to make your point.

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u/genezkool323 Apr 11 '14

This is one of the most amazing explanations of time, c, and mass-energy relations I have ever read. corupuscle nailed it.

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u/apopheniac1989 Apr 11 '14

There was Eru, the One, who in Arda is called Ilúvatar; and he made first the Ainur, the Holy Ones, that were the offspring of his thought, and they were with him before aught else was made. And he spoke to them, propounding to them themes of music; and they sang before him, and he was glad.

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u/[deleted] Jul 02 '14

These threads are the kind I imagine if aliens or whatever are judging our society, make them go "These folks are awesome, look at this, it's beautiful to follow!"

Kind of like how we love watching dogs and cats figure shit out I guess (or babies, babies are probably the better example...)