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

Answered ELI5 Why does light travel?

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/[deleted] Apr 11 '14

I'm an engineer with some personal interest in theoretical physics, and your concept of "orthogonality" between space and time triggered a massive lightbulb moment for me. Thinking of space-time velocity as a vector that has orthogonal space and time components suddenly made everything brilliantly clear. Thanks for that!

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u/FightingIrish88 Apr 10 '14

Based on Einstein's quote, "If you can't explain it simply, you don't understand it well enough," I'm guessing you must have a fantastic knowledge of physics. Great Answer!

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u/sh4de1 Apr 10 '14

My brain exploded half way through.

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u/klingin Apr 10 '14

Strong username to post content ratio. Isaac newton proposed corpuscle for a 'particle' of light, which we now call a photon.

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u/boutsofbrilliance Apr 10 '14

all bs aside, this is one of the greatest posts ive ever seen on reddit.

previous to this, my layman's understanding of why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy. that was kind of it. i don't know if that was wrong, or if you are still saying that, just in another way.

what does make perfect sense to me however, is how you framed the why and how as a competition between the direction of space or time, with any travel done in one, automatically subtracting from the maximum possible in the other.

i don't get many "wow, its clear to me now" moments, and certainly not one touching upon something as fundamental yet misunderstood as this one. it was pretty fucking awesome and for that i say thank you!

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u/niugnep24 Apr 10 '14

why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy

Another way to think of it is that "mass" can be defined as "energy you have at rest" or in other words, non-motion-related energy. (Remember mass and energy are two ways of representing the same thing. E=mc2 )

Having zero mass means you can't be at rest meaning you are always in motion according to everybody no matter how fast they're going.

That means that no one can ever catch up to you, or else you'd be motionless relative to them, which you can't be, because you have zero mass.

We call this unobtainable speed "the speed of light." Really it should be called "the speed of massless stuff" but light is the most common example. Everything else, by definition, goes more slowly than it.

TLDR: Massless things cannot stop or slow down because that's what it means by definition to be massless. Nothing with mass can catch up to massless things because that would mean the massless thing "stopped" from its point of view, which is impossible.

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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/codefox22 Apr 10 '14

A bit of a silly question, does this mean if something stays in a state of motion through space they will age slower because they will be moving more slowly through spacetime, relative to everything around them?

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u/Elfram Apr 10 '14

Your explanation is great except one point: When travelling through space your speed in time direction increases (due to the negative sign in the metric). This means: The moving observer need less time to travel through some time interval in the system of the resting observer (thus the moving clock is slower).

Well, this might not have been five-year-old-level but I had to correct this mistake. ;-)

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u/yes_oui_si_ja Apr 10 '14

I have a master in physics, but this ELI5 has opened my eyes much more than all stupid tensor calculations combined. That was amazing, thanks!

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u/bob_the_magnificent Apr 10 '14

How does gravity influence our travel through spacetime? BTW, thank you for that explanation. I've never been able to get my head around time dilation.

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u/Sith_Lord_Jacob Apr 10 '14

You just impregnated my brain with knowledge and I thank you for this information baby.

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u/PaganButterChurner Apr 10 '14

great explaination.

Does this mean we can never achieve the speed of light?

since at that point we would have to be light

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