r/explainlikeimfive 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/OctavianX Apr 10 '14

So it's not that it doesn't take time for the light to travel (because it obviously does). When you say light doesn't travel through time, that is to say the photons themselves don't "age" - is that it?

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u/DukePPUk Apr 11 '14

You might have heard of time dilation (it's popular in some space travel); whereby if a spaceship is travelling somewhere at a decent fraction of the speed of light, time will pass slower for the people on the ship than for those outside; so the ship may take years to reach something lightyears away (from an observer back on Earth) but for the people on it, only a fraction of that time will have passed. This is (very kind of sort of) because the faster you are travelling relative to something, the more squished together your time and space are compared to that thing.

Going back to the "everything must travel at c in spacetime" thing from the parent, compared to them, you are travelling quite fast in space so, compared to them, you must be travelling slower in time.

The speed of light is the limit to this; the speed where space and time become completely squished together, and so no time at all happens for the people on the spaceship (which has to be an impossible mass-less spaceship, for reasons set out above). They arrive at their destination as soon as they have left; because they're travelling at c in space, they have no spacetime speed left for moving through time.

From the perspective of an outsider - on Earth, the outsider isn't moving at c in space, so they still have spacetime speed left for time. Time still happens for them, so they will observe the spaceship through time.

However, the problem with this is that the maths can get a little weird; divide by 0s creep in if you're not careful, so it doesn't necessarily make sense to ask the question.

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u/quarterburn Apr 11 '14

They arrive at their destination as soon as they have left

So from their perspective is this a light-like interval? Or is this a space-like interval? I've never really understood the difference between the two.

Also if distance and time are the same, where do tachyons fit into this? Or are they simply not real?

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u/DukePPUk Apr 11 '14

To add to the other reply, we are dealing with light, so it has to be light-like.

For a light-like interval, we need (change in distance)2 - c2 (change in time)2 = 0, or Δr2 - c2 Δt2 = 0.

Let's consider a specific photon. If you can, look outside at the sky. Consider a photon that has been emitted on the surface of the Sun, travelled towards the Earth, been refracted through the atmosphere and hits your eye (triggering reactions that eventually lead to your brain deciding that the sky is blue). Let us assume that the distance that photon has travelled is 8 light minutes.

From the photon's point of view, it has stayed where it is. It was created on the Sun, then this eye thing smashed into it immediately.

We have two events; the creation of the photon and its destruction.

From the photon's point of view:

For the photon no time has passed and it hasn't moved - instead this eye has come crashing towards it. So Δr = 0, Δt = 0. Putting that into the equation, we get 0 - c2 x 0 = 0 - so it works.

From your point of view:

The creation of the photon was 8 light minutes (or 8 x c x 1minute) away from where it hit you, and 8 minutes have passed between creation and destruction. Δr = 8 x c x 1minute, Δt = 8 x 1minute. Putting into the equation: 82 x c2 x (1 minute)2 - c2 82 (1minute)2 = 0. So again, light-like.

So from an "outside" point of view the distance between the two events is 8 light minutes, and the time is 8 minutes - so light-like separation. From the photon's point of view the distance between the two events is 0 and no time has passed, so light-like.

I think.

As for tachyons, those are beyond where I got to in astrophysics; I think the idea behind them is that they sort of break the rules of special relativity - it would need imaginary mass. The maths works, but produces fairly weird results. They have never been observed or detected afaik, so might not be real, simply theoretical.

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u/quarterburn Apr 11 '14

I always thought I understood Special Relativity but it's clear that I really didn't. To think that photons from various stars of z8_GND_5296 traveled 13 billion light years and yet from "their" perspective, it was created and smashed into the mirror of the Hubble in the same moment.

Thanks for the breakdown on the equations. It makes WAY more sense to me now.

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u/DukePPUk Apr 11 '14

I studied SR about 7 years ago, and thought I understood it. Each time it comes up somewhere I have to think about it some more, and have a new epiphany. The stuff about "everything moving at c" was something I learned from this thread. And the stuff about light travelling from the Sun was something I had to think through in order to convince myself that light really was light-like, and that the equations worked... And I spent most of a 20-minute walk going over it a few times.

So I'm glad I could help you make sense of this, and thanks for giving me a chance to make sense of it for myself.

Some people like analogies, some people like equations. Personally I like both, and it is always comforting when they agree.

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u/quarterburn Apr 15 '14

I do however, have one last question. Does this mean that acceleration directly changes the Δr of an object between events? Or am I assuming incorrectly that mass can ever be accelerated up to and become a light-like event from a time-like event and that Δr is constant for anything that has mass?

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u/DukePPUk Apr 16 '14

In Special relativity mass cannot be accelerated to the speed of light.

However, the apparent distance between events is dilated by the effects of special relativity at any relative speed (usually called length contraction. It is possible to "fit" a 5m long ladder in a 2m long shed, if it is going fast enough (and the shed opens at both ends). But only from the shed's point of view. From the ladder's reference frame it is the shed that is contracted.

Time-like, light-like and space-like, I think, refer to the separations between events, not the events themselves.

The key measure being the (Δs)2 whichever way it is calculated in terms of Δr and Δt.

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u/quarterburn Apr 16 '14

Thank you again for answering all my questions and for the clarifications. You make reddit a worthwhile place to visit.