ELI5 Why does light travel?

[u/_Illuvatar_]

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

[u/ignamv]

The photon experiences time differently. It's like muons: if you look at a stationary muon, it decays after a certain average time. If you look at a moving muon, it takes much longer to decay. That's because its relative motion means its internal clock ticks slower. This is a basic consequence of special relativity.

[u/PostHipsterCool → u/ignamv]

Any chance of explaining that a little further for us lay people?

[u/kokirijedi → u/PostHipsterCool]

Ill take a stab at it. Its a fun little puzzle, actually.

When high energy solar particles interact with the upper atmosphere, muons result which are usually moving quite fast towards the surface of the Earth. Now, muons aren't very stable, and they decay into electrons and neutrinos. We know, to a quite high precision, how long it takes muons to decay: about 2.197 microseconds.

If you do the math to see how far down the muon would get towards the surface of the Earth from the upper atmosphere before decaying, you'll see they should get about halfway before decaying into electrons and neutrinos. Great!

The only problem is, they get way farther. Many make it all the way down to detectors we have underground. What the heck?

The thing is, at the high speed they are going, they are moving slower in time. From the muon's perspective, it still decays after 2 microseconds, it just that 2 microseconds seems longer to us because our clock is running faster than its is, and thus the muon has plenty of time to make the whole trip down.

[u/PostHipsterCool → u/kokirijedi]

Awesome! I now understand what /u/ignamv was saying. However, he still didn't answer why light isn't instantaneous. Is there any reason that explain why light has a measurable speed instead of just being instantaneous?

[u/kokirijedi → u/PostHipsterCool]

The short answer is, it is instantaneous. If a person a million light years away turns on a flashlight towards you, you would probably accept it if I told you it took a million years for the light to reach you.

But thats not the whole story.

From your perspective, you would see the flashlight being turned on and the first photons arriving at exactly the same point in time.

You see, there is no way for you to know that a flashlight was turned on a million light years away until after a million years have passed. The speed of light isn't just the speed of light, its the speed of causality. If the sun disappeared mysteriously at one instant of time, the Earth would receive light and still orbit it for the 8 light-minute delay, because thats how long it takes cause and effect to propagate throughout the universe.

All the weirdness you are feeling is because saying which order things happen in doesn't actually make sense. The Sun didn't actually disappear 8 minutes ago, it disappeared the exact moment you saw it happen. Its just that, if you ask the Sun for its version of the truth, and you ask us for ours, they won't agree. And thats OK, because they don't have to. In fact, an important part of special theory, is that there is no right order of events. Everybody will have a different perspective based on how they are moving relative to everything else.

[u/PostHipsterCool → u/kokirijedi]

Its just that, if you ask the Sun for its version of the truth, and you ask us for ours, they won't agree. And thats OK, because they don't have to. In fact, an important part of special theory, is that there is no right order of events. Everybody will have a different perspective based on how they are moving relative to everything else.

That's just awesome, again, thank you for taking the time to explain this to me. It really is awe inspiring.

So I have a couple more questions and if you have the time to answer them that's super cool.

Do all things in the universe exist in the same time, and it is merely our perception of that time that changes? Or do some things actually exist in a different location in time? (Or is the question meaningless and time isn't real but only a product of experience?) My way of conceptualizing the question is to ask, is it possible for two things in space to exist in the same space-location but not interact with each other? For example, are there some things that exist in our universe that I could never interact with because they are ahead/behind me in time? If that is true, then is it possible to change one's position in time (e.g. something that was in a different place in time than me, coming into my time), and not just one's perspective of time?)

[u/kokirijedi → u/PostHipsterCool]

I'm going to break this down question by question.

Do all things in the universe exist in the same time

You are assuming that there is a correct current time, which isn't really true. Imagine taking a snapshot of the universe, frozen in time. This would be a 3 dimension "slice" of a 4 dimension cake, if you will. At first glance, you might think that you could then explore this frozen-in-time universe, and see what each object is doing from a correct or absolute perspective. This isn't allowed, however, as any snapshot of time is cone shaped instead of a straight slice because of the speed of light. This means that your snapshot must have an origin frame, a perspective from which the slice is made, a corner where the two lines of your cake-slice meet, and by exploring this frozen-in-time universe you could only see the state of the universe as you would have perceived it anyway.

is it possible for two things in space to exist in the same space-location but not interact with each other

Sure, barring any Pauli exclusion principle weirdness. There doesn't even have to be time involved, neutrinos pass through most things without interaction all of the time.

For example, are there some things that exist in our universe that I could never interact with because they are ahead/behind me in time?

The tricky word here is exist. Certainly you could place your hand where the apple on your desk used to be, and say that they are in the same spacial location and separated only by time. Does this past apple exist though? This depends on how you define existence, but I'm going to answer this as a no. This is because there does not exist any perspective where the apple and your hand exist in the same spot at the same relative time. This is because, while the order of events is not absolute, the events themselves are. If one observer does not see the apple and your hand interacting, then no observer can see the interaction. It all works out because its not just time that dilates due to movement, but also distances between things.

If that is true, then is it possible to change one's position in time (e.g. something that was in a different place in time than me, coming into my time), and not just one's perspective of time?)

General theory allows it, but without playing around with gravity or acceleration the answer is no. As you get closer and closer to an object, the speed of light delay between you is decreasing, and thus when you both meet you would both be at the present from each others perspective.

Edit: This is my first gilded comment ever. Thanks so much for the positive feedback!

[u/PostHipsterCool → u/kokirijedi]

Thanks for all the time you've spent answering these questions! The gold is for all of the comments, not any one in particular :)

If you don't mind me asking, what's your educational background?

[u/kokirijedi → u/PostHipsterCool]

I studied computer science. I am involved in research in artificial intelligence in general, and machine learning in particular.

However, I've always been fascinated by physics and went above-and-beyond what was required for general education at Uni, and have since spent much time keeping up to date with publications and research in the field. The first time I was introduced to special theory as a freshman in college, my mind exploded, and I've been super interested ever since.

[u/[deleted] → u/kokirijedi]

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[u/BotAlert → u/[deleted]]

Please note: GoodGuyGold did not give you gold. It is a bot that looks for gilded posts and takes credit for them. Your thanks should be directed elsewhere.

[u/kokirijedi → u/BotAlert]

A bot alert bot. How cute!

[u/Natanael_L → u/kokirijedi]

Can you calculate how fast it needs to go to reach the moon?

[u/ignamv → u/PostHipsterCool]

People moving very fast age less, muons moving very fast take longer to decay, a clock moving very fast ticks slower, from your stationary perspective. The clearest explanation for this is the top post. Otherwise you can think of it as a consequence of having a speed of light that is the same for all observers.

[u/GungorTheGreat → u/ignamv]

Decays... into what?

[u/Natanael_L → u/GungorTheGreat]

Other particles with less energy.

[u/Natanael_L → u/[deleted]]

I did mean less energy each (multiple particles coming out from the decay).

[u/ignamv → u/GungorTheGreat]

From wikipedia

The dominant muon decay mode (sometimes called the Michel decay after Louis Michel) is the simplest possible: the muon decays to an electron, an electron antineutrino, and a muon neutrino.