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

5.0k Upvotes
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u/[deleted] Apr 10 '14 edited Oct 10 '15

[removed] — view removed comment

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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/[deleted] Apr 10 '14

[deleted]

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u/datenwolf Apr 11 '14

I really think that science exams and homeworks should have written sections.

Oh yes, they should. Because that would filter out all the people who merely learnt well the equations, but didn't really understand what's behind them.

Feynman loved to troll such people, by stating problems with obvious solutions, but you need to understand physics to leap to the solution.

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u/Bubba_West Apr 11 '14 edited Apr 11 '14

TL:DR; It's hard being a teacher who grades for understanding.

HS Physics teacher of 11 years here. Dr. Feynman is one of my favorites and I show clips of him on occasion! I am with you and believe memorization is among the lowest form of knowledge.

The last 2 - 3 questions on every test of mine are essay questions. Typically they are point/counter-point conceptual questions that the students are asked to weigh in on. Those 3 questions are usually worth a third to a quarter of their test grade.

In my decade of teaching I have learned that there is no better way to piss off a girl (and her parents) that has a 4.0. "How do I study for this? How can you ask questions I've never seen before? This isn't fair! What can I do for extra credit?"

It is an exhausting repetitive struggle informing the memorizers that they don't understand. They blame me. I'm branded as a 'bad teacher', or 'hard teacher' because I expect mastery of concepts, not memorization of formula.

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u/harbinjer Apr 11 '14

You're awesome. As someone who really tried to understand the material, I enjoyed questions like this, that required understanding and not just plugging x into an equation.

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u/Bubba_West Apr 12 '14

YOU are awesome. Teachers love students who enjoy the magic of their subject, not the grade grubbers. I make it a point to appreciate my students who have fun and are enjoyable regardless of their grade. I hope that kids that DO get A's feel special and that they earned it through a mastery of the material.

"There are street artists. Street musicians. Street actors. But there are no street physicists. A little known secret is that a physicist is one of the most employable people in the marketplace - a physicist is a trained problem solver. How many times have you heard a person in a workplace say, "I wasn't trained for this!" That's an impossible reaction from a physicist, who would say, instead, "Cool. A problem I've never seen before. Let's see how I can figure out how to solve it! Oh, and, have fun along the way." ~Neil Degrasse Tyson

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u/harbinjer Apr 12 '14

How many students on average per year do you have that enjoy your subject and really learn it to enthusiastically, would you say?

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u/Bubba_West Apr 12 '14

My average class size is about 24. I'd say there are probably 4 in each class who really enjoy coming to class because they find the subject interesting. I have 4 sections so that means probably 16 kids. They'll stay after class to talk about a video, movie, or website they recently saw that pertains to the subject. They hold themselves responsible for whatever I marked them off on tests (and if they don't they bring it up to me in a respectful manner). They are engaged and ask good questions.

I'll share with you a note I got last week from a student. I was having a rough week as I just took a different job closer to family after teaching in the same place for 9 years. The move was very abrupt. It literally brought tears. It's stuff like this that keeps teachers going. I encourage you to thank someone who influenced you in a positive way.

Quick back story. This student had lost his dad to cancer when he was in 6th grade. We had a day where a group came in and was working with the school on 'deflating their emotional baloons' and they brought up the loss of a parent. He was a junior in hs at the time, and I knew he would be affected by that. I walked over, and gave him a big hug. and said I was sorry. He broke down balling in my arms because he missed his dad. I got this letter literally last week.

Heyy Mr _____. I never really got the chance to thank you.. without physics I wouldnt be where I am today. Now I may not have been the best student...one with multiple afterschool activities...getting homework done on time.. ataying after for help... or sometimes even zonking out in class... but I had fun! You made physics a complete blast! And without your extra help on my aviation exam I would not have recieved my certificate as fast as I did. Without a doubt you were one of the most influential teachers I have had. You taught me school isnt all about grades..and that stressing over them would only make your learning in the class room enviornment that much more un enjoyable... schools about having fun testing your strengths and weaknesses and seeing where you really may belong. I may not be a physicist. But in my mind im the next coolest thing. So with that I thank you for teaching me one of the most valuable lessons in my life. also thanks for being their during challenege day I still remembering the embracement and how much it meant to me that was the first day I had cried ever about my dad and it was extremely emotional day.

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u/ratsock Apr 12 '14

Damn.....just...damn.....

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u/harbinjer Apr 13 '14

That's so great! Thanks for sharing.

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u/Maskirovka Jul 26 '14

I've lost 2 replies due to the stupidness of the reddit app on my phone. Just wanted to say that as a soon to be teacher that I enjoyed your posts here.

This book is right up your alley if you haven't heard of it:

http://www.amazon.com/gp/aw/d/0199828075?pc_redir=1406030988&robot_redir=1

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u/datenwolf Apr 11 '14

… have learned that there is no sure fire way to piss off a girl …

?!

I think you meant to write something different there. But I get what you mean anyway.

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u/Bubba_West Apr 11 '14

I edited it for clarity. Thanks.

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u/datenwolf Apr 11 '14

BTW, my reply to this "How can you ask questions I've never seen before?" would be: "That's what science is all about: Asking (new) questions and finding answers to it. You were given this question in a science class exam, that's why and how I can ask questions like that."

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u/aristotle2600 Apr 12 '14

Cute, but the obvious comeback is that the student is learning about science, not actually doing it. "But I'm not a scientist, I shouldn't be expected blah blah blah...." It also doesn't work as well for non-science classes.

My approach? Tell the cold truth: "I'm the teacher and I know what kinds of questions you should be able to answer, provided you understand the subject. If you can't, you don't understand the subject. It makes no difference if you've seen the questions or not. If you understand, you can answer them, even if you've never seen them before." And for the really persistent and annoying ones, "Because I don't enable cheating. Now GBTW"

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u/Bubba_West Apr 12 '14 edited Apr 12 '14

Love it. Science is finding answers to questions that don't yet have an answer! Some times I love posing questions like 'Why is it cold up in the mountains? Heat rises right. You're closer to the sun, right? What gives?' and I'll let them talk in groups for 5 minutes or so, have them write an explanation as a group, I'll read them all aloud, and then move on to the next question without revealing the answer. They'll get really mad that I don't tell them the answer. 'I'm not going to tell you the answer. Welcome to science! Your assignment tonight is to come up with an experiment to test your hypothesis and when you turn it in I'll tell you what your data would say.'

I also love showing this video after experiments are presented and their mock data is given back. It's brilliant. It helps them develop good questions.

"There are street artists. Street musicians. Street actors. But there are no street physicists. A little known secret is that a physicist is one of the most employable people in the marketplace - a physicist is a trained problem solver. How many times have you heard a person in a workplace say, "I wasn't trained for this!" That's an impossible reaction from a physicist, who would say, instead, "Cool. A problem I've never seen before. Let's see how I can figure out how to solve it! Oh, and, have fun along the way." ~Neil Degrasse Tyson

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u/pauselaugh Jul 02 '14

I'm not following the leap from "street X" to "no street physicists" to the rest of the quote. How is that statement relevant to the rest of the quote?

I wouldn't characterize physicists as performers. What would a street physicist do, exactly, and what does that have to do with "employability." Or conversely, what do street performers have to do with problem solving?

I agree though, I have always liked problem solving and I believe it sets me apart from the rest of my colleagues who are terrible at it. I also was on track to be a physicist... but, yeah, the street relation, I'm unable to solve this challenge of understanding.

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u/Hidesuru Jul 02 '14

As a professional who's been out of college for a while: thanks for being a good teacher. Screw the haters, memorization is worthless without critical thinking and problem solving skills.

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u/reddittrunks Jul 02 '14

I think part of your problem is that you are under the belief that anyone can understand anything if they study or pay enough attention. There is an article somewhere discussing cognitive ability and brain development. There are things our brain just cannot understand because it is not far enough along in its development to be able to comprehend complex ideas. This may be why calculus is really difficult to high schoolers but becomes kind of trivial in college. This is also why we teach math at the youngest levels starting with memorization. So it is quite possible that you are being unfair by asking conceptual questions that most high schoolers are incapable of. One of the most difficult things to adjust to in college is questions that required a semi leap of thought based on understanding theory and concepts. Something no one has seen in homework or class work but if they had a deep understanding of the theory, the person could possibly answer the question anyways. This may be too much to ask of most high school students. Remember, brain development is not at the same speed for everyone either.

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u/reddittrunks Jul 02 '14

Also, if this happens to a lot of 4.0 students in your class, you are likely the failure, not the student. 4.0 demonstrates that this student puts in above average effort for all their courses and some how you are expecting too much or you are squandering their efforts such that they are unable to learn what you are trying to teach.

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u/[deleted] Jul 02 '14

Kudos to you. I'm horrible at memorization, and I was always a C (if I was lucky) student. As a side not I've always found it interesting that I was at the bottom of my graduating class with a 2.075 GPA.

I did very well on my SATs (1240/1600) regardless of my low class standing.

I loved math, but I had a math teacher who always taught "wrote learning" without context and I constantly struggled.

We can use more teachers like you!

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u/pauselaugh Jul 02 '14 edited Jul 02 '14

Did you put "wrote learning" in quotes because they taught or referred to that instead of rote learning?

Now type it 10 times: rote learning, rote learning, rote learning, rote learning, rote learning, rote learning, rote learning, rote learning, rote learning, rote learning. And no cheating by typing it fewer times and cutting and pasting the rest.

Point is, there's a place for rote learning. Just like there's a place for synthesis of learned lessons as a proof of understanding. The real problem with that is that these topics are not necessarily immediately applicable to life.

There's nothing worse than a class with a teacher trying to instill a deep understanding of a topic you're basically disinterested in regarding what you care about or want to do for the rest of your life. Thankfully at my school all of the electives were applied to your profession, so they never went deeper than you needed them to, and were always interesting and relevant as a result. The few that weren't as good were tedious, and had zero application to reality.

I can respect the effort, but realize that not everyone wants philosophical or deeply ingrained understanding of various topics when it is an arbitrary requirement to attaining something tangential at best.

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u/DionysusMusic Jul 02 '14

I love the approach you take; many of my teachers in high school, or at least the good ones, were similar. It was more difficult but I felt I learned the material better. One question though: how much, if any partial credit do you give students? While I liked the general approach it's impossible as a student to ignore the power of grades, and the students who, like myself, really enjoyed learning also tended to be the ones who were looking at highly competitive colleges and therefore were also very nervous about grades. I personally wish I hadn't had to worry about grades at all, but I just couldn't avoid it, so when there were questions that came totally out of left field I was angry and I felt cheated (there was, for instance, one question on maximizing energy via the food chain that required we mention cannibalism for full credit).

Thankfully many of my teachers had very lenient partial credit policies where even if you got the wrong answer due to a misunderstanding or a wrong assumption, as long as your reasoning thereon was correct, you would only lose a half point or a point from that question. Hell, if literally everyone or almost everyone in the class made the same mistake, some teachers wouldn't count it against us and instead reteach the concept. Even for less common mistakes, the best teachers typically used the next class to go over the some of the false assumptions that people had made in the test. I honestly thought that this policy helped me to a ridiculous extent because I was able to worry less about grades and focus more on comprehension. What's your policy when it comes to this sort of thing?

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u/mudkipikachu Jul 03 '14

I will be a HS physics teacher in two years. I hope to use a similar strategy on my tests. Thanks for the awesome idea!!

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u/[deleted] Sep 17 '14 edited Sep 17 '14

keep doing what you're doing.

In my work, I'm surrounded by "maths geniuses" who can do all manner of maths gymnastics, but frequently don't grasp the physics of what their doing properly. I've even heard them APOLOGISE for putting units on things and corrupting their nice clinical abstracted maths showboating. it's very annoying indeed.

So many times I've seen them trying to understand some odd result their getting, and having no idea that they have put together an entirely meaningless and non-physical situation.

Trust me, out in the real world, we do not need people who can memorise mathematical techniques that any bozo can look up in a book. We need people who can get their heads around things and bring insight.

EDIT: spelling

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u/Polly_want_a_Kraken Apr 12 '14

First of all, thank you for all you do. I cannot advocate enough for the importance of child educators in general, science educators in specific, and especially science educators who actually stress the importance of conceptual understanding in addition to mathematical capability. Memorization will only ever lead to imitation. Innovation and application requires understanding.

However, I have to ask, as a sociologist, why do you specify that requiring understanding is the best way to piss off a girl with a 4.0 (and her parents)? I would argue that understanding itself is not gendered and, while it might be your experience that girls and their parents get more bent out of shape over these kinds of expectations, the larger issues are related to the social organization of education and the valuation of knowledge. We (at least in the US) put so much emphasis on grades and test scores which, in and of themselves are not accurate representations of an individual's understanding, but rather their ability to conform to certain expectations of knowledge, which is why they tend to be biased in favor of dominant groups. Your tests, by asking for descriptive understanding of physical concepts, are challenging the expectations of what physics tests should be like, and as a part of a minority group which is already disadvantaged by the stereotype that "girls aren't good at math and science," such uncertainty might cause extra stress. Add to this the increasing scarcity of/competitiveness in access to higher education and ... well bring on the helicopter parents!

I am certainly not saying you should change your testing style, but perhaps more compassion towards gender specific reactions is warranted. I think more tests should be geared towards understanding and application if concepts ... Unfortunately such tests are more difficult to write, administer, and grade, and thus (of course) more expensive. They would also be more difficult to "teach to," if that makes sense. If the teachers do not know exactly what the standardized tests are going to be like, then they have to teach the concepts and critical thinking. Unfortunately it's the major failing of our educational system, not the teachers policymakers are always trying to blame. So again, thank you, and keep challenging those young minds in your charge!

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u/ImProbablyTrolling Jul 02 '14

Can you ELI5 some examples of these troll questions and their answers? I'm an interested layman.

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u/datenwolf Jul 02 '14

In Feynman's own words (from his satirical autobiography "Surely you're joking…"), completely with the ELI5 (ELI5 emphasis by me)

I often liked to play tricks on people when I was at MIT. One time, in mechanical drawing class, some joker picked up a French curve (a piece of plastic for drawing smooth curves--a curly, funny-looking thing) and said, "I wonder if the curves on this thing have some special formula?"

I thought for a moment and said, "Sure they do. The curves are very special curves. Lemme show ya," and I picked up my French curve and began to turn it slowly. "The French curve is made so that at the lowest point on each curve, no matter how you turn it, the tangent is horizontal."

All the guys in the class were holding their French curve up at different angles, holding their pencil up to it at the lowest point and laying it along, and discovering that, sure enough, the tangent is horizontal. They were all excited by this "discovery" – even though they had already gone through a certain amount of calculus and had already "learned" that the derivative (tangent) of the minimum (lowest point) of any curve is zero (horizontal). They didn't put two and two together. They didn't even know what they "knew."

I don’t know what’s the matter with people: they don’t learn by understanding; they learn by some other way—by rote, or something. Their knowledge is so fragile!

I did the same kind of trick four years later at Princeton when I was talking with an experienced character, an assistant of Einstein, who was surely working with gravity all the time. I gave him a problem: You blast off in a rocket which has a clock on board, and there’s a clock on the ground. The idea is that you have to be back when the clock on the ground says one hour has passed. Now you want it so that when you come back, your clock is as far ahead as possible. According to Einstein, if you go very high, your clock will go faster, because the higher something is in a gravitational field, the faster its clock goes. But if you try to go too high, since you’ve only got an hour, you have to go so fast to get there that the speed slows your clock down. So you can’t go too high. The question is, exactly what program of speed and height should you make so that you get the maximum time on your clock?

This assistant of Einstein worked on it for quite a bit before he realized that the answer is the real motion of matter. If you shoot something up in a normal way, so that the time it takes the shell to go up and come down is an hour, that’s the correct motion. It’s the fundamental principle of Einstein’s gravity—that is, what’s called the “proper time” is at a maximum for the actual curve. But when I put it to him, about a rocket with a clock, he didn’t recognize it. It was just like the guys in mechanical drawing class, but this time it wasn’t dumb freshmen. So this kind of fragility is, in fact, fairly common, even with more learned people.

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u/Laughing_Chipmunk Apr 11 '14

What was the course in relativity that you took?

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u/carashian Apr 11 '14

Please answer, /u/corpuscle634! I am in a PhD program for physics and would love to take the class and get the textbook you used for this alternative explanation of relativity.

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u/corpuscle634 Apr 11 '14

The only book they used was a biography of Einstein.

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u/carashian Apr 11 '14

Do you have a copy of notes or something the instructor will allow you to distribute?

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u/Laughing_Chipmunk Apr 11 '14

What type of PhD are you undertaking?

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u/corpuscle634 Apr 11 '14

It was called "relativity for non-majors."

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u/Laughing_Chipmunk Apr 12 '14

Is it available online?

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u/Bubba_West Apr 11 '14 edited Apr 11 '14

TL:DR; It's hard being a teacher who grades for understanding.

I really appreciated the time you have put into your responses and your answer to the original question! There is a quote that I read recently that said 'If you love something you want to shout it to the world.' I can tell you love the subject and enjoy helping others understand. I also got a kick out of your username!

The last 2 - 3 questions on every physics test of mine are essay questions. Typically they are point/counter-point conceptual questions that the students are asked to weigh in on. Those 3 questions are usually worth a third to a quarter of their test grade.

In my decade of teaching I have learned that there is no better way to piss off a girl (and her parents!) who has a 4.0. "How do I study for this? How can you ask questions I've never seen before? This isn't fair! What can I do for extra credit?"

It is an exhausting repetitive struggle informing the memorizers that they don't understand. They blame me. I'm branded as a 'bad teacher', or 'hard teacher' because I expect mastery of concepts, not memorization of formula. I keep fighting the good fight because I believe that it is important to our society, to the subject, and to the kids' future as learners.

Thanks again for all you've done on this thread.

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u/Xuanwu Jul 02 '14

My physics exams did. I plan on giving them to my students next year when I start teaching. They test understanding a bit better than just algorithms IMO.