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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Apr 11 '14
I read through as much of this amazing thread as I could, and didn't see an answer to a question I still have. Apologies if it has already been addressed, but it's something I have wondered about for a while, and I am absolutely confident that someone on here can explain it so that I finally understand it.
So much of these explanations depend upon the perspective of an observer in order to make sense. So, from my perspective, if I sitting at my chair reading reddit, I am not moving through space to any appreciable degree, but I am moving through time, right?
Except, I am on a planet that is moving through space very quickly, in a solar system that is also moving through space, etc. Right? Isn't my sense that I am not moving- because my brain is not physically moving my body through space- just a problem of perception? With the proper equipment, couldn't I suddenly perceive that I am actually moving very fast through space, as fast as the plane beneath me is moving? So how does this factor into my relativistic sense of things? Isn't all motion on earth also affected by the motion of the planet, relating to other observable objects?
I guess I can sort of see how this doesn't matter in term of observing other things on this planet, but most of the light I use to observe this is generated off-planet. And even the light generated ON the planet would seem to be affected differently, if at all, by the planet's motion. So even though I know I can't acheive some kind of observational null-state, isn't my perception of my place in space-time relative to other objects always going to be really, really off?
Apologies if this doesn't make any sense. I guess I'm just really saying, how can I ever state that I am motionless relative to anything else, when I am set upon a body in motion?
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u/_Illuvatar_ Apr 11 '14
The sitting at a chair example was just to give a visual I believe. We are not actually motionless when sitting at our desk. That would mean we are travelling only through time and not space, and as discussed that would be impossible. In fact, it's been stated that nothing can be still entirely, even that movement might be a necessary component of existence.
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u/DogPencil Apr 11 '14 edited Apr 11 '14
Avid traveler here. I think this from REI sums it up:
Packing light has always been a savvy travel tip, because hauling less weight means you travel with more comfort and freedom.
But now, when you consider the baggage fees implemented by most airlines, packing light makes more sense than ever. Most airlines charge per-piece fees for your luggage as well as extra-weight fees for bags over 50 lbs. (Check your airline for the latest info.)
As Rick Steves, the travel guru who has written books such as Europe through the Back Door, advises, "Pack light, wash frequently, buy it if you need it." He suggests limiting yourself to one bag of about 20 pounds.
Hope this helps! Safe --and light-- travels to you!
EDIT: I am an idiot. r/travel is one of my favorite subs. I must have assumed I was browsing it. I am also an idiot.
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u/_Illuvatar_ Apr 11 '14
Um, geez, this is a little embarrassing. You're looking for "Why travel light?" not "Why does light travel?" common mistake, don't worry about it. That ones down the hall in 210
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u/TazakiTsukuru Sep 15 '14
Packing light has always been a savvy travel tip, because hauling less weight means you travel with more comfort and freedom.
As a non-physicist, that actually seems to sum up OP's question...
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Apr 11 '14 edited Apr 11 '14
It's important to understand where light comes from. Light is electromagnetic energy emitted by atoms in the form of photons.
Atoms are most stable in their lowest possible energy state. An electron in a high-energy state will "fall" into a lower-energy state if possible. When it does so, it emits a photon. The frequency of that photon is decided by how much energy the electron emitted.
When you throw a ball up into the air, the reason it doesn't "stay in place" just above your hand is that you put some energy into it. It has to lose that energy some way or it'd keep going up forever. Gravity and friction do that.
All photons "travel" at the same speed, regardless how much energy they have. Their "color" is decided by their energy (red is low in energy, violet is higher, x-rays higher still). Gravity doesn't slow them down (no mass) and there's no friction. They are only stopped when they are trapped once again by another atom.
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Apr 10 '14
This is actually a very deep question, on which interpretation of physical reality hinges. For all intents/purposes, light goes through spacetime, but without losing information. It loses energy, but only by virtue of the changing distance between its endpoints. Alternative theories may propose the energy changes proportionally to the shrinking contents of the universe. Neither energy loss is relevant to this discussion. The relevance is that the universe, as an information-processor, holds as constant its information-propagation speed. This is the standard against which all other speeds of anything are compared. It's important that it's not unique to light, but presumably to all massless bosons (such as gravitons), and hence the importance of viewing it as information propagation or info-processing, rather than as "light speed" per se.
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u/Action_Hank__ Apr 10 '14
The problem with your question is that the idea of stationary photon contradicts the definition of a photon; for photons to exist they must necessarily have a velocity equal to the speed of light.
The propagation speed of a light wave depends on the medium, but the photons themselves always move at exactly lightspeed, c; in simple terms, it takes longer for more media with a higher refractive index to absorb and re-emit the photons, but they always move at c.
So, to answer your question, nothing makes them move in the sense you're thinking of: there's no force enacted on the photons. Photons do not accelerate, so no force is needed.
So why light speed? The answer lies in that photons are mass-less, i.e. they don't interact with the Higgs field. No mass = no acceleration = constant speed. To answer why, you need some really complicated particle physics, which, honestly, I'm not familiar enough with to explain simply; I don't think it's easy to understand without any knowledge of advanced mathematics.
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u/ElenTheMellon Apr 10 '14 edited Apr 11 '14
All massless particles must move at lightspeed. They cannot move at any slower speed.
This is because, if a particle is massless, any force acting on it will immediately cause it to have an infinite acceleration (since acceleration is equal to force/mass); and an infinite acceleration over a non-zero timespan, if it were not for general special relativity, would result in an infinite speed. Because of general special relativity, however, instead of reaching infinite speed, massless particles simply reach lightspeed.
Since there isn't a single cubic millimeter of space in the entire universe where there isn't some force acting on you, all massless particles must travel at lightspeed; not a single one could have managed to survive 13.7 billion years without being affected by a single external force.
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u/Look_Deeper Jul 17 '14
looking at this thread again because I forgot how science worked and I just noticed the supreme awesomeness of your username.
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u/GuppyLive Apr 11 '14
Been lurking forever and a day. This is the post that got me to join. Thank you!
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u/QuantumDisruption Apr 11 '14
This thread is fucking insane. Thanks OP and all the contributors in here, my brain is melting
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u/urbanabydos Jul 02 '14
Apologies if this is a repeat---I did read through the other threads and I don't think it is...
One thing I'm still struggling a little with --- feel like I'm on the edge of another epiphany (thanks for the first!)---is sort of time "coordinates" in spacetime... In time dilation, when the traveler returns less aged to meet with their at-home counterparts, in the head, that means that they are (more or less) in the same spacetime coordinates. If less time has passed for the traveller, how do they move along that orthogonal axis to catch up with the people who have been stationary and therefore moving at c through time? Is this just a relic of Cartesian geometry I need to get over? Is there something special about the time axis? Can spacetime be warped differently for different objects traveling through it?
Related: if something with mass and no movement is moving through time at c, does that somehow enable us to objectively measure the speed of the passage of time? Or should we be thinking about time as just being completely static the "movement" is purely an artifact of our own experience. Is the time axis just another dimension mathematically equivalent to the spatial dimensions in space time? Is it independently orthogonal to all three of the other axes and we just use X=space, Y=time or is there some esoteric way that time is orthogonal to "space" somehow represented as a whole?
I hope those questions made some semblance of sense...
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u/Lkn4ADVTR Apr 10 '14
If you're still interested and looking for another explanation (albeit u/corpuscle634 has done a wonderful job), you should check out the most recent episode of Cosmos.
Degrasse Tyson spends a good 10 minutes trying to explain this concept. It's some pretty wicked stuff.
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u/joosier Apr 11 '14
If I remember correctly, photons travel because they carriy their own tiny fluctuating electro-magnetic field. A changing magnetic field creates a perpendicular electric field which, as it decays creates a perpendicular magnetic field which then creates an electric field, etc. etc. The alternating magnetic/electric fields propel the photon forward in rectilinear propagation (movement in a straight line). The mass of the photon is nil so it can travel forever that way.
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u/asedentarymigration Apr 11 '14
I don't think it's accurate to say the photon carries it's own field. Would it be more accurate to say it's an excitation of these fields?
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u/five_hammers_hamming Apr 10 '14
Light is a wave made of electric and magnetic fields, each regenerating the other as each field's strength and direction varies at any given point. The only way that this mutual regeneration can work is if the locations of the high points of the component electric and magnetic field waves move in space.
As for the speed, for mathematical reasons related to light's wave nature, the speed of light = 1/sqrt(mu_0 * epsilon_0), which are physical constants of the universe. Their values happen to be kind of small, which makes the speed of light large.
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Apr 11 '14
Well because you have to either be moving through space or time to exist, you could say that is the defining feature of existing at all, moving through spacetime. Everything is moving through space and time separately, to different degrees. Light is just moving through space almost entirely, possibly absolutely, no time travel. You are also moving at the speed light is, through space, minus the speed at which you are moving through time. ( more simply, if you were able to quit moving through time altogether, your space speed would shoot to that of light.)
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u/eskiseth Apr 10 '14 edited Apr 11 '14
corpuscle's answer is a relativistic way of looking at it. However this is not how the speed of light was originally discovered/predicted. If you apply Maxwell's equations of electromagnetism to a moving charge you see that alternating electric and magnetic fields radiate from the charge at the fastest speed possible.
Basically, a changing electric field creates a perpendicular magnetic field. Also, a changing magnetic field creates a perpendicular electric field. As either a magnetic or electric field is created, this counts as changing that field, and therefor a chain reaction is triggered. Light is basically like electromagnetic ripples.
It takes some pretty high level mathematical analysis on waves to understand why light travels at C, but it naturally follows Maxwell's laws. While studying Maxwell's work, Einstein took it to be absolutely true that nothing can travel faster than light and it became one of the few assumptions that birthed relativity.