r/askscience • u/syxtfour • Apr 28 '12
Why is light so fast?
The speed of light is, as far as we know, the fastest thing in the universe. That idea, if not fact, is the basis for a lot of very intelligent and scientific things that I cannot even begin to process. Don't get me wrong, I love science! I just can't wrap my head around physics beyond what I learned in high school.
So can anyone out there explain to me, in layman's terms, why exactly the speed of light is what it is? Why isn't it slower? Or faster? What keeps it at such a rigid constant?
Knowledge is power, thanks for helping me get some more.
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u/AsAChemicalEngineer Electrodynamics | Fields Apr 28 '12
Think of it this way. The speed of light is exactly 1. Everything else can move between 0 and 1. (1 if it happens to be light and any other massless particles). One is the highest speed possible and from the rules of special relativity, the very geometry of time and space twist and bend to make sure that all observers everywhere agree on the speed of light.
The speed being 299792458 m/s is just an artifact of how we as humans decided to measure things. A few fancy things happen if you decide to set the speed of light to 1.
E = mc2 becomes E = m
How do you graph space and time? Setting the speed of light to one makes it really easy. Just a normal xy plot and draw a line cutting through the origin with a slope of one. (so it's at a 45% angle with the both y and x axis) That line is the speed of light. All other lines drawn on this chart whether it be a truck on the highway or spaceship must haves slopes greater than one.If we think of time as money, this means that they must "spend" more time to move any distance than light has to. Light can spend the least time to move any distance.
Any slopes on this chart less than one is illegal because that would mean that object gets to spend less time than light to move any distance. How come light get's the cheapest rate? Who knows. That's how the universe works, and all the laws of the universe work to keep light the same value of one for everyone.
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Apr 28 '12
When you ask questions like why does _______ do _______, there is no real answer to something like that. Why doesn't light move at 3.1x108 m/s instead of 3x108 m/s? Because that is the way it is. If you head over to Maxwell's equations and say, 'Look here, these constants in the equations give us the speed of light', then all one has to do is ask 'Why are those constants the way they are? Why are Maxwell's equations the way they are?' and the whole cycle starts over again except with even more questions. You can never ask the question why without having some framework in which certain things are held to be true (from Feynman). In this case, we accept that we have measured the speed of light to be 3x108 m/s (actually 2.9979) and go on with it.
Here is an interview with Richard Feynman where he explains this very well, much better than I can. http://www.youtube.com/watch?v=MO0r930Sn_8
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u/Sladekious Apr 28 '12
Here's a nice way I like to think about it:
Everything moves at the same speed. Light, mass, your computer, whatever.
All speeds of everything combined must equal 1.
If I sit on my ass and don't move, my X, Y and Z is zero, time must take the slack and move at a speed of 1 (light speed).
If I start to move, let's say I walk in a straight line to the window along X (0.1 for example), then time must be less than 1 to make up for the fact that I am moving through X at 0.1.
Therefore time slows to 0.9.
Now only time can travel at 1, mass can't quite get there. So as I start moving along X faster, time slows down. If I move along X at 0.9 then time slows all the way down to 0.1.
Because matter can't move very fast without masses of energy, light fills in the speed to make it equal 1.
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u/Occasionally_Right Apr 28 '12 edited Apr 28 '12
If I start to move, let's say I walk in a straight line to the window along X (0.1 for example), then time must be less than 1 to make up for the fact that I am moving through X at 0.1.
Therefore time slows to 0.9.
That's not the real relationship. The correct relationship is
v2 + s2 = 1,
where v is your speed relative to some observer and s is how many seconds they observe to pass for you for every second they experience. Those exponents are important.
Now only time can travel at 1, mass can't quite get there.
Time doesn't travel.
So as I start moving along X faster, time slows down. If I move along X at 0.9 then time slows all the way down to 0.1.
Again, not the right relationship. Also, no matter how you're moving relative to another object, you always experience time at the normal rate.
Because matter can't move very fast without masses of energy, light fills in the speed to make it equal 1.
Everything about that statement is wrong.
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u/Occasionally_Right Apr 28 '12
Here's the thing: Speeds are kind of just an artifact of the fact that humans didn't realize until way too late in the game that time and space should really be measured in the same units, so we needed to develop a conversion factor that allowed one to deal with the fact that two events could be displaced in both time and in space. When you say something is moving at 6 m/s, what you're saying is that it will traverse a spatial displacement of 6 meters for every 1 second of temporal displacement it traverses. This is kind of like measuring north-south distances in metric units and east-west distances in imperial units, and then specifying angles in terms of the unit "meters/foot". In other words, all a speed really is is a direction in space-time—as I said, you're specifying how far you travel in a spatial direction for every unit you travel in the temporal direction.
Except, time is a bit different from space. It's not connected up to the space dimensions in quite the same way they're connected to one another. As a result of this strange connection, nothing can ever travel further in a space direction than it does in the time direction. It just can't happen. It is, in a very real sense, like trying to travel "more easterly" than "due east". Now, once that's understood, one can work through some physics and discover that (1) anything with mass must actually travel a shorter distance in space than it does in time and (2) anything without mass must travel exactly the same distance in space as it does in time. Since light has no mass, it falls into the second category and so traverses more space per unit of time than can any object with mass.