Why is the speed of light constant?

In summary: I'm still not sure about what you are asking here, but I guess you are asking why light travels at the speed it does. In summary, light travels at a constant speed because it is not matter and does not have to overcome friction to move. This question may seem philosophical, but it is actually a physical question that is still being studied and debated by scientists. The speed of light is a fundamental constant in the universe and plays a crucial role in many phenomena. Its value is determined by various factors, such as the size and frequency of atomic structures. Ultimately, the answer to why light travels at the speed it does lies in the laws of physics.
  • #1
physicsnoob12
15
0
why does light travel so fast?
 
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  • #2
Because we live so slow.
 
  • #3
but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.
 
  • #4
physicsnoob12 said:
but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.

So? Even in a frictionless world, objects don't have to travel fast.
 
  • #5
This question is about philosophy, not physics.
 
  • #6
physicsnoob12 said:
but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.

That's a better question. No one knows the answer, but one possibility is that the computational speed of the universe is limited (Wolfram), or even more simply, the universe may evolve in discrete finite time intervals.
 
  • #7
Because we live so slow.
No matter how fast we live, the speed of light is allways the same. And you, the observer, will allways move with zero speed relative to youself, so it allways would seem that the light moves at the speed of light and you are at rest.
 
  • #8
physicsnoob12 said:
why does light travel so fast?

The_Duck said:
Because we live so slow.

cryptist said:
This question is about philosophy, not physics.

I think it is about physics not philosophy and the solution is of the type of The_Duck's.
We live so slow and we are so small because of the size of atoms (not complete explanation) and that depends among other things on the speed of light.
 
  • #9
physicsnoob12 said:
but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.



Light is both a classical and quantum phenomenon and its classical aspect 'prevents' it from being instantaneous, as in the macro realm nothing is instantaneous(which is also forbidden by a postulate of special relativity). It's how nature is.
 
  • #10
epenguin said:
I think it is about physics not philosophy and the solution is of the type of The_Duck's.
Asking that "why light travels at c instead of another speed or instantaneously" is a philosophical question and nothing to do with physics. It is a universal constant. Like pi for example..

If that's not philosophical, then I can ask you; why pi is 3,14... and not something else? Then is this mathematical?
 
  • #11
physicsnoob12 said:
but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.

I'm just spouting stuff here, but it may have something to do with gravity. Light is confined to space-time, so there might be some relevance.
 
  • #12
cryptist said:
Asking that "why light travels at c instead of another speed or instantaneously" is a philosophical question and nothing to do with physics. It is a universal constant. Like pi for example...
I have given a very crude version of what would be a physical argument. When you say the speed of light is so many m/s like any other physical measurement it is a comparison. You can make it into a comparison with e.g. the size of atomic structures and the times of atomic events. As light is more fundamental than the atom the most meaningful comparison is the other way round. It is asking why atoms have the size and frequencies they do, more exactly try an explain why the size is such that light can get back and forth between two adjacent atoms in the time it takes for x flips of a Cs nucleus or something like that, thousands of things like that. Physical questions.


cryptist said:
Like pi for example..

If that's not philosophical, then I can ask you; why pi is 3,14... and not something else? Then is this mathematical?
Not sure what your point is here but yes that is mathematical, once we define pi as twice the circumference of the unit circle. The mathematics answers of is the answer to any 'why' question.
 
  • #13
Pi is the ratio of the circumference of a circle to its diameter. It is 3,14... Asking "why it is 3,14... and not something else" is not a mathematical question. It is a philosophical question. There is no single living thing that can answer why pi is 3,14...
And this is the same case. Pi is a mathematical constant, c is a physical constant. Therefore, asking why speed of light is c is not physical question but philosophical.
Physics and mathematics (in fact any positive science) do not ask "why" questions.
 
  • #14
cryptist said:
Pi is the ratio of the circumference of a circle to its diameter. It is 3,14... Asking "why it is 3,14... and not something else" is not a mathematical question. It is a philosophical question. There is no single living thing that can answer why pi is 3,14...
And this is the same case. Pi is a mathematical constant, c is a physical constant. Therefore, asking why speed of light is c is not physical question but philosophical.
Physics and mathematics (in fact any positive science) do not ask "why" questions.

Puzzling and contradictory IMHO.

It was mathematically proved that pi when defined as you have is that decimal. So it was a mathematical question answered. The mathematical proof is the answer to the why. In ordinary parlance - maybe you can argue deeply that you cannot give a logical sense to the word 'why' that applies here.

Your 'therefore' is completely unwarranted since math and physics are of completely different status and nature - it is not like statements about physics carrying over to chemistry.

At first your use of the word 'philosophical' suggested to me you were one of those scientists who used 'philosophical' as equivalent to 'pointless, unworthy of an adult's attention'. But when I saw 'positive science' I thought you might come from a very definite philosophical position. Unless you just copied the expression.:smile: In any case just one position.

Would you say the Boltzmann constant is just what it is, asking why it has that value is a philosophical question?
 
  • #15
physicsnoob12 said:
why does light travel so fast?

It doesn't travel fast - it's appallingly slow.

If you are designing high speed computers it's annoyingly slow- and if you want to rule a galactic empire it's very frustrating.
 
  • #16
epenguin said:
Puzzling and contradictory IMHO.

It was mathematically proved that pi when defined as you have is that decimal. So it was a mathematical question answered. The mathematical proof is the answer to the why. In ordinary parlance - maybe you can argue deeply that you cannot give a logical sense to the word 'why' that applies here.

I don't think it is contradictory and I don't think the mathematical proof is the answer to the why. "Why" has more deep meaning. Of course we know pi is 3,14... and not something else. But why it is? Can you answer? All you can say is that "Because for any circle circumference over diameter gives 3,14..". Then "Why for any circle circumference over diameter gives 3,14...?". You can ask these but you cannot find an answer. That's why there is a thing called philosophy :)

epenguin said:
Your 'therefore' is completely unwarranted since math and physics are of completely different status and nature - it is not like statements about physics carrying over to chemistry.

I don't think math and physics have completely different status. Physics derived from mathematics. Physics can be considered as applied math. (I think)

epenguin said:
At first your use of the word 'philosophical' suggested to me you were one of those scientists who used 'philosophical' as equivalent to 'pointless, unworthy of an adult's attention'. But when I saw 'positive science' I thought you might come from a very definite philosophical position. Unless you just copied the expression. In any case just one position.

No, I never use "philosophical" as pointless or unworthy. I know the boundaries of philosophy and science, so I am not saying that "Do not discuss why does travel so fast! It is pointless." , I am saying "Quantum Physics forum is not the right place to discuss this kind of why questions". Because it is philosophy.

epenguin said:
Would you say the Boltzmann constant is just what it is, asking why it has that value is a philosophical question?

Every constant in science has a meaning. But "why it has that meaning" is something else.
 
  • #17
No one knows why the speed of light is what it is, nor even why it is fixed; nor why the charge of an electron is what it is, nor why gravity has it's particular value...
 
  • #18
physicsnoob12 said:
why does light travel so fast?

Yes, from "the human perspective" it is fast; 299,792,458 meters per second. But remember that meters and seconds are related to humans, living on the planet Earth.

One meter was originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole.

One second was originally intended to be 1⁄86,400 of the mean solar day (24 x 60 x 60).

Now imagine an alien civilization of terrible large giants visiting our solar system, where the Earth had the size of a head of a pin and the Moon the pin tip, form their perspective (impossible but just for fun). Imagine that these alien visitors saw a laser beam leaving Earth towards the Moon.

– What would that look like to them?

[URL]http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Speed_of_light_from_Earth_to_Moon.gif/400px-Speed_of_light_from_Earth_to_Moon.gif[/URL]
A beam of light is traveling between the Earth and the Moon in 1.255 seconds

As you can see, these giant aliens could most probably walk faster-than-light! :smile:

But this is not possible; it’s just science fiction to give you some "perspective". Absolutely nothing can move faster than the speed of light in space (the expansion of space itself is another story), and this has to do with causality. If you could travel faster than the speed of light (FTL), then you could do time travel, which in turn would cause unsolvable paradoxes with observers going back in time to erase the cause of their own "present" existence, etc.

In the theory of relativity, the speed of light (c) is invariant. If you had an extremely fast "science-fiction-car" doing 0.5 x the speed of light, at what speed would the light of your headlights leave the car? Well the natural answer is 0.5 x the speed of light, as the sum up speed would be 0.5 + 0.5 = 1 x the speed of light, which is maximum speed, right?

But this is wrong! The speed of the light leaving your headlights will always be 1 x the speed of light, no matter how fast you drive, when you measure that speed.

In special relativity, space and time is a unified structure known as spacetime, with c relating the units of space and time, and c = 1, i.e. the speed of light = 1.

physicsnoob12 said:
but why is it not instantaneous because it is not actually matter so it does not have to overcome friction to move.

There is no friction in outer space, whether you are a spaceship or a light beam. If the speed of light was instantaneous, there would not be any time = crazy mess... where everything happens at once...

"The only reason for time is so that everything doesn't happen at once." -- Albert Einstein
 
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  • #19
The problem here is that some people are solving this by just saying " it just does" but imagine if Newton wondered "why did this apple fall?" and then just said " oh yeah, it just does" we have to to a crack at the question.
 
  • #20
don't mind my last comment thanks for the explanation Devilsavacado
 
  • #21
Are you talking to me!? :grumpy:

(:rofl:)


EDIT: Ahhh OK! :blushing: (:wink:)
 
  • #22
epenguin said:
Would you say the Boltzmann constant is just what it is, asking why it has that value is a philosophical question?

cryptist said:
Every constant in science has a meaning. But "why it has that meaning" is something else.

Well the Boltzmann constant comes in a bit everywhere at least in bulk matter theory. So it can have the appearance of a fundamental constant. But if you analyse it it is nothing but a property of water, its solid/liquid/gas states. If you can predict the energy of its melting and boiling you have it. So a property of atoms in the end. I suggest the speed of light is a property of atoms in the same kind of way.
 
  • #23
physicsnoob12 said:
The problem here is that some people are solving this by just saying " it just does" but imagine if Newton wondered "why did this apple fall?" and then just said " oh yeah, it just does" we have to to a crack at the question.

No he went much further, and said "oh yeah, it just does, because of gravity". And we're still at that point now + a few unexplained constants. :P
 
  • #24
physicsnoob12 said:
The problem here is that some people are solving this by just saying " it just does" but imagine if Newton wondered "why did this apple fall?" and then just said " oh yeah, it just does" we have to to a crack at the question.

No, the problem is that we don't actually know WHY light travels so fast, only that it DOES. We can explain how light works to the best of our knowledge, but that's it. It's like the little kid that continually asks why when you explain something too them. Eventually, you HAVE to say "It just does" or "Thats just the way it works". Either because its far to complicated to explain, or because we just don't know.

We can follow up on this with a detailed explanation on an electromagnetic wave and other related subjects, but the first answer might have been enough for the poster.
 
  • #25
I think its a valid question. It could be restated as "why have Earth life forms evolved to deal with only Newtonian physical situations?" (i.e. the speed of light is large compared to any velocities that are relevant to the survival of the organism). It probably has something to do with the fact that life forms cannot be (or have not been) "built" which have "circuits" (neurons) that can send signals at anywhere near the speed of light, nor can they generate forces which would accelerate anybody part to anywhere near the speed of light. If they could, then organisms would have to respond relativistically in order to survive either as predator or prey, and the speed of light would not seem so fast.
 
  • #26
There are many physical constants and most are associated with physical dimensions, e.g. speed = length divided by time.

But there is one fundamental physical constant which is dimensionless – the Fine-structure constant (α).

This seems like a much bigger "mystery" than the speed of light, according to Nobel laureates:
There is a most profound and beautiful question associated with the observed coupling constant, e the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the "hand of God" wrote that number, and "we don't know how He pushed his pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out, without putting it in secretly!

— Richard P. Feynman (1985)
 
  • #27
epenguin said:
Well the Boltzmann constant comes in a bit everywhere at least in bulk matter theory. So it can have the appearance of a fundamental constant. But if you analyse it it is nothing but a property of water, its solid/liquid/gas states. If you can predict the energy of its melting and boiling you have it. So a property of atoms in the end. I suggest the speed of light is a property of atoms in the same kind of way.

This is sort of correct. However, the "everyday use" of kb is to relate temperature to energy (W=kBT) and this is done even for situations where the temperature scale is not defined (meaning you can't relate it to the tripple-point of water) so kb is -when used in this context- certainly fundamental.
It is almost certain that kb will in a few years time be defined to have a certain value, thereby removing all references to the triple point when using the Kelvin. This will be done as part of a larger overhaul of the SI, where the principle of "one unit-one constant" will be used (i.e. c for length, kb for the kelvin, A for mole etc).
 
  • #28
Reading through this fastinating thread makes me wonder: Do at least some of you believe that it is beyond human capability to ever mathematically derive the speed of light, i.e., to explain why it must travel at the speed that it does? And if so, why?
 
  • #29
Oldfart said:
Reading through this fastinating thread makes me wonder: Do at least some of you believe that it is beyond human capability to ever mathematically derive the speed of light, i.e., to explain why it must travel at the speed that it does? And if so, why?

This question has no answer, in the sense that you mean. The speed of light is given as some multiple of a reference distance (e.g. the meter) divided by some reference time (e.g. the second). If we assume that the speed of light is a constant, then we can set its numerical value to anything we want by choosing our reference distance and our reference time. The best we can do is to ask why is its value what it is compared to, say, some other velocity. Then we could take the ratio of those two velocities and get a dimensionless number and about THAT we could ask why. Its pointless to ask why a dimensioned quantity (like the speed of light) has the value it has without inquiring into the reference quantities used to calculate it. The real question is why is the meter as long as it is, and why is the second as long as it is? These are answerable - because they are lengths and times that we as humans need to easily grasp in order to survive - we are evolutionarily programmed to be very concerned about lengths on the order of a meter and times on the order of a second (to within a few orders of magnitude). And velocities on the order of a meter per second. In this sense, the question "why is light so fast?" is a much better question. Saying it another way, we could say "why does the ratio of light velocity to evolutionarily important velocities have the value it does?" And again, that is a ratio of velocities which is a dimensionless number. Only dimensionless quantities (e.g. the fine structure constant mentioned above) have absolute physical meaning, and whose meaning may be questioned.
 
  • #30
Thanks, Rap!

Your explanation is sensible throughout, but seems to sort of duck what I'm asking. Not that you would know that, from my simplistic question. I'll try again...

I got the feeling from some of the posts here that the speed of light is just what it is, sort of a philosophical speed, and was wondering if it might be possible for humans, after a million years of math/phiysics, to change that outlook, to prove that c must be what it is, rather than just accept is as a sort of "god number." The approach to this solution can involve whatever methods you can imagine, ratioing velocities as you cite or whatever. The deep question here is "could this ever happen?" Or to put it another way, is such a feat fundamentally impossible?
 
  • #31
Well, I don't mean to duck the question, I'm trying to explain why its a bad question, and not doing to well, I admit. The short answer is yes, its fundamentally impossible. As a number, the speed of light is not a "god number", because its numerical value depends on your chosen unit of length and your chosen unit of time. The only "god numbers" are dimensionless, like the fine structure constant, because they don't depend on your choice of units. Suppose the speed of light were to suddenly double what it is now. Then all of our physics would change, because some or all of our "god numbers" (fine structure constant and maybe a few more) would change. If the speed of light were doubled, but all other constants were modified too, such that all our "god numbers" stayed the same, then we would live in a world where all the physics is the same. We could detect no change. If, after the change, we chose to define the meter and the second in the same way we do now, the numerical value of the speed of light would be the same as it is now! We could never detect the change.
 
  • #32
Perhaps one reason light travels so fast in comparison to other velocities because it holds no mass and therefore no friction or colliding against other atoms?

Just a little speculation.
 
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  • #33
physicsnoob12 said:
but imagine if Newton wondered "why did this apple fall?" and then just said " oh yeah, it just does"
The whole point of the enlightenment and modern, as opposed to medieval and greek science, is that we can say "it just does".

Newton worked out that objects behave as if there was a force between them that was GMm/r^2 WITHOUT having to know why - there is nothing in Newton's theory that has any clue why things behave like that - he just created a mathematical description to explain how they behaved.

Earlier science was hampered by the need to explain why. So the apple fell because it was made of the same element as earth, or it's natural place was place earth, or because it wasn't godly - none of this reasoning gets you to a law describing how things fall.
 
  • #34
NobodySpecial said:
none of this reasoning gets you to a law describing how things fall.
I don't think any of the reasoning that you mentioned was ever intended to get you a law describing how things fall. Explaining why and describing how are not equivalent tasks.
 
  • #35
cryptist said:
I don't think it is contradictory and I don't think the mathematical proof is the answer to the why. "Why" has more deep meaning. Of course we know pi is 3,14... and not something else. But why it is? Can you answer? All you can say is that "Because for any circle circumference over diameter gives 3,14..". Then "Why for any circle circumference over diameter gives 3,14...?". You can ask these but you cannot find an answer. That's why there is a thing called philosophy :)

I don't think math and physics have completely different status. Physics derived from mathematics. Physics can be considered as applied math. (I think

No, I never use "philosophical" as pointless or unworthy. I know the boundaries of philosophy and science, so I am not saying that "Do not discuss why does travel so fast! It is pointless." , I am saying "Quantum Physics forum is not the right place to discuss this kind of why questions". Because it is philosophy.

Every constant in science has a meaning. But "why it has that meaning" is something else.

I'm sorry, I'm going to have to stop that assumption right there. That is exactly what does NOT progress physics. That claim can be compared to people that have said in the past "everything that has been invented, has already been invented." It is simply your limited perspective.

Learning something such as why light travels at such a magnificent speed would ultimately help push physics further as a science. it would help us unveil the laws of physics as it is. Because truly, we don't really understand much at all.
 

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