Why does light travel at light speed?

[jackpot337]

Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?

 

[mathman]

Photons travel at light speed because that's the way it is. Photons hitting the sun (doesn't matter where they came from) will interact with the material of the sun (electrons, protons, etc.). Being the sun itself is irrelevant.

 

[urtalkinstupid]

It's not as simple as "that's the way it is." Why are they the only things that can travel at c (not including neutrinos which are theorized to travel at c). What property gives them this ability to travel so fast? Is it the no rest mass?

 

[jcsd]

photons are not the only things gthat travel at c (neutrinoes are no longer beleived to travel at c tho'), it's a general property of massless particles.

 

[selfAdjoint]

You can derive the relativitic equation for energy $$e^2 = p^2c^2 + m^2c^4$$. Here p is the magnitude of the three dimensional momentum, m is the invariant mass of the particle, and c, of course is the speed of light. I repeat that this equation follows from the postulates, the basic definition of relativity.

Now suppose p = 0; then the particle has no momentum => you are looking at it in its rest frame and the equation reduces to $$e = mc^2$$ which you might have seen before.

On the other hand suppose m=0; in this case the particle is massless and the equation reduces to $$e = pv$$, the energy is the size of the three-momentum times the speed of light. Plug this into the Lorentz transformation for energy and you get the velocity of the particle is c.

So these two predictions come from the relativistic energy equation, the energy of a particle at rest is $$mc^2$$ and the speed of a massless particle is c.

Of course in more general states you don't have either of these conditions, but the energy equation is still true.

 

[niranjan]

hi,
we know that nothing can travel faster than the speed of light...but what is it about light that gives it this special status?? i mean it could have been sound...could have been some other thing...n if we if we think logically then anything traveling opposite to the direction of light at velocity "v" will see light going away from it at an speed " c+v"...i know this point has been time n again been said to be false when light comes into picture...but some how 1+1 will always be 2!
regards, niranjan

 

[pmb_phy]

Just was wondering if there was something that pushed photons or if it just a property of light. Also what happens to photons from other stars when they reach our sun?

Nothing needs to be pushed for it to move. The speed of light can be calculated from Maxwell's equations. See

http://www.geocities.com/physics_world/em/mawell_eq.htm

It starts with the permittivity and permiability of free space. They appear in Maxwell's equations as the reciprocal of the the product of the two. That is written as the square of a quantity which is labeled "c". The equations for the E and B fields are then shown to be the solution of a wave equation and c is the speed of the wave.

Pete

 

[urtalkinstupid]

photons are not the only things gthat travel at c (neutrinoes are no longer beleived to travel at c tho'), it's a general property of massless particles.

Not true. Photons have a mass, just theoretically no rest mass. So, particles with mass can travel at c as long as they have no rest mass.

niranjan, I like the way you think. Light is relative to the object, but physicists always find some mathematical way to prove it is constant. Simple logic says that if you are moving towards a light source the velocity of light would be dependent on that source.

$$\vec{v}_{light}=c+\vec{v}_{observer}$$

Or if the object is moving away from the light source

[tex]\vec{v}_{light}=c-\vec{v}_{observer}[/itex]

Maybe they are the other way around, but logic says light speed should be dependent on velocity of the source of the observer or light source.

So, neutrinos are said to not travel at the speed of light? So, this means they have a rest mass, right?

Sorry, mentors, if my expression of opinion shouldn't go here but rather in a thread in Theory Development.

 

[marcus]

You can derive the relativistic equation for energy $$e^2 = p^2c^2 + m^2c^4$$. Here p is the magnitude of the three dimensional momentum, m is the invariant mass of the particle, and c, of course is the speed of light. I repeat that this equation follows from the postulates, the basic definition of relativity.
...

On the other hand suppose m=0; in this case the particle is massless and the equation reduces to $$\inline e = pc$$, the energy is the size of the three-momentum times the speed of light...

so at some level there is this basic assumption or observation about the universe:

"there is a certain speed that is the same to all observers"

nothing said about light or massive or massless or any kind of boson, that would be too clunky----just that the universe has a preferred scale of speed that reads the same in all frames

later, after some algebra and thinking about particulars, we learn that massless things actually GO this speed, and that, as it so happens, light goes this speed

Correct me if I am wrong, but it seems as if the basic postulate does not even assume that anything travels at this speed, not to mention saying anything about light. I just says that in the universe there IS this speed which looks the same regardless of your perspective.

and the equations you mention derive from that, by some algebra

I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.

 

[wolram]

By MARCUS.
I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.
----------------------------------------------------------------------------------------------------
So what could this other type of physical quality be, the best i could
think of is a" physical meter", yard stick, universal mile ,what ever,
im sure time is irrelevant to physical quality, so what is missing?

 

[selfAdjoint]

Marcus, I believe you are basically right. Modern texts will say on page 1 or 2, something like "in order to get all the components of our 4-vector into the same units, we have to multiply the time component by something to turn it into a length, like the space components. What do you multiply by a time to get a length? Obviously something with dimensions length over time: a speed! And this speed factor has to be a Lorenz scalar, so that there is only one conversion for everybody. Let's call it c for celeritas (Latin for speed). Hence we get Einstein's second postulate without the light dependence he was forced to by his "operational" framework. (By that last of course i mean he defined the physics in terms of operations people could actually do, an ideal of the radical physicsts, incuding Mach, of the late nineteenth century).

 

[hemmul]

"there is a certain speed that is the same to all observers"

great!

I'm sure this is oversimplifying, but it is a striking idea and immediately prompts one to ask "what if there were two invariant scales instead of just one?" Are there maybe several quantities in the universe that look the same to all observers---a speed and at least one other type of physical quantity.

good question. actually it brought me to a slightly crazy idea right away: from this algebra, it can also be seen, that it is impossible to accellerate a massive particle above c, if initially it was moving slower than c. The same may be true for particles initially moving faster than c - that is they can't be deccellerated to sub-c speeds... Now what if we assume that there is another - greater - velocity? Kinda second "upper-bound" - for super-c particles we can't observe? say this speed equals 2c. and no super-c objects can't be accellerated to super-super-c speeds, and so on... :yuck:
Does it make any sense?
One of the "difficult" questions about that is "why do we live in [0,c] range?"

 

[chroot]

Yes, hemmul, the particles you describe are called tachyons. Their mechanics are already well-understood, but there has been any evidence that they actually exist.

- Warren

 

[chroot]

Simple logic says that if you are moving towards a light source the velocity of light would be dependent on that source.

$$\vec{v}_{light}=c+\vec{v}_{observer}$$

Or if the object is moving away from the light source

[tex]\vec{v}_{light}=c-\vec{v}_{observer}[/itex]

Maybe they are the other way around, but logic says light speed should be dependent on velocity of the source of the observer or light source.

No, logic does not say this, or anything like this. Logic is a tool that allows you to derive conclusions from axioms. Please don't misuse terms. What you mean to say is that common sense says that the velocity of light is dependent on the velocity of the emitter. This concept is called "emitter theory" and has been as soundly defeated as any theory in the history of science. The most damning experiment is one that was done with neutral pions, a type of particle made from two quarks, traveling at nearly the speed of light in a particle accelerator. When these pions decay, they produce gammas -- high-frequency light. The speed of these gammas was measured directly, and was found to be -- drum roll, please -- c. Even though the pions were themselves traveling almost the speed of light, the light they emitted was still going c.

The bottom line is simply that your common sense is based upon observations of things around you (chairs, desks, people and so on) that are not moving very fast at all. You have no common sense about things moving near the speed of light. You cannot therefore rely on common sense beyond its limitations. Experiments show us how the universe works, not common sense.

And, no, your post does not belong here at all. Please don't make a habit of posting non-mainstream theories in the wrong forums here.

- Warren

 

[kurious]

Light speed = electric field / magnetic field
This is considered to be always equal to 3 x 10^8 m/s in a vacuum for all wavelengths of light.

 

[chroot]

Not quite kurious. I think you mean:

$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$

http://www.google.com/search?num=30...nstant+*+the+magnetic+constant)+=&btnG=Search

- Warren

 

[marcus]

Not quite kurious. I think you mean:

$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$

http://www.google.com/search?num=30...nstant+*+the+magnetic+constant)+=&btnG=Search

- Warren

A physicist once explained in my hearing that
$$\mu_0$$
and
$$\epsilon_0$$
are not real physical quantities
they are not actual measurable properties of empty space
but are fictions, their formal existence established by convention within the SI system

therefore according to him it does not explain why the speed of light is c
if one merely demonstrates that putting $$\mu_0$$
and
$$\epsilon_0$$
into Maxwell equations causes one to get out a speed which is c


So I have come to regard this equation:
$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$
as a kind of tautology or a stacked deck

and then the meter got redefined as the distance light in vacuo travels in
exactly 1/299792458 of a Cesium133 atomic clock second which means if you use metric units the speed of light cannot logically be other than
299792458 of those meters per second. It is therefore fruitless to ask why it is what it is. It is the standard speed. (for the metric system and probably for mother universe as well)

I guess maybe the thing to ask is not why is it what it is (why is it 299792458 instead of 299792459?)
but to ask why is it constant? why is it AFAWK the same everywhere in universe? why is it the same for all observers even if the buggers are moving vis-a-vis us? why is there a constant speed standard in the U?

 

[Mike2]

That there is a limit at all suggests that there are properties of spacetime that prohit faster speeds. What properties could this be? They probably have something to do with curvature.

What other properties can spacetime have? What you're suggesting is that anything other than dimensions and how they curve is an artificial device? Can this be right? Can you define fields (scalar or vector) on spacetime without it being a property of the background without being arbitrary? In other words, where could such fields come from if they are not derived from spacetime itself? And if they can not be derived from the spacetime background, can they be real?

 

[urtalkinstupid]

Common sense is a branch of logic. Common sense says speed is dependent on velocity of source or object. My common sense is actually derived from reality based situations. For light to be constant, it would have to have the characteristic of being in two palces at once. Two trains traveling towards each other. Light is coming from two directions. Both light beams hit the two trains. Will the light hit the observers at the same time? It is said yes. Why can it not? Light can not go into the back and front of the train at the same time. By the emitter theory, do you mean how light is emitted and absorbed through different densities?

chroot, take a look at this. I'm sure you won't think much of it. It makes sense to me. After reading Einstein's work and reading this. I've found the link I'm fixing to provide you to make more sense. That's just me though. :rofl: Here it is

 

[Nereid]

Sorry, mentors, if my expression of opinion shouldn't go here but rather in a thread in Theory Development.

And, no, your post does not belong here at all. Please don't make a habit of posting non-mainstream theories in the wrong forums here.

Common sense is a branch of logic. Common sense says speed is dependent on velocity of source or object. My common sense is actually derived from reality based situations. For light to be constant, it would have to have the characteristic of being in two palces at once. Two trains traveling towards each other. Light is coming from two directions. Both light beams hit the two trains. Will the light hit the observers at the same time? It is said yes. Why can it not? Light can not go into the back and front of the train at the same time. By the emitter theory, do you mean how light is emitted and absorbed through different densities?

chroot, take a look at this. I'm sure you won't think much of it. It makes sense to me. After reading Einstein's work and reading this. I've found the link I'm fixing to provide you to make more sense. That's just me though. :rofl: Here it is

I think chroot includes material such as that in 'aliceinphysics' as non-mainstream theories. If you would like to discuss the misunderstanding and misrepresentation of relativity that are evident in 'aliceinphysics', please start a thread in Theory Development.

You might also look through previous posts in PF; IIRC 'aliceinphysics' has been clearly shown to be inconsistent with observational and experimental results.

 

[chroot]

urtalkinstupid,

You absolutely must stop posting personal theories and crackpot websites to the general physics forums. Consider this your final warning.

If you'd like to discuss your train "paradox," please provide a little more detail -- about where the light is coming from, etc. -- and I will be happy to show you how there is no paradox at all.

- Warren

 

[urtalkinstupid]

I'm sure you heard of this "paradox." It really isn't a paradox at all. It was the scenario that Einstein used to support his claims of light traveling at c. It is the old train and lightning scenario. I'll PM you it, if it's cool with you, chroot?

 

[chroot]

No need to PM it. Post it here, and we'll help you understand it.

The simple fact is that tens of thousands of people in the scientific community have thoroughly examined the theory of relativity for over a century, and there are no paradoxes in it. Any paradoxes you claim to find are only evidence of your own misunderstandings.

- Warren

 

[ArmoSkater87]

urtalkinstupid, I don't understand why u are trying to say that light speed isn't constant. It has been proven experimentally sereral times. For example, light coming from stars has been measured from different parts of the world, and every time the speed of light ended up being the same exact number. You might think it might be simply because the Earth's orbiting speed is so little compared to the speed of light...well it doesn't matter because chroot pointed out that light has been measured from pions moving at nearly the speed of light, the speed of light measured was once again the same. If light speed wasnt constant, then the time dilation equation predicted by relativity would be wrong. But the fact is that the model for time dilation is very right and has been proven several times with precise atomic clock. So please, just don't argue about something that has been proven true in so many ways and so many times.

 

[chroot]

ArmoSkater87,

He's arguing against constant c not because he isn't aware of the experimental evidence; he's arguing only because to him it doesn't make "sense." As he's said, he is not concerned with reality.

- Warren

 

[selfAdjoint]

A physicist once explained in my hearing that
$$\mu_0$$
and
$$\epsilon_0$$
are not real physical quantities
they are not actual measurable properties of empty space
but are fictions, their formal existence established by convention within the SI system

therefore according to him it does not explain why the speed of light is c
if one merely demonstrates that putting $$\mu_0$$
and
$$\epsilon_0$$
into Maxwell equations causes one to get out a speed which is c


So I have come to regard
$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$
as a kind of tautology or a stacked deck

Marcus, you can measure $$\epsilon_0$$ and $$\mu_0$$! I don't know where your physicist got this relational stuff, but it would take more than a rumor to convince me they are not as real as the fine structure constant. Maybe you can expain further?

 

[marcus]

Marcus, you can measure $$\epsilon_0$$ and $$\mu_0$$! I don't know where your physicist got this relational stuff, but it would take more than a rumor to convince me they are not as real as the fine structure constant. Maybe you can expain further?

you are closer to being the expert on this so I am hoping you will explain the situation to me!

All i can do is start the ball rolling. I think you will agree that using standard SI metric units one cannot measure the speed of light. It is established by the convention that defines the meter
to be exactly 299792458
Now that is all right because we have to have some convention to define the meter-----and it USED to be measured when the meter was defined as the length of a certain piece of metal in Paris. So everybody is happy with that and we accept the fact that the speed of light is not measurable.

But what are these epsilon and mu naughts? Are they experimentally measurable or not? Do they actually mean anything? Let us go to the National Institute of Standards and Technology (NIST) and check them out.
They may merely be notational conventions---MAY, this is unsubstantiated as you say but let's check it out and try to think what measuring them would amount to.

I'm quite prepared for you to be right (it is conventional to suppose these naughty epsilon and mu things are real physical properties of the vacuum) but I am just dubious or uncertain of that to desire some discussion.

 

[marcus]

Here we are:
http://physics.nist.gov/cuu/Constants/

And here are the universal ones, from that menu

http://physics.nist.gov/cgi-bin/cuu/Category?view=html&Universal.x=80&Universal.y=8

the "characteristic impedance of the vacuum" should be the first
on the list and then there should come epsilon and mu naught

 

[marcus]

Goodness!
It says here that the value of mu_0 is exactly
$$4\pi \times 10^{-7}$$

how can something like 4pi be measurable?
surely this can't be an actual physical property of empty space
what kind of intrument can you picture using to measure it?

 

[russ_watters]

The simple fact is that tens of thousands of people in the scientific community have thoroughly examined the theory of relativity for over a century, and there are no paradoxes in it. Any paradoxes you claim to find are only evidence of your own misunderstandings.

- Warren

Not sure who I would address this to (Einstein's editor?), but names like "train paradox" and "twins paradox" are somewhat misleading. They are only paradoxes when using incorrect physics, ie applying Galilean relativity to situations where it doesn't apply. People read "twins paradox" regarding Relativity and assume its a paradox in Einstein's theory. It isn't. How about "apparent train paradox"? I guess when Einstein was doing his work though, they were still unresolved paradoxes.

I don't understand why u are trying to say that light speed isn't constant. It has been proven experimentally sereral times.[emphasis added]

[/understatement]

 

[zefram_c]

Goodness!
It says here that the value of mu_0 is exactly
$$4\pi \times 10^{-7}$$

how can something like 4pi be measurable?
surely this can't be an actual physical property of empty space
what kind of intrument can you picture using to measure it?

I think I know why it would be said that $$\mu_o$$ and $$\epsilon_0$$ are not physically measurable. They depend on the way we define quantities like one metre, one second, one unit of current, etc. For example, in SI one Ampere is that current that enters the Biot-Savart law so that $$\mu_o$$ is *exactly* as shown. These dimensionful quantities can always be redefined. I have not done the math, but I think we could say that light travels exactly one meter per second, then adjust all other fundamental quantities to recover the same physics. There is a principle that says physics must be independent of the choice of units. On the other hand, no amount of tinkering can let us redefine the value of the fine structure constant, as that is a dimensionless parameter and must be preserved if we redefine our choice of units. That is truly a fundamental parameter of the observed universe.

Not sure who I would address this to (Einstein's editor?), but names like "train paradox" and "twins paradox" are somewhat misleading. They are only paradoxes when using incorrect physics, ie applying Galilean relativity to situations where it doesn't apply. People read "twins paradox" regarding Relativity and assume its a paradox in Einstein's theory. It isn't. How about "apparent train paradox"? I guess when Einstein was doing his work though, they were still unresolved paradoxes.

An excellent point; I prefer to refer to such things as the "twin scenario" or the "twin experiment" to emphasize that there is no unsolved paradox threatening relativity there.

 

[marcus]

...
So I have come to regard this equation:
$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$
as a kind of tautology or a stacked deck
...

the way I originally wrote this, it could be misunderstood to mean
that I'm considering the speed of light to be a tautology, so I edited it
to make clear that it is this equation that represents the stacked deck.

By its defintion the value mu_0 (in SI terms) is exactly
$$4\pi\times 10^-7$$

and epsilon_0 is actually defined in terms of mu_0 and the speed of light.

so if you somehow know what mu_0 is, then you cannot possibly measure epsilon_0, what it has to be is established by convention

$$\epsilon_0 = 1/\mu_0c^2$$

So the value of epsilon_0 (in SI terms) is also conventional and is exactly

$$\frac{1}{299792458^2\times 4\pi\times 10^-7}$$

 

[hemmul]

Yes, hemmul, the particles you describe are called tachyons. Their mechanics are already well-understood, but there has been any evidence that they actually exist.

- Warren

Thanks for You reply, Warren,
well, to say the truth I'm not an expert in tachyons ;) what i posted is just some general "scatch"... actually my "question", stated like this:

One of the "difficult" questions about that is "why do we live in [0,c] range?"

is incorrect - because its basis violates the relativity principle :shy:
it's obvious, that in our RF available-velocity-range will be [0,c], but for tachionic observer this range (still talking about our range) will represent [c,2c]...?
or is it [nobody knows,nobody knows] ? - as, according to Your reply, we have no evidence of tachyons' existence (if i got it right) - so they have no evidence of our existence...

[edit]
putting all this together in a smarter, equivalent, form:
according to our knowledge about tachyons, what is their upper-bound velocity? (relatively to us)
[/edit]

 

[marcus]

I think I know why it would be said that $$\mu_o$$ and $$\epsilon_0$$ are not physically measurable. They depend on the way we define quantities like one metre, one second, one unit of current, etc. For example, in SI one Ampere is that current that enters the Biot-Savart law so that $$\mu_o$$ is *exactly* as shown. These dimensionful quantities can always be redefined. I have not done the math, but I think we could say that light travels exactly one meter per second, then adjust all other fundamental quantities to recover the same physics. There is a principle that says physics must be independent of the choice of units. On the other hand, no amount of tinkering can let us redefine the value of the fine structure constant, as that is a dimensionless parameter and must be preserved if we redefine our choice of units. That is truly a fundamental parameter of the observed universe.
...

I must say, in my turn, that this is an excellent point!
And then the question becomes, how are current and voltage ACTUALLY standardized and how are they actually measured at the national labs and BIPM in paris etc.

And in fact current is not, in practice, standardized by measuring the force between parallel wires but by declaring an exact conventional value for the Von Klitzing constant and using quantum hall effect.
So the current scale is referred to the voltage scale (not to force)

And in fact voltage is measured by Josephson junction and standardized by declaring an exact conventional value for the Josephson constant so the voltage scale, operationally or in practice, is based on frequency----the frequency standard of the atomic clock.

The conventional exact values of Klitzing and Josephson constants
used to establish the CIPM 1990 standard ampere and standard volt are listed at the NIST site and called R _{K_90} and K _{J_90} .


but if volt and amp are measured this way then there is no grounds to have those values we mentioned for the epsilon and mu naught.
It really is a muddle and as twisty as some theological issue like the infallibility of the pope.

I will go get the conventional exact values of R _{K_90} and K _{J_90} .
they should really make them the official definitions of volt and amp and let the chips fall where they may---get it straight and damn the consequences

They are in the "adopted values" menu at NIST
the josephson
K _{J_90} = exactly 483597.9 x 10⁹ Hz V^-1

the von Klitzing resistance is
R _{K_90} = exactly 25812.807 ohms

 

[zefram_c]

Usually the definitions of the fundamental units are set so that they can be easily used as a reference and can be reproduced worldwide. So when the speed of light could be measured more accurately than a "metre" (which I think used to be the wavelength of some atomic frequency times an integer constant), the metre was eventually redefined according to the speed of light measurement.
There is really no a priori reason to set $$\mu_o$$ to what it is; and we could trade in its definition to fix, say, the von Klitzing resistance. Personally I'm all for that, since it's much better than to measure the very small attractive force between two infinitely long wires anyway. As for the Josephson constant, we can trade in the antiquated definition for the kg, which is a prototype somewhere in Paris. But there seems to be no pressing need for such changes. In any event, the system of choice for most theorists is one in which $$h = c = \epsilon_0 = \mu_0 = 1$$, or maybe multiples of pi depending on what equation you want to simplify. Under this choice, the electron charge can't be set to one.