Why doesn't light go faster than c?

[blahsd]

hi,
it's my first post in this forum so I hope I'm in the right section. I've asked three different physics teachers in my high school an explanation for this to no avail. if acceleration is inversely proportional to mass, and photons have no mass, why don't they have infinite acceleration? in other words, what poses a boundary to the speed of light?

 

[Pengwuino]

You're assuming F = ma is applicable to massless particles, which it is not. The quick explanation as to why light has a fixed speed is basically that the laws which govern electricity and magnetism, Maxwell's Equations, show that electromagnetic waves travel at a fixed speed in vacuum. A better explanation would require knowing your level of mathematical understanding.

 

[YummyFur]

In fact it was thought that light traveled at infinite speed once and then came Maxwell's equations as just mentioned, then Einstein pondered what this meant and consequently upset a lot of people.

 

[YummyFur]

@jamietmoore, as said in #2 by Penguwino, to understand the border imposed on a ray of light, a person must acquire the discipline of math first, then understanding comes after.

 

[nitsuj]

@jamietmoore, as said in #2 by Penguwino, to understand the border imposed on a ray of light, a person must acquire the discipline of math first, then understanding comes after.

I couldn't disagree more with that. Seems like complete nonsense.

In no way does someone have to know, or use math, to understand the limit of c.

even the simple equation p=mv has quite a depth to it.

It is the concepts behind the p, m and v that is equal to "understand" math.

From the perspective of describing a limit like c using math is merely the language, an extremely well defined one. If you don't understand the definitions (i.e. p, m, v) then you don't understand why c is the limit.

Other wise e=mc2 is an answer to the question. Or rearranged so c= whatever, I don't know how to rearrange equations.

An answer that maybe as vague as an equation is to the poster. It's because the axis of dimensions can't cross each other, that's just the way taking measurements works.

 

[Naty1]

if acceleration is inversely proportional to mass, and photons have no mass, why don't they have infinite acceleration?

one answer: nobody really knows.


But, in a way they do have "infinite acceleration": immediately upon production, say as a result of an electron energy level transition in an atom for example, all photons are measured at "c"...[they are emitted with different energies (frequencies) but not different speeds.]

not that I'd describe it that way: a better explanation, from quantum mechanics, would note that a photon is not a "particle" in the classic sense, it's a wave quanta...and such electromagnetic waves travel at "c" and "only c"...

So you are really asking about the behavior of electromagnetic waves: why are they always observed at "c"? So another view would be that "Maxwell's equations describe electromagnetic radiation and they show a fixed speed of light", not acceleration from one speed to another. [Not that this mathematics describes photons explicitly.]

Another perspective would be that I don't think anyone understands exactly why an energy level transition results in the electromagnetic form of energy transfer,,,like many things in physics, it's what we observe, what we explain, and we are stuck with it. The electromagnetic wave is one form of energy carrier [field] in this universe, along with the strong, weak, and gravitational 'forces'. As to WHY any of those or all of those are observed, I'm not yet sure anyone has a concrete answer.

 

[sophiecentaur]

As photons are always going at the same speed then is there any way that they could 'accelerate'? When they are created, they are instantly at speed c.
And before you start talking about photons being slowed down within matter, afaik, this can be 'explained' in terms of photons moving in small steps at c within the matter and pausing between their journeys.

 

[DarioC]

If you're still around, how about this:

Your question is based on F=ma, and it doesn't apply. If you want to accelerate a car or an atom you have to push on something. The force has to have something to to push against. There is no "something" there in the sense that the formula applies to only things with mass. Bottom line F=ma doesn't apply to electromagnetic fields which is what light is.

It usually helps to reduce concepts and thinking to basics.

Now, why do electromagnetic waves travel at the velocity C, no one knows.

DC

 

[DaveC426913]

To take the OP's question literally, you actually get the right answer.

Massless particles do have infinite acceleration. When a photon is emitted, it immediately has a velocity of c.

But that's a pretty tortuous and misleading path.

And it has nothing to do with why they don't have infinite velocity.

 

[nitsuj]

I have a hard time accepting the no one knows why c is the limit.

How is something as simple as, "because that is what is observed" not acceptable?

I think an acceptable answer would start with dimensions being at right angles to each other, throw in the concept of relative motion, SR postulates and observations and I'd guess that's well on the way to explaining why c is the "limit", or as I read in another thread, specifically why c is invariant (the speed is arbitrary, I think the OP is fundamentaly questioning why c is finite)

I hope to someday understand SR well enough to actualy provide an answer along those lines lol :shy:, diagrams diagrams diagrams

I noticed the infinite acceleration thing to, but refrained from pointing it because I don't think you can consider acceleration to be "infinite", particularly in this case because it implies that the photon had accelerated. Which I don't think they do.

 

[DarioC]

The question was posed, ...if acceleration is inversely proportional to mass...? To my limited knowledge there is no formula for the acceleration of EM waves.

DC

 

[YummyFur]

I have a hard time accepting the no one knows why c is the limit.

Why? There are many things, in a similar vein that no one knows and probably never will. I dare say all the universal constants are like this.

 

[blahsd]

I think I get it. But then, is light a wave or a particle, or both?

 

[YummyFur]

Either or neither depending on your point of view. It is a quantum object, or a 'wavicle' if you like. However if your question relates to its behaviour, then it behaves according to what the experiment is attempting to detect.

I was listening to one of Feynman's university of Auckland lectures last night and he said that light always comes in lumps. It's a good habit to try and remember all the time that when you are visualising any quantum object, it's only for the purposes of understanding some other characteristic of its behaviour, otherwise it's best to take Heisenberg's advice and not even attempt to visualise it at all.

The reason is that you necessarily only have experience visualising classical objects. Similarly relativity theory is only strange because you have no instinctive understanding or experience of moving at very high velocities.

 

[FireStorm000]

I think I get it. But then, is light a wave or a particle, or both?

It's a touchy subject, but it's really just a matter of semantics; either word, in this case, describes the same behavior. QED(Quantum Electro-Dynamics) and SR(Special Relativity) are weird almost beyond comprehension, but if you really want to understand light, that's where you'll have to look.

To give you something of an answer to why light travels at c, you can think of it like it is the fundamental maximum velocity at which space-time can be traversed. I could throw a dozen formulas at you that show how nothing can ever be observed traveling faster than c, because any massive particles will asymptotically approach it, and mass-less particles simply propagate at it(c). I'm not sure there's really an answer out there that will satisfy you; science isn't really in the business of providing those. Science is great for telling you how things work; the why is slightly beyond it's scope.

 

[nitsuj]

Why? There are many things, in a similar vein that no one knows and probably never will. I dare say all the universal constants are like this.

Dare I say all universal constants are well defined, logically and mathematically.

 

[YummyFur]

Dare I say all universal constants are well defined, logically and mathematically.

But I am referring to the 'why', and no one knows, why.

 

[Pengwuino]

I have a hard time accepting the no one knows why c is the limit.

How is something as simple as, "because that is what is observed" not acceptable?

I think an acceptable answer would start with dimensions being at right angles to each other, throw in the concept of relative motion, SR postulates and observations and I'd guess that's well on the way to explaining why c is the "limit", or as I read in another thread, specifically why c is invariant (the speed is arbitrary, I think the OP is fundamentaly questioning why c is finite)

Except special relativity is based on the postulate of the speed of light being constant. The speed of light being constant is completely an observation of nature. Relativity and spacetime is built up on the idea that the speed of light (technically, any massless particle) is constant. Without a kind of universal speed limit, you have no special relativity nor do you have spacetime in its current form. It is totally an observation (in the sense that Maxwell's Equations were simply observed to be true).

The question was posed, ...if acceleration is inversely proportional to mass...? To my limited knowledge there is no formula for the acceleration of EM waves.

In QED, and quantum mechanics in general, forces make little sense and holding onto them as a tool to explain things like photons and light will cause far more problems than solve.

I think I get it. But then, is light a wave or a particle, or both?

It has properties of both. Again, holding onto the idea that something must be either/or will get you into trouble. A lot of modern physics is simply accepting that the world isn't confined to the ideas present in Newtonian physics.

Dare I say all universal constants are well defined, logically and mathematically.

What does this mean? Most universal constants (with a few exceptions such as the fine structure constant) are just man-made numbers. Hell, knowing mathematicians, they'd be happy to set everything equal to 1 and to hell with what reality says Not to worry, we don't let mathematicians handle the dangerous toys, thankfully.

 

[nitsuj]

Except special relativity is based on the postulate of the speed of light being constant. The speed of light being constant is completely an observation of nature. Relativity and spacetime is built up on the idea that the speed of light (technically, any massless particle) is constant. Without a kind of universal speed limit, you have no special relativity nor do you have spacetime in its current form. It is totally an observation (in the sense that Maxwell's Equations were simply observed to be true).

What does this mean? Most universal constants (with a few exceptions such as the fine structure constant) are just man-made numbers. Hell, knowing mathematicians, they'd be happy to set everything equal to 1 and to hell with what reality says Not to worry, we don't let mathematicians handle the dangerous toys, thankfully.

With your first comment, I'm not sure how your point of view is different from mine. My point is; why isn't the answer "Because it's what is observed." not satisfying enough. I go on to describe what I think would be another acceptable answer (which is fundamentaly exactly the same as the first, just more complicated/descriptive).

My second comment is similar to "Relativity and spacetime is built up on the idea that the speed of light (technically, any massless particle) is constant.". Yes, the logic of SR is built up on the postulates. Those postulates are well defined mathematically by the theory itself. In addition, my comment was worded specifically to be similar to what Yummyfur used (for reader specific context :) I only know the constant c.

I don't know what this means. -> "Most universal constants (with a few exceptions such as the fine structure constant)? are just man-made numbers?."

Re-reading the first comment, It might be that your point is the constancy of c is merely observed (measured + calculated). The postulate addresses this. What ever questions come before this postulate maybe for a different forum, idk.

by the way congratulations on the '11 Best Humor award!

 

[Pengwuino]

With your first comment, I'm not sure how your point of view is different from mine. My point is; why isn't the answer "Because it's what is observed." not satisfying enough. I go on to describe what I think would be another acceptable answer.

My second comment is similar to "Relativity and spacetime is built up on the idea that the speed of light (technically, any massless particle) is constant.". Yes, the logic of SR is built up on the postulates. Those postulates are well defined by the theory itself.

I don't know what this means. -> "Most universal constants (with a few exceptions such as the fine structure constant)? are just man-made numbers?."

You're trying to suggest the use of SR to explain why the speed of light is constant when SR exists because the speed of light is constant. In fact, as you stated, saying it is what we observe is pretty much the answer.

As far as constants, yes, they're all just numbers we use as humans. Hell, HEP guys love setting all the constants to 1 which shows that these constants, while they still have physical meaning (although some would argue different), are things we use to simply keep our system of units in place.

 

[YummyFur]

...we don't let mathematicians handle the dangerous toys, thankfully.

there's my new signature

 

[nitsuj]

You're trying to suggest the use of SR to explain why the speed of light is constant when SR exists because the speed of light is constant. In fact, as you stated, saying it is what we observe is pretty much the answer.

I can't reason what you say.

What we observe is the answer on one hand, well on the other what we observe is described with SR. So yes I am trying to suggest SR explains why the speed of light is constant.

 

[nitsuj]

HEP guys love setting all the constants to 1 which shows that these constants, while they still have physical meaning (although some would argue different), are things we use to simply keep our system of units in place.

Oh, I understood it as the units (here i guess seconds and meters) are already "in place" (note we're talking dimensions here). from there the math is derived from quantitative measurements. 1 is nice and all, but saying time is equivalent to length is way cooler. Ha there is another answer to the OP.

 

[DarioC]

Blahsd,
Particle or wave? It seems to me that what one finds overall is, when you impede a "photon" of light, such as with a detector screen or tube, it appears like a particle. When unmolested it behaves like a wave. The connection appears to be when the radiation's energy is absorbed.

The name photon bothers me some because it conveys a sense of a little sphere that would travel, for example, from a star through space to the earth, but it isn't and doesn't. In transit light is a wave, but the waves CAN be divided into "chunks," for example by a single-photon light source.

When the "wave packet" hits something it behaves like a unit of energy and is therefore particle-like, even though the analogy is inadequate. The amount of energy of each "packet" depends on the wavelength/frequency of the light.

The deeper one digs into this, the more interesting it gets.

DC

 

[YummyFur]

... In transit light is a wave...

Aren't you making the same basic error in your description that you are trying to eliminate?

 

[moatasim23]

hi,
it's my first post in this forum so I hope I'm in the right section. I've asked three different physics teachers in my high school an explanation for this to no avail. if acceleration is inversely proportional to mass, and photons have no mass, why don't they have infinite acceleration? in other words, what poses a boundary to the speed of light?

according to special relativity,(and it agrees with experiments)that speed of light is constt.And every observer should agree on its speed.It can never increase or decrease its speed.If it ever had acc then this theory would be violated.

 

[my_wan]

May sound pedantic but think about the question: Why can't you outrun yourself?

In GR in a certain sense the speed of light can change, just not locally with respect to a local observer. The reason is that clock rates also change for different observers. When the clock rate varies it changes how you measure velocities, which changes how you measure distance, and the changes add up to keep the speed of light the same. You may be tempted to think then that the real velocity does change then and it is only time that makes it look the same. Can't count the number of things that goes wrong with that picture but what does a distance even mean if nothing exist to measure it? What does speed mean without clocks? And how would you correct for clocks if right now means something different for everybody and everywhere?

 

[DarioC]

YF,
My complaint is that teaching that light is little round balls flying through space creates all kinds of conceptual problems.
DC

 

[sophiecentaur]

YF,
My complaint is that teaching that light is little round balls flying through space creates all kinds of conceptual problems.
DC

Hear hear.

 

[Danger]

Not to worry, we don't let mathematicians handle the dangerous toys, thankfully.

One of your best!
I didn't even realize that elections were under way. Congratulations for the Humour badge; it's about time that you received one.

YummyFur, one of the main things that we always used to point out is in FireStorm's comment. Science doesn't ask 'why'. That implies some sort of purpose to things being as they are. Instead, the question is 'how'. (Question marks deliberately omitted to avoid complexity.)

If I dare defy Dave, (and I do )... There is a way that I think about the massless acceleration that just makes it easier for me to understand, and might work for you. I don't accept the idea of 'infinite acceleration' with regard to wave packets because that implies no end to it regardless of how fast they get. To me, the term 'instantaneous acceleration' makes more sense. They start at c and remain there.
By the bye... your name is somewhat disturbing on a couple of different levels, and I can't figure out which one I like best.

 

[Neandethal00]

hi,
it's my first post in this forum so I hope I'm in the right section. I've asked three different physics teachers in my high school an explanation for this to no avail. if acceleration is inversely proportional to mass, and photons have no mass, why don't they have infinite acceleration? in other words, what poses a boundary to the speed of light?

I'm not sure if anyone before pointed out another definition for speed of light which depends on the properties of EMPTY SPACE, permittivity (e_o)and permeability (u_o).

c = 1/SQRT(e_o*u_o) = 3x10⁸ m/sec (current value)

If we half the current values of each, permittivity and permeability,

c = 6x10⁸ m/sec

If we double the current values

c = 1.5 x 10⁸ m/sec

If we lower the current values by 10 times

c = 3x10⁹ m/sec.

In other words, you have to go to another universe with different space-time properties to have higher or lower speed of light in VACUUM.

 

[Danger]

In other words, you have to go to another universe with different space-time properties to have higher or lower speed of light in VACUUM.

That's a cool post, which I can't even begin to understand. Your mention of vacuum, however, demands that you specify which light speed you refer to. Photonic speed and 'group velocity' are different. An individual photon travels at c regardless of the medium. It's the collision with atoms factor that impedes propagation of the 'beam'.

 

[Neandethal00]

Your mention of vacuum, however, demands that you specify which light speed you refer to. Photonic speed and 'group velocity' are different. An individual photon travels at c regardless of the medium.

http://upload.wikimedia.org/wikipedia/commons/2/27/Frontgroupphase.gif

red dot --> front velocity
green dot --> group velocity
blue dot --> phase velocity

I see, physicists have now a new velocity in addition to phase and group velocity, it is 'front velocity', which is 'c' for light. Note, group and front velocity in the picture are equal.
ε_o and μ_o are the lowest in vacuum but higher in transparent objects.

I must admit, I exaggerated a bit when I said 'you have to go to another universe'. Because there are some experiment where they have found the 'space' is not completely isotropic in all directions from earth. Speed of light may be different in different regions inside the universe.

 

[Danger]

It might be because I just finished my 31st beer, but that animation did nothing for me other than to make my eyes go wonky.
The beer store might be closed today, so I have an emergency stash of Ballantine's that I'm about to crack open. Perhaps that will be more conducive to my understanding of the subject.

 

[YummyFur]

@Danger, splendid.

@DarioC, it's not just the little round balls, it's the whole wave/particle concepts. Waves and particles were words coined to describe quantum objects long before the whole concept of a quantum object was even the tiniest spark in someone's mind.

Which is why they hunted the ether, because really speaking a wave MUST have a medium because that is what a wave is. Same with 'particle', it a very very very tiny little bit of something, just as Democritus supposed.

So a quantum object is not a wave at all never was never will be, and as for particle, that's just a word that we call bits of atoms, but it's not the same word in the sense of its meaning as say the other way we use particle in the classical world.

Therefore when someone asks the question is a photon a particle or a wave, they are referring to the classical meaning of the words. They must be because they would not ask the question otherwise.

A photon when it displays an interference pattern is not really even behaving like a wave, because it can't. What it could be said to look as if it's doing is behaving as if it was a classical object in some sort of substance and this substance glowed like light and interfered with itself just like a water wave does. But of course there is no substance that's waving, so it's not like that at all and really speaking it can't be.

What is interfering then? Really no one knows. That is just the same as asking 'what is a photon really'. Which is an invalid question. I might snip out a bit from one of Feynman's lectures and post his answer to the 'is it a wave or particle' question.

 

[sophiecentaur]

@Danger, splendid.

@DarioC, it's not just the little round balls, it's the whole wave/particle concepts. Waves and particles were words coined to describe quantum objects long before the whole concept of a quantum object was even the tiniest spark in someone's mind.

...

That is just the same as asking 'what is a photon really'. Which is an invalid question. I might snip out a bit from one of Feynman's lectures and post his answer to the 'is it a wave or particle' question.

That's all quite true and anyone who demands an answer to advanced questions in elementary terms will usually be disappointed.

But it has to be true to say that the wave model explains a lot of phenomena very well so it is well worth using. (Which is more than can be said for the bullet model).

 

[my_wan]

I must admit, I exaggerated a bit when I said 'you have to go to another universe'. Because there are some experiment where they have found the 'space' is not completely isotropic in all directions from earth. Speed of light may be different in different regions inside the universe.

Yes. In General Relativity the speed of light light is not an absolute constant but in some sense varies with gravitation depth. However, since gravitation depth also determines determines the spacetime metric for local observers the local speed of light is always constant at all gravitational depths. You can also have two observers, both in flat spacetime, which nonetheless have differing gravitational depths. The easiest example is an observer inside a uniform massive hollow sphere such that no gravitational acceleration is locally present. Yet this observer will still be gravitationally time dilated by the same amount as an observer on the surface where gravitational acceleration is at a maximum.

This implies that the observer in another Universe with a higher or lower speed of light may still measure their local speed of light as the same constant we do. No different from the speed variability under General Relativity.

 

[YummyFur]

this is Feynman's take on the wave/particle thing http://youtu.be/7HKOm8fffgU

 

[OCR]

I know that definitions are difficult when discussing this type of subject matter , and maybe this is being 'picky', however...

Speed of light may be different in different regions inside the universe.

Using the word inside, wrt 'the universe', also seems to imply an outside, wrt 'the universe'.

I believe the common thinking is, there is no outside... the universe.

Maybe a better way to express the thought would be...?

"Speed of light may be different in different regions of the universe".




Again, definitions...

Same with 'particle', it a very very very tiny little bit of something, just as Democritus supposed.

A 'bit of something' could depend on how the definition of the word 'particle' is applied.

http://en.wikipedia.org/wiki/Particle

http://en.wikipedia.org/wiki/Elementary_particle

By the bye... your name is somewhat disturbing on a couple of different levels, and

I can't figure out which one I like best

I have absolutely no ambiguity about that... lol



OCR

 

[YummyFur]

A 'bit of something' could depend on how the definition of the word 'particle' is applied.
OCR

Like this for example...

...and as for particle, that's just a word that we call bits of atoms, but it's not the same word in the sense of its meaning as say the other way we use particle in the classical world.

 

[Danger]

I have absolutely no ambiguity about that... lol
OCR

Oboy...
At least one of you had damned well better be female, or I'll never live this down.

 

[YummyFur]

Ok, it's a comic by cartoonist Chester Brown, circa early 90's.

 

[YummyFur]

As opposed to non quantum terms?

 

[Neandethal00]

I know that definitions are difficult when discussing this type of subject matter , and maybe this is being 'picky', however...

Using the word inside, wrt 'the universe', also seems to imply an outside, wrt 'the universe'.

I believe the common thinking is, there is no outside... the universe.

Maybe a better way to express the thought would be...?

"Speed of light may be different in different regions of the universe".


OCR

I hesitated a few seconds before using the words inside universe.
But then again, I'm one of those few people who 'thinks' (not believe)
the universe if finite.

 

[rcgldr]

One puzzling issue is the fact that photons are emitted from electrons at c, inspite of the velocity of the electrons. In the case of lasers, the photons are emitted with the same velocity (and phase?) as the photons that triggered the release.

Regarding acceleration, light curves in gravitational fields, which is acceleration in terms of the direction of velocity so there is acceleration in a Newtonian universe. Can the curving only be explained in GR?

As a side question, how long does it take for an electron to emit a photon (it seems it would be related to the size of a photon / c), and transition to a lower energy state?

 

[Danger]

Regarding acceleration, light curves in gravitational fields, which is acceleration in terms of the direction of velocity

I'm not sure that you can count that as an acceleration, since the light 'perceives' its path as a straight line. It doesn't curve within space; the space itself is curved.