Why doesn't light travel infinitely fast ?

[sikkemike]

Light doesn't have mass so what's stopping it from going infinitely fast ?

 

[Simon Bridge]

Welcome to PF;
Science doesn't really do "why" questions of this form but I'll try to answer you - you see:
the speed of light in a vacuum is a Property of the Universe.

It's not so much that anything is stopping it from going faster so much as that there is no way to make it, or anything, go faster.

Science investigates and attempts to improve our knowledge of the Laws of Nature - we can say what happens and how it happens, and most "why" questions are actually about them ... for instance: "why is the sky blue" is really about how the sky gets it's color. We can then say, "yes, but why!" It's basically because that is how our Universe works.

 

[nitsuj]

I believe it is geometry that "limits" length divided by time.

 

[sikkemike]

I don't really get what you're saying

 

[nitsuj]

either do I in any really deep sense!

but I do know how length & time are defined, and it involves c.

Read about the light-like interval, and how it is distinctly different from space / time like intervals.
There is no proper time / proper length at the speed c, in turn no metric can be described for something traveling at c, at least not according to how length / time is defined by c.

So if I travel at 0.9999... c I can still measure a proper time / length & in turn calculate a speed for what ever I wish to measure. But at c there is no proper time / length. No way to calculate a speed if you can't take those measurements.

 

[Simon Bridge]

He's saying that c is part of the way the Universe works - but he is going into a bit of detail about the "how". That's where the interesting stuff is.

The upshot is he's telling you to learn about relativity.

 

[Simon Bridge]

I don't really get what you're saying

He's saying that c is part of the way the Universe works - but he is going into a bit of detail about the "how". That's where the interesting stuff is.

The upshot is he's telling you to learn about relativity.
If you want to understand the answers, that's pretty much what you need to do.

Meantime - this question gets asked a lot around here.
Have a look at how other people's questions got answered.

 

[Chronos]

The speed of light is determined by the permittivity and permeability of free space.

 

[xAxis]

Also didn't Poincare show that if space is homogenous, then there must exist maximum posible speed in nature, because otherwise the causality would be reversed?

 

[nitsuj]

Also didn't Poincare show that if space is homogenous, then there must exist maximum posible speed in nature, because otherwise the causality would be reversed?

What do you mean by "space is homogenous"?

 

[Mordred]

A quick wiki search will provide that answer, Homogeneous essentailly means the same throughout. In cosmology it essentailly means that at a certain size portion usually at 100 Mpc (megaparsecs). On the average one region of space is the same as another. Isotropic means that there is no preferred direction. regardless of which way you look its all the same.

Obviously we see large scale structures etc in different directions, however their energy densities will average out per region.

A good example of both is spacetime expansion. no matter which region of space or direction. expansion is the same rate provided that region is not gravitationally bound such as around a large scale structure.

 

[nitsuj]

Guess I'm trying to understand what is the same throughout. Beside the constant c I am unsure what it means?

Just don't get what "same" is representing.

 

[Mordred]

Yeah its a little tricky of a term in regards of cosmology this is a cut and paste expaination from "Introduction to Cosmology " by Barbera Ryden.

On large scales, the universe is isotropic and homogeneous
What does it mean to state that the universe is isotropic and homogeneous?
Saying that the universe is isotropic means that there are no preferred directions
in the universe; it looks the same no matter which way you point your
telescope. Saying that the universe is homogeneous means that there are no
preferred locations in the universe; it looks the same no matter where you set
up your telescope. Note the very important qualifier: the universe is isotropic
and homogeneous on large scales. In this context, \large scales" means that
the universe is only isotropic and homogeneous on scales of roughly 100Mpc
or more.
The isotropy of the universe is not immediately obvious. In fact, on small
scales, the universe is blatantly anisotropic.

 

[nitsuj]

Yeah its a little tricky of a term in regards of cosmology this is a cut and paste expaination from "Introduction to Cosmology " by Barbera Ryden.

On large scales, the universe is isotropic and homogeneous
What does it mean to state that the universe is isotropic and homogeneous?
Saying that the universe is isotropic means that there are no preferred directions
in the universe; it looks the same no matter which way you point your
telescope. Saying that the universe is homogeneous means that there are no
preferred locations in the universe; it looks the same no matter where you set
up your telescope. Note the very important qualifier: the universe is isotropic
and homogeneous on large scales. In this context, \large scales" means that
the universe is only isotropic and homogeneous on scales of roughly 100Mpc
or more.
The isotropy of the universe is not immediately obvious. In fact, on small
scales, the universe is blatantly anisotropic.

Ah I see,

I misunderstood homogeneous, always thought of it in the sense of mixtures; and was wondering what are the components that are thoroughly mixed sorry for such bad terminology.

In that text you generously posted, it reads as though it is strictly via comparison that space is called homogeneous.

 

[my_wan]

An interesting side note is that in Newtonian physics if you went an infinite speed you would get there at the same time you left. In special relativity if you could go the speed of light you would get there the same time you left. Only, under relativity, the same time for you could be years later for somebody else.

That can be characterized as a reason why exceeding the speed of light is tantamount to time travel, because it would mean you could get to your destination before you left.

 

[Drakkith]

I don't really get what you're saying

I don't feel that any of the answers here are going to satisfy you, as they do not answer "why" in a fashion you would accept. The only thing I think you can do is to try to accept the following fact.

There is a maximum speed that anything can travel in our universe. That speed is represented by the letter c and is approximately 300,000 kilometers per second. (About 186,000 miles per second)

Why is it like this? We have no idea. But we do know that a great many things would be much, much different if this value were different. Especially if it were infinite.

 

[Mordred]

Ah I see,

I misunderstood homogeneous, always thought of it in the sense of mixtures; and was wondering what are the components that are thoroughly mixed sorry for such bad terminology.

In that text you generously posted, it reads as though it is strictly via comparison that space is called homogeneous.

.

Thats one valid way to look at it. Its also commonly used in models that represent the overall universe, This is largely a means of simplification. If for example we want to find the average vacuum energy of the universe, sample regions of space and apply the mathematics of that region in a manner that represents the average as homogeneous and isotropic. Many of your Freidman equations apply the same methodology.

 

[chill_factor]

light can travel at arbitrarily large values in appropriate units. You can make the numbers as big as you want by changing units.

 

[Drakkith]

light can travel at arbitrarily large values in appropriate units. You can make the numbers as big as you want by changing units.

How does this have any relevance to the thread?

 

[Simon Bridge]

It strikes me that a lot of the answers are just changing the wording of the question.

The speed of light is determined by the permittivity and permeability of free space.

Technically anyone of those is determined by the other two isn't it? Anyway - doesn't that observation just change the question into why the permittivity and permiability are that way?

Also didn't Poincare show that if space is homogenous, then there must exist maximum posible speed in nature, because otherwise the causality would be reversed?

... and this changes the question to "why doesn't causality reverse?" It's the same question in different words.

I do vaguely recall something like that from Poincare - havn't been able to find a reference.
Wikipedia has a mention of something similar.

But I think Drakkith is right here:

I don't feel that any of the answers here are going to satisfy you, as they do not answer "why" in a fashion you would accept.

... I don't see how empirical science can answer this sort of "why" question in the spirit it appears to have been asked. It seems to be a philosophy question more than anything. The exact same question can be asked of any physical constant - or, indeed, all of them together. We see the values we do because we live here. If we lived in a different Universe then perhaps we'd see different values and wonder about them? But there may be limits on the sets of values that make sense - that produce Universes with physicists for example. That's a triple-whiskey discussion right-there!

The what's and hows are usually more interesting - see how engaged OP is with Mordreds posts about the large-scale nature of the Universe.

So there may be a way forward ... the first post suggests that OP is thinking like this:
Since F=ma, the smaller the mass, the higher the acceleration for a given force.
If the mass is zero, then any force produces infinite acceleration, and so an infinite final speed.
Light has zero mass...

If this is, in fact, the reasoning involved then we can answer this in two ways:
1. pointing out that F=ma is incorrect ... only works for small relative speeds.
2. asking how OP imagines accelerating light - and explore the reasoning process more.

 

[Chronos]

Permittivity and permeability are a consequence of quantum field theory. They are also fundamental to things like coulomb charge and magnetic field strength. It is inaccurate to suggest they are just an alternative way to derive the speed of light.

 

[chill_factor]

How does this have any relevance to the thread?

i was trying to make a point that fast is relative, you can make things arbitrarily fast in terms of absolute values simply by changing units.

 

[Simon Bridge]

Permittivity and permeability are a consequence of quantum field theory.

I understood these terms predated field theory?

They are also fundamental to things like coulomb charge and magnetic field strength.

Possibly that the coulomb charge and magnetic field are fundamental to... nah, I'll not go there: sounds like a chicken-and-egg argument to me ... you win.

It is inaccurate to suggest they are just an alternative way to derive the speed of light.

Never used the word "just" I don't think.
Who was it wrote "the speed of light is determined by the permittivity and permeability of free space" anyway? That worthy individual didn't use the word "just" either. Don't know where you get "just" from. <sulks>

To be fair it was a better answer than some of the others ... since $c=1/\sqrt{\epsilon_0\mu_0}$ then c can only be infinite if one of those is zero.
So the question gets converted into things like "why is the speed of electromagnetic radiation in space also the limiting speed in relativity?" But it is still "it's a property of the universe - tough!" answer.

Excuse me I need my coffee... possibly sugar...

 

[Chronos]

Yes, the concepts of ε and μ do predate quantum theory. These constants of nature can be derived independent of the speed of light: ε can be derived from Coulomb's law: \epsilon_0 = q^2/4 \pi F^2. Similarly, μ can be derived from Ampere's law: \mu_0 = 2\pi r B/ I.

 

[Simon Bridge]

... and, similarly, their relationship with the speed of light - which nicely puts light in there with the rest of E-Mag. I know. Mind you, some readers may need to be reminded.

One of the things about this recurring question about the speed of light is that the person asking does not usually distinguish between the speed of electromagnetic waves in a vacuum and the limiting speed in relativity - which are the same. Very often the question can be interpreted as "why is it that the speed of EM waves happens to be the relativistic limiting speed?"

Though, it is usually about special relativity as taught at secondary or freshman-college level... which is how most people have answered this one.

But, this case, I suspect it's more about Newton's Laws not working for high speed.
Could be wrong...

 

[ImaLooser]

Light doesn't have mass so what's stopping it from going infinitely fast ?

If light were infinitely fast and the universe infinitely large then there would be an infinite amount of radiation impinging on everything all the time. That wouldn't work.

 

[Simon Bridge]

@ImaLooser: I like that one ... by that logic then, since we are not all fried then either the Universe is not infinitely large or light does not go infinitely fast or both (you also neglected the middle: maybe only some light goes infinitely fast). Unfortunately that answer begs the question. The question is "what is stopping it from going infinitely fast?" i.e. how come we are not all fried?

See the problem with this sort of question?

 

[HallsofIvy]

i was trying to make a point that fast is relative, you can make things arbitrarily fast in terms of absolute values simply by changing units.

But your "point" is wrong- and a bit silly. I was kind of hoping that your first post was a a joke.

If I am traveling at 50 km/hr, I could say that I am traveling at 50000 m/hr. That's a larger number but I am NOT moving any faster!

 

[Simon Bridge]

@sikkemike : any of this help?

 

[Drmarshall]

Light doesn't have mass so what's stopping it from going infinitely fast ?

Photons do have a little mass.
According to current ideas NOTHING can go faster than light (except things that CANNOT be used to "convey a message")
You have to accept that until people appear on the scene (one generation hence) willing to EVEN DISCUSS alternative hypotheses

 

[Simon Bridge]

Photons do have a little mass.

Astounding claim: how do you know?
Citation please - so we know what you are talking about.

 

[Drmarshall]

Astounding claim: how do you know?
Citation please - so we know what you are talking about.

You don't, until you find out about "radiation pressure"
In the red it is about one pound per Gigawatt

 

[Simon Bridge]

Radiation pressure is evidence of kinetic energy - not mass.
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Radiation_pressure.html
You must have been aware of the standard description of this phenomenon in terms of massless light?

 

[micromass]

You don't, until you find out about "radiation pressure"
In the red it is about one pound per Gigawatt

Please read this: https://www.physicsforums.com/showthread.php?t=511175

 

[FeynmanIsCool]

You don't, until you find out about "radiation pressure"
In the red it is about one pound per Gigawatt

You must discern between mass and Kinetic Energy!

edit* - I did not see the following page where Simon Bridge noted this correction

 

[Simon Bridge]

And I was expecting some reference to "effective rest mass" in a superconductor, or the experimental limits on the photon mass.

I notice that "pounds" is not a unit of pressure here ... in the context of radiation pressure is not even a unit of mass, but iirc of force. Maybe that is the source of the misunderstanding.

1lb of force being the amount of force needed to accelerate 1 pound by 1 foot per second every second? Or is it to accelerate one pound at one gravity? <checks> Ah - it's the gravity one.

Should be written "lbF", shouldn't it, but people often leave the F off.

 

[tms]

One of the things about this recurring question about the speed of light is that the person asking does not usually distinguish between the speed of electromagnetic waves in a vacuum and the limiting speed in relativity - which are the same. Very often the question can be interpreted as "why is it that the speed of EM waves happens to be the relativistic limiting speed?"

Start with \epsilon_0 and \mu_0, particularly the fact that they have the same values for everybody, regardless of how one is moving with respect to anything else. Because their values are universal, the speed of light is also universal. But to get observers moving relative to each other to measure the same speed for the same bit of light, you have to fiddle with lengths and times in different frames. That fiddling necessarily involves the speed of light, because that is what the fiddling is trying to get right. But the same fiddling has to apply to everything, not just light, because it would be ludicrous for lengths to be contracted when dealing with light but not when dealing everything else; how would the meter stick know to contract when measuring a wavelength, but not when measuring a football field? Since the length and time fiddling apply to everything, and since they imply the impossibility of a massive object reaching the speed of light, the speed of light limits everything.

I realize I am speaking a bit glibly, but I think this chain of reasoning makes sense, modulo the glibness.

 

[Simon Bridge]

It's fun isn't it.
What you end up with is an invariant speed in the equations that describe space-time geometry.

But the fool in his heart may still reason (hopefully in his head) that photons may not travel exactly at the invariant speed and, therefore, may have a very small mass. It may be the invariant speed is actually slightly larger than the observed speed for photons since, as you say, originally the invariant speed was postulated to be the photon speed and all the math worked out on that postulate.

Different observers would, indeed, measure a slightly different speed for light if only their equipment were sensitive enough, and relativity would still work. [*]

This would limit the mass of a photon to be certainly below 10^-14eV/c² (depending on the experiment).

The question in post #1 is certainly phrased in terms of the speed of photons - so maybe it can be answered by pointing out that no particle can be accelerated past the invariant speed of the universe, massive or massless, and so, since the slightest force would accelerate it to that speed, we should not be surprised to see all massless particles move at exactly the invariant speed. The name "the speed of light in a vacuum" is an historical accident.

Hmmm ... it's still glib isn't it?

-------------------------

[*] though there is the curious problem of how come all light is in a very narrow range of speeds ... which, in a way, would be the question in post #1.