The Speed of Light, Gravity, and Why it all is what it is.

[Phyzwizz]

I was recently watching part 1 of the Nova special The Elegant Universe based on Brian Greene's book of the same name. It was mentioned that through Einstein we learned that Gravity travels at the same speed of light. What relationship exists between Gravity and Light that makes them travel at the same speed?

Also besides the speed of light relationships between light and gravity, are there any other forces of nature that exist at this same speed? What aspect do all these things possesses that make them travel at such a speed? Is it even known why light travels at the speed that it does?

Thanks I know there are a lot of questions in here.

 

[bcrowell]

The modern way of looking at it is that the c in relativity is fundamentally a maximum speed of cause and effect. It can also be proved to be the speed at which massless particles travel. Since light and gravity waves are both massless, they both travel at c. The gluon is another example.

Is it even known why light travels at the speed that it does?

The value of c has a defined value in the SI. The reason it has this value is that the meter is defined as the distance light travels in a certain fraction of a second.

 

[Pengwuino]

The forces are mediated by massless quanta (although the graviton is still hypothetical). Massless anything travels at the speed of light exclusively. Photons are the only particles we have observed that are massless and are free (gluons have not been observed to be free, but they are massless) so everyone says "speed of light" when in actually, we really mean "speed of massless particles".

 

[Phyzwizz]

Thanks.
And I assume that its unknown why the universe landed on the value it did for the speed of light(I'm not talking about the system of measurement but rather why light travels at 299 792 458 m / s or 670 616 629 mph or 1.07925285 × 109 kph or whatever other system of measurement is chosen, they're all the same speed, why that speed?

 

[Pengwuino]

Thanks.
And I assume that its unknown why the universe landed on the value it did for the speed of light(I'm not talking about the system of measurement but rather why light travels at 299 792 458 m / s or 670 616 629 mph or 1.07925285 × 109 kph or whatever other system of measurement is chosen, they're all the same speed, why that speed?

Yup that's one of those "why?" questions that can't be answered - that's just the way it is.

 

[bcrowell]

Thanks.
And I assume that its unknown why the universe landed on the value it did for the speed of light(I'm not talking about the system of measurement but rather why light travels at 299 792 458 m / s or 670 616 629 mph or 1.07925285 × 109 kph or whatever other system of measurement is chosen, they're all the same speed, why that speed?

The question isn't meaningful. The reason it has the value it does is purely a matter of definition.

Yup that's one of those "why?" questions that can't be answered - that's just the way it is.

I disagree.

There's a good discussion of these issues in this paper: Duff, http://arxiv.org/abs/hep-th/0208093

It makes sense to ask why God made a unitless universal constant, like the fine structure constant, have a certain numerical value. It doesn't make sense to ask why God made a universal constant with units have a certain numerical value. The reason a unitful constant has a particular value is because the system of units was constructed by humans in a certain way.

If God doubled the speed of light at midnight tonight, but also changed e and h so as to keep the fine structure constant the same, there would be no empirically observable consequences.

 

[Pengwuino]

I disagree.

The way I read the question is that is there a reason (if you independently define c = 3x10^8m/s) why it is the value it is and not say, 50m/s. Sure you could talk about a time-dependent structure constant, but you would still run into the problem of why the values are the way they are at some level of the analysis where you simply have to accept what is given to you by nature.

 

[bcrowell]

The way I read the question is that is there a reason (if you independently define c = 3x10^8m/s) why it is the value it is and not say, 50m/s. Sure you could talk about a time-dependent structure constant, but you would still run into the problem of why the values are the way they are at some level of the analysis where you simply have to accept what is given to you by nature.

The reason it's not 50 m/s is that humans didn't define the meter to be the distance traveled by light in 1/50 of a second.

In fact, I could claim that it is 50 m/s today, as opposed to its value yesterday of 299792458 m/s. But it just happens that e and h also changed last night from their previous values to new values that preserve the value of the fine structure constant. As a side-effect, all our clocks have changed the rate at which they tick, and all our meter sticks have changed their lengths. Therefore, we think that c is still 299792458 m/s, even though it's really changed to 50 m/s.

This is the essential point being made in the Duff paper.

 

[Pengwuino]

The reason it's not 50 m/s is that humans didn't define the meter to be the distance traveled by light in 1/50 of a second.

In fact, I could claim that it is 50 m/s today, as opposed to its value yesterday of 299792458 m/s. But it just happens that e and h also changed last night from their previous values to new values that preserve the value of the fine structure constant. As a side-effect, all our clocks have changed the rate at which they tick, and all our meter sticks have changed their lengths. Therefore, we think that c is still 299792458 m/s, even though it's really changed to 50 m/s.

This is the essential point being made in the Duff paper.

No no, that's not what I meant. You could define the meter and second independently of the speed of light. Then you would say the meter is defined as this, the second is this, ergo by observation the speed of light is X. Then one could ask well, is there something special about X and the answer is no, X is simply what we measure the speed of light to be. I'm saying that whatever the speed of light is, it's an empirical observation and can't be derived from anything to say the value X is preferred over some other value Y.

 

[bcrowell]

No no, that's not what I meant. You could define the meter and second independently of the speed of light. Then you would say the meter is defined as this, the second is this, ergo by observation the speed of light is X. Then one could ask well, is there something special about X and the answer is no, X is simply what we measure the speed of light to be.

No, X is what we forced the speed of light to be when we defined (in the original SI) the second to be 1/86400 of a mean solar day and the meter to be the distance bewteen two scratches on a metal bar. If the speed of light changed while keeping the fine structure constant (and all other unitless constants of nature) the same, then X would still have the same value, because the length of the mean solar day would change and the length of the bar would change. (Since the Earth's orbit is controlled by gravity, I suppose you'd also have to readjust things so as to maintain the values of certain unitless constants related to gravity, e.g., the ratio of the mass of the electron to the Planck mass.) Again, all I'm doing is explaining the point of the Duff paper.

 

[atyy]

It can also be proved to be the speed at which massless particles travel. Since light and gravity waves are both massless, they both travel at c.

The forces are mediated by massless quanta (although the graviton is still hypothetical). Massless anything travels at the speed of light exclusively. Photons are the only particles we have observed that are massless and are free (gluons have not been observed to be free, but they are massless) so everyone says "speed of light" when in actually, we really mean "speed of massless particles".

How about phonons? Do they count as massless free particles? If they do, why don't they travel at the speed of light?

 

[Pengwuino]

No, X is what we forced the speed of light to be when we defined (in the original SI) the second to be 1/86400 of a mean solar day and the meter to be the distance bewteen two scratches on a metal bar. If the speed of light changed while keeping the fine structure constant (and all other unitless constants of nature) the same, then X would still have the same value, because the length of the mean solar day would change and the length of the bar would change. (Since the Earth's orbit is controlled by gravity, I suppose you'd also have to readjust things so as to maintain the values of certain unitless constants related to gravity, e.g., the ratio of the mass of the electron to the Planck mass.)

Nooooooooooooooooo I'm not talking about how you could make everything the same by changing everything else. Of course you could! I'm just saying if you just create the world independent of the speed of light, didn't let other constants change, and just observed the speed of light, that whatever you observed the speed to be has no deeper cause than what it is. Nothing tells the speed of light to be what speed it is. Take, for example, a blackbody spectrum. THAT has a deeper theory that tells you why the blackbody spectrum is the way it is. That is unlike the speed of light which is empirical; it has no deeper theory as far as we know.

 

[bcrowell]

How about phonons? Do they count as massless free particles? If they do, why don't they travel at the speed of light?

They're a vibration of a medium that has mass.

Nooooooooooooooooo I'm not talking about how you could make everything the same by changing everything else. Of course you could! I'm just saying if you just create the world independent of the speed of light, didn't let other constants change, and just observed the speed of light, that whatever you observed the speed to be has no deeper cause than what it is. Nothing tells the speed of light to be what speed it is. Take, for example, a blackbody spectrum. THAT has a deeper theory that tells you why the blackbody spectrum is the way it is. That is unlike the speed of light which is empirical; it has no deeper theory as far as we know.

The speed of light isn't empirical. It's defined. So are e and h and G. Their numerical values are all defined, directly or indirectly, by the system of units we use. The purpose of talking about changing the numerical values of the constants is to show that you can never determine them empirically, you can only define them. If you could measure them empirically, then you could tell when they changed over time. But you can't tell when they change over time.

 

[Pengwuino]

The speed of light isn't empirical. It's defined. So are e and h and G. Their numerical values are all defined, directly or indirectly, by the system of units we use. The purpose of talking about changing the numerical values of the constants is to show that you can never determine them empirically, you can only define them. If you could measure them empirically, then you could tell when they changed over time. But you can't tell when they change over time.

We seem to be at a disagreement on what it means to define something vs. observe it and in what sense the OP is questioning what 'c' is. I know exactly what you mean, but I don't think that is the sense the OP is wondering why 'c' is what it is.

 

[bcrowell]

We seem to be at a disagreement on what it means to define something vs. observe it and in what sense the OP is questioning what 'c' is. I know exactly what you mean, but I don't think that is the sense the OP is wondering why 'c' is what it is.

I think what we're disagreeing on is not what the OP was asking but whether the question the OP asked was meaningful.

 

[atyy]

They're a vibration of a medium that has mass.

But is the vibration itself massless?

 

[GrayGhost]

Thanks. And I assume that its unknown why the universe landed on the value it did for the speed of light(I'm not talking about the system of measurement but rather why light travels at 299 792 458 m / s or 670 616 629 mph or 1.07925285 × 109 kph or whatever other system of measurement is chosen, they're all the same speed, why that speed?

The question isn't meaningful. The reason it has the value it does is purely a matter of definition.

Bcrowell,

When you say c is a matter of definition alone, I take it you mean the 1-way speed of light, yes? I mean, we've measured the 2-way speed at only c. We don't know why it's 2-way speed is precisely its value, nor do we know why it is invariant. However, we do know that the value of c is determined. The design of SR reveals the symmetry of space and time required (ie Lorentz symmetry) if an invariant light speed exists, but that's like the chicken vs the egg deal. We first assume an invariant c, then we design the required math model for that to be the case. Then we (or many) claim that the reason light's speed must be invariant is because of the inherent symmetry in space and time. There is a reason for everything, whether we know why yet or not. There's something that causes both the invariance and symmetry to be as they are.

My position is that the question is meaningful. In fact, it's no less meaningful a question than had Plato (hypothetically) asked whether the Earth might not be flat. IMO, this question will likely be answered only because the right individual thinks about the question long enough ... why is c it's present value? Maybe the better question is this ... why does light move at all, let alone at c? If gravity moves at c, then it stands to reason that the precise value of c is as much about the medium as it might be about the light. I'm convinced the question will eventually be answered to most everyone's satisfaction, and likely lead to major advancements in cosmology and physics, as it may be key to a complete unification. Answer that question, and we may be led to answers of other yet unanswered important questions. In fact, it may well be ranked as one of the greatest answers of all time when it's done and said. On the other hand, maybe there is no reason for the value of c whatever, and we will never know why :) That would be somewhat depressing in my view.

GrayGhost

 

[Dale]

I agree fully with bcrowell. Any dimensionful universal constant has its value due to the choice of units. They essentially function as unit conversion factors.

The meaningful question is why does the fine structure constant have the value it does. And that one is indeed unknown.

 

[bcrowell]

When you say c is a matter of definition alone, I take it you mean the 1-way speed of light, yes? I mean, we've measured the 2-way speed at only c.

What I'm saying applies to all measurements of c.

We don't know why it's 2-way speed is precisely its value, nor do we know why it is invariant.

I would claim that we do know why it has the value it does -- because of our choice of units. Depending on what assumptions you start from, its invariance can be either an unprovable postulate (in Einstein's 1905 axiomatization) or a provable proposition ( http://arxiv.org/abs/physics/0302045v1 ).

 

[tkav1980]

I think the question being debated back and forth is why is C a universal constant. What in the make up of space-time itself causes C to be C and remain constant for all zero mass particles/waves/ anything.

The way i always thought about C was little packets of vibrations of a specific frequency and amplitude "vibrating" through a medium. Sound travels through pure iron at a verry specific speed, Light does the same with the universe. Light would travel at this velocity with or without our observation or measurement, it simply wouldn't be labeld c or measured as roughly 300000km/s.

I think the op was asking if we understand the mechanism of space-time itself that created this constant. Similar to the idea that sound moves through steel at velocity x, through water at velocity n... and so forth. We know the reason soundwaves change depending on medium is density of that medium. But I've never seen an answer to the question of C being constant because space has a specific density, or some mechanism that is measurable and appropriate for space...etc.. I myself would love to be able to understand this.


I hope my rambling made some sence. It doesn't always come out on the screen the way its ordered in my head.

 

[GrayGhost]

I agree fully with bcrowell. Any dimensionful universal constant has its value due to the choice of units. They essentially function as unit conversion factors. The meaningful question is why does the fine structure constant have the value it does. And that one is indeed unknown.

Indeed, any dimensionful universal constant has its value due to the choice of units. However, no matter what unit of measure one elects, a specific value is determinable (sometimes based upon assumptions). Wrt the OP, it seemed to me that the question was not as to why c is 299,792.458 m/s vs (say) 6.7 x 10⁸ mi/hr. As I read it, the OP's question was as to why it possesses its unique value "for any specific units elected", versus some other specific value. Yes?

GrayGhost

 

[bcrowell]

I think the question being debated back and forth is why is C a universal constant. What in the make up of space-time itself causes C to be C and remain constant for all zero mass particles/waves/ anything.

No, this isn't the issue. Based on the symmetry properties of spacetime, c must be a universal, frame-invariant constant: http://arxiv.org/abs/physics/0302045 In that sense, we do understand why c is a universal constant.

The way i always thought about C was little packets of vibrations of a specific frequency and amplitude "vibrating" through a medium. Sound travels through pure iron at a verry specific speed, Light does the same with the universe. Light would travel at this velocity with or without our observation or measurement, it simply wouldn't be labeld c or measured as roughly 300000km/s.

No, light isn't a vibration of a medium.

Indeed, any dimensionful universal constant has its value due to the choice of units. However, no matter what unit of measure one elects, a specific value is determinable (sometimes based upon assumptions). Wrt the OP, it seemed to me that the question was not as to why c is 299,792.458 m/s vs (say) 6.7 x 10⁸ mi/hr. As I read it, the OP's question was as to why it possesses its unique value "for any specific units elected", versus some other specific value. Yes?

The reason it has the specific value of 299792458 m/s is because the meter is defined as 1/299792458 of a second.

 

[Dale]

As I read it, the OP's question was as to why it possesses its unique value "for any specific units elected", versus some other specific value. Yes?

There is no other reason besides the choice of units.

 

[pervect]

Thanks.
And I assume that its unknown why the universe landed on the value it did for the speed of light(I'm not talking about the system of measurement but rather why light travels at 299 792 458 m / s or 670 616 629 mph or 1.07925285 × 109 kph or whatever other system of measurement is chosen, they're all the same speed, why that speed?

A second is a second because of historical accident, as other posters have mentioned. The same with the meter.

We could ask instead about the relationship between the time scale of the hydrogen atom, the characteristic time for one cycle of the lowest radiation, the "size" of the hydrogen atom, as measured by its "bohr radius", and the speed of light. What if we expressed 'c' in these hydrogen-atom based units?

I'm not sure if you'd be interested in that sort of answer, but physicists are. If you are too, we could go into more details - I'll give a gentle spoiler by saying it leads to something called the "fine structure constant" - which is dimensionless, so it avoids any squabble over unit-dependence.

 

[tkav1980]

Bcrowell, The arXiv paper you linked me to is a bit above, ok i lied, its way above my head. I am quite a few years removed from anything besides basic math and cannot really understand it. I understand that light isn't actually a wave through a medium, I guess my attampt at an analogy failed miserably due mostly to my severe lack of knowledge. However, your explinations in this thread does clear a lot up for me.

 

[GrayGhost]

The reason it has the specific value of 299792458 m/s is because the meter is defined as 1/299792458 of a second.

OK then, if I may rephrase ...

Why is the speed of light 299792458 m/s versus say 199792458 m/s?

Why is the meter 1/299792458 of a sec (by light) versus say 1/199792458 of a sec (by light)?

GrayGhost

 

[khemist]

It is what it is. We just live here.

 

[GrayGhost]

It is what it is. We just live here.

That's one answer, and maybe easiest one at that. This is unacceptable though. There is an answer for everything, whether we know the answer as yet or not. Simply because the answer does not come easily, does not lead we should quit or assume it does not exist.

GrayGhost

 

[Quickless]

This might shed some light on your query - http://www.phys.unsw.edu.au/einsteinlight/jw/module3_Maxwell.htm#light

 

[Dale]

Simply because the answer does not come easily, does not lead we should quit or assume it does not exist.

Simply because the answer does come easily does not mean that we should assume it is wrong. The answer has been given: because that is how we chose to define the units.

 

[pervect]

I should probably expand on my earlier response a bit...

Point 1. It's just as important to describe what you aren't changing as what you are. For instance, $c = 1 / sqrt\left(\epsilon_0 \, \mu_0 \right)$. So, obviously if you're changing 'c', you can't keep both these fundamental constants the same, and you need to specify if you're changing only one, or if you're changing both, how they change. It's also not entirely clear what other physical constants you may or may not be holding constant when you "vary c", the procedure operationally isn't well defined.

Point 2. You might be making a simple "scale change", without even realizing it. For instance, suppose your change of 'c' changes the size of every atom in the Earth uniformly. Maybe what looks like a change will vanish if you define a "new meter" that has the same number of atoms laid end-end as the old meter. This is assuming you are using the old-fashioned meter stick standard rather than the SI defintion, which you probably are, because if you were using the modern defintion you wouldn't be talking about varying it, because the new defintion just assigns 'c' a constant value.

 

[Dale]

To further what pervect said, here are a couple of old posts where I went through this kind of exercise in quite a bit of detail:
https://www.physicsforums.com/showpost.php?p=2011753&postcount=55
https://www.physicsforums.com/showpost.php?p=2015734&postcount=68

Also, here is a FAQ entry on the subject that I like:
http://math.ucr.edu/home/baez/constants.html

 

[nitsuj]

Bcrowell, The arXiv paper you linked me to is a bit above, ok i lied, its way above my head. I am quite a few years removed from anything besides basic math and cannot really understand it. I understand that light isn't actually a wave through a medium, I guess my attampt at an analogy failed miserably due mostly to my severe lack of knowledge. However, your explinations in this thread does clear a lot up for me.

Don't let someone elses comment like that discourage your train of thought. Absoulutley electric-magnetic wave forms could be discribed as vibrating.

A quick snap of "No, that's wrong", doesn't add any wieght. Especialy when the responding poster doesn't even read your comment correctly.

You said through a medium. He responded, "No, light is not a vibration of a medium." which may be true, but not even remotely close to what you had said, "light is a vibration through a medium".

So no, your analogy about light being a vibration through a medium is not way off. It is a "vibration" of electro-magnetic waves through a medium.

 

[GrayGhost]

I should probably expand on my earlier response a bit...

Point 1. It's just as important to describe what you aren't changing as what you are. For instance, $c = 1 / sqrt\left(\epsilon_0 \, \mu_0 \right)$. So, obviously if you're changing 'c', you can't keep both these fundamental constants the same, and you need to specify if you're changing only one, or if you're changing both, how they change. It's also not entirely clear what other physical constants you may or may not be holding constant when you "vary c", the procedure operationally isn't well defined.

OK, very good. Now then ...

Why are the electric and magnetic constants for vacuum the values they are, versus different values that might lead to (say) 199792458 m/s versus 299792458 m/s ?

What is it about the nature of the very medium such as to produce the electric and magnetic constants for vacuum at the precise tune they exist today?

GrayGhost

 

[bcrowell]

Why are the electric and magnetic constants for vacuum the values they are, versus different values that might lead to (say) 199792458 m/s versus 299792458 m/s ?

This has been answered over and over.