well, in that context, "why" could indicate purpose


have a good day all!

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-work in progress--

sabianq,

I really like the viewpoint you express in this thread. When you drew an
analogy between understanding why the speed of light is what it is, and
understanding why the speed of sound is what it is, I immediately thought
"Ah! Excellent analogy!" Even though the differences between the natures
and behaviors are many, great, and diverse, the relevant similarity makes
a useful analogy possible. In the case of sound we can search for and
find reasons why it has the speed that it does. Same with light.

It was suggested in a post above that the speed of light also applies to
some things other than light. That is true, and some posters who know
more about the subject than I, who I hope will contribute to this thread,
have said that even if there were no such thing as light, the speed of
light would still be a fundamentally important fact describing how matter
and spacetime behave.

The original question is a very good one. The answer, as always,
depends on what kind of answer you want. Sometimes korjik's answer
of "c=1/sqrt(epsilon0*mu0)" is sufficient. But we can go on to ask why
epsilon0 and mu0 have the particular values that they do. Korjik's
answer is good if you want to do a calculation. But it is only a small
part of the answer if you want to understand how the world works.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Wierd, it is like someone already posted that in this thread

To expand on the last paragraph:

You could look at the constants (epsilon0 and mu0) and look at what causes them to take those values, then look at what causes the cause, then look at what causes the cause of the cause, but at a certain point you will run into the fact that certain values for certain numbers were set at the creation of the universe. Pi, the fine structure constant, e, c, h, the masses of particles, and so on. Physics cannot tell you the 'why' of those values, it cannot know, and it cannot tell. Both are outside of what physics is.

Beyond what point?

Why at the creation of the Universe? Why not earlier, or later? Why
not different values at different times, or in different places? How do
you know that they were "set"? What set them? How were they set?

Why those values?

Why not?

Why can't it know?

Why?

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Actually there is a big difference between dimensionless constants like pi and e (natural log base, not electron charge) and dimensionful constants like ep0, mu0, h-bar, c, G, etc.

The former are something fundamental. The latter actually just reflect our choice of *units*. Planck units demonstrate this nicely. This gets very deep mind you and you're not going to appreciate fully without a lot of study, but when you do, it all clicks nicely. Indeed, in Planck units, *everything* is of the same dimension, '1'. We measure mass, electric charge, length, time, the "fundamental units" in the *same unit*.

Now, that just flies in the face of dimensional analysis, something all physics students are taught by heart. It is very important to understanding. But it shows us something. Nature doesn't really care about units. That's a crutch *we use* in our models of nature to help us get things right. That's all it is.

The funny thing is, in order to appreciate why you don't really need units, you've got to understand units and dimensions very well. Dimensional analysis helps show, to use the common cliche, "you can't add apples and oranges" (but you can take their ratios and products...). And that's a very important point for a student to get in his head.

Well, it turns out you can add apples and oranges, you've just got to make sure you do it right. Nature herself, through her physical laws tells us how many apples are in an orange so to speak.

If we wish to use indepedent units, we've got to use those dimensionful constants. Many people go no further than this with units and dimensions. That's where it stops. But you if go farther, down deep, you'll see how it's really indeed just a crutch, just a convention we use to better keep the bookeeping straight.

For that reason, many including me half whimsically call Planck units "God's units" -- the point is that there you're looking at the very architectural level of the universe. You're truly, for the first time, discovering there is a natural notion of what "big" and what's small.

You'll find that we humans on our terrestrial scales are pitifully weak (what we think are large forces and accelerations), are pitifully slow (what we think is a short unit of time), and are pretty darn big and clumsy (what we think is a small distance). We can't see the architectural scale of the universe with our eyes. It's like something the size of the Milky Way trying to ponder a wristwatch.

-Richard

EM units in regard to the speed of light are a good example of how our "crutch" of units and dimensionful constants can lead us astray.

Looking at Maxwell at first blush we tend to think there are two degrees of freedom there, two "force constants", ep0 and mu0 that are indepedent and their product is c. Change one and you change c. Ergo, 'c' is a fundamentally electromagnetic property that has to with the electromagnetic properties of the universe, we tend to naively conclude.

And that gets it exactly backwards. 'c' is a more fundamental thing, it has to do with how space and time "mix", which mixture we call space-time.

It's exactly the opposite. The relation between the electric and magnetic, the relation between the force constants, is due to the properties of space-time.

We actually have only on degree of freedom in the EM force constants -- the other is immediately determined by c. Gravity itself does a (non-linear) wave dance, propagating at 'c'. In the field equations of GR, this is expressed explicity in terms of G and c. One force constant, plus c. And the other two forces have a "magnetic like" part as well -- they have to to be relativistic but it's not considered important in most analyses. Ie, when viewed from a moving frame of reference, the color force would have to have a magnetic-like acting part. And that would be related by 'c' the same way the magnetic is to the electric.

EM can be formulated in the exact same way. Gaussian units do that. We could do it in SI as well, speaking of ep0 and c.

But, you can go even further, and the framework of EM "Duality" illustrates this beautifully. You might tend to think of the electric as being the fundamental field, the fundamental force, and the mangetic as being some relativistic effect of that, due to c and the way space-time works. However, Duality shows this is not the case. You can easily consider the magnetic to the fundamental.

Neither is fundamental. They are just different aspects of the same thing. The relation between them is determined by 'c' and space-time.

We get one degree of freedom to choose how much one unit of charge (or one unit of current) is.

-Richard

Thanks for all your answers and I apreciate your input.

Does my theory have any strength at all, my theory being that the universe is made up of a "quark canvass" it would explain alot of things and in the comparison between light and sound certainly fits, as if there was nothing in a vacume then surely light would have infinite speed?

By the way, you should be careful. This is a family-oriented board.

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As above, so below

If we don't know why the speed of light is what it is, then how do we know it can't change? If not why, then the question, I guess, is how did the speed of light come to be what it is?

We assume our constants and values are constant because we have yet to observe any evidence to the contrary. That's how they have always been in this universe, but doesn't the fact that Big Bang has this annoying seemingly inexplicable singularity mean that we don't know enough about the creation of this universe to definitively say that all of our constants are just that? Since we don't know how they became the way they are, then how do we know they really are constant?

The limit is only apparent to our frame. If you're traveling near the speed of light, light will still appear to be traveling at the speed of light with respect to you.

Strange eh?

I personally enjoyed "E=mc2" done by Nova. I think you can watch it online on pbs.org. It helped me visually and it was fun seeing the story they did from Energy, Mass, Speed of light and squaring and wrapping it up all together with enough explanations to actually make things click a bit more.

Two great posts, Richard, thanks.

Yeah but but ...how (not why) can this be? I will forever bow to the one who can explain a way to grok! that. I understand about the raisins in rising bread. And lots of popular cosmic analogies. Is there a way to get ones head around this one or is it like quantum weirdness...just don't go there?

BTW does your statement (which i'm sure is true) imply that the apparent light speed measured by observers moving at some other speed relative to the pair moving at 0.5 c would differ from their measurement?

OOPs, I think I was composing my previous note while you posted this one. Thanks I'll check out the PBS site! And strange YES, a lifetimes worth of puzzlement.

I think I read somewhere that a photon crosses the universe in zero time, from its point of view, to us (any observer) it travels at the speed of light. Not sure if that is 100% true, but it seems to make sense from an Einstein perspective.

If Aristotle knows, "The sun rises because the earth spins."
Does Aristotle necessarily know, "Why does the earth spin?"

Oops.. I almost had a heart attack there.. I really thought you said there was no absolute speed of light, then I reread it and my heart rate came back down... heh.. whew.. close one..

I don't think it matters what the actual values are as long as everything changes in the same proportion. If it were some other value, then that is what we would be using. I think as long as everything works as it should we'll be okay. If it starts to change then scientists will try to figure out why.. and if it changes to where our universe implodes, we won't be here to worry about it.

If I remember right, the singulerity is due to the fact that we don't yet have the math to explain the conditions inside one. It's not necessarily a singulerity because it literally is INFINITELY MASSIVE, our current math just breaks down there, sort of like when God divides by zero and makes a new black hole.

Or am I just totally wrong here?

I think (very basically) it has to do with how as you gain speed you get shorter in the direction you are travelling (to an observer in a different inertial frame) and you gain mass which makes time move slower for you so it takes you longer to move the same distance, so it takes you longer to reach the speed of light. Thus the speed of light is the same no matter how fast you travel.

The shorter compression thing is a simplification of the train/station platform example: You are on a train going past a train station platform, the guy on the platform sees a shorter version of the train go by, you see a shorter version of the platform go by, but neither one of you is shorter (in your own frame).

How badly did I butcher this? (heh)

This is related to quantum physics, namely, both Planck length and Planck time.

Planck length is the distance a photon will travel in a vacuum over a duration of Planck time.

Planck units are key to several governing reasons as to why c, the speed of light, is exactly 299,792,458 m/s in a vacuum. It's exactly that fast, because the speed of light itself is what's used as the standard for a meter.

Planck units themselves are inextricably intertwined with both c and the gravitational constant, G, which is 6.674 28(67) × 10^-11 m³ kg⁻¹ s⁻². Additional Planck units include time, mass, charge, and temperature. The Planck time and temperature are both critical for the unification of GUT and gravity.

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Why are photons limited to light speed? The question needs to be re-addressed because its the other way around, and "light speed" is calculated (originally) from the measured speed of photons (light).

This result will vary according to any criteria thats "variable" in the calculating constraints.

MORE IF INTERESTED,

I understand half of this but I think you make the point that most posters were missing. In special relativity, "the speed of light" is not a matter of the nature of light, it is the nature of spacetime. The material universe has a "speed limit" which is the fastest that anything can go (other than the expansion of the universe), and you probably need to read Einstein himself to understand that. So go learn German for ten years :-) And it's the nature of light to move at the universal limit speed. This also should be true of gravity waves, if we can ever detect them.

For us layman's on this forum i think you get it with this. The very nature of the dynamics of space-time determine C. It was mentioned earlier that for a photon itself no distance or time has been traveled yet it travels at a constant of around 300,000 kps from which ever source it came from and can be measured to be this by any relative observer regardless of their speed or motion. The very fact that the speed remains constant but the distance and time measure different for different frames of reference proves that C is a fundamental dynamic of space-time. If the "fabric" of space-time was different in its earlier existence then maybe the constant of C measured differently then.
(the maximum warp to zero velocity
My line of thought would be that C is the warping of space-time to zero for any given in motion object. And thats the reason that an object of mass cannot reach C because the object its self would need to warp space time to zero but then there would be no volume for that object to exist. Hence the theoretical possibility of a singularity In a sense matter expands space-time to create a volume for its existence. Maybe the Planck length and time values could be considered the shortest distance/time in which volume can be measured in units (3 spacial dimensions)?

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ooops..... wrong forum!!!

Length is relative. Time is relative.

How can you relate the speed of light, a constant, with someone that fluctuates, saying these fluctuating quantaties determine the speed of light?

I see a disconnect.

Examine the equations for time dilation and length contraction. It appears c is a constant. But is it really? Is it merely the speed of light in a vacuum, or the speed of light both in a vacuum and absent any force of gravity? Gravitational time dilation is real. And if gravity affects time, then it also affects c, thus c isn't as constant as we'd like, either.

Now, examine the equations for the basePlanck units. Both G and c are used, always in some sort of ratio. Sometimes it's G/c³, while other times it's G/c⁵ or just G/c (or c/G). If it were always the same ratio, it wouldn't be telling. But because these terms appear in several different powers, and the results have been repeadly verified in the laboratory, it's fairly clear that both G and c are constants, and that any affect of length contraction or time dilation is not the result of a change in either G or c, but rather, the result of another mechanism, notably, the warping of spacetime itself.

By "How can you relate the speed of light, a constant, with someone that fluctuates, saying these fluctuating quantaties determine the speed of light?" I take it that you meant "something," rather than "someone."

Time and length don't "fluctuate," which implies a cyclical effect. Rather, they dilate or contract in response to both relative velocity with respect to both c and G, specifically a ratio of G/c² which is a part of the Schwarzschild Radius of the mass inducing the gravitational tensor.

The final equation is t₀=t_f*sqrt(1-r₀/r), where r₀=2GM/c² (which is the Schwarzschild Radius).

That and the length contraction and time dilation equations of Minkowski spacetime is how I relate c with time and length.

What disconnect are you seeing?

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Apparently one of Einstein's thought experiments is "What would it be like to ride on a photon?" but that may be one that doesn't have a satisfactory answer. A photon doesn't experience time. Or a photon is a packet of energy like a letter in an envelope sent through the mail, or a wrapped parcel; when it's delivered to the destination, it's opened or unwrapped and you can dispose the contents however you please, but in between time it's inviolable.

Sometimes the point of a thought experiment is to expose defective assumptions - that was what Schrodinger's cat was supposed to be getting at.