# Why is there a universal speed limit, c, and why is it what it is?

by CosmicVoyager
Tags: limit, speed, universal
P: 159
 Quote by Naty1 As noted there is no known answer........any more than we know why an electron and proton have the charge we observe.... but is likely due one of the following reasons: (a) nature "selected" this UNIQUE factor because it's a constant that makes our unique and one time one of a kind of universe (and hence us) possible, (b) It's one of an infinite number of possible situations among an infinite number of universes..and this one happened to give rise to us.... (c) "God", Allah, Budda, or anoher designer of your choice, picked it....and maybe us, too...
I am not asking something like you describe above. I am not asking something like why an electron has the charge is does.

What am asking is the same way I would ask why is there a limit to how much mass an object can have, or why is there a limit to ow big and object can be, or why is there a limit to how fast something can spin? It seems there is not reason there should be limits, ad if there is what is limiting factor? What is causing the restriction?
P: 159
 Quote by CosmicVoyager Originally Posted by jambaugh View Post " A better understanding of c, is not so much as the speed limit but as a unit conversion factor. The E=mc^2 equation is an identity "mass is energy" plus a unit conversion. 1kg = c^2 joules. In that context, also note that "speed" is just a slope in space-time and unitless when working in common units. I find this helps to understand the "speed limit" as a geometric phenomenon instead of a dynamic one." This is excellent. I think this is headed in the right direction. Conceptually, the important form of the equation e=mc^2 is c=squarroot(m/e). The real question is why is the speed limit of the universe equal to the square root of the mass of any object divided by it's energy. I get the feeling that it must be, by the very definitions of what mass and energy are. I don't see it yet though. I think this is very close. Something to do with the fact that energy is motion. I will think about it. If anyone can illustrate why that must be so, please do. The square root of mass divided by energy. Hmm...

I hypothesize that it is not a random coincidence that the square root of (m/e) exactly equals the speed limit. I think there is some connection between matter and energy and the speed limit.
P: 3,187
 Quote by CosmicVoyager I am not asking something like you describe above. I am not asking something like why an electron has the charge is does. What am asking is the same way I would ask why is there a limit to how much mass an object can have, or why is there a limit to ow big and object can be, or why is there a limit to how fast something can spin? It seems there is not reason there should be limits, ad if there is what is limiting factor? What is causing the restriction?
Another way to look at it is that everything has wave nature (read up on De Broglie, http://en.wikipedia.org/wiki/Matter_wave). Therefore nothing can go faster than the speed of light, just as no sound wave can go faster than the speed of sound.

Note that different people give different answers to that question, and probably nobody can prove to really have the right answer.

Harald
Emeritus
PF Gold
P: 5,597
 Quote by harrylin Another way to look at it is that everything has wave nature (read up on De Broglie, http://en.wikipedia.org/wiki/Matter_wave). Therefore nothing can go faster than the speed of light, just as no sound wave can go faster than the speed of sound.
This argument doesn't make sense. There is no (direct) logical connection between the speed of an electron wave and the speed of a photon wave.

 Quote by harrylin Note that different people give different answers to that question, and probably nobody can prove to really have the right answer.
This is incorrect. Once you fix a set of postulates, there is a right answer. See http://arxiv.org/abs/physics/0302045v1

 Quote by CosmicVoyager I hypothesize that it is not a random coincidence that the square root of (m/e) exactly equals the speed limit. I think there is some connection between matter and energy and the speed limit.
See Einstein, "Does the inertia of a body depend upon its energy content?," Annalen der Physik. 18, 639: 1905. English translations are available online.
 P: 80 My answer here: http://www.physicsforums.com/showthr...=478783&page=2 in CosmicVoyager's similar thread explains why c defines a limit. Why c holds the value it does, though, is a different matter entirely. Naturally, the units used are meaningless since metres are derived from the observed speed of light in vacuo and the second relates to Caesium decay. This means that the 2.99897(or whatever) ms^-1 number is not significant as a quantity, but as a unique value, like PI (which is ultimately defined by the curvatre of spacetime), or the the natural logarithm/root, or the fine structure constant, or the universal gravitation, it has a special characteristic of being a constant (to our current understanding, theories of c variance notwithstanding) in our universe. Regardless of the wavelength of light, how much energy it has, it maintains the constant speed. Unlike massive objects for which the greater their mass, the more energy is needed. That bosons have a fixed speed, DeBroglie showed how E is related to wavelength as well as mass, giving a means to describe massless objects and waves with momentum equivalent to their massive counterparts. Introduxing Planck's constant, quantised the relativistic energy and allowed working with manifest in the same terms of energy and momentum as massive entities. ______ c=\/h^2.f.L That frequency and wavelength are related according only to c, c is directly dependant only on the value of another constant.
P: 848
 Quote by CosmicVoyager I am extremely confident that I will understand with there is a speed limit, and when I do I will be able to explain it in a few sentences. And I will ask why someone else who understood couldn't have done that. I know because it keeps happening. There is an explanation for why there must be a limit and why it is what it is.
It depends on who you are explaining it to. If he doesn't understand your language and terms you are using, you first have to teach him that. If you will first learn a little analytic geometry (affine coordinates, how to graph functions, etc.) and the Pythagorean theorem, then you will understand it after looking at my post #5 along with a simple two-line derivation of the Minkowski metric.
P: 848
 Quote by CosmicVoyager I hypothesize that it is not a random coincidence that the square root of (m/e) exactly equals the speed limit. I think there is some connection between matter and energy and the speed limit.
You are not even close.
P: 848
 Quote by CosmicVoyager From another thread: This is excellent. I think this is headed in the right direction. Conceptually, the important form of the equation e=mc^2 is c=squarroot(m/e). The real question is why is the speed limit of the universe equal to the square root of the mass of any object divided by it's energy. I get the feeling that it must be, by the very definitions of what mass and energy are. I don't see it yet though. I think this is very close. Something to do with the fact that energy is motion. I will think about it. If anyone can illustrate why that must be so, please do. The square root of mass divided by energy. Hmm...
You are going down the wrong path. First figure out what you have to go off and study to understand bcrowell's first post (he has written a first class book on gerneral relativity, but it may be advanced from where you seem to be with your background). Google special relativity and also spacetime diagrams and see what you come up with.
Emeritus
PF Gold
P: 5,597
 Quote by bobc2 First figure out what you have to go off and study to understand bcrowell's first post (he has written a first class book on gerneral relativity, but it may be advanced from where you seem to be with your background).
He/she might find this easier going: http://www.lightandmatter.com/html_b...ch01/ch01.html A similar treatment is Palash B. Pal, "Nothing but Relativity," http://arxiv.org/abs/physics/0302045v1
P: 3,187
 Quote by bcrowell This argument doesn't make sense. There is no (direct) logical connection between the speed of an electron wave and the speed of a photon wave.
That wasn't implied; instead it implies that an electron will be increasingly light-like at higher velocities, and cannot possibly move faster than light.
 Once you fix a set of postulates, there is a right answer.
The OP asked "What is causing the restriction". I interpreted this as a physical question, not a question about human postulates that formulate the restriction!

CosmicVoyager, did I understand you correctly?

Harald
P: 159
 Quote by harrylin That wasn't implied; instead it implies that an electron will be increasingly light-like at higher velocities, and cannot possibly move faster than light. The OP asked "What is causing the restriction". I interpreted this as a physical question, not a question about human postulates that formulate the restriction! CosmicVoyager, did I understand you correctly? Harald
Yes, I mean what is holding the light back. It is like an invisible hand jumps up in front of things at c and stops them from going faster.

I suspect my paradigm might be completely wrong and I need to shift, in which case I need why it is wrong explained. It seems the same as if I were adding more and more to an object to increase it's mass, then suddenly the mass stopped increasing, and I think "What the heck is going on?"
Emeritus
PF Gold
P: 5,597
 Quote by harrylin The OP asked "What is causing the restriction". I interpreted this as a physical question, not a question about human postulates that formulate the restriction!
It's impossible to do logical reasoning without starting from some assumptions. Failing to spell out one's assumptions is simply sloppy reasoning.
Emeritus
PF Gold
P: 5,597
 Quote by nitsuj CosmicVoyager, consider the relationship between time and speed. It is as much a "speed limit" as a "time limit". Or an "activity limit". to your own point space is a medium, "C" is the bandwidth of that medium.
You haven't defined "activity," "bandwidth," or "medium" in this context.

 Quote by nitsuj I fall back to thinking about the theory of the universe expanding. I belive it is expanding at the speed of light.
This is incorrect.

 Quote by nitsuj I also belive it is the medium itself that is expanding.
What medium are you talking about?

 Quote by nitsuj Perhaps that theory comes before the "speed limit".
What you've said above isn't a theory.

 Quote by nitsuj Said differently, maybe something can't travel faster then the meduim is being "built".
This doesn't make sense, because the speed limit exists in special relativity and also in spacetimes in GR that are not expanding. It also doesn't make sense because the speed limit is a universal local property of all spacetimes, independent of any features that differentiate one spacetime from another.
 P: 1,098 All that I said was after my comment: CAUTION: LOTS OF HOT AIR FROM AN ARMCHAIR 2nd, the OP is playing imagination games, not condutcing physics experiements. The OP wasn't satisfied with replies that answered the question well, from a physics perspective. Also the question itself suggests a limited physics background. Therefore he must be asking from some other perspective, hence just some fun food for thought. Tearing apart my laughable post, is equaly laughable. It wasn't posted to see if it holds up to peer review. seems Im as bad at physics as you are with...
P: 1,098
 Quote by bcrowell This doesn't make sense, because the speed limit exists in special relativity and also in spacetimes in GR that are not expanding.
I googled it and the first page that came up explained how GR included something called the "cosmological constant". Which compensates for the expansion of the universe. Off topic but still interesting, the article mentioned how Einstien called it his "greatest blunder".
Mentor
P: 41,436
 Quote by nitsuj All that I said was after my comment: CAUTION: LOTS OF HOT AIR FROM AN ARMCHAIR
That's no excuse.
 2nd, the OP is playing imagination games, not condutcing physics experiements. The OP wasn't satisfied with replies that answered the question well, from a physics perspective.
And yet, this is a physics forum.
P: 159