Register to reply

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

by CosmicVoyager
Tags: limit, speed, universal
Share this thread:
CosmicVoyager
#19
Mar10-11, 07:27 AM
P: 159
Quote Quote by Naty1 View Post
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?
CosmicVoyager
#20
Mar10-11, 07:33 AM
P: 159
Quote Quote by CosmicVoyager View Post

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.
harrylin
#21
Mar10-11, 07:36 AM
P: 3,187
Quote Quote by CosmicVoyager View Post
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
bcrowell
#22
Mar10-11, 12:24 PM
Emeritus
Sci Advisor
PF Gold
bcrowell's Avatar
P: 5,597
Quote Quote by harrylin View Post
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 Quote by harrylin View Post
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 Quote by CosmicVoyager View Post
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.
_PJ_
#23
Mar10-11, 01:30 PM
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.
bobc2
#24
Mar10-11, 01:47 PM
P: 848
Quote Quote by CosmicVoyager View Post
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.
bobc2
#25
Mar10-11, 01:49 PM
P: 848
Quote Quote by CosmicVoyager View Post
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.
bobc2
#26
Mar10-11, 01:54 PM
P: 848
Quote Quote by CosmicVoyager View Post
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.
bcrowell
#27
Mar10-11, 03:09 PM
Emeritus
Sci Advisor
PF Gold
bcrowell's Avatar
P: 5,597
Quote Quote by bobc2 View Post
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
harrylin
#28
Mar11-11, 03:45 AM
P: 3,187
Quote Quote by bcrowell View Post
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
CosmicVoyager
#29
Mar11-11, 04:53 AM
P: 159
Quote Quote by harrylin View Post
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?"
bcrowell
#30
Mar11-11, 09:44 AM
Emeritus
Sci Advisor
PF Gold
bcrowell's Avatar
P: 5,597
Quote Quote by harrylin View Post
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.
nitsuj
#31
Mar11-11, 09:59 AM
P: 1,098
Quote: bcrowell
"Failing to spell out one's assumptions is simply sloppy reasoning"

Or sloppy social skills, in comprehension.


CAUTION: LOTS OF HOT AIR FROM AN ARMCHAIR

I think it looks like cosmicvoyager is looking for an answer that works for him, a subjective one that works for his current understanding of his question.

Many of the posts here more then answer his question. Chicken / egg sums it up.

Here is my best attempt.

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.

Why does the medium have a limit?

I fall back to thinking about the theory of the universe expanding.

I belive it is expanding at the speed of light. I also belive it is the medium itself that is expanding. Perhaps that theory comes before the "speed limit".

Said differently, maybe something can't travel faster then the meduim is being "built".

From there, let's turn the idea on its head.

At rest we are travelling at the limit (with space itself as it expands) and its movement that reduces this effect. So from an observer perspective as something travels, towards "C", the limit, it is actualy, relative to the medium, traveling towards zero, also were time / activity stops. So an object moving at "C" is moving as fast as the medium (space).

From the perspective of the medium "C" is not the maximum, its the minimum. And zero is the maximum.

To reiterate points already made lets assume my suggested answer is fact. Your question still stands, however shifts to "why does expansion of the medium itself have a "speed limit"?". Anwsers have to have a context, and your question didn't provide one. Without context it can always be asked, "yea, but why?".
bcrowell
#32
Mar11-11, 11:18 AM
Emeritus
Sci Advisor
PF Gold
bcrowell's Avatar
P: 5,597
Quote Quote by nitsuj View Post
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 Quote by nitsuj View Post
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 Quote by nitsuj View Post
I also belive it is the medium itself that is expanding.
What medium are you talking about?

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

Quote Quote by nitsuj View Post
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.
nitsuj
#33
Mar11-11, 12:02 PM
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...
nitsuj
#34
Mar11-11, 12:33 PM
P: 1,098
Quote Quote by bcrowell View Post
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".
Doc Al
#35
Mar11-11, 12:55 PM
Mentor
Doc Al's Avatar
P: 41,439
Quote Quote by nitsuj View Post
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.
CosmicVoyager
#36
Mar11-11, 01:32 PM
P: 159
Quote Quote by nitsuj View Post
Quote: bcrowell
"Failing to spell out one's assumptions is simply sloppy reasoning"

Or sloppy social skills, in comprehension.


CAUTION: LOTS OF HOT AIR FROM AN ARMCHAIR

I think it looks like cosmicvoyager is looking for an answer that works for him, a subjective one that works for his current understanding of his question.

Many of the posts here more then answer his question. Chicken / egg sums it up.

Here is my best attempt.

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.

Why does the medium have a limit?

I fall back to thinking about the theory of the universe expanding.

I belive it is expanding at the speed of light. I also belive it is the medium itself that is expanding. Perhaps that theory comes before the "speed limit".

Said differently, maybe something can't travel faster then the meduim is being "built".

From there, let's turn the idea on its head.

At rest we are travelling at the limit (with space itself as it expands) and its movement that reduces this effect. So from an observer perspective as something travels, towards "C", the limit, it is actualy, relative to the medium, traveling towards zero, also were time / activity stops. So an object moving at "C" is moving as fast as the medium (space).

From the perspective of the medium "C" is not the maximum, its the minimum. And zero is the maximum.

To reiterate points already made lets assume my suggested answer is fact. Your question still stands, however shifts to "why does expansion of the medium itself have a "speed limit"?". Anwsers have to have a context, and your question didn't provide one. Without context it can always be asked, "yea, but why?".
I like this sort of creative thinking :-) It might not be true, but having the idea expands one awareness and you can keep it in your bag of tricks for future explanations.

"Your question still stands, however shifts to "why does expansion of the medium itself have a "speed limit"?"."

It wouldn't for me. It does not have a limit. That speed is the result of whatever propelled the expansion. It could have been faster or slower.


Register to reply

Related Discussions
Universal speed of light General Physics 3
Limit of a mountain using universal gravitation Introductory Physics Homework 1
Violation of Universal Speed Limit Cosmology 3
Question - Universal Speed Limit General Physics 4
Light speed relative to universal speed... General Physics 9