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

bcrowell

I disagree. Depending on what you take as your postulates, there can be a known answer. If you choose the postulates used in this paper Palash B. Pal, "Nothing but Relativity," http://arxiv.org/abs/physics/0302045v1
, for example, then the existence of a frame-invariant speed limit is a theorem that can be proved from those postulates.

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?

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.

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.

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

Harald

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.

This is incorrect. Once you fix a set of postulates, there is a right answer. See http://arxiv.org/abs/physics/0302045v1

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_

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

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

You are not even close.

bobc2

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

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

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

CosmicVoyager

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

It's impossible to do logical reasoning without starting from some assumptions. Failing to spell out one's assumptions is simply sloppy reasoning.

nitsuj

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

You haven't defined "activity," "bandwidth," or "medium" in this context.

This is incorrect.

What medium are you talking about?

What you've said above isn't a theory.

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

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

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".