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

[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 believe it is expanding at the speed of light. I also believe 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 traveling 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 let's 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]

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.

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

I believe it is expanding at the speed of light.

This is incorrect.

I also believe it is the medium itself that is expanding.

What medium are you talking about?

Perhaps that theory comes before the "speed limit".

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

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]

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 I am as bad at physics as you are with...

 

[nitsuj]

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]

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]

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 believe it is expanding at the speed of light. I also believe 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 traveling 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 let's 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.

 

[nitsuj]

Phew, thank you (i mean it sincerely) Cosmicvoyager for your reply. It demonstartes, to me, that my response was close to the right context for your question.

"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." Cool, I think that means you picture it the way I was trying desperatly to describe it.

 

[DrGreg]

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.

Maybe you are thinking, why can't you travel at 99% the speed of light, then go just 2% faster and find yourself at 101% the speed of light? Well, when you are traveling at 0.99c your notions of distance and time change. To the outside observer it looks like you have only 1% further to go, but from your own point of view you still have 100% to go -- the speed of light relative to you is still 299792458 m/s and you are no nearer to it than when you started. To look at this mathematically, velocities don't "add" as u+v but as

\frac{u+v}{1+uv/c^2}​

In the example above, your final velocity isn't 1.01c but

\frac{0.99c+0.02c}{1+0.99\times0.02}=0.99039c​

Another way of looking at this: the usual way to measure speed is to take distance on the observer's ruler divided by time on the observer's clock. But there is another way: take distance on the observer's ruler divided by time on the traveller's clock. This method is called "celerity" (or "proper velocity", a name I don't like). It turns out that the celerity of light is infinite, so if you translate your question from speed into celerity, "why is the universal celerity limit infinite?" it's a bit of a non-question.

You may well ask, "Why don't speeds add up" or "Why is the celerity of light infinite yet its speed is finite?" and most books will send you in a circle: "Because the speed of light is invariant" (the same for all observers), so it hasn't really answered your question. That's just the way the Universe is, who can say why?

 

[nitsuj]

Doc Al,

"That's no excuse." That's subjective.

"And yet, this is a physics forum." Actualy it's a physics forum.

 

[Ferrus]

http://golem.ph.utexas.edu/category/2006/08/letter_from_grothendieck.html

'One story has it that Grothendieck is now convinced that the Devil is working to falsify the speed of light. Schneps ascribes his concerns with the speed of light to his anxiety about the methodological compromises physicists make. He talks constantly, however, about the Devil, semi-metaphorically, sitting behind good people and nudging them in the direction of compromise, of the fudge, of the move towards corruption. ‘Uncompromising’ is the expression Schneps favours.'

 

[pip1974]

I think I understand what CosmicVoyager is getting at. It is fascinating and illuminating to start gaining an understanding of the implications of relativity from basic principles, i.e. the experimentally verifiable facts that light is observed to have the same speed for every observer and also that every frame of reference is equally valid. From that, we can explain and describe almost everything we can observe. But what's missing is the 'why' bit. And whether you like it or not, it's the 'why' bit that is the fundamental part of human nature that drives us to discover more and more about the universe.
CosmicVoyager is just wondering what it is about the fabric of the universe that makes things behave like this.
Nitsuj talked about some 'medium' expanding, but actually it is space that is expanding in every direction.
Isn't it the case that if time and space began with the big bang and has been getting bigger ever since, that the space I'm sitting in right now is an inflated bit of space that was once, and still is part of the big bang?
It isn't unreasonable to muse on the possibility that this stretching of space time in all directions has something to do with what we observe as a speed limit.

 

[Calrid]

Why is any constant what it is fine structure constant or c?

It's a superbly interesting question with no real coherent answer, only plenty of good ideas.

"There is only one thing faster than the speed of light and that is the speed of rumour."

Terry Pratchett, I'm paraphrasing.

Limits are troubling they make fools of us all I think.

 

[Dale]

But what's missing is the 'why' bit. And whether you like it or not, it's the 'why' bit that is the fundamental part of human nature that drives us to discover more and more about the universe.

hi pip1974, welcome to PF

Perhaps it is just the fact that I have young kids, but IMO "why" is generally not that important a question. any answer to a why question can be, and often is, simply followed by another why question. It is a game that any four-year old can play expertly. also, in my experience, generally a why question is asking for a bedtime story rather than a scientific experiment.

 

[Calrid]

hi pip1974, welcome to PF

Perhaps it is just the fact that I have young kids, but IMO "why" is generally not that important a question. any answer to a why question can be, and often is, simply followed by another why question. It is a game that any four-year old can play expertly. also, in my experience, generally a why question is asking for a bedtime story rather than a scientific experiment.

Why?

 

[Dale]

Once upon a time ...

 

[Calrid]

Once upon a time ...

Oooh! Does it have dragons!

 

[harrylin]

Why?

Yes exactly, why!
I agree with you. Children who never ask asking why (each time one level deeper) become scientists!

PS: the original popular reply to the "why" is because of the existence of a universal medium (Lorentz etc.), and the most popular reply today is because of the existence of a physical spacetime (Minskowski etc.). And likely there are numerous variants on those ideas and many other (such as post #16).

 

[pip1974]

hi pip1974, welcome to PF

Perhaps it is just the fact that I have young kids, but IMO "why" is generally not that important a question. any answer to a why question can be, and often is, simply followed by another why question. It is a game that any four-year old can play expertly. also, in my experience, generally a why question is asking for a bedtime story rather than a scientific experiment.

"Why?" is the most important question! And in science leads to the next level of understanding and more questions, that normally begin with "Why...".
Like, "Why do the planets appear to move the way they do?" Kepler already had equations to describe the motion of the planets and still Newton asked the question "why" because he felt that there were links to be made, patterns to be found and ideas to be brought together.
As I said before, it is human nature to ask the magic question "why" and to seek patterns in the things we experience and it is at the root of all scientific discovery. It is the same evolutionary trait that allowed our ancestors to be better hunter gatherers and to avoid predators that drives us to ask questions like "Why is there a universal speed limit".
Actually the simple answer to this particular question is that we don't know. Yes, we know that in the representation of the universe as we understand it that there is a speed limit, that there has to be one, and that it is based on the assumption that quite basic facts are true, but we do not know the real underlying nature of this speed limit or what the actual fabric of space itself is really like. We are still coming up with theories that try to model what the universe is really like deep down in the finest detail, but as we know the best theories are incompatible with each other.
And I think when kids want a story, and they ask "why this" or "why that" it's because we were all little scientists when we were kids. At some point, some people feel they have enough rules established to allow them to get by in their particular environment, whether they are a banker, a truck driver, an engineer or a physics teacher. But some people, bless them, just can't let things lie and keep on asking "why" and end up being scientists, philosophers, priests or artists.

 

[bobc2]

...It seems there is not reason there should be limits, and if there is, what is limiting factor? What is causing the restriction?

The reason there is a limiting factor for speeds is that the ever increasing speeds lead to a convergence of the X4 and X1 dimensions as they rotate toward each other (see sequence of sketches below). Then, the next "why question" is, "Why does the X1 coordinate rotate so as to maintain symmetry with X4 about the photon world line (note that this symmetry, with 45 degree photon world line, results in the same speed of light for all observers)?" A possible answer:

Nature wanted all observers to live with the same rules of physics, and this was the only way to do it (thus, nature contrived coordinates with invariance under Lorentz transformations)

 

[nitsuj]

pip1974 - "Nitsuj talked about some 'medium' expanding, but actually it is space that is expanding in every direction."

The "medium" I talked about is space/time.

To say it differently I was describing space/time as a medium.

It is not just space that is expanding, time is as well.

 

[Dale]

"Why?" is the most important question! And in science leads to the next level of understanding and more questions, that normally begin with "Why...".

This is a very common misunderstanding. Science is fundamentally incapable of addressing "why" questions. Science can only answer questions of the following form: "Does the mathematical model X correctly predict the observation Y in experiment Z to within experimental error?". Even if the answer is "yes" the scientific method does not and cannot answer the question "why" the model works.

 

[bobc2]

This is a very common misunderstanding. Science is fundamentally incapable of addressing "why" questions...

DaleSpam, what is your assessment of the "Theory of Everything"--assuming Stephen Hawking is proven correct in his pronouncements?

 

[Dmitry67]

DaleSpam is right.
But just 2 cents.
Science is capable of answering "Why" questions (only to some extent) - using the Anthropic principle

http://en.wikipedia.org/wiki/Anthropic_principle

 

[nitsuj]

c

DaleSpam" "Why?" is the most important question! And in science leads to the next level of understanding and more questions, that normally begin with "Why...". "

"This is a very common misunderstanding. Science is fundamentally incapable of addressing "why" questions. Science can only answer questions of the following form: "Does the mathematical model X correctly predict the observation Y in experiment Z to within experimental error?". Even if the answer is "yes" the scientific method does not and cannot answer the question "why" the model works. "

You set the context to "Science is fundamentally incapable of addressing "why" questions". Well sticking to that, it is also incapable of "answering" questions of anyform. To your point science is fundamentally mathematical models. It is people who, motived by the question of "why", pursue an answer we can all agree on. Numbers, and inturn mathematical models are a perfect tool for this pursit of "why". To me I see,

science is fundamentaly the question of why. Or editing your text,

Science is fundamentally addressing "why" questions.

 

[pip1974]

The "medium" I talked about is space/time.

I know what you meant! Yes, it's all expanding, we think.

This is a very common misunderstanding. Science is fundamentally incapable of addressing "why" questions. Science can only answer questions of the following form: "Does the mathematical model X correctly predict the observation Y in experiment Z to within experimental error?". Even if the answer is "yes" the scientific method does not and cannot answer the question "why" the model works.

I don't want to be involved in a semantic debate. Surely you can't claim you don't know what I'm talking about? There's more to science than the scientific method! I'm talking about what drives us to discover things; what makes us engage in the scientific method in the first place.
The great thinkers of history were most definitely not inspired to do what they did by thinking one day "I would quite like to devote the rest of my life to working out whether the mathematical model X correctly predicts observation Y". No! They were inspired by looking at the world and thinking "I wonder why that happens".
Why is the sky blue? Can science address that question? Yes!
Why do I come back to Earth when I jump in the air? Can science address that question? Yes! Of course it can.
Why does water feel wet?
Why is the grass green?
Why does the sun rise in the morning and set in the evening?
I've already said I do not want to get into a semantic debate, but for the sake of clarity: No, you cannot apply the scientific method, as defined, directly to one of the questions above and come up with an answer, but the question sparks the imaginative process and the subsequent investigation in which the scientific method can be applied in order to come up with an explanation. The scientific method bit is a valuable tool but the question is the thing that compels us to use it.
I'll leave it there, because we're not talking about physics anymore and that's why I came to this thread in the first place.

 

[nitsuj]

I wonder, if in the "science community" there is a saying simular to "that guy's head is so far up those mathematical models..."

 

[Huttate]

I think that there are two kinds of scientists.

Those who know it! - "I have been taught why this is so and I can prove it to you"

And those who think about it! - "Why is that so?" and then go on to discover why it is so.

Unfortunately the world has a surfeit of the former and a deficit of the latter.

 

[Calrid]

I wonder, if in the "science community" there is a saying simular to "that guy's head is so far up those mathematical models..."

I think there is a group name its called string theory.



The fact is if you go too far into why something is you might end up compactifying dimensions so much you will end up with your head up your bum.

 

[Dmitry67]

Why is the sky blue? Can science address that question? Yes!
Why do I come back to Earth when I jump in the air? Can science address that question? Yes! Of course it can.
Why does water feel wet?
Why is the grass green?

No.
It answers other questions.

What is a color of the sky? (calculate the spectrum of the sky)
Calculate the trajectory when you jump in the air.
Etc.

That version of the question looks equivalent to yours, beginning from WHY. However, the 'Why' questions work only when they explain something complicated based on the behavior of something basic.

Why water is... because it consists of H20.
Why H2O is stable? Because atoms...
Why atoms... because quarks and electrons, based on QFT...

But now this WHY sequence breaks, because we hit the fundamental level. It is like you can prove a theorem based on the axioms. But you can't 'prove' an axiom.

Why 1+1=2?
Can you answer the question?

So listen to DaleSpam - aside of AP comment, hi is absolutely right.

 

[Dmitry67]

P.S.
String theory does not change anything, it just adds 1 more level.