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Author Topic:   Speed of Light Barrier
Son Goku
Inactive Member


Message 46 of 178 (224639)
07-19-2005 2:00 PM
Reply to: Message 41 by Ben!
07-19-2005 10:27 AM


Re: Why 3x10^8m/s?
quote:
What I don't get is why the length of that vector is the length that it is. Or another way to restate the same question (I think) is, why does OUR space-time necessitate a "c" of 3x10^8m/s in order for
the space-time interval to be invariant?
This is actually what I'm talking about. I'm not saying if you choose any arbitrary measurement system you can get any speed you want.
quote:
although it's possible to measure the speed of light as 1(length unit "A")/(time unit "B")
This is essentially the crux of the problem.
In special relativity there is no space and time, only spacetime.
Similarly velocity should be measured not in space and time units but in a single spacetime unit.
Of course this unit can be arbitrary, just as space and time units can be arbitrary, but the point is that there is a single unit for spacetime.
Making a space unit and a time unit separates them out and creates constants like "c".
If we measure light's speed in a single arbitrary spacetime unit it is always 1.
"c" can be best thought of as a constant which modifies our inadequate Newtonian space and time units.
This message has been edited by Son Goku, 07-19-2005 02:01 PM
This message has been edited by Son Goku, 07-19-2005 02:34 PM

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 Message 41 by Ben!, posted 07-19-2005 10:27 AM Ben! has not replied

  
NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 47 of 178 (224641)
07-19-2005 2:08 PM
Reply to: Message 45 by Percy
07-19-2005 12:20 PM


Re: Why 3x10^8m/s?
I've never seen a calculation but I understand that when Maxwell formulated his famous electricity and magnetism equations and from them realized that they suggested a EM wave propagating and used them to calculate the velocity of propagation he got the answer of c. So it seems it has something to do with how E and M relate.
Don't know any more than that.

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Tony650
Member (Idle past 4032 days)
Posts: 450
From: Australia
Joined: 01-30-2004


Message 48 of 178 (224930)
07-20-2005 3:37 PM
Reply to: Message 16 by cavediver
07-15-2005 4:02 AM


Hi cavediver.
cavediver writes:
Heh, no worries. I wasn't being critical, just emphasising the important points as I see them.
No problem. You're relatively new around these parts so I just wanted you to know that I don't really debate or argue here. I don't post a great deal but, when I do, my only purpose is to learn.
If you're interested, I've previously started a number of threads on various topics of interest to me. Dimensional Discourse you already know about. There is also Relative Motion (A Thought Experiment), First confirmed image of extrasolar planet, and The resilience of matter's fundamental components. Not that I'm trying to monopolize your time or anything. You're not the first educated person I've latched onto and said, "Teach me!"
cavediver writes:
This is the trouble with fundemental physics. It has no obvious physical description. The analogies are the best you will get. And they become poorer the deeper you go...
Yes. The deeper I dig, the more I get the impression that common intuition simply can't take you beyond a certain point. For those of us that don't have the requisite grounding in mathematics, things make progressively less and less sense the further you go. I don't know if this is different for those like yourself, but I would imagine that, at the very least, the concepts that they come up against aren't totally opposed to their intuition precisely because part of their way of thinking is based on the math.
cavediver writes:
...ever considered how you get attraction by particle exchange?
Yes! I have, indeed! Unsuccessfully... grr!!
I seem to recall, once, reading an "explanation" of the exchange of gravitons. It used the analogy of two people standing on skateboards (or in boats, or some such...too long ago to remember exactly) throwing a ball back and forth to each other. The point was to demonstrate that this will cause them to gradually move apart and, thus, show how one body can affect another by exchanging particles.
Now, if you're sitting there rolling your eyes, don't worry; I can see that this is a pretty lousy analogy, for numerous reasons. Not the least of which is the fact that it doesn't actually explain gravitational attraction. In fact, I'm not even sure it would make a decent analogy of repulsive forces. I somehow doubt that repulsion between, say, bodies of alike electric charge is caused by the emanation of the carrier particles physically nudging the bodies apart in a Newtonian "equal-and-opposite-reaction" kind of way.
cavediver writes:
Well, the only obvious comment I had on the whole thing was on the justification for extra dimensions.
Well, I suppose since the universe feels no need to conform to our intuition in so many other aspects, why should its dimensional properties be any different?
cavediver writes:
Hmmm, if that's your ultimate goal, then what are you going do about all the dimensions above 4? My first work was in bosonic strings and there we work in 26 dimensions
Ha! You know, even as I typed that, I thought I should have clarified it more!
Let's just say that it's my current ultimate goal (oxymoron alert ).
Or, to put it another way, I currently have my sights set on four. If such a time comes that I achieve my comprehension of that, I will re-evaluate and, perhaps, aim for the same understanding of five...and so on, indefinitely.
Of course, this is hardly an urgent concern in my mind as I am, thus far, in absolutely no danger of acquiring the comprehension that I seek. In all honesty, not to be pessimistic, but I have come to accept that I may never achieve this particular goal. That won't stop me trying, of course, but it seems that there are very few who can do it. And, from what I've seen, they always seem to be either physics or mathematics majors (or both) which doesn't bode well for the chances of a layman like myself.

This message is a reply to:
 Message 16 by cavediver, posted 07-15-2005 4:02 AM cavediver has replied

Replies to this message:
 Message 50 by cavediver, posted 07-22-2005 1:01 PM Tony650 has replied

  
Tony650
Member (Idle past 4032 days)
Posts: 450
From: Australia
Joined: 01-30-2004


Message 49 of 178 (224933)
07-20-2005 3:52 PM
Reply to: Message 23 by cavediver
07-15-2005 4:58 PM


cavediver writes:
So as far as you are concerned, given a spacecraft with sufficient thrust and fuel, you can visit any part of the universe in as short a period of time as you like. Just don't expect anyone to be around when you get home...
Or your destination to be there when you arrive.
I've always considered our universe to have a kind of sick sense of humour, in this regard. "Sure, you can travel to the other side of the cosmos in less than a day ship time. Of course, the universe itself will have experienced heat death by then... but hey, at least it's only been a day for you!"
Lousy universe.

This message is a reply to:
 Message 23 by cavediver, posted 07-15-2005 4:58 PM cavediver has not replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 50 of 178 (225480)
07-22-2005 1:01 PM
Reply to: Message 48 by Tony650
07-20-2005 3:37 PM


If you're interested, I've previously started a number of threads on various topics of interest to me. Dimensional Discourse you already know about. There is also Relative Motion (A Thought Experiment), First confirmed image of extrasolar planet, and The resilience of matter's fundamental components.
Some cool stuff there, especially in that first topic. Personally, I think Ned's comment is great
Again, I offer some advice: going on about these kinds of things when we know pretty much zip about them is a waste of time. You may ask a few questions and hope someone who can talk sensibly about them comes by. Just making up stuff using some of the words you've picked up here and there is just gibberish.
Unfortunately this is true. To think sensibly about these topics requires a certain level of background to keep you on the straight and narrow and away from "crackpot" land! There was definitely a deal of that in those threads
BUT, the questions posed (in the main by you) were excellent, and very relevant.
To answer your first thread - take the earth (spherically symmetric ball) in an otherwise empty universe. Translational motion is trivial, in that you can always jump to a moving frame and view the earth as "moving". It has no consequences. Acceleration is always detectable, and would have a definite effect on the space-time. There is the "C-metric" which essentially describes two objects in an otherwise empty universe accelerating away from each other. Rotation in an empty universe is rather un-Machian in Relativity - it has definite consequences despite no "distant stars": there is the frame-dragging. Now, the two planets orbiting each other is interesting... I'm tempted to say that at distance it would appear to be a rotating point source and so there will be evident frame-dragging, and hence a frame in which the planets were not-rotating and supposedly in-falling would be invalid.
One other point... objects in orbit (if "small" enough) are precisely those that are NOT accelerating!

This message is a reply to:
 Message 48 by Tony650, posted 07-20-2005 3:37 PM Tony650 has replied

Replies to this message:
 Message 62 by Tony650, posted 08-01-2005 11:57 AM cavediver has not replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 51 of 178 (225496)
07-22-2005 1:49 PM
Reply to: Message 38 by Son Goku
07-17-2005 1:37 PM


Re: Why 3x10^8m/s?
Ok, fine... let's use geometric units: c=1, G=1, h_bar=1.
Now, instead of why 3x10^8, let's have...
why is alpha 1/137?
why is Mp/Me 1836?
why is Me:Mm:Mt = 1:207:3477
etc, etc...
(Mp: mas of proton, and electron , muon, tauon)

This message is a reply to:
 Message 38 by Son Goku, posted 07-17-2005 1:37 PM Son Goku has replied

Replies to this message:
 Message 52 by Son Goku, posted 07-22-2005 2:11 PM cavediver has replied

  
Son Goku
Inactive Member


Message 52 of 178 (225505)
07-22-2005 2:11 PM
Reply to: Message 51 by cavediver
07-22-2005 1:49 PM


Re: Why 3x10^8m/s?
I was only speaking of the constants that appear when the Newtonian framework is kept in other theories.
i.e., the coloumb, "c", "G", "k", e.t.c
In other words the most common constants, relation ones.
Not literally every constant such as the fine structure constant.
I'm aware they aren't explained.

This message is a reply to:
 Message 51 by cavediver, posted 07-22-2005 1:49 PM cavediver has replied

Replies to this message:
 Message 53 by cavediver, posted 07-22-2005 2:40 PM Son Goku has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 53 of 178 (225513)
07-22-2005 2:40 PM
Reply to: Message 52 by Son Goku
07-22-2005 2:11 PM


Re: Why 3x10^8m/s?
Not literally every constant such as the fine structure constant.
To me, a change in c is a change in alpha. Otherwise it is just an unobservable scale-change. Which I guess is what you were arguing...

This message is a reply to:
 Message 52 by Son Goku, posted 07-22-2005 2:11 PM Son Goku has replied

Replies to this message:
 Message 54 by Son Goku, posted 07-22-2005 2:52 PM cavediver has replied

  
Son Goku
Inactive Member


Message 54 of 178 (225519)
07-22-2005 2:52 PM
Reply to: Message 53 by cavediver
07-22-2005 2:40 PM


Re: Why 3x10^8m/s?
No, I'm not arguing anything.
I'm just saying in the context of special relativity "c" can be explained by the fact that light forms a 45 degree Minkowski angle to the any axis of any observer.
There are arguements as to why a change in the fine structure constant wouldn't literally translate as a change in "c" and it isn't determined if alpha has actually changed, but this is the domain of quantum field theory and I'm only discussing Special relativity, in which "c" has no meaning.

This message is a reply to:
 Message 53 by cavediver, posted 07-22-2005 2:40 PM cavediver has replied

Replies to this message:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 55 of 178 (225592)
07-22-2005 7:40 PM
Reply to: Message 54 by Son Goku
07-22-2005 2:52 PM


Re: Why 3x10^8m/s?
I'm just saying in the context of special relativity "c" can be explained by the fact that light forms a 45 degree Minkowski angle to the any axis of any observer.
This is a convenience... it is not real. What is "45 degrees" in the t-x plane of Minkowski space? You think "45 degrees" as a result of embedding the t-x plane in a positive definite space... i.e. a sheet of paper/diagram. You can compare scales among the spatial dimensions because you can rotate a space-like vector from say pure-x to pure-y. You cannot rotate a time-like vector into a null-vector or a space-like vector to compare scales. Remember, orthogonality and "angles" do not retain their usual sense in pseudo-Riemannian geometry.
I'm only discussing Special relativity, in which "c" has no meaning.
True, but the original question was asked in respect of our universe, where you certainly need to consider the quantum aspects.

This message is a reply to:
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Replies to this message:
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Son Goku
Inactive Member


Message 56 of 178 (225594)
07-22-2005 7:53 PM


Yeah, I know. I was just saying that for shorthand.
Technically light's speed comes about from the fact that light is described with a null vector.
Since I was discussing Minkowski space orthogonality is largely preserved so I was just using the common "Schutz" description.
As for Quantum Theory, it isn't really shown yet that it changes the picture of light's velocity's numerical value, so I choose to leave it out.

Replies to this message:
 Message 57 by cavediver, posted 07-22-2005 8:49 PM Son Goku has not replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 57 of 178 (225598)
07-22-2005 8:49 PM
Reply to: Message 56 by Son Goku
07-22-2005 7:53 PM


orthogonality is largely preserved so I was just using the common "Schutz" description.
I'm not sure I follow... and I could be wrong, but I don't think it's down to Bernard that we draw null rays at 45 degrees.

This message is a reply to:
 Message 56 by Son Goku, posted 07-22-2005 7:53 PM Son Goku has not replied

  
Son Goku
Inactive Member


Message 58 of 178 (225656)
07-23-2005 11:41 AM


Yes, I know it isn't because of him.
It's convention, but it is usually termed the Schutz description.
Which doesn't imply he came up with it, it just arises from the popularity of his book as an undergraduate text.
I wasn't claiming that "Bernard" causes light to be a null ray, I was saying that the Euclidean representation of Minkowski space is called the Schutz description.
Basically because the basis vectors are constant I can use this Euclidean terminology, even though it is incorrect.

Replies to this message:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 59 of 178 (225710)
07-23-2005 2:54 PM
Reply to: Message 58 by Son Goku
07-23-2005 11:41 AM


I wasn't claiming that "Bernard" causes light to be a null ray, I was saying that the Euclidean representation of Minkowski space is called the Schutz description.
No, I appreciate what you were saying... I just hadn't heard it called the "Schutz description".
Is his book still that popular? It was good, but D'Inverno's was a huge improvement and there must have been several in the last decade of which I am unaware...
But I must say that his "Geometrical Methods of Mathematical Physics" is what helped turn me from an astrophysicist into a mathematical physcist.

This message is a reply to:
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Son Goku
Inactive Member


Message 60 of 178 (225939)
07-24-2005 12:19 PM


Yeah, due to the fact that it is a rare mix of a physical and a mathematical view on the subject.
D'Inverno is definitely better if you want physical exposition though.
Carroll and Hartle are the two major new ones.
Hartle emphasizes physics and Carroll emphasizes maths.

  
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