Constancy of the speed of light

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Offline Solvay_1927

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Constancy of the speed of light
« on: 17/11/2005 20:55:37 »
Just a quickie - can anyone tell me what hard evidence we have for the constancy of the speed of light? (i.e. evidence that different people/instruments will always measure c - in a vacuum - to be the same speed, irrespective of their motion relative to the source.)

I'm familiar with the Michelson-Morley experiment (which is pretty convincing evidence, I know).

And I think I sort of understand why J. Clerk-Maxwell's equations imply that c must be a universal constant for all observers (although I'd appreciate any further specific explanations or links anyone has on this).

But is there any further experimental evidence or theoretical reasons why c must be constant?  Any help would be much appreciated.  Many thanks.

Solvay (Paul)

P.S. I do accept relativity theory - the evidence supporting it is fairly compelling - but I'm just curious about this specific postulate.


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Offline Solvay_1927

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Re: Constancy of the speed of light
« Reply #1 on: 21/11/2005 00:16:57 »
Huh!  I'll take that as a "no", then!  [:(]

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Offline turbo-1

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Re: Constancy of the speed of light
« Reply #2 on: 23/11/2005 23:39:24 »
I believe that the speed of light in a vacuum is dependent on the density of the vacuum.  The Pioneer anomaly is a quantifiable measure of this.  EM is returning faster than we expect, which is why (if we think the speed of light in a vacuum is constant) some people think that the probes are slowing down.  The probes are not slowing down, but the EM is returning sooner than we expect.  This is an indication that light travels faster the farther the probes get from the Sun.
 

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Offline Solvay_1927

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Re: Constancy of the speed of light
« Reply #3 on: 24/11/2005 19:48:25 »
Turbo - I'm not familiar with the topic you describe - can you give more info / a web link?
Thanks.
Paul.
 

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Dr. Praetoria

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Re: Constancy of the speed of light
« Reply #4 on: 26/11/2005 22:19:03 »
One interesting sidelight to this constancy of light's velocity, is that in a sense Einstein never said that some object couldn't travel faster than "c" only that if it did, it could never go slower than "c" that is, it would have to always travel at a velocity greater than "c". Also, there is some discussion going on now that delineates a concept that perhaps, all physical constants have changed or were of a different value in past, cosmic evolution.
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Offline turbo-1

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Re: Constancy of the speed of light
« Reply #5 on: 02/12/2005 15:35:30 »
quote:
Originally posted by Solvay_1927

Turbo - I'm not familiar with the topic you describe - can you give more info / a web link?
Thanks.
Paul.

I'm working on a model of quantum gravity, initially motivated by optical effects like gravitational lensing.  It didn't take me too long to bump into Einstein's 1920 Leyden address and later, his 1924 paper "On the Ether".  In both of these he insists that GR's curved space time is a description of an ether pervading all space, and possessing physical qualities that are conditioned by the matter and energy embedded in it.  Here is a non-technical overview, including a list of testable predictions by which the model can be falsified.  The Pioneer anomaly has confirmed on of the predictions - that the speed of light in space is dependent on density of the local vacuum fields (GR ether) through which it propagates.

http://www.bautforum.com/showthread.php?t=35191
 

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #6 on: 02/12/2005 18:23:21 »
quote:
This is an indication that light travels faster the farther the probes get from the Sun.


Why was that unexpected? I would have thought it entirely logical. The closer to the sun you are, the more intense the solar wind hence more particles for photons to interact with.
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Offline turbo-1

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Re: Constancy of the speed of light
« Reply #7 on: 02/12/2005 22:23:58 »
quote:
Originally posted by DoctorBeaver

quote:
This is an indication that light travels faster the farther the probes get from the Sun.


Why was that unexpected? I would have thought it entirely logical. The closer to the sun you are, the more intense the solar wind hence more particles for photons to interact with.

In my model, the speed of light is dependent on the density of the vacuum through which it propagates.  It has to do with the density of the quantum vacuum, which falls off very smoothly the farther you get from the sun.  EM propagates as waves through a transmissive medium, not as corpuscular particles hurtling around on geodesics of curved space-time.
 

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Offline Solvay_1927

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Re: Constancy of the speed of light
« Reply #8 on: 06/12/2005 01:21:20 »
Turbo,
I've now read your paper (in the above link), and I like it.

Unfortunately I know precious little about physics - and even less about cosmology - so I can't really comment on whether your theory is robust or not.  But the concept is powerfully simple, which I find attractive.

I have a few questions related to the paper:

1. This is just a general question (not specifically about your paper):  How do we know the ZPE field exists?  Has the Casimir effect been demonstrated other than in the vicinity of matter (metal sheets or whatever)?  Does it prove that there is virtual particle-antiparticle creation & annihilation throughout space, not just in the vicinity of matter?  (I accept that this is suggested by QFT, but I've never really thought about it in this way before - how can you tell it happens even in a "vacuum" several metres away from any matter?)

2. Why do the pairs want to borrow the least energy that allows them to exist for the longest time?  Why such constraints?  (Again, it's probably just my ignorance of Quantum Theory - I didn't realise it couldn't be just any combination of time and energy that satisfies the Heisenberg Uncertainty Principle.)

3. Why would antiparticles be more attracted to matter than particles would?

4. Are you saying that the speed of light measured on earth would be different depending on whether you measured it parallel to the earth's surface or perpendicular to it (as the density of the ZPE field would reduce perpendicularly to the earth's surface)?  So that the Michelson-Morley experiment might not have given a null result if the apparatus had been set up perpendicularly to earth's surface rather than parallel to it?

5. Has your paper been submitted for review to any physics departments, and have you got any feedback?

Please let me know, I'm interested.  Many thanks.
Paul.
 

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #9 on: 06/12/2005 03:09:32 »
I've started reading it & so far it seems quite intriguing. I'll go through it properly when I'm not so tired
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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #10 on: 06/12/2005 17:26:51 »
Interesting stuff but one of the real tests of variable speed of light ideas is how they get roind the resultant changes in the fine structure constant which can be determined with great accuracy all over the universe.

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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #11 on: 06/12/2005 17:40:48 »
For more detailed information look at  

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

Essentially fine stucture is a critical feature of energy levels in an atom and can be observed to be constant to a high degree of accuracy over the whole obdervable universe.  It is a dimensionless number that links the charge on the electron the speed of light and Plancks constant and implies thart if the fine stucture constsnt is constasnt that other fundsmentla constsnrs in the universe must vary in synchtonism to avoid all sorts of anomalies in observed facts.

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Offline realmswalker

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Re: Constancy of the speed of light
« Reply #12 on: 07/12/2005 02:20:26 »
hmm waht if C was only constant relative to the space directly around  it.
so if u had a patch of universe already moving and light is traveling inside of that, to its surroundings it would be traveling at the speed of light, not faster, but it would be moving faster that the light outside,here ill use a metaphor:
penguins(photons) can slide at a speed of 10feet/second, all penguins do.
THey live on the ice(the space around it).
When sliding on the ice in a race there are marks every 10 feet.
2 penguins take off, your watching the race from directly above.
acording to the constant speed the penguin should cross a mark every 1 second.
Unfotunately one penguin is on a piece of ice 100 feet long when it breaks off and its floating forward at a rate of 10 feet per second also.
now to the other penguin the one on the broken piece of ice seems to be going 20 feetper second! but from above he is still only crossing 1 mark every second.
The ice reconects to the main ice block just as the penguin is about to slide off the edge.
that penguin is now ahead by 50 feet,but never violated the law that he can only cross 1 mark per second.
can this be applied to light? i believe so.
is it easier to invision the concept with a 2 dimensional world (flat ice w/ penguins) being viewd from a 3 d world (ure above them) than it is to invision with a 3d world (light and the space its in)? yes, but it still holds true in our world (u would have to observe it from a 4/th dimensional view to see that it does not violate the law that light travels at a constant speed though...)


« Last Edit: 07/12/2005 02:21:25 by realmswalker »
 

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #13 on: 07/12/2005 13:37:19 »
Realmswalker - That does appear to be so. Galaxies on opposite sides of the universe can move apart at a rate greater than the speed of light.
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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #14 on: 07/12/2005 16:31:41 »
Realmswalker  Three important questions

How can you tell one piece of space from another?
What happens when one piece of space detaches itself and moves with respect to another piece of space?
What causes this to happen?


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Offline Solvay_1927

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Re: Constancy of the speed of light
« Reply #15 on: 09/12/2005 23:39:32 »
Oh well, if Turbo isn't going to come back, can anyone else (Ian??) help with my first question above?
quote:
Has the Casimir effect been demonstrated other than in the vicinity of matter (metal sheets or whatever)? Does it prove that there is virtual particle-antiparticle creation & annihilation throughout space, not just in the vicinity of matter? (I accept that this is suggested by QFT, but I've never really thought about it in this way before - how can you tell it happens even in a "vacuum" several metres away from any matter?)
 

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #16 on: 09/12/2005 23:56:48 »
extract from  http://www.calphysics.org/zpe.html

quote:
There is a force associated with the electromagnetic quantum vacuum: the Casimir force. This force is an attraction between parallel metallic plates that has now been well measured and can be attributed to a minutely tiny imbalance in the zero-point energy in the cavity between versus the region outside the plates. This is not useful for propulsion since it symmetrically pulls on the plates. However if some asymmetric variation of the Casimir force could be identified one could in effect sail through space as if propelled by a kind of quantum fluctuation wind. This is pure speculation


I take what I've highlighted in bold above to mean that the Casimir effect only happens in the gap between the plates.  However, I don't think it really answers your question as it doesn't say whether any fluctuations take place outside the plates.
« Last Edit: 10/12/2005 00:03:59 by DoctorBeaver »
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Offline ukmicky

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Re: Constancy of the speed of light
« Reply #17 on: 10/12/2005 04:33:33 »
PAUL
A LIST OF  EXPERIMENTS[:)]

HOPE IT HELPS
 



http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#3.%20Tests%20of%20Einstein's%20two%20postulates



Michael                                      
« Last Edit: 10/12/2005 04:42:33 by ukmicky »

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #18 on: 10/12/2005 13:59:28 »
Good link! Have a Brownie point
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Offline Solvay_1927

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Re: Constancy of the speed of light
« Reply #19 on: 11/12/2005 23:57:40 »
Michael, that's a really useful link about evidence for Special Relativity, many thanks.  It's put my mind at rest.

Eth, that's a really useful link (with lots of other links within the calphysics site) about the ZPE field, many thanks.  (Incidentally, I now realise that the thinking behind my original question is bordering on the stupid - you can't measure whether quantum fluctuations are occurring far from any matter, because any measuring device you use would be made of matter.  It's a bit like asking "how can you know which path a photon has really taken through a vacuum?"  You can't - you can only measure the location of a photon when it interacts with matter - you can never know for sure where the photon has been other than at the points where you emit it or measure it.)
 

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #20 on: 12/12/2005 00:48:39 »
quote:
you can only measure the location of a photon when it interacts with matter - you can never know for sure where the photon has been other than at the points where you emit it or measure it


A bit like knowing where your cat (or Schrodinger's) is when it comes home but you have no way of knowing where it's been all night
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Offline tequalseminuseminusone

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Re: Constancy of the speed of light
« Reply #21 on: 16/12/2005 23:46:47 »
Sorry to hijack this topic, but the original post WAS about the constancy of c.

I would just like to point out to the original poster that the hard evidence that exists to support the constancy of c may be flawed.

I have been struggling with this recently while checking out VSL theory:

http://frontwheeldrive.com/joao_magueijo.html

However, I find that there exists a setting where both SR and VSL fit, which involves a bit of a paradox.

A relative contstant? Is that possible?

VSL speculates that c was faster at the beginning of the universe. How could this be and ALSO include SR with a constant c?

I believe the relativity of c has to do with observers at different time-energies. relative yet constant.

an observer at the beginning of the universe measures c and gets 300,000km sec.

an observer today measures c and gets the same results

an observer today measures c based on light being emitted from the beginning of the universe and gets the smae results.

**now the interesting part**

an observer today measures the SAME burst of light that the observer at the beginning of the universe measured and gets a result greater than c.

(this is a philisophical discussion at this point)

Why?

If everything is decaying at the same rate (t), then t becomes the constant and c becomes relative.

However, basd on SR, we know that t is relative as well, which again makes the paradox of the relative constant. Observable time is relative, yet universal time is absolute?



 

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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #22 on: 16/12/2005 23:58:47 »
I've been keeping an eye on joao's work for some toime now.  He is a respected worker in the field.  The important thing to realise in this area is tha varying the speed of light in exteremis is no big deal in fact most cosmologists would agree that at some stage most of the laws of physics are up for grabs.  the interesting bit is how and why they change and why did they get to bee the seemingly rather improbable values that have to be precisely obeyed to get our universe to work and generate complex lifeforms!

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #23 on: 17/12/2005 01:10:17 »
quote:
VSL speculates that c was faster at the beginning of the universe. How could this be and ALSO include SR with a constant c?


Energy levels in the very young universe were of an almost unbelievable magnitude. We don't really know how the laws of physics, as we understand them, stand up in such an environment. SR & GR may well only apply in the universe the way it now is.
No-one has yet come up with a reason why universal constants have the values they have; so how can we know they have always been the same?
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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #24 on: 17/12/2005 11:43:24 »
The interesting and important feasure about the actual values of the critical physical constants that make our universe the way it is, is that they are very finely balanced to appear to ensure that complex objects and life can exist.  Even very tiny changes result in for example there being no atoms much more complex than hydrogen or helium.

Some people can see this as evidence of an outside influence (or God!)  designing a universe that can include life like us. Alternatively  I tend to prefer to see it as evidence that some sort of evolutionary process has already taken place as the physical laws "settled down" during the initial stages of the big bang.  This process maximises the complexity and longevity of any universe as it is formed.

Look at my website for a bit more explanation of physical evolution processes.

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Offline DoctorBeaver

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Re: Constancy of the speed of light
« Reply #25 on: 17/12/2005 14:30:06 »
quote:
Alternatively I tend to prefer to see it as evidence that some sort of evolutionary process has already taken place as the physical laws "settled down" during the initial stages of the big bang. This process maximises the complexity and longevity of any universe as it is formed.


That's more-or-less what I was saying, although I wouldn't use the word "evidence". We can only reverse-engineer the universe according to our current understanding of the laws that govern the way it is today (or as far back in time as we can see); and there are some fairly large holes in that knowledge anyway. Take inflation theory. That may help to explain 1 question but it raises other questions that we can't answer.
With doubt now being cast upon the absoluteness of the fine structure constant, there may well be a case to be made for other universal constants not always having had the values they have now.
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Offline Searcher

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Re: Constancy of the speed of light
« Reply #26 on: 28/12/2005 22:40:34 »
Solvay 1927 asked:
 
quote:
can anyone tell me what hard evidence we have for the constancy of the speed of light? (i.e. evidence that different people/instruments will always measure c - in a vacuum - to be the same speed, irrespective of their motion relative to the source.)

But is there any further experimental evidence or theoretical reasons why c must be constant? Any help would be much appreciated. Many thanks.


I have a theory on this subject and it goes like this:-

Back to basics, velocity = distance travelled / time taken.

Assume the distance is a fixed amount say 300,000 Km therefore 300,000Km /sec = speed of light.

If the velocity changes then time changes. But what if time cannot be changed then the velocity is fixed.

My theory is that at the big bang time was the first thing to come into existence since I have not heard of any matter existing outside time. Time then moved out and created the space for the cosmos to expand into. Therefore time itself must have a velocity and since nothing can exist in front of time nothing can travel faster than time moving out creating space.  It follows that time is moving out at the speed of light hence nothing can travel faster.

I am preparing a paper on the subject at the moment and would welcome any help or criticism. The mathematics is quite daunting.  
 

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Re: Constancy of the speed of light
« Reply #27 on: 29/12/2005 10:18:02 »
Thought Albert Einstin sorted that out quite adequately a century ago.

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Offline DocN

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Re: Constancy of the speed of light
« Reply #28 on: 29/12/2005 21:43:02 »
I have read that this early time dimension could be "space-like"--that is, giving us four space-like dimensions at the big bang--with no time element present at this event.  I wonder what event set off the formation of such a construct?  I know it's difficult enough to understand the instant and short period after  the big bang but even more interesting is what was happening before it.
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Re: Constancy of the speed of light
« Reply #29 on: 29/12/2005 22:44:03 »
OK, I am about to demonstrate my total ignorance in these matters (as if I had not amply demonstrated sufficiently already).

I understand (or at least, have been able to look up – which is probably well short of claiming to understand) that the difference between spacelike and timelike relate to Minkowski space.

First problem is that although at a very superficial level I can understand what a matrix is, but understanding the nomenclature of matrix manipulations is beyond me.

Maybe if I could get over that hurdle, I could understand what the rest of this was about, but can someone explain to be in experiencial terms what would be the effect of a universe with 4 spacelike dimensions.

To my tiny mind, time is simply the way we measure change.  If we have no time, then how would something be able to change, so how could you change from a situation without time to one with time if there was no time over which things changed?

Sorry if this is getting seriously naïve, but I am seriously getting to the point where I feel I have to put my hands up and surrender.

Ofcourse, maybe the answer you will come back with is that it will take a 3 year degree course to explain it all, and we don't have 3 years to do it in.

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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #30 on: 29/12/2005 23:23:19 »
You should read the book "the end of time" by Julian Barbour a respected cosmologist who explains a lot of quantum wierdness by suggesting that we should think of a timeless universe in which all the possible sequential configurations occur at all times and our observation creats a set of world lines witihn this much vaster universe of quantum potential.  It is quite readable and does not make very great mathematical demands on the reader

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Re: Constancy of the speed of light
« Reply #31 on: 30/12/2005 01:02:44 »
quote:
Originally posted by Soul Surfer

You should read the book "the end of time" by Julian Barbour a respected cosmologist who explains a lot of quantum wierdness by suggesting that we should think of a timeless universe in which all the possible sequential configurations occur at all times and our observation creats a set of world lines witihn this much vaster universe of quantum potential.  It is quite readable and does not make very great mathematical demands on the reader

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OK, book on order from Amazon – I'll see if it makes the difference.  Some of the reviews seem a bit mixed, although they seem to indicate more that they disagree with the conclusions of the author, but nonetheless seemed to be a good way of clarifying a number of concepts along the way.

I should probably also find myself a simple book on understanding matrix manipulation beyond simply multiplying matrices (which is as far as my comprehension presently goes).

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Offline realmswalker

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Re: Constancy of the speed of light
« Reply #32 on: 30/12/2005 01:28:43 »
what if the speed of light is relative to the time of the universe...
the universe is expanding, so is it possible to say that the time that it takes a beam of light to travel across the universe is the constant, not the speed its self. just a notion
 

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Re: Constancy of the speed of light
« Reply #33 on: 30/12/2005 11:21:11 »
This cannot be true because observations of remote objects clearly indicate that the speed of light a long time ago wa exactly the same within many decimal places what it is today.  Even variable speed of light models only allow tiny changes within the easily observable universe

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Re: Constancy of the speed of light
« Reply #34 on: 30/12/2005 11:26:39 »
I have already mentioned Roger Penrose's "Road to Reality" elsewhere on this forum.  This starts with an assumption of very little mathematical knowledge and takes you all the way to quantum gravuty and string theory including tensor calculus etc.  It's the best and most readable maths book that I know.

Learn, create, test and tell
evolution rules in all things
God says so!
Learn, create, test and tell
evolution rules in all things
God says so!

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Re: Constancy of the speed of light
« Reply #35 on: 30/12/2005 14:44:41 »
quote:
Originally posted by Soul Surfer

I have already mentioned Roger Penrose's "Road to Reality" elsewhere on this forum.  This starts with an assumption of very little mathematical knowledge and takes you all the way to quantum gravuty and string theory including tensor calculus etc.  It's the best and most readable maths book that I know.

Learn, create, test and tell
evolution rules in all things
God says so!



Curiously enough, Amazon claim that is not yet published – due out in February.

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Offline Soul Surfer

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Re: Constancy of the speed of light
« Reply #36 on: 30/12/2005 20:35:05 »
That's probably the paperback version I bought the hardback earlyish 2005 shortly after it was published. and spent a lot of the summer reading it.

Learn, create, test and tell
evolution rules in all things
God says so!
Learn, create, test and tell
evolution rules in all things
God says so!