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# Speed of Light

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Author Topic:   Speed of Light
Percy
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 Message 6 of 268 (471934) 06-19-2008 10:39 AM Reply to: Message 4 by Hoof Hearted06-19-2008 10:17 AM

I was hoping you'd stick around for an answer to this other issue you raised:

 Hoof Hearted writes:Another concept I am given to understand is that the speed of light CAN vary according to the density of the medium in which it is travelling.

The question this raises is that if the speed of light is a fundamental constant, c, then how can it vary according to medium? Once this question is raised the answer is usually expressed more concisely, that c is actually the speed of light in a vacuum.

But this isn't a very satisfactory answer, either. If one could shrink to atomic levels and climb between the molecules in a medium such as glass, would one really measure a slower speed of light in the space between molecules, which must also be a vacuum, just like the vacuum of space? Of course not.

So why does a photon of light take longer to pass through a pane of glass than through the same distance in a vacuum? The answer is that it doesn't. The original photon entering the glass is not the same one that emerges. The molecules of a transparent or translucent medium absorb the photon, rise to a higher energy level, then retransmit a photon and drop back to their normal energy state. The precise molecular structure governs the direction of the new photon. In high quality optical glass, the direction is the same as the absorbed photon, except at the interfaces between mediums, such as that between glass and air. In translucent materials the direction of retransmission is somewhat more random. The extra time light takes to travel through a medium like glass is due to the time it takes for absorption and retransmission of photons. But in the space between molecules the photons of light travel at c, which is a fundamental constant.

--Percy

 This message is a reply to: Message 4 by Hoof Hearted, posted 06-19-2008 10:17 AM Hoof Hearted has taken no action

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 55 of 268 (538099) 12-03-2009 3:21 PM Reply to: Message 54 by Son Goku12-03-2009 2:23 PM

Re: Special Relativity and geometry
There are often many ways of saying the same thing, and many ways of drawing a perspective on a concept or idea. Regardless of whether you give Pope or Bondi or Minkowski or whoever priority for making the observation that c can be considered as how many spatial units are in the standard temporal unit, it seems obvious. Why does this observation have any particular value?

It feels like mileage. With most motor vehicles you measure it as distance per volume, but with some motor vehicles (like tanks) it's more convenient to look at it as volume per distance. So what.

Note that I'm not arguing that the observation isn't valuable. What I'm trying to point out is that if this observation about c is truly valuable then it is possibly in a way that isn't obvious to the layperson (or at least to me) and needs to be explained. Otherwise you and CaveDiver and Viv Pope may as well be speaking ancient Greek as far as the rest of us are concerned.

--Percy

PS - I'm still moderating this thread as Admin. If active moderation becomes necessary then I'll drop out as Percy.

Edited by Percy, : Minor correction.

 This message is a reply to: Message 54 by Son Goku, posted 12-03-2009 2:23 PM Son Goku has replied

 Replies to this message: Message 56 by Son Goku, posted 12-03-2009 4:05 PM Percy has replied Message 59 by cavediver, posted 12-04-2009 5:31 AM Percy has replied

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 57 of 268 (538106) 12-03-2009 5:08 PM Reply to: Message 56 by Son Goku12-03-2009 4:05 PM

Re: Special Relativity and geometry
Hi Son Goku,

Thanks for trying to bring this down to my level. There's something I still must not be grasping, but I understand the explanation and maybe the significance will become apparent to me over time.

But more importantly, most physicists would agree with this particular observation by Viv Pope, and there's also agreement that this is a significant observation, but doesn't Viv Pope believe that most physicists fail to appreciate how truly significant, revolutionary even, this observation is? If so, what's he see in this observation that they don't?

--Percy

 This message is a reply to: Message 56 by Son Goku, posted 12-03-2009 4:05 PM Son Goku has replied

 Replies to this message: Message 60 by Son Goku, posted 12-04-2009 5:40 AM Percy has replied

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
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 Message 61 of 268 (538145) 12-04-2009 6:28 AM Reply to: Message 59 by cavediver12-04-2009 5:31 AM

Re: Special Relativity and geometry
Hi Cavediver,

I started looking up the symbols and terms you used, but this is a workday and I have to move on. If you have time, the information I'm missing is an understanding of permitivity and the magnetic constant (permeability), and what the symbol E represents.

--Percy

 This message is a reply to: Message 59 by cavediver, posted 12-04-2009 5:31 AM cavediver has replied

 Replies to this message: Message 83 by cavediver, posted 12-05-2009 2:38 PM Percy has seen this message

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 62 of 268 (538147) 12-04-2009 6:35 AM Reply to: Message 60 by Son Goku12-04-2009 5:40 AM

Re: Special Relativity and geometry
Thanks for the info. The explanation I once heard that I like best, maybe it was via Greene, is that we move through space/time at a constant velocity. When stationary in space you're moving through time at c, and when traveling through space at c you're stationary in time, and at velocities between 0 and c you're moving through time at a rate given by the Lorentz factor. It's been a few years, I wonder if you find the way I remember it now accurate or garbled.

So what we're interested in knowing from Viv Pope is what he finds in this that makes it a revolutionary observation, and what new predictions and advances in understanding this might bring. In other words, what are their expectations for its contributions to science.

--Percy

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 Replies to this message: Message 63 by cavediver, posted 12-04-2009 6:46 AM Percy has seen this message

Percy
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From: New Hampshire
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 Message 65 of 268 (538161) 12-04-2009 9:31 AM Reply to: Message 64 by Viv Pope12-04-2009 7:55 AM

Re: Moderator On Duty
Hi Viv,

How is this different from Minkowski?

--Percy

 This message is a reply to: Message 64 by Viv Pope, posted 12-04-2009 7:55 AM Viv Pope has replied

 Replies to this message: Message 70 by Viv Pope, posted 12-04-2009 1:20 PM Percy has replied

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 67 of 268 (538164) 12-04-2009 10:13 AM Reply to: Message 66 by Viv Pope12-04-2009 9:33 AM

Re: Special Relativity and geometry
Hi Viv,

I think Son Goku was saying that to him your ideas are not noticeably different from Minkowski. Rather than reiterating that your ideas are indeed different from Minkowski, I think what might be most helpful would be to explain how they are different.

--Percy

 This message is a reply to: Message 66 by Viv Pope, posted 12-04-2009 9:33 AM Viv Pope has taken no action

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 69 of 268 (538186) 12-04-2009 11:55 AM Reply to: Message 68 by Viv Pope12-04-2009 10:36 AM

Re: Response to arrogance
 Viv Pope writes:Anyway, to stick to the real point of this thread, your claim that light can interfere with light in the vacuum of space has been roundly discussed elsewhere – in the PIRT conferences at Imperial College, London, for instance – where it was generally agreed to be on very dodgy ground.

I can't comment on the PIRT conference, but Cavediver was only pointing out something about your point 8 from Message 34:

 Viv Pope in Message 34 writes:8. Can light be scattered by light, as some experimenters have claimed? If a powerful laser-beam is shone across the path of another, do their ‘photons’ collide or their ‘waves’ interfere? In a simple experiment devised and carried out at Brunel university, in 1980 [ ], two powerful lasers were beamed across each other’s paths and also shone head-on at each other. No blocking or interference whatever was detected. If any such interference were to take place, then that light would suffer dispersion. Considering the amount of light that is allegedly ‘criss-crossing’ around, it would be amazing if visual acuity were possible over the length of a single metre. All the light that is allegedly shooting around in all directions would be as much a barrier to vision as the densest fog that can be imagined. The fact, then, that there are photographs of the farthest galaxies that display awesome clarity militates against the validity of any such experimentalist claim.

Cavediver was pointing out that such interference is incredibly rare, so rare that light couldn't possibly interfere with itself to the degree you describe in point 8.

If as you argue in point 5 that it is meaningless to think of c as a velocity in space:

 5. If c is interpreted as a ‘velocity in the vacuum of space’ (as Einstein’s Second Postulate states), then in a vacuum to what can that ’velocity’ possibly be referred, constant or otherwise? So the concept of light as having a ‘velocity in space’ is just another absurdity.

Then do you believe it is wrong to describe light traveling from the distant galaxies as having traveled the distance in a certain time, which is the definition of velocity?

--Percy

 This message is a reply to: Message 68 by Viv Pope, posted 12-04-2009 10:36 AM Viv Pope has replied

 Replies to this message: Message 72 by Viv Pope, posted 12-04-2009 3:39 PM Percy has replied Message 85 by cavediver, posted 12-05-2009 3:01 PM Percy has replied

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 71 of 268 (538209) 12-04-2009 2:01 PM Reply to: Message 70 by Viv Pope12-04-2009 1:20 PM

Re: Moderator On Duty
 Viv Pope writes:I trust that answers your question on this particular point regarding Minkowski.

You say that Minkowski didn't go far enough, that he didn't follow the implications, but you don't describe the implications or what they lead to, so I'm still left wondering how your ideas differ from Minkowski.

--Percy

 This message is a reply to: Message 70 by Viv Pope, posted 12-04-2009 1:20 PM Viv Pope has replied

 Replies to this message: Message 73 by Viv Pope, posted 12-04-2009 5:07 PM Percy has replied

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
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 Message 77 of 268 (538237) 12-04-2009 5:55 PM Reply to: Message 72 by Viv Pope12-04-2009 3:39 PM

Traveling Light
 Viv Pope writes:My aim, here, is by no means to castigate Cavediver or even challenge him for his belief that contemporary quantum physics ‘says it all’...etc...

I wasn't making a point about Cavediver. Someone made a point that you misconstrued. The someone happened to be Cavediver, but if mention of his name causes you to write about him instead of about his point then forget I ever mentioned him.

The point that was made was that the interference you described in point 8 is very rare, so rare that light couldn't possibly interfere with itself to the degree you describe in point 8.

 Viv Pope writes:So, from where do we get the idea that the ,light has taken that time to travel? Again, it is from nothing more than the information itself. We know that the optical distance of a body, in metres divided by the dimensional constant. c ( pace Bondi) is a time in seconds, and from that knowledge we extrapolate the time, relative to us, at the source of those quanta when we receive them, which may be millennia in some cases. But none of that implies that light ‘travels’.

I'm still trying to grasp what you're saying. So when we bounce, say, radio waves off the moon, are you saying that the electromagnetic waves don't travel to the moon and back?

--Percy

 This message is a reply to: Message 72 by Viv Pope, posted 12-04-2009 3:39 PM Viv Pope has replied

 Replies to this message: Message 82 by Viv Pope, posted 12-05-2009 2:05 PM Percy has replied

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 78 of 268 (538239) 12-04-2009 5:59 PM Reply to: Message 73 by Viv Pope12-04-2009 5:07 PM

Discussion Wanted
 Viv Pope writes:the whole thing is laid out in full in the websites, books and things I have liberally mentioned.

I'm here for discussion, not reading assignments, plus as the Forum Guidelines say:

1. Bare links with no supporting discussion should be avoided. Make the argument in your own words and use links as supporting references.

I'm interested in discussing your ideas, but if you to prefer to instead just provide references then I think, having done that, that you're done here.

--Percy

 This message is a reply to: Message 73 by Viv Pope, posted 12-04-2009 5:07 PM Viv Pope has taken no action

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
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 Message 84 of 268 (538316) 12-05-2009 2:53 PM Reply to: Message 82 by Viv Pope12-05-2009 2:05 PM

Re: Traveling Light
Hi Viv,

How does a chain of instantaneous quantum interactions equate to the passage of time for a macro observer? It makes sense that the (in your view) non-existent photon traveling at c would not experience any passage of time, and perhaps whatever the thing is (in your view) that allows objects at a distance to influence each other is everywhere at the same time from its own perspective, but I don't see how that affects the fact that it is traveling from an outsider's perspective.

Spacecraft travel to the moon at speeds well below c, but let's say we could send them at greater and greater speeds. First we send a spacecraft to the moon at .9c, then another at .99c, then another at .999c, and so forth. They *do* travel to the moon, despite traveling at a speed very close to c. But according to you light at 1.0c doesn't travel to the moon, it gets there by instantaneous quantum interaction. But at the lowest levels everything is just quantum interactions, so what you're really telling us is that nothing travels anywhere. Right?

Ultimately the question will become what predictions your ideas make that differ from those of standard relativity, but I'm still trying to make sense of your explanations.

--Percy

 This message is a reply to: Message 82 by Viv Pope, posted 12-05-2009 2:05 PM Viv Pope has replied

 Replies to this message: Message 93 by Viv Pope, posted 12-06-2009 8:13 AM Percy has seen this message

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 86 of 268 (538324) 12-05-2009 4:33 PM Reply to: Message 85 by cavediver12-05-2009 3:01 PM

Re: The funny thing is...
Hi Cavediver,

I understand that light doesn't travel from the perspective of itself, but it does travel from the perspective of us, doesn't it? Relativity, right? My latest guess is that Viv is trying to say that light doesn't travel from our perspective, either, but I'm just casting about trying to find the revolutionary aspect of Viv's way of looking at things. I'm entering this discussion under the assumption that Viv is on to something and that his ideas are not just an unfamiliar way of saying what is already widely accepted within physics. Of course, it would be better if you or Son Goku were doing this, but maybe the level of detail of your knowledge in this area pushes Viv's views too far into the realm of the ridiculous.

You gotta hand it to Viv, though. He's quite the master of the understated insult, though interestingly, it doesn't seem to work in his favor. It's sending everyone's bullshit detectors through the roof.

--Percy

 This message is a reply to: Message 85 by cavediver, posted 12-05-2009 3:01 PM cavediver has replied

 Replies to this message: Message 87 by cavediver, posted 12-05-2009 4:53 PM Percy has seen this message Message 100 by cavediver, posted 12-06-2009 11:35 AM Percy has seen this message

Percy
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Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 212 of 268 (540437) 12-25-2009 9:15 AM Reply to: Message 211 by cavediver12-25-2009 2:40 AM

Re: Spin and Perception
Now *I'm* confused.

--Percy

 This message is a reply to: Message 211 by cavediver, posted 12-25-2009 2:40 AM cavediver has replied

 Replies to this message: Message 213 by AZPaul3, posted 12-25-2009 2:50 PM Percy has seen this message Message 214 by cavediver, posted 12-25-2009 6:04 PM Percy has seen this message

Percy
Member
Posts: 20746
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.2

 Message 215 of 268 (540554) 12-26-2009 7:02 AM

AZPaul3 and Cavediver
Got it - thanks!

--Percy

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