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Author Topic: What do we mean by "propagation"?  (Read 3758 times)

Offline Geezer

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What do we mean by "propagation"?
« on: 19/02/2010 07:44:38 »
As usual, I'm confused.

Propagation in horticulture means one thing, but in physics I always thought it referred to some form of wave energy passing through a medium of some sort or another. However, I think I've seen the term being applied to particles travelling through "nothing". To me, that does not sound like propagation at all.

Is this a "word trick"?


 

Offline JP

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What do we mean by "propagation"?
« Reply #1 on: 19/02/2010 07:55:41 »
Propagation is waves traveling in space.  That space can be vacuum or it can be a medium.
 

Offline Geezer

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What do we mean by "propagation"?
« Reply #2 on: 19/02/2010 08:08:22 »
How can anything propagate through a vacuum? (I suppose it depends on the definition of a vacuum.)

If a vacuum is truly "nothing", propagation is meaningless in that context. Propagation can only occur through "something".

If energy can be transmitted through "nothing" without a transfer of mass, would that not violate some rather fundamental laws?
« Last Edit: 19/02/2010 08:39:14 by Geezer »
 

Offline JP

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What do we mean by "propagation"?
« Reply #3 on: 19/02/2010 08:17:15 »
Are you concerned with the definition or the physics?  By definition, propagation can take place in a vacuum.  The physics of it requires a longer explanation.  
 

Offline Geezer

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What do we mean by "propagation"?
« Reply #4 on: 19/02/2010 08:49:13 »
Frankly, I'm not sure!

It sounds to me like the term "propagation" has been redefined.

I think at one time propagation referred to wavelike energy transfer. If it is now being used in the context of particle energy transfer, I think that could be confusing. Perhaps different terminology would be appropriate.

I suppose I should ask who defined that propagation can take place in a vacuum.

« Last Edit: 19/02/2010 08:53:34 by Geezer »
 

Offline JP

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What do we mean by "propagation"?
« Reply #5 on: 19/02/2010 08:52:38 »
But you can have waves in a vacuum, e.g. light.  Do you agree with that?  (I'm talking purely classical here.  There's no need to go to quantum mechanics to define propagation.)
 

Offline Geezer

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What do we mean by "propagation"?
« Reply #6 on: 19/02/2010 09:17:12 »
That's the essential dichotomy.

It's fairly certain that energy is transferred by electromagnetic means, whether in the visible spectum or not.

If the energy transfer is through "nothing" there has to be a transfer of "something".

If the energy transfer is through "something" there is no need to transfer anything.

Propagation strongly suggests the second case.
« Last Edit: 19/02/2010 09:21:43 by Geezer »
 

Offline JP

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What do we mean by "propagation"?
« Reply #7 on: 19/02/2010 09:44:55 »
Although it might suggest the second case, it doesn't require it by definition.  Electromagnetic waves can travel through a vacuum, and therefore by definition they propagate through a vacuum.  What is moving is the waves themselves along with the energy they carry.
 

Offline Geezer

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What do we mean by "propagation"?
« Reply #8 on: 19/02/2010 09:53:46 »
My education is sadly lacking, but I am highly skeptical. Who's definition are you referring to?
 

Offline JP

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What do we mean by "propagation"?
« Reply #9 on: 19/02/2010 10:22:38 »
Unfortunately I didn't bring my intro-level physics textbooks with me to Singapore, so they're all back in storage in the US.  My grad level textbooks don't bother defining it, since they assume you know it.  It's one of those things that are common knowledge to people in the field who often work with them, so no one bothers to define or cite definitions.

From the Oxford English Dictionary, "The passage of movement, energy, a vibration, a wave, etc., in a particular direction or through a medium; the progression of a crack through a solid."

(In a particular direction is what happens in a vacuum.) 

And I hate to link to books I don't have, but the best I could turn up on a quick Google books search is here.

If you happen to have any intro physics books that deal with optics, there should be a similar definition there.

What's part of the problem, I think, is that people started using the term to describe waves prior to anyone realizing that light can move in a vacuum, but the term was already there so now it's used both in a medium and in a vacuum.
 

Offline Geezer

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What do we mean by "propagation"?
« Reply #10 on: 19/02/2010 17:40:31 »
Thanks JP!

I suppose it's too late to change the definition  ;D
 

Offline yor_on

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What do we mean by "propagation"?
« Reply #11 on: 21/02/2010 19:57:30 »
The medium then?

If you say that waves propagate through a medium the next question should be if space is a medium?
Is it?

Or does it only become a medium when there are particles present in it?
 

Offline norcalclimber

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What do we mean by "propagation"?
« Reply #12 on: 21/02/2010 20:14:00 »
If space is substantial enough to be curved by gravity, doesn't that mean it is substantial enough to be at least loosely defined as a medium? 
 

Offline yor_on

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What do we mean by "propagation"?
« Reply #13 on: 21/02/2010 21:11:03 »
Nice one, don't know? Depends on how you define a vacuum I would say.
To me it seems to be 'distances' as measured by an observer from his frame of reference.

Then we have 'matter' aka invariant mass, waves aka energy aka mass too, depending on definition. And Vacuum which classically is a 'nothing', Quantum mechanically as a 'something'. Containing an 'energy' that either is 'neutralized' as a vacuum in whatever frame you ever will be in, accelerating or not, or expressing itself as 'heat bath and as 'real particles'.

Let me point out one thing here that seems painfully obvious, to me at last. If the idea of Rindler observers observing a constant 'heat bath' due to uniform acceleration (? G constantly) then that 'energy' isn't the vacuum they move through. The energy and the vacuum can't be the same, and the same goes to the idea of 'virtual particles'.

If you have a 'heat bath' as observed from one frame that from another simultaneous frame still is observed as 'virtual' at the same time as you, from your frame of reference at rest relative Earth, observe the uniformly accelerating rocket moving in what you observe to be a vacuum, and they too observe that vacuum, then energy and vacuum isn't the exact same. Maybe too many words here :) but I hope my view still is clear enough to understand.

One could possibly present an idea where acceleration 'compresses' a vacuum due to uniform acceleration, expressing itself as a 'heat bath'? Naah..
« Last Edit: 21/02/2010 21:18:06 by yor_on »
 

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What do we mean by "propagation"?
« Reply #13 on: 21/02/2010 21:11:03 »

 

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