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Author Topic:   Relative Motion (A Thought Experiment)
RingoKid
Inactive Member


Message 31 of 86 (127460)
07-25-2004 9:05 AM
Reply to: Message 28 by coffee_addict
07-25-2004 2:33 AM


Re: Two slit experiment
I've seen it argued that light is not a wave...
...it merely acts like one when travelling through a medium

This message is a reply to:
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coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 32 of 86 (127493)
07-25-2004 1:53 PM
Reply to: Message 31 by RingoKid
07-25-2004 9:05 AM


Re: Two slit experiment
What about experiments done in a vacuum?

The Laminator
For goodness's sake, please vote Democrat this November!

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Maxwell's Demon
Member (Idle past 6229 days)
Posts: 59
From: Stockholm, Sweden
Joined: 05-09-2004


Message 33 of 86 (127494)
07-25-2004 1:56 PM
Reply to: Message 30 by coffee_addict
07-25-2004 3:25 AM


Re: Two slit experiment
The double slit experiment was devised by Thomas Young if I'm not mistaken (it's often referred to as Young's Double Slit Experiment).

"tellement loin de ce monde..."

This message is a reply to:
 Message 30 by coffee_addict, posted 07-25-2004 3:25 AM coffee_addict has replied

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coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 34 of 86 (127496)
07-25-2004 2:03 PM
Reply to: Message 33 by Maxwell's Demon
07-25-2004 1:56 PM


Re: Two slit experiment
I wasn't asking about the Young experiment. I was talking about the concept of field. If you read the post I made before that, it clearly stated that Young started the experiment.

The Laminator
For goodness's sake, please vote Democrat this November!

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Maxwell's Demon
Member (Idle past 6229 days)
Posts: 59
From: Stockholm, Sweden
Joined: 05-09-2004


Message 35 of 86 (127497)
07-25-2004 2:05 PM
Reply to: Message 34 by coffee_addict
07-25-2004 2:03 PM


Re: Two slit experiment
Oh. Sorry 'bout that. I have this nasty habit of skipping posts in long topics.

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


Message 36 of 86 (127501)
07-25-2004 2:58 PM
Reply to: Message 31 by RingoKid
07-25-2004 9:05 AM


Wave?
I've seen it argued that light is not a wave...
It isn't a wave; it isn't a particle. These are analogies that we use to describe what we observe. It has characteristics which the mathematics of waves can be used to describe. It has other characteristics that are particle like. It has characteristics which are neither.

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Eta_Carinae
Member (Idle past 4375 days)
Posts: 547
From: US
Joined: 11-15-2003


Message 37 of 86 (127504)
07-25-2004 3:02 PM
Reply to: Message 36 by NosyNed
07-25-2004 2:58 PM


Exactly
Light is light. Period.
We describe it mathematically by models. Sometimes we get the correct prediction of experiment by using the wave picture. At other times the particle picture is more appropriate.
But just as my name on a sheet of paper is not me, then describing light as a particle or a wave is not light.

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Hangdawg13
Member (Idle past 751 days)
Posts: 1189
From: Texas
Joined: 05-30-2004


Message 38 of 86 (127557)
07-25-2004 9:18 PM
Reply to: Message 21 by NosyNed
07-24-2004 4:01 PM


Re: Relative orbiting
In his book The Science of Mechanics (1893), Ernst Mach put forth the idea that it did not make sense to speak of the acceleration of a mass relative to absolute space. Rather, one would do better to speak of acceleration relative to the distant stars. What this implies is that the inertia of a body here is influenced by matter far distant.
from: Mach's Principle -- from Eric Weisstein's World of Physics
So does this mean that space is a product of the matter that is in it? If Mach's principle is not true, and the lone planet or tidally locked system is the only thing in a universe of the same characteristics of our own, then it seems like this little thought experiment says that space is a 'thing' that motion is relative to.
Just thinking out loud, but if massive objects tend to pull space with them (I think we recently put a satellite up to test this) and motion is relative to space, then might it be possible to verify this experimentally? A massive spinning planet or star would spin space a little with it reducing the actual centripetal acceleration from the theoretical by a very slight amount?
Maybe I'm just crazy.

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crashfrog
Member (Idle past 1467 days)
Posts: 19762
From: Silver Spring, MD
Joined: 03-20-2003


Message 39 of 86 (127571)
07-25-2004 10:18 PM
Reply to: Message 38 by Hangdawg13
07-25-2004 9:18 PM


A massive spinning planet or star would spin space a little with it reducing the actual centripetal acceleration from the theoretical by a very slight amount?
Isn't that exactly what we sent up that satellite to test?

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 Message 38 by Hangdawg13, posted 07-25-2004 9:18 PM Hangdawg13 has replied

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sidelined
Member (Idle past 5908 days)
Posts: 3435
From: Edmonton Alberta Canada
Joined: 08-30-2003


Message 40 of 86 (127668)
07-26-2004 6:14 AM
Reply to: Message 38 by Hangdawg13
07-25-2004 9:18 PM


Re: Relative orbiting
HD13
So does this mean that space is a product of the matter that is in it
No this means that space has no existence independant of reference to bodies within it. There is no absolute space. In other words it is meaningless to assign a motion to a single body in an otherwise empty void.
You are going to be learning to fly jets and if you do it will be necessary for you to develop a keen sense of your surroundings which you can only do by maintaining a reference frame by which to relate to "other bodies around you" and by that know your own motion relative to the space defined by those other bodies.

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


Message 41 of 86 (127810)
07-26-2004 1:49 PM
Reply to: Message 18 by coffee_addict
07-24-2004 3:34 PM


Oh, whew! *wipes sweat off forehead* Don't do that to me, Lam! For a minute there, I thought I was going to have to learn that stuff all over again!
So what I thought originally was correct then? Ok, cool. Thanks for clearing that up.
Lam writes:
Yes, you are correct. However, do you really know what the duality of particle/wave nature of light is? It's an interesting concept, but it can only be shown mathematically and no more, just like the dimensions beyond our space-time.
I'd be lying if I said I completely understand it but I do have some idea. I understand that light is observed to have both particle-like and wave-like properties and I'm familiar with Young's two slit experiment.
However I am not familiar with the underlying mathematics. As is the case with most of my scientific "understanding", my knowledge of this is limited to the concepts themselves and not the mathematical theory behind them. I just hope that this won't hold me back from fully appreciating the concepts.
Lam writes:
No, you have not misunderstood.
Thanks again for clearing that up. For a minute there, I really did think I was going to have some serious "unlearning" to do.
Tony650 writes:
Please feel free to set me straight on this.
Lam writes:
You mean you're gay?
Heh, never miss a trick, do you?
What's funny is that as soon as I posted that I realized the irony of using that expression in a reply to you. I thought you'd pick up on it. Well done. *chuckle*
Oh and to answer your question...I used to be, but I'm not quite as cheerful these days. You see, I've been under a lot of stress lately and...Oh! Gay! Right...Gotcha!
No, I'm not. Don't worry, though. For what it's worth, I can assure you that you'll receive absolutely no judgement or hatred from me for being gay, Lam.

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


Message 42 of 86 (127816)
07-26-2004 2:07 PM
Reply to: Message 19 by :æ:
07-24-2004 3:53 PM


Tony650 writes:
I truly didn't intend it to be a trick question.
:: writes:
I know you didn't, and forgive me if you feel accused.
Heh, it's ok. Little Mr. Sensitive here takes things the wrong way sometimes.
I understand that on forums like this, you kind of need to be on "troll alert" and I just wanted it to be clear that my questions are always sincere. I don't handle heated arguments well at all, which is why I tend to avoid participation in the "controversial" topics.
But I digress.
:: writes:
To inhabitants on the earth, it would have no meaning. To them, it wouldn't appear that there is any orbiting at all, and more or less all motion *IS* is a matter of appearances.
Yes, and this, I think, is what's causing my dilemma. Motion has real-world consequences which can be tested for and detected. How, then, can something with objective consequences be defined by completely subjective frames of reference?
If the people on our hypothetical Earth test for, and find, signs that the Earth and moon are in orbit around each other, then what does this mean? In orbit relative to what? Not to each other. From either's perspective, the other doesn't move at all (if they are tidally locked). And if there is nothing else, what meaning can their "orbiting" possibly have?
Hence my earlier suggestion that they may in fact be orbiting each other relative to space. But is this possible? Or would it violate relativity's principle that there can be no truly "absolute" frame of reference?
:: writes:
I think it's best regarded as an abstraction. It's the continuum of associations that we make in our minds between our memories of the past and our observations and the present.
Hmm...True to a point, but then, doesn't it have real-world consequences which can be objectively detected? This is one of my points of confusion: Does our lone system of tidally locked planet and moon suddenly lose its ability to be in orbit because of the lack of relative reference frames? I find this hard to believe.
But then, if this isn't the case, doesn't that imply that motion is some kind of absolute quality which can exist without anything to be relative to? I find this equally hard to believe.
So it seems that I'm stuck between those two frustrating places which are, as the saying goes, "hard".
:: writes:
If we didn't know that they were orbiting (which, since you stated the they were, is what made your question tricky ) there would be no way to discern that they were moving absent some additional point of reference moving non-uniformly to the earth-moon system.
Yes, I thought that might have confused things a bit.
How about this? Let's say that the "Earth" is inhabited by a society which, as yet, has no advanced knowledge of astrophysics. How could they tell whether their universe was one in which massive bodies orbit each other, or one in which they are "fixed" in the heavens, not affecting each other in any way?
Now, I assume they could perform tests which would determine this (Coriolis effects on the Earth for instance) and if they found no indicators of motion then problem solved; there is no motion. But if they did then I am forced back to my original question; moving relative to what?
Tony650 writes:
Since they are the only points of reference that exist, and neither of them moves relative to the other, can the concept of them orbiting each other have any meaning?
:: writes:
I'd say no.
At this point, so would I. But if the rules of gravitation work the same as they do in our universe does this mean that the gravitating bodies will inevitably fall together?
If their "orbiting" has no meaning without at least one external frame of reference, then there is no distinction between a tidally locked orbiting system with nothing to relate its orbit to, and a system which is simply fixed in its place, not moving at all.
This is my other major dilemma: Does motion simply "turn off" without a relative reference frame? Or "turn on" with a relative reference frame?
If someone gets in a ship, flies away from our hypothetical Earth, and looks back when they are in a position such that they can see the Earth and moon orbiting, they have effectively "created" a relative frame of reference.
So do all the laws of motion suddenly "kick in" now that the ship has broken uniform motion with the Earth/moon system? And do they "switch off" again when he returns to the Earth, reintegrating that relative frame of reference back into the Earth/moon system's uniform motion?
I find this thought quite ludicrous, myself. But I don't see how it can be avoided unless their orbiting motion is permitted without any relative frame of reference. But if this is permitted then I am again forced back into a corner regarding my original question: Orbiting relative to what?
I apologize for being so repetitive. I realize that I'm kind of arguing in circles here but I'm afraid this one really has me flamboozled. I seem to be faced with a paradox no matter which way I go.
Also, I hope what I've written is intelligible enough. I'm having almost as much trouble putting my thoughts into words as I am with the problem itself. I hope my posts are at least partially comprehensible.

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


Message 43 of 86 (127820)
07-26-2004 2:13 PM
Reply to: Message 21 by NosyNed
07-24-2004 4:01 PM


Re: Relative orbiting
NosyNed writes:
This shows a spinning merry-go-round. On it there is no relative motion between the participants. But the ball they throw behaves very different than if the platform is stationary. I think, maybe, the same effect would be detectable in the case of the lone earth-moon system.
Thanks for the link, Ned!
And yes, I would agree with you. But this isn't really my problem.
In this example, the merry-go-round is spinning relative to the ground beneath it, the trees around it, the other rides, the people, etc. It has a clearly definable relative motion. The Earth/moon system (or the sole "Earth") in our thought experiment does not.
I'm not saying that you couldn't detect such effects in our hypothetical universe. What I'm trying to figure out is: If you were to detect such things in a system totally isolated from any relative frame of reference, what would it mean?
Would it mean it was moving? Again, relative to what? Does the concept of one body's motion actually mean anything without a second body moving relative to the first? Isn't that precisely what motion is?
NosyNed writes:
However, there is also the idea of Mach's principle.
Yes, I am vaguely (very vaguely) familiar with this. Is Mach's principle generally believed to be true?
For that matter, is there any way we can even test it? By its nature, it would seem to be a difficult idea to either confirm or disprove. As we are stuck in this universe, how does one go about demonstrating that a body's inertial properties are different in a universe with a different overall mass? Or perhaps I misunderstand the principle?
If I understand it correctly, am I right in thinking that, in the original thought experiment (with only the Earth), motion would in fact not be possible, as there is no mass outside of the Earth itself?
Or would it mean that the Earth could (possibly) move but it would simply feel no inertial effects of its movement? Perhaps, then, the Earth in this scenario could move but you could never know because it would show none of the physical characteristics of motion.
Unfortunately this, in my opinion, makes things even worse. What meaning can the "motion" of the Earth have if it not only has no relative frame of reference, but also shows no physical evidence of motion?
Of course I may be completely misunderstanding (or misapplying) Mach's principle. Please feel free to correct me (or point me in the right direction) if I am wrong on this.

This message is a reply to:
 Message 21 by NosyNed, posted 07-24-2004 4:01 PM NosyNed has replied

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


Message 44 of 86 (127823)
07-26-2004 2:22 PM
Reply to: Message 23 by sidelined
07-24-2004 4:44 PM


sidelined writes:
If we are on the surface of earth aboard a train travelling at 60 mph we can use the train as a fixed reference point and state that the earth is moving relative to our reference point on board.
Exactly! This is what I'm confused about; it's all relative. If the Earth is all that exists in the universe then there is nothing to relate it to. So in what sense can it "move"?
sidelined writes:
It is not indecisive but relative to the reference frame.
Yes, that was a poor choice of words, on my part. I apologize. Your phrasing ("relative to the reference frame") cuts right to the point, I think. I've been fumbling my words a bit and this may help me make myself a little clearer. Thank you.
In the original example there is one frame of reference (the Earth). Now, what this ultimately comes down to (I think) is whether or not one reference frame can move without another. If only one frame of reference exists and no other, can it be said to "move" or is this a meaningless concept?
sidelined writes:
They would each orbit about the barycenter of their gravitaional system.
Yes but relative to what? This is what I can't figure out.
sidelined writes:
One would orbit the other in reference to the other one.
In a tidally locked system wouldn't they be motionless, relative to each other? From the Earth, the moon would appear motionless, and from the moon, the Earth would appear motionless, correct? If there is no other frame of reference to relate them to then how can they meaningfully be said to have any motion, orbital or otherwise?
I apologize for all these questions. I'm not trying to be confrontational, I'm just having trouble getting this. I hope I'm not coming off as a know-it-all.
I'm well aware that most of the people here are far more educated in these subjects than I am and it's not my intention to disagree with anyone. I'm just having trouble reconciling some of this with my layman's understanding.
Anyway, I'm sure that the problem lies in my own misunderstanding or lack of education so please don't think that I'm ever being argumentative. I genuinely want to understand this and I very much appreciate everyone's patience.

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


Message 45 of 86 (127826)
07-26-2004 2:26 PM
Reply to: Message 43 by Tony650
07-26-2004 2:13 PM


Mach's Principle
I think the whole first half of your post (to the second quote of me) is a restatement of Mach's Principle.
I have not idea if it is true or not. I think the problem is, as you've stated, it testability.
If MP is true then the merry go round would not see correolis forces alone in the universe, I think. But it would see them if MP isn't true.
Got a spare Merry-go-round? Got a spare empty universe?

This message is a reply to:
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