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Author | Topic: Purple dosn't beleve in relativity | |||||||||||||||||||||||||||||||
PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
OK then Percy. Since you asked so nicely, let's get down to the fun stuff.
First up we need to look at an example of why time has to be a variable under experimental (theoretical anyway) conditions. Go to about half way down this page and you will see such an example. However I will use my own version of it here. This is how it was explained to me in college.A fast moving spaceship of some sort is moving from point A1 towards point A2 at some significant portion of light speed. Onboard this spacecraft is a light clock in which a beam of light is fired (at point A1) directly downwad toward the mirror. The mirror is actually part of the light clock and moves with it but for the sake of keeping the diagram simple I have drawn it as one continuous line or plane. For an observer in the spacecraft, the trajectory of the light beam will appear to be as shown by t1. ie. straight down then straight up again and will take exactly one second to return to its starting point. (this is an unrealistic time I know but that is entirely beside the point) However, to an observer at position B, the light beam will appear to travel along trajectory t2 which is obviously a much greater distance and so will take considerably longer than the one second observed by A. We know that the light has actually travelled distance t2 for both observers so the only conclusion is that time must be compressed for observer A with respect to (relative to) observer B This all seems logical and as a "proof" of the principals of relativity in action, it is quite convincing and is often used to educate the "uninitiated" However, Let's take the example one step further. (this is never done by physics professors to my knowledge).Take this diagram.. Now both observers have a light clock and are moving at a constant speed (wrt each other) in opposite directions.As before, observer A measures his light speed as c, his elapsed time as 1 second and the distance traveled by the light beam as t1. Observer B still sees observer A's light beam as travelling a greater distance than A does. Conclusion: As before, observer B determines that observer A's time must be compressed relative to his own due to the greater distance that A's light beam travelled. Now here comes the bit that has always thrown me. Observer B sees his own light beam travel distance t3 in one second while observer A sees observer B's light beam travel distance t4 in considerably more than 1 second.As light speed is constant, observer A concludes that observer B's time has to be compressed relative to his own. Now we have the paradox. Both observers see the other observer's time as being compressed relative to their own. That is clearly not possible so there has to be something wrong with the original hypothesis. No accelerations to explain it all away this time. How can this be possible? PY
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PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
But newtonian physics can actually be tested under some conditions. In fact it works completely under most conditions as far as I am aware. (If I am wrong here then please tell me so.)
That means that it is testable and is therefore falsifiable and valid under those conditions. STOR is not 100% accurate under any conditions. PY
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jar Member (Idle past 394 days) Posts: 34026 From: Texas!! Joined: |
Now we have the paradox. Both observers see the other observer's time as being compressed relative to their own. Where is the paradox? I don't really see it there? Aslan is not a Tame Lion
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PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
Jar
Where is the paradox? I don't really see it there?
That is a response I didn't expect. Seems simple enough to me. A's elapsed time is shorter than B's but B's is also shorter than A's. How can you have two identical peices of string that are both the shorter of the two? If that isn't a paradox then I'm living in the wrong universe PY
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PaulK Member Posts: 17822 Joined: Member Rating: 2.2 |
Newtonian mechanics can only work EXACTLY if all objects are stationary relative to each other. If there is ANY relative movement at all Special Relativity gives more accurate results.
The REASON Newtonian mechanics works is because it is a very close approximation unless the relative speed is an appreciable fraction of c. I'm sorry but if you don't get that then I'm afraid that you have no business trying to critique General Relativity.
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PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
Paul writes: I'm sorry but if you don't get that then I'm afraid that you have no business trying to critique General Relativity. Who says I have to be an expert in the field in order to ask pertinent questions and examine the logic behind the answers? I have never claimed to know everything about it. I am merely offering observations making logical conclusion.That kind of answer is usually one given by somebody who is absolutely unwilling to even examine their own frame of reference (wanted to avoid the word beleifs here) Also, by the way you put it, newtonian physics is also untestable. I admit to being completely unaware that it was invalid if any relative motion is present between the bodies involved. So we live in a universe defined by vaguaries and approximations on every side do we? near enough is good enough is it? What the heck is reality then? PY
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PaulK Member Posts: 17822 Joined: Member Rating: 2.2 |
What I'm saying is that if you don't realise the limits of sciences ability to test theories - and if as a result you make unreasonable demands for "proof" of theories you disagree with you are wasting everyones time.
And Newtonian mechanics isn't completely wrong - far from it. It's every bit as good as we need for most applications on Earth. Thats why it is still taught - very few people will go into particle physics or astrophysics where they need Relativity. Special Relativity is even more accurate and General Relativity is better still.
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jar Member (Idle past 394 days) Posts: 34026 From: Texas!! Joined: |
A's elapsed time is shorter than B's but B's is also shorter than A's. The elapsed time for A is the elapsed time for A. The elapsed time for b is the elasped time for b. How that appears for another entity depends of their relationship. You are looking at it from three different POVs, A, B and some independant and unspecified observer that sees only t2 and t 4. A sees all of the events from his perspective. The same is true of B. They see t1 and t4, and t3 and t2 respectively. And the unspecified outside observer sees only t2 or t4. Aslan is not a Tame Lion
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PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
Paul writes: What I'm saying is that if you don't realise the limits of sciences ability to test theories - and if as a result you make unreasonable demands for "proof" of theories you disagree with you are wasting everyones time. If you recall, this was just a side point anyway. I ended up off at a tangent to the real point anyway. My final point along this path is that there must be a science somewhere that can give 100% accurate results 100% of the time. We just don't have it yet. If the universe operates on a set of rules (which I would say it has to) then these rules cannot be "approximate". They must be precise.Relatively may be mostly right and so might newtonian physics. Ether way it is neither here nor there. They don't explain everything so I am looking for the thing that does. And you are right that this tangent is a waste of time. PY
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Jazzns Member (Idle past 3911 days) Posts: 2657 From: A Better America Joined: |
I know you all said you weren't going to talk math but I thought this was simple enough to bring up.
Forgive my ignorance of physics if I am wrong and this dosen't apply to this scenario. In math: (A < B) and (B < A) => (A = B) Which seems to make sense if you were a third observer in this example. Their times are really the same. I don't know if this helps.
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PaulK Member Posts: 17822 Joined: Member Rating: 2.2 |
Ever heard of the Heisenberg Uncertainty Principle ?
The more accurately you know a particle's position the less accurately you can know its momentum. And vice versa. And then theres the problem that we can' measure anything with infinite precision. So even if there were an exact theory which described everything we could never know if we had it or not. And this tangent isn't a waste of your time - you should be learnimg from it. It just would have been better if you actually had found out about it for yourself.
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JonF Member (Idle past 168 days) Posts: 6174 Joined: |
A's elapsed time is shorter than B's but B's is also shorter than A's. How can you have two identical peices of string that are both the shorter of the two? What you are missing is identifying the observer or the reference frame in your statements and questions. One thing relativity tells us is that identifying the reference frame is crucial. It also tells us that measurements in different reference frames may come out differently; time looks different to observers that are moving relative to each other. You have not identified the frames properly. In A's frame A's elapsed time is shorter than B's. In B's frame B's elapsed time is shorter than A's. There is no paradox. The measurements are made in different frames and need not agree. If you use the equations that relativity provides to transform the results from one frame to another you get consistent, agreeing answers. But your second picture is drawn from the point of view of another observer, call her C, in a third frame. You didn't draw in observer C or identify her. Unintentionally introducing new and unidentified frames is a common cause of confusion in relativity. In C's reference frame, A is moving to the right and B is moving to the left, and each is moving with the same speed. From C's point of view, A's elapsed time is the same as B's elapsed time, and both are slower than C's elapsed time. In your second picture, t1 and t3 are both C's time, t2 is A's time, t4 is B's time, and t1 = t3 and t2 = t4 > t1. If you want to just stick to A and B, you have to use either the first picture you posted (which is how the situation appears in B's frame) or use this one, a re-labeled mirror, that shows how the situation appears in A's frame:
Here we see the full symmetry between your first picture and this one; t5 = t2 and t6 = t1. Always identify the frame or point of view when you make a statement. You can have two pieces of string that are identical when viewed in a reference frame in which they are not moving relative to each other, but piece A is shorter when viewed from a reference frame in which piece A is moving and piece B is not, and piece B is shorter when viewed from a frame in which piece B is moving and piece A is not. {edited to correct a "B" to "A"} This message has been edited by JonF, 12-07-2004 02:32 PM
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PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
The elapsed time for A is the elapsed time for A. The elapsed time for b is the elasped time for b. How that appears for another entity depends of their relationship. You are looking at it from three different POVs, A, B and some independant and unspecified observer that sees only t2 and t 4. Only 2 POVs actually. I didn't specify a third observer in this particular example. That is besides the point though. So, are saying is that because these two observers are in different frames of reference then the entire example is pointless?If that is the case then why do they continue to teach the first part of my example as a "proof" for way relativity works. The link that I provided also sites this as an example, just worded somewhat differently. It also would mean that there can never be an outside observer in any relativity example that would be meaningful in any way whatsoever. I'm sorry but this sounds like an even bigger paradox. How can you ever prove something like this seeing as how the person observing the event is always in a different frame of reference from the event he is observing so his observation is, of necessity, wrong and unreliable. "something happens -----> makes something else happen. Somebody else sees something different happen. But what the heck. We can never observe it accurately anyway so who cares?" I am starting to seriously wonder whether anything exists at all. PY
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PurpleYouko Member Posts: 714 From: Columbia Missouri Joined: |
And this tangent isn't a waste of your time - you should be learnimg from it. It just would have been better if you actually had found out about it for yourself. Just did didn't I? Also please try to remember that I am playing devil's advocate here (as stated in my first post in this thread) PY
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JonF Member (Idle past 168 days) Posts: 6174 Joined: |
Only 2 POVs actually. I didn't specify a third observer in this particular example Ah, er, yes, you did. You probably didn't realize it, and you certianaly didn't explicitly specify it; but your second picture implicitly requires a third observer as I discussed in the message I just posted. Without a third observer your second picture makes no sense.
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