Discussion in 'Physics & Math' started by renislaj, Apr 18, 2013.

1. ### renislajRegistered Member

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There is a human body suspended horizontally by two wires in a train car moving on the tracks. The body is positioned symmetrically wrt to the center of the car. The wires are simultaneously cut (in the frame of the car). Obviously, the body falls parallel to the car floor hitting the floor simultaneously with all its points (in the car frame).
But in the track frame the two wires are not cut simultaneously and the body does not fall parallel to the car floor : one end of the body will hit the floor before the other end. This will result into one end (say, the head) absorbing the full impact. The same experiment has differing results in the two frames. Hence there is a paradox in special relativity.

3. ### Markus HankeRegistered Senior Member

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There are no paradoxes in special relativity. It is a trivial matter to proof that this is true :

http://www.thescienceforum.com/physics/29958-general-proof-special-relativity-self-consistent.html

You have taken this scenario word for word from a thread on The Science Forum - not only is it highly questionable that you are copying content such as this without providing a reference to the source, but you also neglected to present the various solutions to this apparent "paradox" which were explained on that thread.

The basic reasoning is as follows ( this is my version ) :

1. In the train frame the body falls straight down and hits the floor flat
2. In the embankment frame, one of the wires is cut earlier, so a torque is introduced around the point still attached
3. When the body starts falling a phenomenon called Thomas precession comes into play, which introduces a rotation of the same magnitude as above rotation, but with opposite sign. This is a special relativistic phenomenon which also affects gyroscopes and particles with intrinsic spin.
4. Thus, despite the wires not being cut at the same time in the embankment frame, the body nevertheless falls down straight in both frames.

There are other possible solutions to this scenario as well; the main point is that Special Relativity does not produce paradoxes under any circumstances so long as only approximately inertial frames are involved.

5. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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What is a "train frame" and "embankment frame" and what is the difference between the train frame and embankment frame? How do you determine which frame you are in?

At 12:00:00 on my watch there is an embankment and a train in reality. How do I determine if they are in the same frame or different frames?

7. ### Markus HankeRegistered Senior Member

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Isn't that obvious ? The train frame has a dead body hanging off the ceiling, the embankment frame doesn't.
In other words, the dead body is at rest relative to the train frame, but not relative to the embankment frame.

8. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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No, because you haven't explained it yet.

I asked you what a train frame and embankment frame were, but you failed to respond to that, and then used the very terms again in your reply. I ask again, what is a train frame and embankment frame and how do you know which one you are in?

9. ### eramSciengineerValued Senior Member

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I wonder what a human body was doing suspended on a train?

So the body falls straight in both frames, that sounds reasonable.

10. ### Markus HankeRegistered Senior Member

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The answer was already provided; unfortunately you seem unable to comprehend even a simple scenario such as this, so it looks like I have to dumb it down a little more for you :

1. The train frame is the frame which is at rest relative to the train
2. The embankment frame is the frame which is at rest relative to the embankment
3. You are in the train frame if the dead body is at rest relative to you
4. Your are in the embankment frame if the dead body is in uniform motion relative to you

No, because you are ignorant of basic physics. The train thought experiment is standard, elementary relativity - anyone who has studied the field knows about it, and whoever does not know about it needs to go and study it first before entering into discussions about it.

11. ### Markus HankeRegistered Senior Member

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It's a circus train, and he/she strangled himself/herself doing acrobatics practice

12. ### wellwisherBannedBanned

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Special relativity has three equations, one each for distance, time and mass. Most SR examples tend to look at only distance and time, which can lead to paradox problems, since these 2 out of 3 variables are relative. But mass is not relative, due to energy conservation. If we include mass we resolve the problem.

As an example, a person on a moving train may feel he is stationary and that the person standing at the station is moving. While the person at the station may assume he is stationary and the person on the train is moving. For many calculations these references are relative, using only distance and time.

If we include the mass variable, we now need to do a mass/energy balance. Mass does not change with reference, like distance and time. Using mass/energy we can prove that only one of these references is real and other is an illusion.

The person on the train, will see the entire earth appear to move if they look out the window. The person at the station will only see the train move. What each person sees, adds to different amounts of energy. We can infer, based on mass/energy, the train is moving even if the person on the train is tripping out and thinks he is stationary because of 2 out 3 tradition.

Most relativity examples, violate energy conservation leading many people sensing something wrong that they can't put their finger on, since they attempt use the 2 out of 3 convention, also. The paradox is not that relativity is wrong, rather it is the use of space-time without mass that is violating how relativity was designed by Einstein.

Do an energy balance from each reference and and compare to the 2 out of 3 assumption and notice the latter often creates and/or destroys energy, thereby allowing magic tricks that make relativity appear to be wrong.

13. ### eramSciengineerValued Senior Member

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I had something more macabre in mind.

14. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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I'm wondering who is actually doing the "dumbing down" here?

1. What is a frame and and how do you determine it is "at rest?"
2. There is no motion that I can detect at 12:00:00. It seems to me that at 12:00:00 there are objects in reality that have specific locations and specific distance from each other. The big problem is, in order for me to be able to detect a motion, some time has to elapse, so it is no longer 12:00:00.
3. At 12:00:00 according to my watch, all objects are in the same frame because there is no motion at 12:00:00.

God is a standard term in religion, too. God is talked about, worshiped, praised, written about, and yet there are actually people that think there is no God? Can you believe it? All that is talked about God, and there are doubters???

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16. ### Markus HankeRegistered Senior Member

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A frame is a local coordinate system.
At rest relative to what ? You have to be more specific here.

And why, exactly, is it a "big problem" that time should elapse for you to be able to detect motion ? Can you give us an example of a situation where time has stopped elapsing ?

You could have simply said that you don't believe in relativity. It's shorter and more concise.

17. ### Markus HankeRegistered Senior Member

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Not so easy if Motor Daddy is involved. Think back to the original post - has he made any attempt at addressing the issue at stake ?

18. ### Markus HankeRegistered Senior Member

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Not my kind of movie

But then again, she didn't end up suspended from the ceiling, as in the OP...

19. ### ash64449Registered Senior Member

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i agree with you.

20. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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Oh good! So at 12:00:00 all objects use the same coordinate system, so at 12:00:00 the train and the embankment are in the same coordinate system!

You claimed "at rest" now define it, "at rest" compared to what?

Because motion does not occur instantly, so at 12:00:00 according to my watch, all objects are in the same frame (same coordinate system.)

You could have simply replied, "thanks for enlightening me," but you didn't. You trying to tell me what to say, now?

21. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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There is nothing to "determine" about the reference frame's state of motion: by definition, it is at rest relative to itself. Motor Daddy may be trying to squeeze into the discussion his belief in absolute motion, not realizing it isn't relevant here. Instead it just makes it look like he has trouble understanding basic physics.

And by the way, this problem has nothing specifically to do with SR. Signal delay paradoxes are a Galilean Relativity issue.

22. ### originTrump is the best argument against a democracy.Valued Senior Member

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Interesting - good post.

Motordaddy is one of our local relativity deniers. He pops up every now and then with arguments that are shown to be wrong but he never learns. Talking to him is like trying to reason with an artichoke.

23. ### Markus HankeRegistered Senior Member

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Motion, as you say yourself, is not instantaneous, it is a change in position over time. In this case we are comparing two frames of reference.
You don't detect motion at 12:00, that is intrinsically impossible. All you do is take note of the spatial positions of objects in the other frame at that time, and then you let time elapse. At, for example, 12:01 you note the spatial positions of the other frame again, and compare them to the ones noted before. If there is any difference in readings, then there is motion relative to you. If not, then the other frame is at rest relative to you.

Of course all you are doing here is trying to deliberately create confusion, in order to detract from the fact that you don't know the answer to the puzzle in the OP. Or would you like to present such an answer here for us ?

And why did you not answer my question about giving an example for a physical scenario where time has stopped elapsing ?

Why would you need a watch if it always shows 12:00 anyway ?