
#1
Apr2712, 04:54 PM

P: 4

Chapter 9 on the theory of relativity discusses a thought experiment (http://www.bartleby.com/173/9.html) about a passenger on a train that claims that
it is impossible to say in an absolute sense whether two events occur at the same time if those events are separated in space. I have devised a modification to this thought experiment to disprove this. Assume you have two long ropes laying sidebyside with the tracks, both going from point A to point B. You have a watchman standing at point A, and a watchman standing at point B. They will pull their rope when they see lightning strike their point. The passenger can see the grooves of the rope shift when it is pulled. If the passenger sees both ropes get pulled simultaneously at the midpoint, he will see them get pulled simultaneously anywhere. ... This is because all points of the rope get pulled instantaneously. There is no waiting time like with the traveling of light. For example, consider a rope that is one lightyear long. When the rope gets pulled from one endpoint, a person at the other endpoint a lightyear away will see the rope's end move with no delay, due to the laws of matter. Even though no actual material is traveling faster than the speed of light, the knowledge that the rope was pulled is traveling faster than the speed of light. So if you consider when the rope gets pulled to correspond to when the event of lightning occurs, there seems to be an absolute sense of simultaneity no matter where the train is or how it is moving. 



#2
Apr2712, 04:56 PM

Mentor
P: 16,477

This is a pretty common suggestion, so we have a FAQ that explains why it doesn't work:
http://www.physicsforums.com/showthread.php?t=536289 



#3
Apr2712, 05:04 PM

P: 2,045

Basic flaw: all segments of the rope don't react simultaneously.




#4
Apr2712, 05:13 PM

P: 4

found a way around relativity of simultaneity
Well what about this:
Since relativity of simultaneity ultimately leads to proof that no object can travel faster than light, you can't assume beforehand that no object can travel faster than light. So shoot something that travels instantaneously to signal that lightning struck the point 



#5
Apr2712, 05:15 PM

Mentor
P: 5,341





#6
Apr2712, 05:25 PM

Mentor
P: 16,477

It is a little rude to not even bother to read a link that answers your question. 



#7
Apr2812, 03:19 AM

P: 215

I know that there is nothing like faster than speed of light. The rope pulling wave has very less speed than light. But, vdub has a fair point.
Suppose, that pulling wave of rope travels at speed of sound. The two lightning events is simultaneous for platform observer and not for train observer. And platform observer also confirms that the two events are not simultaneous for train observer, whereas train observer confirms that the two events are simultaneous for platform observer. Both observer are agree that the two events are simultaneous for platform observer and not for train observer. Now, if two events occurs simultaneously for platform observer, and he confirms that the two events are not simultaneous for train observer. So, he also confirms that rope pulled by watchmen are simultaneous and the effect of the wave reaches to train observer would also be simultaneous. Because, speed of pulling wave is not related with direction of motion of train. So, what platform observer confirms that is true for train observer, so train observer also see simultaneous pulling wave and unsimultaneous lighting events. This seems paradox. We have to solve this. 



#8
Apr2812, 06:59 AM

Mentor
P: 16,477

Please demonstrate the paradox mathematically.
When you do so you will find that this statement is false: 



#9
Apr2812, 12:18 PM

Physics
Sci Advisor
PF Gold
P: 5,507





#10
Apr2812, 12:57 PM

Sci Advisor
Thanks
P: 2,951

In the context of OP's SoWrong(tm) suggestion that "relativity of simultaneity ultimately leads to proof that no object can travel faster than light" this is about a thirdorder nitpick  I'm offering it up because it might matter to someone else wandering through this thread and trying to understand what is premise and what is conclusion in SR. 



#11
Apr2812, 01:36 PM

PF Gold
P: 5,685

vdub, when you come up against something that flies so utterly in the face of established science, it is not a good idea to start off reaching different conclusions and stating them as correct but rather to start off with the assumption that you have made a mistake somewhere and try to find out where it is. If you have NOT made a mistake you will find the flaw in the established science, but that is very unlikely to happen. If you start off thinking that you have overturned established science you are likely to just end up embarrassed.




#12
Apr2812, 02:17 PM

P: 27





#13
Apr3012, 06:42 AM

P: 215

Suppose, train running from left to right. Platform observer sees that two events occurs simultaneously at both end of train. Platform observer confirms that information of left event reaches to train observer lately than right event, because speed of light from both direction is same for platform observer. Platform observer confirms that events is simultaneous in platform frame, and if he transforms the timing of events into train frame, he also confirms that the events is not simultaneous in train frame. Train observer sees both events one by one. Speed of light is same from both direction for train observer. So, train observer confirms that the events is not simultaneous in train frame. But, train observer can calculate that timing of the events in train frame. And if train observer transforms the timings into platform frame, train observer also confirms that the events is simultaneous in platform frame. Now, platform observer sees that both rope is pulled by watchmen simultaneously in platform frame. The pulling wave speed is faster from left end to middle than right end to middle for platform observer. Platform observer sees that both pulling wave reaches to train observer simultaneously. The train observer also sees that pulling wave reaches to him simultaneously. But, beam of lightning reached to him unsimultaneously. Suppose, length of train is 2 ls. Ans speed of train is 0.6c. If the two events occurred in platform frame at [itex]t_{p}=0[/itex], [itex]x_{pl}=1[/itex] and [itex]x_{pr}=1[/itex]. We get [itex]t_{tl}=0.75[/itex] and [itex]t_{tr}=0.75[/itex] in train frame after lorentz transformation. Now, suppose that platform observer sees that watchmen pulling rope at [itex]t_{pulling\text{ }rope}=0[/itex], [itex]x_{left\text{ }pulling\text{ }rope}=1[/itex] and [itex]x_{right\text{ }pulling\text{ }rope}=1[/itex] in platform frame. The rope will reach at middle at some [itex]t_{reaching\text{ }rope}=t[/itex] and [itex]x_{pulling\text{ }rope}=x[/itex] in platform frame. If we transform the [itex]t_{reaching\text{ }rope}=t[/itex] and [itex]x_{pulling\text{ }rope}=x[/itex], we will get some [itex]t'_{reaching\text{ }rope}=t'[/itex] and [itex]x'_{pulling\text{ }rope}=x'[/itex] in train frame. Train observer sees pulling wave reaching to him simultaneously. The main cause of the paradox is pulling wave and light is not behaving same. 



#14
Apr3012, 07:03 AM

Mentor
P: 16,477





#15
Apr3012, 07:16 AM

P: 215





#16
Apr3012, 08:44 AM

Mentor
P: 16,477

This statement ...




#17
Apr3012, 08:56 AM

P: 215

I am taking about lightning events which is unsimultaneous in train frame in first statement.
Now, does this seem paradox? 



#18
Apr3012, 10:47 AM

Mentor
P: 16,477




Register to reply 
Related Discussions  
Relativity of Simultaneity  Special & General Relativity  317  
How Is The Relativity Of Simultaneity Consistent With Physics And Relativity  Special & General Relativity  47  
Relativity of Simultaneity  Special & General Relativity  2  
Simultaneity in Relativity  Special & General Relativity  12  
Relativity of Simultaneity  Special & General Relativity  25 