# Found a way around relativity of simultaneity

by vdub
Tags: relativity, simultaneity
 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 side-by-side 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.
 Mentor P: 16,945 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
 Sci Advisor P: 2,659 Basic flaw: all segments of the rope don't react simultaneously.
 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
Mentor
P: 5,403
Sorry I have no idea what you are trying to say here.
 Quote by vdub 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.
Huh?
 Quote by vdub So shoot something that travels instantaneously to signal that lightning struck the point
There is nothing that travels instantaneously.
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P: 16,945
 Quote by vdub 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
Like what? Did you even bother to read the FAQ? It explains why there is no such thing.

 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.
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P: 16,945

When you do so you will find that this statement is false:
 Quote by mananvpanchal Because, speed of pulling wave is not related with direction of motion of train.
Physics
PF Gold
P: 6,029
 Quote by vdub 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.
Relativity of simultaneity is a *consequence* of the fact that the speed of light is finite and nothing can go faster; it is not a premise used to prove it.
Thanks
P: 3,420
 Quote by PeterDonis Relativity of simultaneity is a *consequence* of the fact that the speed of light is finite and nothing can go faster; it is not a premise used to prove it.
I'd rather say that relativity of simultaneity and the the presumption that nothing can go faster than light are *both* consequences of the more fundamental premise that the speed of light in vacuum is the same in all reference frames.

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 third-order 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.
 PF Gold P: 6,080 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.
P: 27
 Quote by phinds 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.
This is probably the best advice you can possibly find on PF if one desires to become a theoretical physicist.
P: 215
Quote by DaleSpam

When you do so you will find that this statement is false:
 Quote by mananvpanchal Because, speed of pulling wave is not related with direction of motion of train.
This is strange statement by me. I was actually trying to say that if the two rope pulled simultaneously in platform frame, then the pulling waves reach to train observer simultaneously in platform frame. Whereas in case of lightning, two stroke occurs simultaneously in platform frame, the beam of the lightning reaches to platform observer simultaneously in platform frame, but the beam of the lightning will not reach to train observer simultaneously in platform frame. So platform observer confirms that the two events wouldn't occur simultaneously in train frame.

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 $t_{p}=0$, $x_{pl}=-1$ and $x_{pr}=1$. We get $t_{tl}=0.75$ and $t_{tr}=-0.75$ in train frame after lorentz transformation.

Now, suppose that platform observer sees that watchmen pulling rope at $t_{pulling\text{ }rope}=0$, $x_{left\text{ }pulling\text{ }rope}=-1$ and $x_{right\text{ }pulling\text{ }rope}=1$ in platform frame. The rope will reach at middle at some $t_{reaching\text{ }rope}=t$ and $x_{pulling\text{ }rope}=x$ in platform frame. If we transform the $t_{reaching\text{ }rope}=t$ and $x_{pulling\text{ }rope}=x$, we will get some $t'_{reaching\text{ }rope}=t'$ and $x'_{pulling\text{ }rope}=x'$ 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.
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P: 16,945
 Quote by mananvpanchal 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.
Are the watchmen and ropes at rest wrt the train or wrt the platform?
P: 215
 Quote by DaleSpam Are the watchmen and ropes at rest wrt the train or wrt the platform?
Both watchmen are situated on both end of train. They are at rest in train frame and moving in platform frame. The both rope are also at rest in train frame and moving in platform frame.
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P: 16,945
This statement ...
 Quote by mananvpanchal the events is not simultaneous in train frame.
and this statement ...
 Quote by mananvpanchal Both watchmen are situated on both end of train. They are at rest in train frame and moving in platform frame. The both rope are also at rest in train frame and moving in platform frame.
 Quote by mananvpanchal The train observer also sees that pulling wave reaches to him simultaneously.
Since the distance is the same and since the mechanical wave speed is the same then if they are pulled non-simultaneously then they will necessarily reach him non-simultaneously also.
P: 215
I am taking about lightning events which is unsimultaneous in train frame in first statement.
 Quote by DaleSpam Since the distance is the same and since the mechanical wave speed is the same then if they are pulled non-simultaneously then they will necessarily reach him non-simultaneously also.
But, rope pulling events is simultaneous in platform frame, so it would reach to train observer simultaneously.