Relativity thought experiment

Z

Zeno

Registered Senior Member
There are two ships of equal length each moving past each other at high velocity and each ship agrees to fire a powerful laser at the other ship and destroy it when its nose reaches the tail of the other ship. So, from the frame of reference of ship1, ship2 is contracted and the nose of ship1 reaches the tail of ship2 before the nose of ship2 reaches the tail of ship1. Ship2 is therefore destroyed.

<---------------Nose[-----------body of ship------------]Tail<------Ship1
--------------->Tail[body of ship]Nose----------------------------->Ship2

But, ship2 can say exactly the same thing as ship1 and therefore ship1 is destroyed first. So how can both situations be equally valid?
 
Because of simultaneity, time dilation only occurs for observed independent, not your own frame of reference. From either ship , the other ship is appearing to move away from you at the product of the velocities of each ship, while you appear to yourself to be moving zero velocity.
 
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Zeno said:
There are two ships of equal length each moving past each other at high velocity and each ship agrees to fire a powerful laser at the other ship and destroy it when its nose reaches the tail of the other ship. So, from the frame of reference of ship1, ship2 is contracted and the nose of ship1 reaches the tail of ship2 before the nose of ship2 reaches the tail of ship1. Ship2 is therefore destroyed.

<---------------Nose[-----------body of ship------------]Tail<------Ship1
--------------->Tail[body of ship]Nose----------------------------->Ship2

But, ship2 can say exactly the same thing as ship1 and therefore ship1 is destroyed first. So how can both situations be equally valid?
Click to expand...

For the sake of simplicity, assume that the laser is located at the nose of each ship, it fires at the tail of the other, and there is zero distance between the respective noses and tails when the lasers fires.

That being said, ship 1 fires at ship 2's tail and the laser instantly hits the tail. But the effect of the laser cannot propagate to the nose of the ship faster than the speed of light. Thus the time it takes for this effect to reach the nose of ship 2 (according to ship 1) is the length of ship 2 divided by the difference between c and the relative velocities of the ship. If you work it out, you find that this effect will not reach the nose of ship 2 until the nose of ship 2 reaches the tail of ship 1, and ship 2 fires its laser. Both ships are destroyed according to observers traveling alongside each ship.
 
Hi Zeno,
From where is each laser fired, and how long do they take to reach their target?

There's no immediate problem with a frame dependent difference between which event happened first... as long as those events are causally independent. The order of events is only necessarily fixed if one of them can affect the other.

For example, the events:
  • Ship1's laser is destroyed, and
  • Ship1's laser is fired
Are obviously causally dependent. They have to happen in the same order in all reference frames, right?

Now, for your problem, "Ship1 is destroyed" is not a single event, because Ship1 is not a small object. "Ship1's tail is destroyed" is a different event to "Ship1's nose is destroyed" for example.

So, you should decide where the lasers are, and see what relativity predicts about when the lasers are fired and destroyed in each reference frame.
 
Pete said:
Hi Zeno,
From where is each laser fired, and how long do they take to reach their target?

There's no immediate problem with a frame dependent difference between which event happened first... as long as those events are causally independent. The order of events is only necessarily fixed if one of them can affect the other.

For example, the events:
  • Ship1's laser is destroyed, and
  • Ship1's laser is fired
Are obviously causally dependent. They have to happen in the same order in all reference frames, right?

Now, for your problem, "Ship1 is destroyed" is not a single event, because Ship1 is not a small object. "Ship1's tail is destroyed" is a different event to "Ship1's nose is destroyed" for example.

So, you should decide where the lasers are, and see what relativity predicts about when the lasers are fired and destroyed in each reference frame.
Click to expand...

Pete; you are aware that neither Lorentz nor Einstein specified causality, but, rather, later Relativists added the Weak Causal Principle and the Causal Principle.

Right?
 
I think this is another example of A happens before B and B happens before A. There was another one a while back called "The Cylnder and the Nail". IMHO it indicates that time is a subjective experience rather than objective reality.
 
CANGAS said:
Pete; you are aware that neither Lorentz nor Einstein specified causality, but, rather, later Relativists added the Weak Causal Principle and the Causal Principle.

Right?
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Relevance?
 
The subject is the order in which two events occurred.
Causality is relevant, but the relevance of whether or not Einstein or Lorentz specified causality is not clear.

Please be less obscure.

It would also help to explain what you mean by the causal principle... I don't think you're talking about Aristotle.
 
Pete said:
The subject is the order in which two events occurred.
Causality is relevant, but the relevance of whether or not Einstein or Lorentz specified causality is not clear.

Please be less obscure.
Click to expand...

Obscurity is in the (mis)perception and (mis)understanding of the hearer, not in the message.

In Newton relativity causality is a slam dunk. In Einstein (or Lorentz) Relativity causality is up for grabs. How quickly you forget Einstein's Relativity of Simultaneinity. Causality principles were added ex post facto by later Relativitists trying to save Einstein Relativity from fatal criticism based on causality paradoxes.

In any discussion based on Einstein Relativity, in this forum or anywhere else, it is not proper to assume Newton Relativity causality. It is proper only to very carefully tiptoe while scrupulously examining the logic to see that it makes sense in light of the Relativity of Simultaneity.

My first post was just a friendly and helpful reminder to not take causality for granted in an obviously Einstein Relativistic scenario.

If you understand and like my advice then take it with my welcome.

If you don't understand or don't like my advice then put it out of the sunshine and ignore it.
 
Thanks for the clarification.

If we do take causality for granted (as Einstein was inclined to do), then SR implies that faster than light communication is impossible. Conversely, if SR holds, and FTL communication is impossible, then causality holds.

What's the problem? Do you think there is some FTL communication going on in Zeno's scenario?
 
Pete said:
Thanks for the clarification.

If we do take causality for granted (as Einstein was inclined to do), then SR implies that faster than light communication is impossible. Conversely, if SR holds, and FTL communication is impossible, then causality holds.

What's the problem? Do you think there is some FTL communication going on in Zeno's scenario?
Click to expand...

I was not, for purely personal reasons, not interested especially in the thread start post, although I do not withold praise for it being a good idea for a good thread. It is just not my personal cup of tea. So I am not going to involve myself much in it specifically. Which means that I don't want to do much that would possibly be a diversion.

It might not be a perfectly kept secret that I am generously skeptical about the overall validity of Lorentz/Einstein Relativity, so the possibility of FTL communication, whether of raw velocity of objects or or signals of information, does not send me into a paroxysm of hysteria. And, I have the personal opinion that much and perhaps all of the relativistic transformations of time, space and mass may be explained in less arcane models than Relativity's Gamma.

I have the very strong personal opinion that causality is absolute, and that therefore any overall theory must blatantly uphold causality in every case without any propping up or any ifs, ands, or buts. I believe that the need for latter-day Relativists to prop up Einstein Relativity with the Weak Causal Principle and the Causal Principle is a strong warning sign.

It is my personal opinion that a perfectly careful examination of the derivation of the Einstein Relativity transformation of length as a result of relative velocity to the observer will provide a thoughtful person with a strong reason to doubt that the length contraction mentioned in this thread start could be a real thing in the real world.

However, after my polite responding to your last post with this post of mine, in the interest of not wanting to divert a good thread, I will not furthermore respond in this thread, but will just wish it the best of success from afar.
 
We need more parameters ...
Is the course parallel ..... is the point of reference looking at the other ship , the same as the point of the laser .....where is that point located .... what angle are you looking at .....what angle is the laser pointing ....what is the speed of the ships ......

If the course is parallel , the points of reference, looking at the other ship is in the nose, directed at an angle of 90 degrees from the course of your own ship - then the ships will see, the tail to nose event, at exactly the same time ......
 
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CANGAS said:
I have the very strong personal opinion that causality is absolute, and that therefore any overall theory must blatantly uphold causality in every case without any propping up or any ifs, ands, or buts.
Click to expand...

So there's no problem then.
You're skeptical of relativity because you have a feeling that there's a chance that it might violate causality (which would upset your strong personal opinion).

I'm still interested in the Causal Principle you say was developed by relativists. The only Causal Principle I'm aware of goes back to Aristotle.
 
Sputnik said:
If the course is parallel , the points of reference, looking at the other ship is in the nose, directed at an angle of 90 degrees from the course of your own ship - then the ships will see, the tail to nose event, at exactly the same time ......
Click to expand...

Hi Sputnik,
There are two tail to nose events. One is the tail of ship1 to the nose of ship2, the other is the tail of ship2 to the nose of ship1.

Strangely enough, these events will not occur at the same time according to intelligent observers at rest with respect to either ship.

Even more strangely, the order of the two events will be reversed for the two sets of observers. This is the issue that Zeno addressed.

My response is that it doesn't matter. Neither of the events can change the other event, unless they can communicate at faster than light speed.
 
Hi Zeno. I think the resolution is to ask what an outside observer will see. I suspect the answer is that, supposing both ships decide to fire their lasers, both ships will be destroyed. From Ship 1's frame, it sees Ship 2's length contracted, but an observer on Ship 2 will see Ship 1 contracted by the same amount. Anyone second this?

Will have to think about this some more. I think that there are paradoxes similar to this proposed by Einstein and other early relatavists.
 
Hi Ben,
The location of observers doesn't matter... only their velocity.

All intelligent observers that have the same velocity as Ship1 (no matter where they are) will determine that Ship2 is shorter than Ship1.

All intelligent observers that have the same velocity as Ship2 (no matter where they are) will determine that Ship1 is shorter than Ship2.


A side note about observers:
"Intelligent observer" is a loaded term in these kinds of problems... it's assumed that they never use measuring tools (clocks and rulers) that are moving with respect to themselves, and that they always account for signal delays when taking measurements.

In practice, an observer can make measurements in a different frames of reference by using measuring tools with different velocities.
 
If that wasn't clear, it is what I meant. I was assuming that there was an observer on Ship 1, an observer on ship 2, and a stationary outside observer.
 
Pete said:
So there's no problem then.
You're skeptical of relativity because you have a feeling that there's a chance that it might violate causality (which would upset your strong personal opinion).

I'm still interested in the Causal Principle you say was developed by relativists. The only Causal Principle I'm aware of goes back to Aristotle.
Click to expand...

Pete, the past century of Relativity literature is abundant with the additions of Weak Causal Principle and Causal Principle. It has not happened in a closet and it is as openly available in texts as 1+1=2 is available in arithmetic texts.

I am concerned that you are in a position of some substantial power in the fate of this Forum but you do not know about such a basic fact as this.

So, tell us once again that you have never heard of the Weak Causal Principle and the Causal Principle.

Then say bye, bye.
 
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