# Standard SRT problem need a standard SRT solution.

Discussion in 'Physics & Math' started by Quantum Quack, Mar 26, 2005.

1. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
SL, I am sure all these questions have been asked before and resolved but if we take a look at this next diagram.

Please Register or Log in to view the hidden image!

It shows a total of 5 objects.

According to C object A and C1 are co-moving and objects B and C2 are also co-moving.
Object C can ascertain that C1 and C2 are pulsing in phase, in other words they share the same tick rate. He also knows that A and B are also in phase.
Object C can clearly see that the same would apply for objects A and B.

Now this is where it gets sticky.

Object A and object B are unaware of C, C1 and C2.

Object A according to SRT will state that object B has all the velocity thus his light pulses on his reflector [remember the two ship scenario posted earlier] will be out of phase.

Now clearly there is a conflict between what C knows as true and A and B know as true.

The question is : Who is correct? Is C correct? Or is either of A or B correct?

to hide all adverts.
3. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
another are I have jsut thougth of that may lead SRt astray is that even if a GPS has supposedly got >V than a surface clock it's orbit may actually be slower than that of the surface due to the movement of teh solar system within the galactic ballet.

In other words the illusion of greater velocity could give totally wrong dilation results.

How fast is the Earth travelling contra to the GPS orbit?

to hide all adverts.
5. ### superluminalI am MalcomRValued Senior Member

Messages:
10,876
Yes there is and this is not a problem.

There is no absolute truth. Reality is subjective and always has been. It just gets a bit more confusing when the oddities of light are thrown in. Each is correct IN THEIR OWN FRAME. If a kid hits a baseball and I'm 1000ft away and you're standing 100ft away, and we both wear blindfolds, we'll both record different times based on the sound.

Last edited: Apr 10, 2005

to hide all adverts.
7. ### superluminalI am MalcomRValued Senior Member

Messages:
10,876
No. All that matters is the relative velocity between observers. The solar system could be spinning wildly through the galaxy (hmmm... I guess it is...) and it makes no difference to the measurements between ourselves and a sat in a stable orbit.

All of your statements so far have an underlying assumption of some reference other than the frames you are talking about. This gets you nowhere.

8. ### superluminalI am MalcomRValued Senior Member

Messages:
10,876
At some point you just have to accept that there is no absolute reference and no absolute "reality" or "truth" as proven by experiments which match theory exquisitely. As the observer, truth is what you say it is. And if you switch your POV, truth switches with you.

When you accept this, these problems will become trivial to handle.

9. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
Good, and I'm waiting with anticipation!

Of course! But what the observer sees is subject to the "IF" factor. IF the observer had different motion, they would observe things differently, yes?

It just occurred to me that perhaps you haven't thought about relativity from the perspective of being a change to the observer rather than the other way around (fundamentally, it's not either... but it helps to think about it in that way at some stage).

10. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
Movement is relative, just like "here" is relative.
Imagine there is a tree on A.

Where is the tree? Here or There?

A says "here", and is correct.
B says "there", and is correct.

So which is the real truth? Is the tree Here or There???

The problem with your animated image is that you have explicitly specified that A and B are both moving against some fixed background. This is a natural intuitive position - you are imagining some type of "Ether"... some fixed background that forms an absolute rest frame.

The difficulty is that the fixed background is undectable if it exists at all! So SRT simply suggests that it doesn't exist, and finds a consistent model without it.

Another way of approaching your animated image is to imagine that the backdrop is infinite... if it's infinite, can you tell if its moving? The moving "window" you've placed on the backdrop is arbitrary... you could just as easily have reversed the window's movement, right?

11. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
[ Edit - Please ignore the following two lines. Thanks to superluminal for pointing out my silly error. ]
Please!! That's not what comoving means!
Comoving means stationary with respect to each other.

Not so! I explained this already. If A and B are approaching each other at 0.8c (in A's frame or B's frame), then A will receive B's pulses 3 times faster than A sends pulses. This is true in ALL reference frames. A, B, and C all agree on this!

The interpretation they each make of this (the original pulse rates) will be different, but that's OK - the original rates are relative measures, just like "here" and "there".

Last edited: Apr 11, 2005
12. ### superluminalI am MalcomRValued Senior Member

Messages:
10,876
Pete, I think in his animation the objects he is referring to are comoving.

13. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
Thanks superluminal,
You're right, and I am embarrassed.

Sorry, QQ, I misread. Forgive me?

14. ### James RJust this guy, you know?Staff Member

Messages:
31,445
Pete's right. All are correct. Each is correct in his own reference frame. Nobody has a "privileged" view of what's going on.

15. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
Actually Pete this is a good example of how easy it is to be mistaken on the internet.
Originally I made a mistake with the statement that they were at rest. SL corrected me and said they were co-moving [two ships at equal v coming towards each other] And then I used this error later which you corrected me on, That they were not co-moving and so I corrected for the last diagram but you had failed to see my correction.....sheesh!!! What fun we have.....ha

Please Register or Log in to view the hidden image!

Last edited: Apr 11, 2005
16. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
JamesR, Thanks for your response and I agree that this would be the SRT position.
However and there is always a however, I would question the merit of a theory that allows C to observe A as being in synch with B yet A observes that he is not in synch with B.

Just to make my hesitation very clear:

C sees A is in Synch with B
A sees he is out of synch with B

So maybe instead of claiming all views are correct we should be claiming all views are Incorrect.

Please Register or Log in to view the hidden image!

edit: keeping in mind that what C sees is what A sends to him to see. [light pulses] C gets a light signal that tells him that A and B are in Synch according to A. Yet simultaneously A is also claiming that he is out of synch with B.

Intuitively one would have to consider that there is a problem....With the fact that A is stating he has two observations. To me it demonstrates a hard syntax error in how we are applying SRT to this scenario.

Last edited: Apr 11, 2005
17. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
I'd like to qualify that:

C determines that A is in Synch with B
A determines that he is out of synch with B
[/quote]

I think it is vital to distinguish these determinations from actual observations.

According to C's observation:
A's reflector accumulates three times as many pulses from B as from A.

According to A's observation:
A's reflector accumulates three times as many pulses from B as from A.

Do you see?

18. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
Pete, two things I wanted to raise with you form an earlier post:

Ther distinction between a prediction and an observation needs to be brought to the fore here.

IN the scenario I showed two ships coming together at v=0.8c. There are no other references to draw upon. You have stated that whilst hey ca not assume their v is relative zero, they can use the SRT model to predict what the othership will see and to do so it has to be assumed that one has all the velocity. This is s bit of a logic loop or circular reasoning....IMO

Can't asssume he is at rest but must do so to predict what the other is seeing......circular for sure.....

The issue is not about prediction but more about inherant symmetry when two ships are approaching each other and they are considered the only two objects in their universe. Whilst each frame is at rest to itself this does not mean that the ships rest frame is at rest relative to the other ship.

The other thing:

Whilst my debate is essentially about just this, I am not in any way attempting to disprove SRT. I accept that SRT will stand or fail on it's own merit.

What I am interested in is how SRT deals with what is apparently an inconsistancy in that in the case of our scenario A's observation is one for himself and one for C. when both observations are generated by A.

19. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
Actually, neither observer can draw on the other ship's velocity as a direct observation. They can only predict what that velocity is based on their observations.

They can use any model and data they want to predict what the othership observes, but only a good model together with good data will give predictions that correspond to the actual observations of the othership.

If an observer assumes that they have a velocity of zero relative to the othership, then they're working with bad data, so their predictions will not give good results.

He can assume he is at rest. In fact he must, with no other reference to go by. He can't assume he is at rest relative to the othership.

20. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
If you find a true inconsistency, then this disproves SR. Part of 'standing on it's own merit' is producing consistent results.

Do you accept that the prediction of SR is that A observes B's pulses arrive three times faster than A's pulses leave, and that this prediction is consistent in all frames?

Last edited: Apr 11, 2005
21. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
Sorry Pete, I am a little confused [must be obvious hey?]

When you say that both ships will see their light pulses on their reflectors as a ratio of 1:3 does this include dilations and the superluminal effect combined?

Please Register or Log in to view the hidden image!

22. ### Quantum QuackLife's a tease...Valued Senior Member

Messages:
18,311
if so then what information do you predict C will recieve from A? 1:3 as well?

Please Register or Log in to view the hidden image!

23. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
You might wonder what I'd personally consider to be a "true inconsistency".

Well, I have something I borrowed from Schrodinger called "the classical cat test".

If it is possible to describe valid initial conditions including a cat, such that two valid applications of some model are inconsistent in their prediction of the cat's fate, then a true inconsistency has been discovered and the model in question can not correspond to reality.

For example, place a cat in ship A together with some lethal apparatus triggered by some sort of detector. If some observer validly uses SRT to predict that the apparatus will fire and the cat dies, while some other observer validly uses SRT to predict that the apparatus will not fire, then SRT is shown to be truly inconsistent.

Note that this test is a classical test - I don't attempt to apply it to quantum models.