Which Train Explodes First in the Time Dilation Paradox?

[PAllen]

The observer's perception corresponds to his position and his local clock. He is his own frame of reference. He can't make an observation from another frame of reference moving relative to himself or use the time of that frames clock.

So, if you are riding a bicycle, you are not entitled to think you are moving; you must insist that the ground is moving? [edit: and if, while riding a bike, I look at a clock tower, I must ignore its time because it is not in my frame.]

More germane to physics, for all these years of colliding particles or ions into stationary targets, physicist were acting incorrectly when they chose to record observations and do calculations in the COM frame rather than the lab frame?

 

[harrylin]

Well, I meant to point out that light seen by one observer will not be seen by another. [..] I guess i should've just said observer instead of frame and save us all from this waste of time.

[..] If a blink of light propagates spherically, all observers will eventually see it, but they are not seeing the same patch of light. If i send a laser beam into your eye, you are the only one who will see it. And this has nothing to do with relativity or the topic anymore.

[..] Your reminder that two can't occupy the same place at the same time is all we need to forbid any notion of light seen in another observer's frame. In real life, that light can never be directly a part of our data, we can only know of its existence through secondary effects, like "yeah, i saw that too!"

Evidently you recognized that:
- saying "frame" for "a light detector" (or a person who observes) is a mix-up;
- and "the same patch of light" has nothing to do with relativity or this topic;

Thus it's a mystery to me why you continue introducing these things...

 

[bgq]

hmmm, only #5 and #21 seem to have some concrete stuff; too bad they had been buried under all the "after talk."


I like this paradox, let's call it the "Action Movie Train Scene" paradox.

I am very happy to know that you like this paradox; however, the "after talk" here is very valuable, I have learned from them much although I am not qualified enough to participate in these discussions.

 

[Dale]

The fact that 2 observers can't occupy the same spatial position at the same time, requires that their descriptions of an event differ.

That was my point a few posts back, but you articulated it way better. Your reminder that two can't occupy the same place at the same time is all we need to forbid any notion of light seen in another observer's frame.

Welcome to the discussion phyti, you are making the same mistake that altergnostic is making. You can pat each other on the back all you want for being wrong together. The rather trivial fact that two observers cannot occupy the same place at the same time has absolutely nothing whatsoever to do with the question of which reference frames are admissible.

Their description of a given event certainly CAN use the same reference frame, despite the fact that they are in different locations and potentially moving wrt each other. If I get pulled over for going 100 mph then I am perfectly justified in saying that I was going 100 mph, I am not required to say that I was at rest and the officer was going -100 mph.

I am not required to use a reference frame where I am at rest, and I am not required to use a reference frame where I am at the origin.

He can't make an observation from another frame of reference moving relative to himself or use the time of that frames clock.

Yes, he can. GPS satellites do it all the time.

If a single photon is emitted, only the observer in line with its path will detect it.
No one else detects it!

Sure. But that observer detects that photon regardless of what coordinate system you choose to use to analyze the detection. You and altergnostic both seem to confuse the physical observer/detector and the purely mathematical construct of a coordinate system.

 

[phyti]

altergnostic: #90

... Your reminder that two can't occupy the same place at the same time is all we need to forbid any notion of light seen in another observer's frame.

That would not be true if the event in common involved multiple photons. In Einsteins train scenario, both observers saw the flashes because they were multiple photon events. My statement was a logical basis for expecting different descriptions of events from 2 different observers. If 10 people circled a statue and each took a photo, each would have a different perspective as evidenced in the photos, yet they would agree there was only 1 statue. That's a simple case with no motion.

In real life, that light can never be directly a part of our data, we can only know of its existence through secondary effects, like "yeah, i saw that too!"

Not sure of what you are saying here, but we perceive the universe indirectly. Light is the messenger and via sensory input, says 'something happened over here', and indicates a direction.
From other posts:
When you discuss A and B observing each others clock running slower than their own, remember each is comparing his current clock event with a clock event in the others past. Does that have any useful meaning?

 

[phyti]

PAllen #91

So, if you are riding a bicycle, you are not entitled to think you are moving; you
must insist that the ground is moving? [edit: and if, while riding a bike, I look at a
clock tower, I must ignore its time because it is not in my frame.]

If you read post 88 carefully, it doesn't imply any of what you said. It was just a
response to all the confusing 'frame' talk.
On a bike, on a park bench, it's irrelevant, because all things are moving. SR theory
just says if you are moving, you may choose to assume a pseudo rest frame, and
regardless of which you choose, you use a local clock. In your scenario, which was
not the intended range of application, there would be no significant difference in
using the bikers watch or the clock tower (assuming synchonization).With reference
to the observers frame, A.E. states in the 1905 paper, "It is essential to have time
defined by means of stationary clocks in the stationary system,"

More germane to physics, for all these years of colliding particles or ions into
stationary targets, physicist were acting incorrectly when they chose to record
observations and do calculations in the COM frame rather than the lab
frame?

That's just tranforming from one origin to another, which is what SR does. I don't
see any problem there.

 

[phyti]

For those who may not have accepted the geometry based example, and motivated by ghwells example using doppler methods, this is the math for the general case of two observers in relative motion, sending each a signal at a predetermined local time. The signals will arrive at the same local time for each.
https://www.physicsforums.com/attachments/51567

 

[Dale]

The signals will arrive at the same local time for each.

Do you think that any reference frame claims otherwise? If not, then no observer is required to use any particular frame to get the correct answer.

 

[phyti]

Do you think that any reference frame claims otherwise? If not, then no observer is required to use any particular frame to get the correct answer.

It's intended for bgq, altergnostic, or anyone else who thinks they have to start with a symmetrical situation, i.e. equal speeds in opposite directions.
As for 'frames', I'll wait for a sound/consistent definition, if there is one.

 

[Dale]

As for 'frames', I'll wait for a sound/consistent definition, if there is one.

The "official" definition of a frame is given here: http://en.wikipedia.org/wiki/Frame_fields_in_general_relativity

But I usually use the term as though it were synonymous with "coordinate system".

With either definition it should be clear that any observer can use any frame to predict the outcome of any experimental measurement.