Simultaneity and t=0 - SRT

Quantum Quack

Quantum Quack

Life's a tease...
Valued Senior Member
HI guys,

The question that I was pondering upon is:

"If we have two or more observers [ RF's ] at relative v is t=0 for light events simultaneous for both (all) of them?"

I was under the impression that t=0 is relative and not absoute, according to SRT but I have reason to believe I am mistaken.
care to discuss?:)
to avoid confusion the following gedanken:

gedanken:

we have a single wave of light heading away from a source.
5 ships hit that same wave all travelling at relative velocity to each other,

question:
Do all ships hit the same wave at the same time [ t=0 ] from all perspectives including that of the photon waves'?
according to SRT?
Click to expand...
 
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Remember that what you are calling t = 0 is simply a point in time. What relativity talks about is events, and in particular, separation of events. You can only do an experiment that can detect differences in time intervals. Usually you would define an event to be at t = 0 in all frames, like the first door closing in the ladder in the barn thought experiment and the following time intervals are different for different observers.
 
prometheus said:
Remember that what you are calling t = 0 is simply a point in time. What relativity talks about is events, and in particular, separation of events. You can only do an experiment that can detect differences in time intervals. Usually you would define an event to be at t = 0 in all frames, like the first door closing in the ladder in the barn thought experiment and the following time intervals are different for different observers.
Click to expand...
can we agree that all events can only occur at t=0 according to SRT?
I understand that this question may appear simplistic on the surface but from experience with these sorts of things I know that it is not that simple.
 
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Quantum Quack said:
HI guys,

The question that I was pondering upon is:

"If we have two or more observers [ RF's ] at relative v is t=0 simultaneous for both or all of them?"

I was under the impression that t=0 is relative and not absoute, according to SRT but I have reason to believe I am mistaken.
care to discuss?:)
Click to expand...
t=0 is a completely arbitrary choice for any observer in any situation. It's whenever you want it to be.
Generally, it's convenient to set t=0 to be the start of some time period of interest (eg when timing a running race, you set t=0 to be when the starter's gun fires).

Quantum Quack said:
can we agree that all events can only occur at t=0 according to SRT?
Click to expand...
No. For any event considered in isolation, any observer can set any value of t at all. It's a completely arbitrary choice.
 
Pete said:
t=0 is a completely arbitrary choice for any observer in any situation. It's whenever you want it to be.
Generally, it's convenient to set t=0 to be the start of some time period of interest (eg when timing a running race, you set t=0 to be when the starter's gun fires).


No. For any event considered in isolation, any observer can set any value of t at all. It's a completely arbitrary choice.
Click to expand...
yeah I am getting that impression from other past expereince I thought this was the case however...it appears that there may be an issue communication.

A light event such as a photon particle or wave actually striking an object of mass occurs at t=? ?

meaning that I am suggesting that from a photons perspective it is always t=0

btw , Hi pete:) been a while....
 
Quantum Quack said:
A light event such as a photon particle or wave actually striking an object of mass occurs at t=? ?

meaning that I am suggesting that from a photons perspective it is always t=0
Click to expand...
My naive understanding is that the time elapsed between the beginning and end of a photon's path in vaccuum from a photon's perspective is zero. But this doesn't mean that t=0 at the end, unless t=0 at the beginning... and that's an arbitrary choice. You could just as validly have the beginning and end at t=100, or t=-16.2, or whatever.

btw , Hi pete:) been a while....
Click to expand...
Hi to you too!
 
Pete said:
My naive understanding is that the time elapsed between the beginning and end of a photon's path in vaccuum from a photon's perspective is zero. But this doesn't mean that t=0 at the end, unless t=0 at the beginning... and that's an arbitrary choice. You could just as validly have the beginning and end at t=100, or t=-16.2, or whatever.


Hi to you too!
Click to expand...


hmmmmm....ok...shall think on it some and maybe other posters would care to join.

I will I think be referring to the light cones later to try and clear this up.
 
Pete said:
My naive understanding is that the time elapsed between the beginning and end of a photon's path in vaccuum from a photon's perspective is zero. But this doesn't mean that t=0 at the end, unless t=0 at the beginning... and that's an arbitrary choice. You could just as validly have the beginning and end at t=100, or t=-16.2, or whatever.


Hi to you too!
Click to expand...
pete, have amended the OP to include direct reference to light events
 
Just to help clarify the question:

gedanken:

we have a single wave of light heading away from a source.
5 ships hit that same wave all travelling at relative velocity to each other,

question:
Do all ships hit the same wave at the same time [ t=0 ] from all perspectives including that of the photon waves'.
 
Pete said:
My naive understanding is that the time elapsed between the beginning and end of a photon's path in vaccuum from a photon's perspective is zero. But this doesn't mean that t=0 at the end, unless t=0 at the beginning... and that's an arbitrary choice. You could just as validly have the beginning and end at t=100, or t=-16.2, or whatever.


Hi to you too!
Click to expand...

If 't' can equal say 10 minutes from now when we get to 't' isn't it t=0?

if 't' equals 10 minutes from now then that event is in the future and is yet to occur.
if 't' equals -10 minutes from now then that event is in the past and is no longer observable except as a historical record.

but even so t= -10 must have occurrered at t=0


does that make sense?
and then if you can understand it, is it a correct understanding?
 
pete said:
My naive understanding is that the time elapsed between the beginning and end of a photon's path in vaccuum from a photon's perspective is zero. But this doesn't mean that t=0 at the end, unless t=0 at the beginning... and that's an arbitrary choice. You could just as validly have the beginning and end at t=100, or t=-16.2, or whatever.
Click to expand...

This is my understanding.
 
Quantum Quack said:
If 't' can equal say 10 minutes from now when we get to 't' isn't it t=0?

if 't' equals 10 minutes from now then that event is in the future and is yet to occur.
if 't' equals -10 minutes from now then that event is in the past and is no longer observable except as a historical record.

but even so t= -10 must have occurrered at t=0


does that make sense?
and then if you can understand it, is it a correct understanding?
Click to expand...
An event can have as many values for t as you like, but they are only meaningful when compared with the value of t for other events using the same system.

So, yes. Any time is t=0 time, if you want it to be. You can push the start button on a stopwatch whenever you like. You could have billions of stopwatches that start at different times if you want, and thus have billions of different concurrent time measurement systems with different times for t=0.

Values of t are all about measuring time elapsed. About the length of periods between various events.
 
Pete said:
An event can have as many values for t as you like, but they are only meaningful when compared with the value of t for other events using the same system.

So, yes. Any time is t=0 time, if you want it to be. You can push the start button on a stopwatch whenever you like. You could have billions of stopwatches that start at different times if you want, and thus have billions of different concurrent time measurement systems with different times for t=0.

Values of t are all about measuring time elapsed. About the length of periods between various events.
Click to expand...
Yes I have got confirmation from other sources as well as to the SRT position on 't'
However I trust yours the most....thanks Pete.
 
In order to use the Lorentz transform of time between two different coordinate systems, you have to have one common reference point. The common reference point can be any one of the many reference points contained in the two systems. Arbitrary choice can lead you to t=0, or t=-198, or t=987,654,321.0123, or any other time to be the one common reference point in the two compared systems.

So, you set t=-198, as my arbitrarily chosen example, as the common time in two systems. t=-198 is not arithmetically changed when applying the transformation by multiplying by GAMMA. All other time numbers are appropriately arithmetically changed.

The person comparing two sets of observation time numbers in two coordinate systems has total freedom of choice about which time number to set as the common time coordinate. Once chosen, that time number cannot be subsequently transformed whereas all other time numbers must be properly transformed arithmetically.
 
prometheus said:
Remember that what you are calling t = 0 is simply a point in time. What relativity talks about is events, and in particular, separation of events. You can only do an experiment that can detect differences in time intervals. Usually you would define an event to be at t = 0 in all frames, like the first door closing in the ladder in the barn thought experiment and the following time intervals are different for different observers.
Click to expand...

Ladder in the barn paradox, also known as Train in the tunnel paradox, Poland Barn Paradox, and such like, has unfortunately been abused by some people seeking to either prove it resolved, or, unresolved , by their improper choice and abuse of a common reference point.

The proper choice and use of a common reference point makes it blatantly plain whether or not the Poland Barn Paradox et al disproves Special Relativity.
 
Hello Pete and Quantum, et al -- ha ha -- I can't believe there is a current SR thread going! (Light-speed-invariance, etc.) I saw MacM and Superluminal in a thread not too long ago, and it brought back memories. Something about MacM getting a patent on an absolute velocity device? Thread was locked, but it's all good. Good times, good times.

Anyway, let's assume a group of spacecrafts all happen to hit the wavefront of a light ray (ie laser beam) in the same place, at the same time. This event can be set to time t=0, and spacial coordinates x,y,z = 0.

Even though the crafts are moving at different velocities (inertially, and relative to each other), these are the coordinates agreed upon by all parties for the "light on" event.

It's almost meaningless unless we consider another event, at "t not=0" and "x,y,z not=0".

Anyway, all bets are off after that. The clocks and measuring rods are all frame-dependent, so the only thing they can all agree on is that the light beam went from here to there, and all frames agree on that. But, everyone disagrees as to time and distances elapsed. Weird.
 
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Neddy Bate said:
......Anyway, let's assume a group of spacecrafts all happen to hit the wavefront of a light ray (ie laser beam) in the same place, at the same time. This event can be set to time t=0, and spacial coordinates x,y,z = 0.

Even though the crafts are moving at different velocities (inertially, and relative to each other), these are the coordinates agreed upon by all parties for the "light on" event.
Click to expand...

The whole point though is to show that even though they all hit the same light front at exactly the same time they all have different time tick rates,
which helps prove this diagram:
10.gif


The diagram is relatively old so please excuse it's uhmmm....use of the word NOW.

But what the issue for me is to try and demonstrate that t=hsp is common to all reference frames regardless of relative velocity. Therefore the NOW would be a universal constant. Which of course can not be correct as SRT requires non-simultaneity of *t=hsp [ NOW ]

However this is off topic and a new thread will be needed to clarify or pursue this issue if desired.

Another similar image along the same lines
11.gif


* hsp= hyper surface of the present refer light cones
 
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Quantum Quack said:
The whole point though is to show that even though they all hit the same light front at exactly the same time they all have different time tick rates
Click to expand...

Yes, at that instant of time (the NOW), they all have different tick rates on all of their clocks, but that is small potatoes! There is so much more weirdness in SRT. The poor saps on these space crafts, who happen to be coasting along in relative motion to each other, also disagree about spacial units (ie. length along one axis) and clock-synchronization (again, along one axis, seemingly a resultant of vectors).

To unabashedly quote myself, "Weird."

:idea:
 
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