question about simultaneity (1 Viewer)

[martin310015]

explain what does it mean. THANX

 

[Huy]

Two or more events that are simultaneous for one observer are not necessarily simultaneous for observers in different inertial frames of reference.

That's all I remember, sorry (it's been a while since my exam).

 

[martin310015]

does inertial frame of reference the same as different point of view

 

[Huy]

Originally posted by martin310015
does inertial frame of reference the same as different point of view

An inertial frame of reference is one that is moving with constant velocity (non-accelerating).

A non-inertial frame of reference is one that is accelerating.

 

[Ragerunner]

Originally posted by Huy
An inertial frame of reference is one that is moving with constant velocity (non-accelerating).

You forgot to mention it's not only constant velocity but also at rest.

Simultanenity refers to different observers viewing the events differently. This is because they are in a different inertial frame of reference.

If you recall the thought experiemtn about the train moving at the speed of light you'd find that is mainly what it's about.

 

[Huy]

Originally posted by Ragerunner
You forgot to mention it's not only constant velocity but also at rest.

Correct, the two being indistinguishable, with mechanical experiments not being able to be performed in such frames of reference.

Originally posted by Ragerunner
If you recall the thought experiemtn about the train moving at the speed of light you'd find that is mainly what it's about.

I was going to explain this to martin, but decided not to.

I've forgotten all about Physics.

 

[Ragerunner]

I know my Physics even more than when I did the HSC.

 

[...]

yea..

imagine it as this..a train travelling at the speed of light..(not mathematically possible, but meh!) and inside the train theres a light bulb...at both ends of the train, there are doors that are light sensitive..that is..as soon as they feel light, they open.

so k, ur the train is in a tunnel and travelling the speed of c...the guy inside the train turn on the light and to him, both doors open...(think special relativity theory)

however, if u r obsreving outside the train, what will u see...u will only see the back door open while the front door remains close...(draw a diagram, makes more sense)

so there are "two" events..one that both doors open, the other, only one..

whether the door "actually' open in reality, is not required to know...

 

[martin310015]

but isn't the person in the train experience a shorten in the lenght of time so therefore he would see the front door open then the back door. correct me if im wrong thanx

 

[Dash]

Originally posted by martin310015
does inertial frame of reference the same as different point of view

You could say that in a way, but we don't say that person A standing on the left of you is in a different frame of reference to person B is standing over on the right.
Frames of references are used to tell us whether a person is accelerating or at constant velocity.
A person subject to acceleration is in a different frame of reference to someone who is stationary or moving with a constant speed.
It is important to consider frames of references when discussing simultaneity.
The reason being... as Huy stated:

Originally posted by Huy
Two or more events that are simultaneous for one observer are not necessarily simultaneous for observers in different inertial frames of reference.

So the time interval between two events depends on the observer's frame of reference (ie intertial or non-intertial) when considering relativistic speeds and the theory of relativity.

edit

Originally posted by martin310015
but isn't the person in the train experience a shorten in the lenght of time so therefore he would see the front door open then the back door. correct me if im wrong thanx

I'm a little rusty on the train thought experiment... so I'll wait for sumone to answer it

 

[martin310015]

ok thanx

 

[zeropoint]

Both observers mutually observe each other's clock to slow down. In the frame of the train, time flows normally and the person outside experiences time dilation. In the outside frame, time flows normally and the person in the train experiences time dilation.

This can be explained by the fact that any inertial observer can be regarded as stationary by undergoing a suitable change of coordinates. For example, in outside frame, the outside observer is stationary and the train is moving in the positive x-direction with velocity v. However in the frame of the train, the observer on the train is stationary and the outside observer appears to be moving in the negative x-direction with velocity −v.

Originally posted by martin310015
but isn't the person in the train experience a shorten in the lenght of time so therefore he would see the front door open then the back door. correct me if im wrong thanx

As far as the person on the train is concerned, the train is stationary so the light will reach both ends of the train simultaneously. The person outside the train will observe the train to be moving to the right with speed v. However, since the speed of light is constant in all inertial frames (fundamental postulate of SR) the light will reach the back door, which is moving towards the light source, before reaching the front door, which is receding from the light source.

 

[...]

quote :

As far as the person on the train is concerned, the train is stationary so the light will reach both ends of the train simultaneously. The person outside the train will observe the train to be moving to the right with speed v. However, since the speed of light is constant in all inertial frames (fundamental postulate of SR) the light will reach the back door, which is moving towards the light source, before reaching the front door, which is receding from the light source.


exactly...

but front door will not open to the outsider..

cos the train is travelling at c...and inside the trian, the light cannont catch up with the front door...so the door will be stationary(front door)
however, if the train is travelling at 0.9c, then back door opens first, then front

 

[Rahul]

relativity of simulataenity basically means that an event for one person wont necessarily be the same for another person, with one of them in an accelerating frame of reference.

you must keep in mind the word relative when thinking of the gendakens. if one ship is still or moving with a constant velocity [inertial F.O.R] and another is accelerating or decelerating, negative acceleration, [non inerial F.O.R], there is relative acceleration between the two ships. but if you are in either ships, you cannot do any mechanical experiments by which you can deduce which ship is in which F.O.R.
thus, the person in ship A, will say that he is still and ship B is moving past him. similarly, the person in ship B will say that ship A is moving.
which one is correct? both. that is because there is a non-inertial F.O.R involved.
an observer will notice the moving ship to be experiencing length contraction and time dilation. so try to remember for questions, the one that is moving will experience the relativistic phenomena, but it depends on where, or what F.O.R the observer is in.

 

[juggernaut]

oh nah, wrong thing

 

[zeropoint]

Originally posted by ...
quote :
but front door will not open to the outsider..

cos the train is travelling at c...and inside the trian, the light cannont catch up with the front door...so the door will be stationary(front door)

I was under the assumption that the speed of the train v < c. It is meaningless to consider the train having velocity v greater than or equal to c as it invokes zero or imaginary mass.

Originally posted by Rahul
relativity of simulataenity basically means that an event for one person wont necessarily be the same for another person, with one of them in an accelerating frame of reference.

Time dilation in SR depends only on the instantaneous velocity of observer; it is completely independent of the acceleration. Inertial observers moving with uniform velocity relative to each other will disagree on the coordinate of an event in space-time.

Originally posted by Rahul
you must keep in mind the word relative when thinking of the gendakens. if one ship is still or moving with a constant velocity [inertial F.O.R] and another is accelerating or decelerating, negative acceleration, [non inerial F.O.R], there is relative acceleration between the two ships. but if you are in either ships, you cannot do any mechanical experiments by which you can deduce which ship is in which F.O.R.

Doubly incorrect. First, Uniform velocity is relative in SR, acceleration is not. Acceleration in SR is frame invariant under Lorentz transformation. Second, there are any number of experiments (mechanical and electromagnetic) that can trivially distinguish between the coordinate systems you describe.

 

[...]

Originally posted by zeropoint
I was under the assumption that the speed of the train v < c. It is meaningless to consider the train having velocity v greater than or equal to c as it invokes zero or imaginary mass.

exactly, i stated its not possible earlier..but for a veryclear cut imagine of sim. incident..its good to imagine it as v=c

 

[Rahul]

zeropoint, thanks for picking up that mistake.

i got the description of inertial/non-inertial FOR and special relativity mixed up.