simultaneity (2 Viewers)

[table for 1]

ok, i need someone with a lot of patience to explain simultaneity to me. i know what it refers to, but i'm having a hard time understanding it. i just don't understand how the one event can be observed differently for different frames of reference. how can two people from different frames of refernce see different events [or order of events] if it's just the same thing that's happening? it just doesn't make sense to me... !!

thanks for trying, if you do. [ps. don't bother using much jargon, it'll just get me more confused. simple language works best =D ]

 

[wanton-wonton]

ok, i need someone with a lot of patience to explain simultaneity to me. i know what it refers to, but i'm having a hard time understanding it. i just don't understand how the one event can be observed differently for different frames of reference. how can two people from different frames of refernce see different events [or order of events] if it's just the same thing that's happening? it just doesn't make sense to me... !!

thanks for trying, if you do. [ps. don't bother using much jargon, it'll just get me more confused. simple language works best =D ]

Good day,

Well.......imagine there are two points, A and B, 100 metres away. Now, there are two people in the middle of the two points. Both are 50m away from A and B.

Suddenly, a lightning stikes on both end of the points at the same time (at the same time means simultaneous because both observers are in the same framer of reference) Now as you know, it takes time for both the ray to reach to the centre.

Imagine if one of the observer was on a train, heading towards point B. Now, the lightning strike from point B would take less time to reach the person on the train than lightning from point A, since there is less distance to be covered, so in this case, for person B, the lightning strike is not simulatenous.

However, for the other person, who is standing still, he see's the two lightning strikes at the same time, since both have the same distance to cover.

That should be it....?

 

[table for 1]

ok, that example sounds much better than the one i have. much easier to understand, but still a bit confused, still trying to process the information. thanks !

damn physics. i want to cry now

 

[zenger69]

Simulatneity is just that things simultaneous in one frame of reference will be different in other frames of reference. So simultanoues even become relative.

Hence the title Relativity of Simulataneity.

And the thought experiment by Mr Wanton is pretty good.[/SIZE]




-------------pardon my spelling------- -------------too tired------------------------------------

 

[table for 1]

i got the 'train with light operated doors' thought experiment. and that just really confused me because it's saying how the person inside the train carriage saw the doors open simutaneously, whereas the person outside saw the doors opening at different times. that just really confused me because how can the doors be both opening at the same time and not? argh ! this is really confusing

 

[zenger69]

yeh the two people represent the two frames of reference.

As the person in the train see them go simultaneously but becuase the train is moving, the person outside see one them the other.

 

[table for 1]

i know....but i just can't accept that the doors can open simutaneously, but not simutaneously. i just can't accept it !! with the example wanton-wonton gave, that i find more easy to understand because it's the person's eyes who sees it differently [rather than the doors opening differently. ok really, there shouldn't be much of a difference, but with the way my brain thinks, i just ... argh !! this sucks]

 

[tennille]

i know....but i just can't accept that the doors can open simutaneously, but not simutaneously. i just can't accept it !! with the example wanton-wonton gave, that i find more easy to understand because it's the person's eyes who sees it differently [rather than the doors opening differently. ok really, there shouldn't be much of a difference, but with the way my brain thinks, i just ... argh !! this sucks]

Physics is confusing in that way. Just try and accept it...well, when you do your exam, just say they're not simultaneous or whatever. lol.

 

[wanton-wonton]

i know....but i just can't accept that the doors can open simutaneously, but not simutaneously. i just can't accept it !! with the example wanton-wonton gave, that i find more easy to understand because it's the person's eyes who sees it differently [rather than the doors opening differently. ok really, there shouldn't be much of a difference, but with the way my brain thinks, i just ... argh !! this sucks]

I haven't been taught the opening door example, the example I gave I saw on a video and was more ... 'clearer' so if I were you, I'd just forget about that opening door example and use mine as an example....

I'll look up the 'opening door' example tonight and see if it makes sense.

 

[zeropoint]

i know....but i just can't accept that the doors can open simutaneously, but not simutaneously. i just can't accept it !! with the example wanton-wonton gave, that i find more easy to understand because it's the person's eyes who sees it differently [rather than the doors opening differently. ok really, there shouldn't be much of a difference, but with the way my brain thinks, i just ... argh !! this sucks]

I personally don't like wanton-wonton's explanation because it gives the impression that the relativity of simultaniety is an optical illusion rather than an actual physical consequence. If you position observers at each end of the train in the train's frame of reference, the time they measure for each flash can be used to verify that the flashes actually took place at different times in the train frame. If you find relative simultaneity difficult to accept, then why do you find it any less difficult to accept relative velocity? Remembering that velocity is just the ratio of distance to time, it makes sense that distances and times will shrink and expand to keep the speed of light constant for all inertial observers.

 

[Xayma]

The main point to grasp simultaneity, I think, is that while the speed of light is constant to an observer in each frame of refrence, the speed of light relative to another object (seperate from the observer) in a different frame of refrence is not constant.

This means that you may see that the speed of light relative to the front of a moving train is 2*10⁶ ms^-1 while the back will be moving faster (if the source was in the train). Now on the train the speed of light relative to the moving train is 3*10⁸ms^-1. Meaning that any light trigged events, such as doors opening due to light triggers, may appear simultaneous in one frame of refrence because they view the speed of light relative to those objects as constant, but in another frame of refrence, the speed of light relative to the objects will be different.

I hope that made sense.

 

[zeropoint]

The main point to grasp simultaneity, I think, is that while the speed of light is constant to an observer in each frame of refrence, the speed of light relative to another object (seperate from the observer) in a different frame of refrence is not constant.

This is certainly not correct. The speed of light relative to every inertial observer has the same value c. What special relativity postulates is that no light beam may travel at any speed other than c. If you fire two laser beams towards each other, the distance between the tips of the light beams decreases at a rate of 2c. Nowhere in this experiment has the relative velocity of either beam had any value other than c.

This means that you may see that the speed of light relative to the front of a moving train is 2*10⁶ ms^-1 while the back will be moving faster (if the source was in the train). Now on the train the speed of light relative to the moving train is 3*10⁸ms^-1. Meaning that any light trigged events, such as doors opening due to light triggers, may appear simultaneous in one frame of refrence because they view the speed of light relative to those objects as constant, but in another frame of refrence, the speed of light relative to the objects will be different.

Actually, the speed of light relative to the embankment is c. The speed of light relative to the front and the back of the train is also c. It makes no sense to talk about the speed of light measured by one observer relative to another observer since you're implicity assuming the Galilean transformation of velocites which breaks down at high fractions of the speed of light.

Hope this helps.

 

[Xayma]

I was talking about from an observer, the speed of light relative to another object may not be c.

I knew it wouldnt sound right.

Basically an observer may see a photon of light moving relative to an object at a speed past c. (S)he would see the light travelling at c and then the object at a speed. This becomes a relative speed above c. Now if there is an object moving in the same direction as the wave, it will have a speed relative to that object (from an observers view) of less then c. Hence any light trigged events will appear to occur at different times, if the wave was released from the middle.

If you were travelling at the same velocity of the objects however, you will see the wave travelling at c and nothing else. It will hit both objects at the same time.\

Hope that made more sense. I agree with everything you said zeropoint but my wording was confusing.

 

[table for 1]

I personally don't like wanton-wonton's explanation because it gives the impression that the relativity of simultaniety is an optical illusion rather than an actual physical consequence.

oh yeah, good point. but to me it makes more sense...?

If you find relative simultaneity difficult to accept, then why do you find it any less difficult to accept relative velocity?

i don't know, i guess to me, something shrinking and expanding sounds more believable than an event being seen differently. [ i mean, its the same event, it just doesn't make sense if it can be seen differently... i'm just a confused person].


ok, you two lost me somewhere there. how about we just focus on the train carriage with light-operating doors example, and try to explain it until i get it. [which will be a long time... told you that you needed a lot of patience]

it goes something like this :
A train carriage has light-operated doors. The light is in the middle of the carriage. When the light it switched on, it travels equal distances to each door and opens them at the same time. At least, that's how someone inside the train sees things happening. The train is at rest relative to this observer.

Hoever, a person outside the train sees the train moving. When the light is turned on it travels out towards each door, but at the same time, the train moves forward. So the light reaches the back door sooner than it reaches the front door because the back of the train has moved closer to the light rays approaching it.

The front door is moving in the same direction as the light, so the light has to to travel further to reach it - it takes longer to get there. The back door opens first. The two events [the doors opening] do not occur simutaneously in this observer's fram of reference.

i get why the doors don't open at the same time to the observer outside the carriage, but the the main problem i have is understanding how the doors can open and not open at the same time. i think i just think weirdly, and that's just why i can't accept it. anyways, i must be really annoying, still not getting it, over something that kind of sounds really trivial to the overall senario. i think in the end, i'm just gonna have to accept whatever the textbook says, and write whatever the textbook says in the test.

 

[zeropoint]

You certainly don't think weirdly, you think just like your experiential senses have brainwashed you to think . As for understanding relative simultaneity, let me see if I can put it in a way that you're already familiar with. I think you can understand where the relativity of lengths and time intervals stems from. If you think of the speed of light as c = (light path) / (time interval) where c is a universal constant of nature. It is clear that you cannot change distance without also changing time. If you understand the relativity of time intervals, then you can see that if two clocks are initially synchronised, they will lose their synchronisation if they are put in relative motion. If you understand this much, that's great! If your mind is filled with troubled thoughts then you haven't quite understood time dilation. Let me know and we'll see if we can de-brainwash you . Suppose one clock flies off at high speed towards the east and the other remains at home. Imagine yourself in the inertial frame of the moving clock. As you wisp along at some good fraction of the speed of light, you notice at the very moment your clock strikes noon, a dove is released from a church in the distance. However, we know from time dilation that the moving clock will appear to run slow relative to the stationary clock. Thus the events of dove release and coincidence of the hour and minute hand with the 12 on the clock face are no longer simultaneous. So you can see that the relativity of simultaneity is a direct consequence of the relativity of time intervals.

 

[2confused!]

damn you ppl know too much! but if zeropoint or someone else wants to explain time dilation to me id be reeaally happy, cos i still havent got that figured out! i agree with table for 1, i dont get how all that stuff can change just because of your frame of reference. i guess ill just have to believe it. but yeah, if someone can help me out without using too many words i dont understand, thanx

 

[table for 1]

You certainly don't think weirdly, you think just like your experiential senses have brainwashed you to think . As for understanding relative simultaneity, let me see if I can put it in a way that you're already familiar with. I think you can understand where the relativity of lengths and time intervals stems from. If you think of the speed of light as c = (light path) / (time interval) where c is a universal constant of nature. It is clear that you cannot change distance without also changing time. If you understand the relativity of time intervals, then you can see that if two clocks are initially synchronised, they will lose their synchronisation if they are put in relative motion. If you understand this much, that's great! If your mind is filled with troubled thoughts then you haven't quite understood time dilation. Let me know and we'll see if we can de-brainwash you . Suppose one clock flies off at high speed towards the east and the other remains at home. Imagine yourself in the inertial frame of the moving clock. As you wisp along at some good fraction of the speed of light, you notice at the very moment your clock strikes noon, a dove is released from a church in the distance. However, we know from time dilation that the moving clock will appear to run slow relative to the stationary clock. Thus the events of dove release and coincidence of the hour and minute hand with the 12 on the clock face are no longer simultaneous. So you can see that the relativity of simultaneity is a direct consequence of the relativity of time intervals.

i think i kind of got that.... =D !!
see ! i get other examples, i just don't get the one i was originally given. can time dilation really be applied to the 'light operated train carriage doors' example? if so, how? i don't quite get it. oh screw...i think i should just forget about that one then.


to 2confused: i'd like to help you, but considering i'm so lost about physics... and there are much more experienced physicists in these forums, so i'd better not confuse you to begin with

 

[wanton-wonton]

The main point to grasp simultaneity, I think, is that while the speed of light is constant to an observer in each frame of refrence, the speed of light relative to another object (seperate from the observer) in a different frame of refrence is not constant.

Errr....WHAT?! The speed of light is constant in ALL frames of reference....

This means that you may see that the speed of light relative to the front of a moving train is 2*10⁶ ms^-1 while the back will be moving faster (if the source was in the train). Now on the train the speed of light relative to the moving train is 3*10⁸ms^-1. Meaning that any light trigged events, such as doors opening due to light triggers, may appear simultaneous in one frame of refrence because they view the speed of light relative to those objects as constant, but in another frame of refrence, the speed of light relative to the objects will be different.

I hope that made sense.

No, not quite.......Hmm.....

 

[wanton-wonton]

Actually, the speed of light relative to the embankment is c. The speed of light relative to the front and the back of the train is also c. It makes no sense to talk about the speed of light measured by one observer relative to another observer since you're implicity assuming the Galilean transformation of velocites which breaks down at high fractions of the speed of light.

Hope this helps.

I don't know what you're talking about, but when referring to relative velocity of high velocities, e.g. near the speed of light, you use the equation as follows:

combined velocity = (v1 + v2)/(1+v1v2/c^2)

E.g., if a space ship is travelling at 0.9 towards another space ship also travelling at 0.9c, the relative speed of one space ship to another is not 1.8 c, instead, use the equation.

 

[zeropoint]

i think i kind of got that.... =D !!
see ! i get other examples, i just don't get the one i was originally given. can time dilation really be applied to the 'light operated train carriage doors' example? if so, how? i don't quite get it. oh screw...i think i should just forget about that one then.

This thought experiment has a slightly different resolution but it's nothing too dificult to grasp. To understand this experiment you should first note that the ``light operated doors '' are not an essential feature of the experiment. If the train rider decided to tint all of the carriage windows, the embankment observer would have no knowledge of the cause of doors opening, however the effect would be the same. The point of the experiment is that two separate events (the opening of each door) occur simultaneously in one frame and at different times in another frame. To simplify the experiment (and make it more conceptually feasible) we'll remove the doors completely, and outfit the train with a pair of flashtubes on either end of the carriage. The flashtubes are designed to emit very brief intense flashes of light when a switch is thrown at the midpoint of the carriage. When the train operator activates the flashtubes, wavefronts of light depart from each end of the train and arrive at the train observer at the same time. There are three important events you should consider; the emission of the rear flash, the emission of the front flash and the reception of both flashes at the midpoint of the train. The first two events occur simultaneously in the train frame, the third event is common to all frames, since you know that all events must have an observer independant existence. Our aim is to prove that the former events occur at different times in the embankment frame. Try imagining the situation from the embankment frame, we know that the wavefronts from both flashes must arrive at the centre of the carriage simultaneously in both frames, however we also know that the embankment sees the train carriage moving with speed v. This means that at the very moment both wavefronts arrive at the centre of the carriage, the centre will have moved due to the motion of the train as the light waves approach the point of reception. This means that the flashes of light must have been emitted at different times in order for both waves to reach the centre at the same time in the embankment frame. If for example, the train moves to the right, the rear flash will occcur first, since the light from that flash must travel a longer distance than the light from the front flash, in order to reach the centre of the train at the same time as the front flash.

Hope this helps.