Real life examples of simultaneity

[goodabouthood]

Can you give me some "real life" examples of simultaneity?

For instance I know the one about the train and the lighting strikes but I was under the impression that this only holds up if the train is moving close to the speed of light.

Thanks.

 

[nitsuj]

Can you give me some "real life" examples of simultaneity?

For instance I know the one about the train and the lighting strikes but I was under the impression that this only holds up if the train is moving close to the speed of light.

Thanks.

A muon in a particle accelerator and an inertial observer would disagree on the timing of events, such as it's half life.

Predicting mercury's orbit is more accurately calculated with GR I think,

GPS is an example of both types, and shows that even tiny variations can have "real life" effects (sufficient accuracy).

 

[BruceW]

In special relativity, if two events are simultaneous according to one frame, then the difference in the time at which they happened according to some other frame is:
\frac{\beta}{\sqrt{1- \beta^2}} \Delta x
Where \Delta x is the distance between the two events according to the first frame and \beta is the relative speed of the two reference frames (as a fraction of the speed of light) (and assuming the relative movement of the two frames happens in the same dimension as the distance between the two events - i.e. the x direction).

So this means that for the principle of relativity of simultaneity to become apparent, we need both a spatial separation of the two events and we must be considering two reference frames with relative speed which is a significant fraction of the speed of light.

 

[goodabouthood]

So I take it that simultaneity is not really observed on day to day life?

Could this be an example? If I was on a large field inside a house and my friend was on the same field but standing 100 miles away he would should disagree about the time I turned a light on in my house.

I should see the light turn on in my house slightly before he sees it. Considering he is only 100 miles away it seems that this wouldn't make much of a difference I suppose.

But let's say this was a really big field and he was standing 186,000 miles away. Would that mean he sees the light turn on 1 second after I do?

 

[HallsofIvy]

Be careful of your words! "Simultaneity" is witnessed all the time. "Dependence of simultaneity on frame speed" could only be witnessed by two observers moving at near light speed relative to one another.

 

[nitsuj]

So this means that for the principle of relativity of simultaneity to become apparent, we need both a spatial separation of the two events and we must be considering two reference frames with relative speed which is a significant fraction of the speed of light.

I think all that is required for the principle of relativity of simultaneity to become apparent is two observers that measure time, measure it differently. The rest is implicit no?

 

[goodabouthood]

Is this example correct though?

Person A and B are on an extremely large imaginary field. Person A is inside a house with a light switch and Person B is on the other side of the field 186,000 miles away.

When Person A turns on the light switch he should see it right away but Person B should see it one second later.

----

I'm not really sure if I understand this stuff correctly.

 

[nitsuj]

So I take it that simultaneity is not really observed on day to day life?

Could this be an example? If I was on a large field inside a house and my friend was on the same field but standing 100 miles away he would should disagree about the time I turned a light on in my house.

I should see the light turn on in my house slightly before he sees it. Considering he is only 100 miles away it seems that this wouldn't make much of a difference I suppose.

But let's say this was a really big field and he was standing 186,000 miles away. Would that mean he sees the light turn on 1 second after I do?

Thats not an example of the principle of relativity of simultaneity or what ever it`s called.

Time dilation / length contraction cause issues with what observers agree happened simultaneously.

The relative speeds / gravitational potential don`t have to be significant fractions of c to become apparent, your measurements have to be more accurate.

However I do think it is kinda related to distance, because c is invariant.

 

[robphy]

www.scientificamerican.com/article.cfm?id=time-dilation

describes a high-precision clock...

 

[BruceW]

The example is not correct. According to both person A and person B, the light is switched on at the same time, since the people have no relative motion.

You only need to worry about the relativity of simultaneity when objects are moving at a significant fraction of the speed of light. Two good examples are particles and satellites.

So you are right in thinking relativity of simultaneity doesn't matter in 'every day life'. If we consider only the reference frames of people, then simultaneity is approximately absolute, because all people have very small relative speeds (as a fraction of c). So if two events on Earth are simultaneous according to me, then they are approximately simultaneous according to everyone else on earth.

Nitsuj is right that simultaneity is always relative, its just that we don't notice it in every day life because the effects are tiny compared to the precision our measuring equipment.

Edit: of course, robphy's link seems to show some measuring equipment that is precise enough. So it takes very precise equipment to show that absolute relativity is only an approximation.

 

[goodabouthood]

The example is not correct. According to both person A and person B, the light is switched on at the same time, since the people have no relative motion.

You only need to worry about the relativity of simultaneity when objects are moving at a significant fraction of the speed of light. Two good examples are particles and satellites.

So you are right in thinking relativity of simultaneity doesn't matter in 'every day life'. If we consider only the reference frames of people, then simultaneity is approximately absolute, because all people have very small relative speeds (as a fraction of c). So if two events on Earth are simultaneous according to me, then they are approximately simultaneous according to everyone else on earth.

Nitsuj is right that simultaneity is always relative, its just that we don't notice it in every day life because the effects are tiny compared to the precision our measuring equipment.

Edit: of course, robphy's link seems to show some measuring equipment that is precise enough. So it takes very precise equipment to show that absolute relativity is only an approximation.

But I thought that if someone is 186,000 miles away they would see the Light come to them one second later because light travels at 186,000 miles/second. I don't think this would really depend on motion.

 

[BruceW]

Relativity of simultaneity refers to 2 events. What are the events in your example? I'm guessing one event is the first person seeing the light at the house and the other event is the other person seeing the light? These two events happen in different places and at different times.

So the example isn't really an example of relativity of simultaneity.

 

[goodabouthood]

What if person A and person B were talking on the phone?

Would they still agree?

 

[BruceW]

It looks like you're trying to make sense of 'relativity of simultaneity' by making a line of reasoning using the fact that light propagates at c according to all observers.

It is possible to make a line of reasoning this way. One way to do it is by the mind-experiment of the train and the lightning strikes (as you already mentioned).

But the example you're developing doesn't make a similar line of reasoning. In your example, the reason the people experience the flashes of light at different times is simply because they are in different places.

On another note - my equation in post #3 is written in natural units.

 

[goodabouthood]

I guess I am not clearly understanding the theory. I am fairly new to all of this.

I just thought people can't agree on when an event happened (the light bulb turning on) is because they are separated by a distance.

Are you sure it's only dependent on motion?

 

[BruceW]

An event is different to two events.

 

[BruceW]

People can't agree on when an event happened because they have relative speed.

 

[BruceW]

And this is not an example of relativity of simultaneity because we are only talking about one event.

 

[goodabouthood]

So would it become two events if they both shined a light?

Now if we add an observer at in the middle of them, he could see them both shine a light at the same time but the person at A would say he shined his light before the person at B.

 

[goodabouthood]

And person B would also say he shined his light before the person at A but the person in at M would say he saw them at the same time.

Correct?

 

[bobc2]

Here's a real life example: Bill and Ruth walk past each other on the street. Since they are in motion relative to each other, they are each living in a different instantaneous 3-dimensional world, each one being a different cross-section of the 4-dimensional universe. Thus, in Bill's world a meeting is taking place in the Andromeda Galaxy in which it is being decided whether or not to attack earth. However, at the instant Bill and Run are passing each other, Ruth is living in a different 3-dimensional world from Bill (an instantaneous cross-section of the same 4-dimensional universe) in which the Andromeda leaders have already made their decision and the Andromeda Space Fleet has already been launched and is heading toward earth.

This is of course the example posed by the renowned physicist, Roger Penrose ("The Emperor's New Mind"), known as the Andromeda Paradox.

Disclaimer: Bill and Ruth passing each other, living in two different 3-D cross-sections of the universe is a real everyday happening. But, the attack of the Andromeda fleet is made up (I hope).

 

[PatrickPowers]

So I take it that simultaneity is not really observed on day to day life?

Could this be an example? If I was on a large field inside a house and my friend was on the same field but standing 100 miles away he would should disagree about the time I turned a light on in my house.

I should see the light turn on in my house slightly before he sees it. Considering he is only 100 miles away it seems that this wouldn't make much of a difference I suppose.

But let's say this was a really big field and he was standing 186,000 miles away. Would that mean he sees the light turn on 1 second after I do?

Yes that is correct.

I have just completed reading Einstein's original relativity paper from 1905. I'd recommend it. It is written quite clearly, and this particular question is his starting point.

If you want my probably-distorted version:

In your field there is no problem with simultaneity. Your friend can find out how far away he is from you, he knows the speed of light, and he can correct for all this. So simultaneity is easy and makes sense.

Now somebody moving at half the speed of light is watching you two guys. He sees you do your corrections to get simultaneity. But to him your procedure seems to be wrong! What is going on? Einstein figures out a transform to restore sense to this crazy situation.

 

[BruceW]

And person B would also say he shined his light before the person at A but the person in at M would say he saw them at the same time.

Correct?

not correct. All three would agree that they shined their lights at the same time.

 

[goodabouthood]

Let me rephrase this again.

There is a large field with Person A separated from Person B at 186,000 miles (which is the speed that light travels in one second). Person M is located in the middle of AB.

Now person M sees lighting bolts hit A and B at the same time.

Would person A still see the lightning bolts at the same time as the person at B? I would think each sees their lighting bolt and then one second later sees the other persons lightning bolt.

 

[BruceW]

Yep. each person would see their own lightning bolt, then 1 second later see the other lightning bolt. Therefore the time at which the lightning bolts struck the ground was the same, according to both observers.

 

[goodabouthood]

What about the guy in the middle?

Wouldn't he see both at the same time?

 

[nitsuj]

Let me rephrase this again.

There is a large field with Person A separated from Person B at 186,000 miles (which is the speed that light travels in one second). Person M is located in the middle of AB.

Now person M sees lighting bolts hit A and B at the same time.

Would person A still see the lightning bolts at the same time as the person at B? I would think each sees their lighting bolt and then one second later sees the other persons lightning bolt.

Its clear what you are saying. An other way to put it is the moon is over a light second distance away. Whatever could possibly happen on the moon, Earth wouldn't know of it until 1 second later at the earliest. That is not the same as "relativity of simultaneity". In the above example the frame of reference of the moon measures 1 second the same as the Earth FoR.

Now that being said, gravity on the moon is less then Earth so I imagine time on the moon is slightly faster then on Earth. That slight difference in measuring time equates to differences in determining simultaneous events.

 

[vela]

What about the guy in the middle?

Wouldn't he see both at the same time?

When A, B, and M see the light from the lighting bolts striking is different than when the lightning bolts strike. Because they are all at rest relative to each other, they will all agree that the bolts hit at the same time after accounting for light travel time. For example, A does not see the light from B until t=1 s, but because A knows B is one light-second away, A concludes that the bolt hit B at t=0 s.

If there were another observer, C, moving relative to them, even after accounting for the time it took the light to travel, C would say that the bolts hit at different times.

 

[goodabouthood]

But what about the person situated in the middle of AB?

Wouldn't he see the lightning bolts at the same time?

 

[nitsuj]

I guess I am not clearly understanding the theory. I am fairly new to all of this.

I just thought people can't agree on when an event happened (the light bulb turning on) is because they are separated by a distance.

Are you sure it's only dependent on motion?

They are only separated in spatial dimensions, their clocks tick at the same rate, so they measure distance (spatial dimensions) the same. They can measure & calculate simultaneous events and will agree .

 

[CLSabey]

Consider lightning strikes at points A and B and for an observer at midpoint M on the embankment (reference frame K) the events occurred simultaneous because the light beams reach him or her at the same time.

Suppose when lightning strikes at A and B for an observer who when the events occur at A and B is at midpoint M but moving at 1/2 c (speed of light) toward point B, which is moving towards M' at c. Figure c = 300,000km/s. At what point on x can we identify M' to meet with B'? So B' = c and M' = 1/2c or c/2. They move toward each other in a straight line in vacuum at these velocities. What point do they meet. Convert to meters when appropriate to do so. I will be very suprised if anyone's wisdom and analysis can find the answer number to this riddle. The meaning of this is speaks to the relativity of simultaneity.

 

[goodabouthood]

When A, B, and M see the light from the lighting bolts striking is different than when the lightning bolts strike. Because they are all at rest relative to each other, they will all agree that the bolts hit at the same time after accounting for light travel time. For example, A does not see the light from B until t=1 s, but because A knows B is one light-second away, A concludes that the bolt hit B at t=0 s.

If there were another observer, C, moving relative to them, even after accounting for the time it took the light to travel, C would say that the bolts hit at different times.

I thought the guy in the middle would see them at the same time.

 

[vela]

But what about the person situated in the middle of AB?

Wouldn't he see the lightning bolts at the same time?

Yes, he would see the bolts hit A and B at the same time. The light from both would reach him when t=0.5 s. Knowing he was right in the middle of A and B, he would then conclude that the bolts hit A and B at t=0 s.

 

[goodabouthood]

Yes, he would see the bolts hit A and B at the same time. The light from both would reach him when t=0.5 s. Knowing he was right in the middle of A and B, he would then conclude that the bolts hit A and B at t=0 s.

Now wouldn't that be relative simultaneity because person M sees both lighting bolts at the same time but person A and person B see them at different times?

Person A and B will not agree with the person M when the bolts striked.

At least that is what I thought simultaneity was.

 

[PatrickPowers]

Consider lightning strikes at points A and B and for an observer at midpoint M on the embankment (reference frame K) the events occurred simultaneous because the light beams reach him or her at the same time.

Suppose when lightning strikes at A and B for an observer who when the events occur at A and B is at midpoint M but moving at 1/2 c (speed of light) toward point B, which is moving towards M' at c. Figure c = 300,000km/s. At what point on x can we identify M' to meet with B'? So B' = c and M' = 1/2c or c/2. They move toward each other in a straight line in vacuum at these velocities. What point do they meet. Convert to meters when appropriate to do so. I will be very suprised if anyone's wisdom and analysis can find the answer number to this riddle. The meaning of this is speaks to the relativity of simultaneity.

In the stationary frame they meet at 1/3 the distance between M and B.

If you want A' too, that meets at 2 times the distance between M and A.

 

[CLSabey]

Yes, he would see the bolts hit A and B at the same time. The light from both would reach him when t=0.5 s. Knowing he was right in the middle of A and B, he would then conclude that the bolts hit A and B at t=0 s.

You are presuming the line AB to be 300,000km then. Time is only significant when we know the reference frame for the events which we speak of. Here we could use any value for time so long as the beams of light resulting from the bolts reached M at same time, whatever that time may be from t=0 which coincides to origin of events A and B (lightning bolts). Whatever the t'= x then you can determine distance of lines AM and BM by dividng time in half and multiplying by distance in speed of light (use seconds for t).

Let's say it takes one second for light beams from A and B to reach you. The line AB is 600,000 km/s.

 

[vela]

No, they all agree that the bolts hit at t=0 s, even though they didn't know about the strikes until some time later when the light was able to propagate from the event to the observer.

The relativity of simultaneity happens between observers moving relative to each other. Since A, B, and M are all at rest to one another, they all agree that the bolts hit at the same time, even though they didn't see the bolts hit until later. Again, when an observer sees the event and when the event actually happened are two different things.

Say you had an observer C going from A to B who passes M at t=0 s, the time the lightning bolts strike. From A, B, and M's point of view, the light from B will reach C before the light from A does because C is moving toward the light coming from B and away from the light coming from A. C also agrees that the light from B reached him before the light from A, but he also knows that A and B were at the same distance from him. If the light from B reached him first, it must be because the lightning hit B before the lightning hit A. To C, the events weren't simultaneous.

 

[CLSabey]

In the stationary frame they meet at 1/3 the distance between M and B.

If you want A' too, that meets at 2 times the distance between M and A.

I want when M' and B' meet. What specifically is the number of the point they meet. No ratios or relations or descriptives just the number.

 

[vela]

You are presuming the line AB to be 300,000km then.

Yes, because that's the set-up goodabouthood made up. Something happens at A and B sees it one second later. M being in the middle would be a half light-second away from A and B.

 

[CLSabey]

Yes you are right

 

[goodabouthood]

No, they all agree that the bolts hit at t=0 s, even though they didn't know about the strikes until some time later when the light was able to propagate from the event to the observer.

The relativity of simultaneity happens between observers moving relative to each other. Since A, B, and M are all at rest to one another, they all agree that the bolts hit at the same time, even though they didn't see the bolts hit until later. Again, when an observer sees the event and when the event actually happened are two different things.

Say you had an observer C going from A to B who passes M at t=0 s, the time the lightning bolts strike. From A, B, and M's point of view, the light from B will reach C before the light from A does because C is moving toward the light coming from B and away from the light coming from A. C also agrees that the light from B reached him before the light from A, but he also knows that A and B were at the same distance from him. If the light from B reached him first, it must be because the lightning hit B before the lightning hit A. To C, the events weren't simultaneous.

This actually helps. The guy at C actually experiences the event before the others. The event of seeing the lightning B actually happens for him first.

 

[nitsuj]

Now wouldn't that be relative simultaneity because person M sees both lighting bolts at the same time but person A and person B see them at different times?

Person A and B will not agree with the person M when the bolts striked.

At least that is what I thought simultaneity was.

Well it depends on context.

With that perspective their experience is what is used to define "right Now". And your right they didn't experience each lighting strike at the same time, and in that sense wasn't simultaneous.

After the group measures how much time (this is key, they measure time the same) elapsed between events, they measure the distance between themselves and using their time and distance measurements, A, B & M calculate that the lighting strikes flashed at the same time.

 

[goodabouthood]

That is the train example. It's most likely a better example than the one I tried to give.

So events actually happen for some FoR than they do for other FoR.

 

[goodabouthood]

So let's say that the guy on the platform has a clock and the girl on the train also has a clock that are synchronized.

Special relativity tells us that even if they are reading synchronized clocks the events that happen would be different for them at particular times. They couldn't agree.

I understand this more now. Thanks.

 

[BruceW]

good. I think in your example before, you were confusing two different events: the lightning bolt hitting the ground, and the light from it going into someone's eye. These two events are different because light travels from one place to another. They are not different due to relativity of simultaneity.

 

[nitsuj]

So let's say that the guy on the platform has a clock and the girl on the train also has a clock that are synchronized.

Special relativity tells us that even if they are reading synchronized clocks the events that happen would be different for them at particular times. They couldn't agree.

I understand this more now. Thanks.

If their clocks are synchronized (adjusted for differences in measuring time) they would agree on the time of events, like with the GPS.

Another way to say it is SR/GR redifined the concept of simultaneous with the postulate c is invariant.

 

[ghwellsjr]

So let's say that the guy on the platform has a clock and the girl on the train also has a clock that are synchronized.

Special relativity tells us that even if they are reading synchronized clocks the events that happen would be different for them at particular times. They couldn't agree.

I understand this more now. Thanks.

If their clocks are synchronized (adjusted for differences in measuring time) they would agree on the time of events, like with the GPS.

Another way to say it is SR/GR redifined the concept of simultaneous with the postulate c is invariant.

In the example of the train and the platform, it is not possible to adjust the clocks so that they remain synchronized. Identical, unadjusted clocks will run at different rates in a symmetrical, reciprocal way, making it impossible to tweak them so that they can run at the same rate. The platform observer sees the train clocks ticking slower than his own and the train observer sees the platform clocks ticking slower than her own.

This is the point that has been repeated many times on this thread: clocks moving inertially (in a constant direction at a constant speed) with respect to each other run at different rates and cannot be synchronized, and Frames of Reference moving with respect to each other will have different definitions of the synchronization of the clocks at rest in their respective frames.

It's a different story with the time dilation caused by gravity where clocks at different altitudes and/or in orbit run at different rates, but there the relationship is not symmetrical and reciprocal which allows them to be tweaked so that they can display the same time and remain "synchronized". So, for example, the atomic clocks at Greenwich, England, run at a different rate from identical clocks at Boulder, Colorado, but they both agree on the difference because its not symmetrical and reciprocal, so they can both be used as standards for the second here on Earth with appropriate tweaking.

And goodabouthood, you're still confused on the meaning of "event" as used in relativity. Please study this recent https://www.physicsforums.com/showthread.php?t=543416" and see if it helps you.

 

[bobc2]

Here is a space-time diagram to help you visualize what special relativity is representing in the train-lightening example. Try to imagine a 4-dimensional universe with 4-dimensional objects (the train passenger car, the observer on the platform and the observer in the train car--these are all 4-D objects). These objects extend into the 4th dimension in the sketch below. A 4-D object corresponding to motion along X1 is slanted in the 4th dimension (the 3-D representation of the object moves along its 4th dimension at the speed of light). Google "space-time diagram" and "Block Universe" to get insight into these concepts. It's likely that you will not really get a good grasp of special relativity until you can comprehend a space-time diagram.

 

[ghwellsjr]

Wow--if I thought I had to comprehend space-time diagrams before I could really get a good grasp of special relativity, I'd have given up a long time ago, especially when you promote this 4-D "Block Universe" version which is not mainstream SR but rather a philosophical speculation. Why don't you stick with the legitimate 1-D version as described in the wikipedia "Minkowski diagram" article? And since they are inherently 1-D, how can they illustrate the difference in length contraction along a second dimension?

 

[goodabouthood]

1. My idea of an event is pretty much an instant in time where something happens.

2. The idea in my head about all of this is that the observer on the platform and the observer on the train experience one of the events together. That would be the lighting bolt that hit B.

The lightning bolt hitting A would happen at the same time as B for the observer on the platform so he would think that these events were simultaneous. The observer on the train is moving towards B so the lightning bolt from A has to catch up to him so he would not think these events were simultaneous.

Is this correct?