# On simultaneity

Let's modify Einstein's thought experiment on simultaneousness. A person G on ground sits in the middle of two fireworks. The fireworks are wired (equal distance) to an ignitor just in front of the person G. A person T on a train of constant velocity V passes by the ignitor and pushes the button to light up the two fireworks. Then, person G will see the two fireworks light up simultaneously. However, person T will see them light up not simultaneously because he is racing away from one firework to another firework. Since the wires' equal distance to the two fireworks will guarantee they are lit up simultaneously (absolute time), then person T see the events not simultaneously (reference frame dependent time), does it mean anything to you folks? Or I get the logics wrong here?

Yes you just explained Einstein's thought experiment. What is your point though?

PeterDonis
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2020 Award
Moderator's note: Thread level changed to "I".

PeterDonis
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Since the wires' equal distance to the two fireworks will guarantee they are lit up simultaneously (absolute time)

No. There is no such thing as "absolute time". The fireworks will go off simultaneously in the rest frame of person G. They will not go off simultaneously in the rest frame of person T. That's all there is to it.

Yes you just explained Einstein's thought experiment. What is your point though?
My point is that the wires and the ignitor mechanism will guarantee the fireworks be lit up simultaneously. and Person T will disagree with a sure thing. Is there anything wrong with person T's time?

PeterDonis
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My point is that the wires and the ignitor mechanism will guarantee the fireworks be lit up simultaneously.

No, they won't. They will only guarantee that the fireworks be lit up simultaneously in the rest frame of person G.

Your belief that the wires and mechanism impose some kind of "absolute" simultaneity is because you are failing to recognize that your statement of the scenario makes hidden assumptions that are not absolutely valid; they're only valid in the rest frame of person G.

Nugatory
Mentor
My point is that the wires and the ignitor mechanism will guarantee the fireworks be lit up simultaneously.
They don't guarantee that, except in the frame where the fireworks and igniter mechanism are at rest.

Assume that the ignition signal is travelling at speed ##c## in the wires (that's not necessary, but it simplifies the analysis). In a frame in which the fireworks and igniter mechanism are moving, one of the two fireworks is moving towards the approaching signal and closing the distance while the other one is moving away from its signal and increasing the distance. Thus, in that frame the time for the signal to reach one is different than the time for it to reach the other - so the mechanism does not guarantee simultaneity.

Dale
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Assume that the ignition signal is travelling at speed ccc in the wires (that's not necessary, but it simplifies the analysis).
@Jianping Zhang if you assume that the signal moves at some v<c with respect to the wire then relativistic velocity addition makes sure that it also works out.

They don't guarantee that, except in the frame where the fireworks and igniter mechanism are at rest.

Assume that the ignition signal is travelling at speed ##c## in the wires (that's not necessary, but it simplifies the analysis). In a frame in which the fireworks and igniter mechanism are moving, one of the two fireworks is moving towards the approaching signal and closing the distance while the other one is moving away from its signal and increasing the distance. Thus, in that frame the time for the signal to reach one is different than the time for it to reach the other - so the mechanism does not guarantee simultaneity.
Thanks all for your kind reply. Very inspiring. I also know that Person T will see the two fireworks happened at different time. But I kind of believe it is his transformed view of the world. Not the true world reality. For me it would be very hard to believe that if I drive a car, then the who universe changes just because I drive my little car, even if I have energy to boost my car to 0.98C, I still have no energy to change the whole universe to my special one. But of course, my view to the universe will be transformed due to my speeding. If this is understandable to you all, then, why there is no such a thing as true reality? If that is really relativity theory is about, then it is very similar to the quantum multi-world picture. In short, I still believe that you drive your little rocket, you can change your view of the universe, but you cannot change the universe reality.

jbriggs444
Homework Helper
I still believe that you drive your little rocket, you can change your view of the universe, but you cannot change the universe reality.
You are correct that changing your view of the universe -- i.e. selecting a coordinate system in which you are currently at rest is not really changing the universe. However, that is not the same thing as saying that there is one true and correct coordinate system. All coordinate systems are arbitrary. None are singled out as the one true and correct coordinate system.

Nugatory
Mentor
But I kind of believe it is his transformed view of the world. Not the true world reality.
Tell me how you determine that T is the one who is really moving while G is really not moving, as opposed to T being at rest while G is moving? Before you answer that question, consider that although G is standing still on the surface of the earth, that surface is moving with the earth's rotation and orbit around the sun, and that the sun is moving with the galaxy's rotation and drift through interstellar space... so why is G any more at rest than T?

If you cannot tell me which of T and G are really moving, you will not be able to say which of them has the real view of the world and which has the transformed one. The key insight that both viewpoints are equally real and neither is to be preferred over the other dates all the way back to Galileo.

Dale
You are correct that changing your view of the universe -- i.e. selecting a coordinate system in which you are currently at rest is not really changing the universe. However, that is not the same thing as saying that there is one true and correct coordinate system. All coordinate systems are arbitrary. None are singled out as the one true and correct coordinate system.
If no one can change the universe by speeding, then why there is no such a universe reality where in that reality things happen in its true order?

This is even more like quantum world now. You look, you change the world.

Tell me how you determine that T is the one who is really moving while G is really not moving, as opposed to T being at rest while G is moving? Before you answer that question, consider that although G is standing still on the surface of the earth, that surface is moving with the earth's rotation and orbit around the sun, and that the sun is moving with the galaxy's rotation and drift through interstellar space... so why is G any more at rest than T?

If you cannot tell me which of T and G are really moving, you will not be able to say which of them has the real view of the world and which has the transformed one. The key insight that both viewpoints are equally real and neither is to be preferred over the other dates all the way back to Galileo.
I need not to tell who is at rest. everyone is in a jar, moving at his frame. so he will have his view of the world. but what is the jar's view of the world?

phinds
Gold Member
If no one can change the universe by speeding, then why there is no such a universe reality where in that reality things happen in its true order?
There IS NO "true order". That's what everyone is trying to help you understand.

Nugatory
Mentor
I need not to tell who is at rest. everyone is in a jar, moving at his frame. so he will have his view of the world. but what is the jar's view of the world?
But there is no jar.... if there were it would be another thing in space so would be at rest relative to itself, moving relative to other things.

It is very tempting to return to your triggered fireworks experiment and say that because both wires have the same length, the frame in which the explosions happen simultaneously is the real one, the one that is truly at rest, the one that we can use to define the point of view of the hypothetical jar. But we can set up two such experiments, one on Mars and one on Earth. Mars and Earth are moving at several kilometers per second relative to one another. There's a frame in which the two explosions on Mars are simultaneous and the two on earth are not, and vice versa (as well as many more frames i which neither are simultaneous). There is no frame in which both are simultaneous. So how would ever determine when we were in the frame in which the jar was at rest?

Dale
There IS NO "true order". That's what everyone is trying to help you understand.
you mean everyone sees an image through his distorted view, but there is not a real object for those images?

phinds
Gold Member
you mean everyone sees an image through his distorted view, but there is not a real object for those images?
You are misinterpreting what I said. You need to study the relativity of simultaneity. Objects are real and events are real but the ORDER of events is frame dependent.

For example, take the two events "An anvil drops on your head" and "a bomb goes off next to me". ALL frames of reference will agree that both events happened, but the ORDER in which they happened will be frame dependent.

Dale
But there is no jar.... if there were it would be another thing in space so would be at rest relative to itself, moving relative to other things.

It is very tempting to return to your triggered fireworks experiment and say that because both wires have the same length, the frame in which the explosions happen simultaneously is the real one, the one that is truly at rest, the one that we can use to define the point of view of the hypothetical jar. But we can set up two such experiments, one on Mars and one on Earth. Mars and Earth are moving at several kilometers per second relative to one another. There's a frame in which the two explosions on Mars are simultaneous and the two on earth are not, and vice versa (as well as many more frames i which neither are simultaneous). There is no frame in which both are simultaneous. So how would ever determine when we were in the frame in which the jar was at rest?
could space itself be the jar?

PeterDonis
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2020 Award
Objects are real and events are real but the ORDER of events is frame dependent.

More precisely, the order of spacelike separated events is frame dependent. The fireworks going off in this scenario are spacelike separated events, which is why their order is frame dependent.

phinds
Gold Member
could space itself be the jar?
No.

Nugatory
Mentor
then why there is no such a universe reality where in that reality things happen in its true order?
The "true order" is that all causes must happen before their effects. If you try playing with the relativity of simultaneity, you will find that all observers everywhere agree about this order (as long as nothing can travel faster than light, which we already know is impossible).

For example, different observers may disagree about which explosion happens first in your setup, but all observers will agree that the signal leaves the center point before either explosion.

PeterDonis
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2020 Award
could space itself be the jar?

There is no jar.

But I kind of believe it is his transformed view of the world. Not the true world reality.

Your belief is wrong. There is no "true world reality" in the sense you are using the term. There is a "true reality", but in that true reality, the ordering of some pairs of events (any pair of events which are spacelike separated) will depend on the observer's state of motion.

This is even more like quantum world now. You look, you change the world.

Observers in different states of motion are not changing the world by looking. They are just seeing the same world--the same spacetime--from different vantage points. It's like two people looking at the same object from different directions and seeing the object from different perspectives. The object itself doesn't change.

The difference between relativity and the ordinary change in perspective I described just now is that in relativity, you "change your perspective" by changing your state of motion; and the different perspectives on the same spacetime include changes in things that, in the ordinary case of a change in perspective, would not change--like the ordering of spacelike separated events. It's counterintuitive, but experiments have shown that that is, indeed, the way things work. So we just have to learn to deal with it.

Dale
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2020 Award
But I kind of believe it is his transformed view of the world. Not the true world reality.
How do you know his view is not correct and the ground is the transformed one. After all, we know that the ground rotated around the earth’s axis and the earth revolves around the sun and the sun revolves around the galaxy and so forth. So what justification do you have for claiming that the ground is true reality and the train is not?

The fact is that there is no such justification. The laws of physics are equally valid in all inertial frames. No experiment can distinguish one inertial frame from another in an absolute sense, despite more than a century of attempts to measure such an effect with ever more precise experiments.

If you believe that there is a “true reality” (personally I think it is a scientifically useless concept) and if you stipulate that “true reality” cannot depend on the reference frame then the correct conclusion is that simultaneity is not part of “true reality”. Instead, causality is what “true reality” uses.

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