Relativity of simultaneity and the balance of the systems

[PeterDonis]

this is more of a semantics than physics

If you think that, you need to really take a step back and review your physics.

A group of people jumping at the same time is one event

No, it isn't. An "event" is a point in spacetime. Each individual person's jump takes place at a different point in spacetime. Therefore they are different events.

This is not just "semantics" because the separation in spacetime between different events has physical implications: it means that there cannot be any direct "connection" between them. Any "connection" between different points in spacetime can only propagate causally, i.e., at the speed of light. That means that two events that are spacelike separated--which any pair of events which are simultaneous in any frame must be--cannot be causally connected.

Your entire analysis ignores this critical limitation. Ibix's responses are valid and you should really take some time to read and understand them.

Think about the energy produced and the way it is distributed.

Exactly: think about how the energy produced at one event (one individual person jumping) will propagate through spacetime. It must do so no faster than the speed of light. Again, take some time to read and understand Ibix's analysis; he is doing exactly what you describe, but he is doing it correctly, taking into account the separation in spacetime between different events.

 

[Nugatory]

Therefore it is my understanding that simultaneous events should be treated as one event.
Well, this is how I see it, but I'm here to learn, not to teach

An event is defined as a single point in spacetime. Thus, if two things happen at the same place and time that counts as one event. However, if they happen in different places that's always going to be two events; if the two events happen at different places but have the same time coordinate in some frame then we say that they are simultaneous in that frame.

 

[Sisoeff]

Thanks for the time taken to answer, Peter.
I'm not going to argue whether simultaneity is one or more events.
I just gave my understanding about it, and I apologize if my comment appeared like I was trying to teach you or enforce my opinion on you.
Lets focus on my question, and if you can help me, I'll be gone for the next few years

Would you tell me what do you think about this:
Simultaneous events in one inertial frame are producing energy.
The simultaneity is not present in another inertial frame.
Where does the energy go?

Ibix answer was:

If they "produce energy" in one frame they produce energy in all. Unless you have a detailed countexample in mind...?

It is a bit vague answer, but if understand it correct, then simultaneity is present in all frames. It is just seen differently. Which makes the relativity of simultaneity false.
Is that what are you guys saying?

 

[PeterDonis]

Would you tell me what do you think about this:

I think just what I said before: you are misunderstanding the physics because you are not taking into account the spacetime separation between events.

if understand it correct, then simultaneity is present in all frames. It is just seen differently. Which makes the relativity of simultaneity false.
Is that what are you guys saying?

No. We are saying that there is not one "energy" being produced by all the different people jumping at different events. If there are N people jumping, then there are N "packets" of energy being produced, each one at a different event in spacetime. Those N packets of energy are separate and you can't think of them as one thing; they aren't. The fact that the jumps are all simultaneous in one particular frame is irrelevant; they are still N separate things.

 

[Sisoeff]

Well, let me put it in a simple way, as I understand it, and then you tell me where am I wrong.
I see energy as information.
Different amount of energy transferred in certain time holds different information.
If in inertial frame A two simultaneous events release 2 joules of energy, and they are transferred to another inertial frame B in 1 second, that information would reveal part of the properties of frame A. That property would be: Inertial frame A releases 2 joules of energy per second.
Note that if frame B receives 1 joule per second because simultaneity does not exist in this frame, that information would be different and won't be part of the property of frame A.
If the simultaneous events in frame A do not exist in frame B, but frame B still gets the same information (2 joules per second), that would mean, that simultaneity exist, but is not seen in as such.
Hope that this make sense, and you'll have an easy way to guide me to better understanding.

 

[PeterDonis]

If in inertial frame A two simultaneous events release 2 joules of energy, and they are transferred to another inertial frame B in 1 second

Energy doesn't get transferred between frames; that makes no sense. Frames are not physical things; they're abstract tools we use to analyze the behavior of physical things. Energy gets transferred between events. Those events can be described in frame A or frame B.

that information would reveal part of the properties of frame A

Energy is not a property of a frame; frames don't have "properties" in this sense. Energy is a property of an object. If an event releases energy, that energy gets carried by something: a moving billiard ball, a pulse of radiation, etc. The "transfer" of energy is just the motion of whatever is carrying the energy.

If the simultaneous events in frame A do not exist in frame B

Events always "exist" in every frame; they just aren't simultaneous in every frame.

Hope that this make sense

Not really. See above.

you'll have an easy way to guide me to better understanding

The best advice I can give you is to stop thinking in terms of "frames" and start thinking in terms of objects and events and how they are related in spacetime. If we have two events that each release a pulse of energy, what carries that energy? Where does it go? What other objects will receive it? And how are all these things connected in spacetime? (Hint: two events being simultaneous in some particular frame is not a "connection" in this sense; it has no physical meaning.) If you analyze things in these terms, you will see that energy never appears from nowhere or disappears into nowhere; all it does is change form.

 

[andrewkirk]

If we put back the garage in an inertial frame

I think a tractable, reasonably similar, experiment can be constructed using only inertial frames.

Consider the garage sitting in free fall in empty space, far from any gravitational field. Call the inertial frame in which the garage is initially stationary F. Then at at time t1 in frame F, both garage doors open. Let's assume they slide up (in positive y direction), rather than rotating, to make the motion as simple as possible. Then the garage will start moving downwards (in negative y direction) in F and come to a rest (in F) at time t2 in F when the doors come to rest fully open. The centre of mass of the garage will not have moved in F, and its orientation will not have changed at any point - it will always be parallel to the x axis. But after the door-opening the garage floor will have a lower y coordinate than it originally had.

Now consider the inertial co-moving frame F' of a body (a ladder, if you like) that is flying past the garage (no need to go through it - it makes no difference) in the positive x direction. In that frame the two doors do not start opening simultaneously, but instead start opening at times $t'_{front}$ and $t'_{back}$ where $t'_{back}>t'_{front}$. The door that has a larger x coordinate is 'front'. The doors finish opening at times $t'_{front}+\gamma(t2-t1)$ and $t'_{back}+\gamma(t2-t1)$ respectively.

My initial expectation from that would be that, in frame F', the garage appears to tilt, with the front end first tilting down, then it starts tilting back when the other door starts opening, and ends up with no y-direction velocity in F' and parallel to the x-axis in F'.

That is, it ends up with the same configuration in F' as it would have if the two doors opened simultaneously in F'. But its velocity, angular velocity and location is different between when the first door starts opening and second door finishes opening.

That sounds a bit weird. I expect I've got something wrong, because things like tilting seem less likely when both frames are inertial.

But I've got to run now, so I'll have to leave it to later to think more about what would really be going on.

 

[Sisoeff]

Peter, there is a better way to answer such a simply put explanation, which if not else, it gives you the idea.
Diving in details to show how much I am far from the scientific terms, after I already said that I'm not a physicist, brings the feeling of trolling (no offense, just saying)
Thank you for giving me some understanding, which helps me to correct the way for presenting the idea.

I know that frame are not physical things :) By saying "transfer to another frame" I refer to whatever is that moving object. I thought that this is acceptable way of expression and if not correct it is at least easy to understand.
I'll change that.
You don't agree with "simultaneous events do not exist", but then you say that the events exist but not as simultaneously. Isn't that the same thing?
OK, then I'll use "simultaneity" instead.
You say that the energy is not a property of a frame, but I did not say that.
I said that the property is "releases energy". Or perhaps I should use another word instead of "property"? Perhaps you could help me with that, please.
Furthermore, I'd like to thank you for your advise, and to mention that I know about energy transformation and energy preservation.
I don't understand why must I stop thinking in terms of frames if the core of the problem (which I see as such for now) is in the simultaneity in different reference frames.

So, let me clean up my presentation:

I see energy as information.
Different amount of energy released in certain time holds different information.
If in inertial frame A two simultaneous events release 2 joules of energy per second, this is an information which reveals part of the properties of frame A. That property would be: Inertial frame A releases 2 joules of energy per second. That information must be present for inertial frame B.
Note that if frame B receives/has information for 1 joule per second because simultaneity does not exist in this frame, that information would be different and won't be part of the property of frame A.
If the simultaneity in frame A do not exist in frame B, but frame B still gets the same information (2 joules per second), that would mean, that simultaneity exist, but is not seen as such.

Tried to correct all mentioned flaws in the scientific terms used for this explanation.
And I really, really appreciate Peter's notes and help.
Thanks, Peter.

 

[Ibix]

Would you tell me what do you think about this:
Simultaneous events in one inertial frame are producing energy.
The simultaneity is not present in another inertial frame.
Where does the energy go?

Ibix answer was:

If they "produce energy" in one frame they produce energy in all. Unless you have a detailed countexample in mind...?

It is a bit vague answer, but if understand it correct, then simultaneity is present in all frames. It is just seen differently. Which makes the relativity of simultaneity false.
Is that what are you guys saying?

I meant that your definition of simultaneity is bizarre, and at odds with the basic physics: if two things produce energy they produce energy, no matter how you are moving when you look at them (which is one way of looking at a choice of frame).

"Simultaneous" just means that two events happen at the same time. Special relativity makes this more complex because not everyone agrees on what "the same time" means. That's all. It's absurd to ask "where does the energy go" because either the two events are producing energy (whether you see them as simultaneous or not) or they are not (whether you see them as simultaneous or not).

 

[Ibix]

If in inertial frame A two simultaneous events release 2 joules of energy per second, this is an information which reveals part of the properties of frame A. That property would be: Inertial frame A releases 2 joules of energy per second. That information must be present for inertial frame B.
Note that if frame B receives/has information for 1 joule per second because simultaneity does not exist in this frame, that information would be different and won't be part of the property of frame A.

It isn't at all clear where you are getting these numbers from, or what process you think you are modelling. Energy measurements can be different between frames, but this is not a consequence of simultaneity or lack thereof, but the frame does not release energy or do anything with energy. A frame is just a choice of coordinates. A choice of coordinates can can't release energy.

 

[PeterDonis]

Diving in details to show how much I am far from the scientific terms, after I already said that I'm not a physicist, brings the feeling of trolling (no offense, just saying)

Whether or not you're a physicist is irrelevant; we're discussing physics here, so we use physics to evaluate people's proposed thought experiments. That's what I'm doing. The issue isn't with your terminology; the issue is with your understanding of the physics.

By saying "transfer to another frame" I refer to whatever is that moving object.

What moving object? You haven't included any actual moving object in your description of the scenario. All you've included are two "frames" and a bunch of events at which some energy is "released".

I don't understand why must I stop thinking in terms of frames if the core of the problem (which I see as such for now) is in the simultaneity in different reference frames.

Simultaneity is not a physical thing. Focusing on it is only confusing you. You should stop thinking about simultaneity and start thinking about events and objects and how they are connected in spacetime.

Tried to correct all mentioned flaws in the scientific terms used for this explanation.

You didn't; I see no substantive difference between your "cleaned up" version and your previous version. It still has all of the same problems.

 

[PeterDonis]

in inertial frame A two simultaneous events release 2 joules of energy per second

Where does this energy come from? Does it get created out of nothing? Or is it just that some energy that already existed changes form? (Hint: the correct answer is the latter.) And if energy didn't get created out of nothing, but just changed form, what does that say about what energy is "present" at a given instant of time in any inertial frame?

 

[Sisoeff]

@Ibix sorry I didn't answer your comments until now, but I have the feeling that you don't want to look at the idea, and instead you are enlightening the unimportant details.
Any way, let's give you the idea.

There is output from every event.
Should I say, every event as cause, produces an effect.
You got the idea, right?
Two simultaneous events can produce one output (simultaneously touching two sides of a metal plate acts like switch and puts on the lights)
If the events are not simultaneous, no light.
So, in frame A, two simultaneous events, put on the lights.
Lights are part of the properties of frame A.
They also serve as information about the identity of frame A.
The lights also tell you that there are simultaneous events on frame A, which are the cause for the light being present.
If you see the lights, you know that this is frame A. The lights are kind of its signature.
If they are seen in B, then simultaneity is present in frame B, but not seen as such.
If the lights are not seen in B, then B does not see frame A, but C, D... or any other frame.
Why? Simply because in frame A the lights are on.

Please let me know if you understand the idea.
If you understand the idea, but don't like the way I put it together, please help me put it together in more understandable from scientific point of view way.
If you don't understand it, I'll try again, but I don't have too much time

 

[Ibix]

If the lights are on they are on. Your choice of frame does not and cannot change this. This is a fairly straightforward concept.

Two observers might have different reasons for explaining why the light is turned on or they might not - it depends on details of the experiment that you have not given. However, that the lights turn on or not is a frame-independent fact.

None of this is "unimportant details". It's requiring that your scenarios have basic self-consistency. If it doesn't bother you that your scenarios make no sense then that's fine - but it's not science you are talking about.

 

[PeterDonis]

Two simultaneous events can produce one output

No, they can't. They produce two causal "signals", which both propagate to some other location where the output is produced. In your example, the two events would be two ends of a switch touching the two metal plates to close a circuit. But closing the circuit doesn't magically turn on the lights; all it does is allow energy to flow through the circuit from the power source to the lights. That takes time, and the switch has to stay closed while it's happening. So your analysis of this scenario is very incomplete; you haven't taken into account the flow of energy in the circuit that actually turns the lights on.

 

[Sisoeff]

Well, it is logic, and science requires logic.
You didn't tell me whether you understand the idea.
Saying that "If the lights are on they are on." does not gives me any understanding.
What do you mean by that?
What is missing in the scenario.
The fact that the observer does not know why the lights are on, or that they should be on, doesn't change the given scenario.
I already set it for you, and you know that lights goes on as a result of two simultaneous events.
I started from the ladder paradox, which explains relativity of simultaneity, and I'm explaining why I see problem in that explanation.
For better understanding we changed the scenario few time, not in great details, but I thought that it is enough for you guys to understand it.
Non of you even told me what you don't understand, and I don't feel like you have the intention to help me.
Sorry!

 

[Sisoeff]

No, they can't. They produce two causal "signals", which both propagate to some other location where the output is produced. In your example, the two events would be two ends of a switch touching the two metal plates to close a circuit. But closing the circuit doesn't magically turn on the lights; all it does is allow energy to flow through the circuit from the power source to the lights. That takes time, and the switch has to stay closed while it's happening. So your analysis of this scenario is very incomplete; you haven't taken into account the flow of energy in the circuit that actually turns the lights on.

Are you serious?
Wow!

 

[russ_watters]

Well, let me put it in a simple way, as I understand it, and then you tell me where am I wrong.
I see energy as information.
Different amount of energy transferred in certain time holds different information.
If in inertial frame A two simultaneous events release 2 joules of energy, and they are transferred to another inertial frame B in 1 second, that information would reveal part of the properties of frame A. That property would be: Inertial frame A releases 2 joules of energy per second.
Note that if frame B receives 1 joule per second because simultaneity does not exist in this frame, that information would be different and won't be part of the property of frame A.
If the simultaneous events in frame A do not exist in frame B, but frame B still gets the same information (2 joules per second), that would mean, that simultaneity exist, but is not seen in as such.
Hope that this make sense, and you'll have an easy way to guide me to better understanding.

I think what you are missing is that the observation that two events are simultaneous can be agreed upon by all if everyone understands how relativity works. If an observer at A detects two simultaneous events and releases some energy, an observer at B will be able to use his/er understanding of relativity to calculate that two events which B did not observe simultaneously A did observe simultaneously.

Your way of thinking is a bit like standing at a train station confused about how a train can get there at 10:00 if it is somewhere else at 11:00 -- as long as you know how to read the schedule, it shouldn't confuse you!

 

[PeterDonis]

Are you serious?

Yes, I'm quite serious. If you are going to really try to think about how "energy" is accounted for in different frames, you have to actually account for all of it, where it is and how it flows. Anything less will lead you to erroneous conclusions--as you have illustrated multiple times now in this thread.

The correct conclusion is that energy is locally conserved, i.e., it can never be created or destroyed at any point in spacetime. That means that in any frame, the "total energy", meaning the energy at each spatial point added up at a given instant of time in that frame, is conserved. But you have to actually look at where each piece of energy is and how it moves, in spacetime, to arrive at this correct conclusion.

 

[PeterDonis]

I don't feel like you have the intention to help me.

We would like to help, but you need to pay attention to what we are actually saying. What we are actually saying is that the way you are looking at things is fundamentally wrong. (The fact that you had to ask me whether I was "serious" about my description of the light switch scenario, when my description was a simple and obvious consequence of relativity, is an indication of this.) I realize this is not what you wanted to hear, but unfortunately that's what we are saying.

A few small tweaks to your conceptualization of your scenario won't help. You need to forget everything you think you know about "frames" and "simultaneity" and so forth, and start fresh, focusing on the actual physics: actual, physical objects that contain and transport energy, and where they are and how they move in spacetime.

 

[jartsa]

Haha
As I already said, there are countless examples (at leas to my understanding of the problem) that can be used.
Let's focus on only one

Let's focus on the example with a space station with unfolding solar panels.

Ah but now I want to change it to a space station with panels that are first transparent and then become opaque, liquid crystal panels or similar.

May I do this last change pretty please?

The panels bend a little bit when they become opaque and start absorbing photons. One panel bends first, then the other, then the panels are equally bent, that's what happens in the frame where the space station is in motion.

The photons impart momentum to the panels, that momentum travels to the space station through metal rods or something.

Ok now I have a question:

If in the frame where the space station is in motion two photons hit simultaneously the two panels, does the space station feel the two impulses simultaneously or non-simultaneously?

 

[Sisoeff]

I think what you are missing is that the observation that two events are simultaneous can be agreed upon by all if everyone understands how relativity works. If an observer at A detects two simultaneous events and releases some energy, an observer at B will be able to use his/er understanding of relativity to calculate that two events which B did not observe simultaneously A did observe simultaneously.

Your way of thinking is a bit like standing at a train station confused about how a train can get there at 10:00 if it is somewhere else at 11:00 -- as long as you know how to read the schedule, it shouldn't confuse you!

Thank you, Russ.
You are right that an observer in B can calculate and find out that there are two simultaneous events which release energy, or put the lights on.
(Let's use "lights on" instead of energy, please. Peter made me feel uncomfortable with "release of energy" output)
Must I understand that it is OK for science to have an effect without having corresponding cause in the reference frame?
When we observe the Universe and we don't understand certain effect, can we assume, that the cause of the effect lies in another frame of reference, but which one we would never know.
To me it looks like an open door for great speculations and miss interpretations.

 

[Ibix]

Well, it is logic, and science requires logic.
You didn't tell me whether you understand the idea.
Saying that "If the lights are on they are on." does not gives me any understanding.
What do you mean by that?
What is missing in the scenario.
The fact that the observer does not know why the lights are on, or that they should be on, doesn't change the given scenario.
I already set it for you, and you know that lights goes on as a result of two simultaneous events.
I started from the ladder paradox, which explains relativity of simultaneity, and I'm explaining why I see problem in that explanation.
For better understanding we changed the scenario few time, not in great details, but I thought that it is enough for you guys to understand it.
Non of you even told me what you don't understand, and I don't feel like you have the intention to help me.
Sorry!

I explained what was wrong with your analysis of your balanced barn problem in my first post on this thread. Peter has pointed out the problem with your "touching two sides of a piece of metal" version of the problem. The general point is that it takes time for the effects from an event to propagate, and that ends up meaning that consequences such as "does the barn tip?" are frame indepedent. Everyone agrees that the barn does not tip. They don't necessarily agree on the sequence of all the things that lead to that result. But they do agree on what all those things are, and they agree on the final consequence. If the barn tips according to one observer it tips according to all. Or in my slightly pithier language from earlier, if it tips, it tips.

The general issue with all of your scenarios is that you seem to think that you can build something that will only react to simultaneous events. You can't. You can pick a frame and build something that will only react to events that are simultaneous in that frame. However, if you analyse the device in any other frame you will find that it reacts to events that are separated by some time $\Delta t$, and that $\Delta t$ is exactly the time diffrrence you find in this frame between events that are simultaneous in the first frame.

So yes, I understand your scenario descriptions. I also understand what is impossible about them. I already explained the correct analysis of one of them and Peter has explained another. We are happy to help, but you have to read the help we provide.

 

[Ibix]

Thank you, Russ.
You are right that an observer in B can calculate and find out that there are two simultaneous events which release energy, or put the lights on, which he doesn't see.
(Let's use "lights on" instead of energy, please. Peter made me feel uncomfortable with "release of energy" output)
Must I understand that it is OK for science to have an effect without having corresponding cause in the reference frame?
When we observe the Universe and we don't understand certain effect, can we assume, that the cause of the effect lies in another frame of reference, but which one we would never know.

Every effect has a cause. Observers don't necesarily agree on the order of all of the events leading up to something (except those that are causally related themselves), but they agree that all of the same things happened.

Your question about which frame something happened in has no meaning. A frame is just a point of view. Things happen. They don't "happen in a frame". It's like asking whether that chair exists when I'm moving. Or implying that it exists for you (because you are stationary) but not me because I'm walking. That's not a description of the universe we live in.

Please see my last post. You seem to think itvis possible to detect simultaneity. You can only pick a frame and detect whether two events were simultaneous in that frame.

 

[A.T.]

does the space station feel the two impulses simultaneously or non-simultaneously?

What does this mean?

 

[Nugatory]

Must I understand that it is OK for science to have an effect without having corresponding cause in the reference frame?
When we observe the Universe and we don't understand certain effect, can we assume, that the cause of the effect lies in another frame of reference, but which one we would never know.
To me it looks like an open door for great speculations and miss interpretations.

You are misunderstanding what a frame is - a frame is no more than a convention for assigning coordinates to events. When I say that a chair is one meter to my left, and someone else says that the chair is two meters to the right of the table, we are using different frames. Thus, it makes no sense to say that something "lies in another frame", or is "in" this frame but not that one. Everything is always "in" all frames always, just as the chair is there no matter what numbers I use to describe its position.

 

[Nugatory]

If in the frame where the space station is in motion two photons hit simultaneously the two panels, does the space station feel the two impulses simultaneously or non-simultaneously?

The impulses travel through the structure of the space station at the speed of sound. In general, they will reach different points in that structure at different times because they have to travel different distances from the points of impact.

However, if both impulses arrive at a given point at the same time according to measurements carried out using anyone frame, they will arrive at that point at the same time using measurements carried out using all frames.

 

[jartsa]

What does this mean?

By this question:

If in the frame where the space station is in motion two photons hit simultaneously the two panels, does the space station feel the two impulses simultaneously or non-simultaneously?

I mean this:
We have a moving rod. Two sound waves start to propagate towards the center of the rod when the two ends of the moving rod are hit simultaneously by two photons, simultaneously in the frame where the rod is moving. The two hits are not simultaneous in the rod frame, because that frame where the hits are simultaneous is the frame where the rod is in motion. Do those sound waves that started off at different times in the rod frame reach the center of the rod at the same time?

As the answer to that question is no, we conclude that the two sound waves spend different times traveling from one end of the rod to the center of the rod, in that frame where the two ends where hit simultaneously, and as we remember that frame was the frame where the rod was in motion.

 

[russ_watters]

Must I understand that it is OK for science to have an effect without having corresponding cause in the reference frame?
When we observe the Universe and we don't understand certain effect, can we assume, that the cause of the effect lies in another frame of reference, but which one we would never know.

Of course! Most of what happens in the universe doesn't happen here, we only observe it from here!

To me it looks like an open door for great speculations and miss interpretations.

I don't see how - as long as you keep the reference frames straight and don't play loose with the wordings of the descriptions, it really is not difficult to interpret relativity of simultaneity. Most of your scenarios suffer from the same problem: you are wording them poorly which makes them tough to interpret. What bothers me is that you don't seem to care about getting them right (and are actually getting upset that people are trying to help you correct them! ) before jumping to the next poorly worded scenario.

 

[Nugatory]

By this question:I mean this:
We have a moving rod. Two sound waves start to propagate towards the center of the rod when the two ends of the moving rod are hit simultaneously by two photons, simultaneously in the frame where the rod is moving. The two hits are not simultaneous in the rod frame, because that frame where the hits are simultaneous is the frame where the rod is in motion. Do those sound waves that started off at different times in the rod frame reach the center of the rod at the same time?

Whereever in the rod the sound waves meet, it will be the same in all frames. See post 57 above.

Do remember, however, that the speed of the sound waves is different in different frames.