# The Relativity of Simultaneity

Discussion in 'Physics & Math' started by Motor Daddy, May 12, 2010.

1. ### Motor DaddyValued Senior Member

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Light travels independently of the train, at c. The time it takes the light to travel to the train observer's midpoint position is different for each strike. It is true that the train observer remains at the midpoint of the train, but when opposite ends of the train have light sources, and the lights are simultaneously activated (easily done with two lights and a common switch at the midpoint, activated by the observer), during the time it takes for light to travel the half length of the train, the observer also travels away from one light, and towards the other. That creates a situation that the light from one source has less distance to travel than the light from the other source, until it impacts the observer.

I've already showed why synchronized clocks will appear different to the observer on the train, which is a red flag as to the observer's actual velocity, unlike the embankment observer's actual zero velocity. You can not say the train is at rest and that it is actually the embankment that has a velocity (or any other combination of the two), due to the fact that the strikes occurred as the points lined up, and the light from the strikes impacted the embankment observer at the same time, and the train observer at different times. That is not a reversible situation. The points MUST be lined up. I explained the difference with the clock phenomena, which is also a true indicator of a zero velocity.

Last edited: May 12, 2010

3. ### rpennerFully WiredValued Senior Member

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Inability to comprehend relativity is the inability to put oneself intellectually into the shoes of another and working out the consequences. It seems to me that this belongs on the non-clinical side of a spectrum of sociopathic disorders. http://www.squidoo.com/the-sociopath-next-door

Motor Daddy, The embankment is moving from:
1) The train's perspective
2) The detailed perspective of people in the next county
3) The obvious perspective of people in the antipodes.
4) The perspective of the Moon
5) The perspective of Mars
6) The perspective of the Sun
7) The perspective of the center of the Galaxy
8) The perspective of a hypothetical observer who observes no dipole anisotropy in the cosmic microwave background.

So billions of people on Earth and nearly any other hypothetical observer attached to a massive object anywhere in the universe believe the embankment to be in a non-rest state of motion. Why do you prefer one observer's absolutely over the other's? Their local experiments do no reveal their state of motion and their situations differ only in degree.

Indeed, if you are willing to replace the train and embankment with equal-mass planets counter-rotating and making a close approach, the relationship can be made as arbitrarily symmetrical as desired, and your choice of one observer other the other becomes not only unphysical but specious.

5. ### Motor DaddyValued Senior Member

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Can we stick to Einstein's example?

Are you saying that the light from the back of the train to the observer travels the same distance as the light from the front of the train to the observer?

7. ### rpennerFully WiredValued Senior Member

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I'm saying different folks will have different evidence-based viewpoints what the distance between events is, and that special relativity not only tells you that all inertial evidence-based viewpoints are equivalently valid but how to transform one viewpoint to another -- to place one observer in the other observer's shoes.

Further, it works better as demonstrated by experiments as far back as 1859 than the previous notion of Galilean Relativity.

8. ### Motor DaddyValued Senior Member

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The train observer's viewpoint of the strikes occurring at different times is invalid. That is simply impossible, as the points have to be lined up when the strikes occur, and there is only ONE point in time when they line up. In order for the strikes to have occurred at different times, the points would have to have two different reference points, and that simply isn't the case. The points were lined up when both strikes occurred. That is an indisputable fact for any observer in this universe.

Sure, the lights hit the train observer at different times, but that is not when the strikes occurred, that is after the strikes occurred.

9. ### rpennerFully WiredValued Senior Member

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Nope. You are asserting absolute time but not arguing for it. Neither are you making an evidence-based argument.

You may be confused about the content of Einstein's argument.

All observers agree that events that happen at the same place and same time happen at the same place and at the same time. (The opposite would be maddening if not madness.) Coordinate-wise, these two things happen at the same coordinates in space an time. By a slight misuse of ordinary language we can refer to any quadruple of space-time coordinates as an "event" even if nothing happens there. But events that one observer sees happen in the same place at different times, another may say happen in different places with a non-zero distance between them for a second observer. (Even Galileo agreed with this.) And since 1859 we have experimental evidence that says two events which happen in different places may legitimately be seen by one observer to happen at the same time may equally as legitimately be seen by another observer to happen at different times.

But while neither the distance between two events, $\Delta x$, nor the time interval between two events, $\Delta t$ is seen the same by all observers, the spacetime interval $\left( \Delta x \right)^2 - \left( c \Delta t \right)^2$ between the two events is universally agreed upon. (You should be able to do the math that this statement also says all observers agree on the speed of light. )

10. ### Janus58Valued Senior Member

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2,285
All the points line up at one instant only according to an observer at rest with respect to the embankment. There is another effect of Relativity in play here" Length contraction. Anything with a motion relative to your own will be contracted in length along the line of motion.

Thus, if the Train were at rest with the embankment, it would be noted that it longer than the distance between the points on the ground where the lightning strikes occurred. While it is moving, it is measured as contracted from the embankment, causing it to fit exactly between the strike points.
However, from the train, it is the embankment that is contracted, and the distance between the strike points are closer together than the length of the train.
So when the front end of the train reaches the forward strike point, the rear of the train has not yet reached it corresponding strike point yet.

The simultaneous line up of all the points is not an indisputable fact for all observers.

11. ### Janus58Valued Senior Member

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2,285
You haven't shown anything. You are making a claim based on how you believe things would behave. Your argument is basically " Relativity of Simultaneity is wrong because Simultaneity is absolute."

Einstein uses a different set of postulates regarding the behavior of light. The concept of the Relativity of Simultaneity is a consequence of those postulates.

Put simply, you are saying that the universe behaves in one way, while Relativity say it behaves in another.

So, how do we know which is right? By performing an actual physical experiment. For example, we could take two synchronized clocks, but thema at the ends of a fast train and see if an observer sitting in the middle actually reads the same time on both or not.

Now this particular experiment might not be easy to pull off, but there are other, easier experiments that can test the same thing. These experiment have been performed with increasing accuracy since the 1800's, and in every case, the result come down on the side of Relativity.

So you have a choice:
You can stick to your guns even though every piece of actual physical evidence in existence says that you are wrong.
Or
You can man-up, admit to being wrong, and try to wrap your mind around what it means to live in a Relativistic universe.

12. ### AlphaNumericFully ionizedRegistered Senior Member

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6,702
I was speaking more generically, in that when a nay sayer makes his case its in terms of things which are pretty basic, they never phrase their arguments in terms of more advanced mathematics or physics. This suggests that they are either a) unaware there's more to relativity than just doing coordinate transformations, b) that they've only just come across relativity and so haven't advanced that far or c) they don't understand more advanced material, though often they'll claim the opposite.

For instance, Jack_ throws around words like 'decidable' yet can't use the word 'proof' properly and all his algebra is 1st year stuff (at best). He claims to understand the posts I make in regards to bundles yet he never uses the much more powerful formalism of bundles to make his case. Why make life harder for yourself if you have the tools at your disposal to strengthen and streamline your argument? Unless, of course, he doesn't understand.

This is false. As already explained to you such a result is in direct manifest contradiction to one of the postulates of SR and thus SR doesn't predict that the clocks will be different. And its a good job it does because that's in agreement with what experiments say!

Velocity with respect to whom? The ground? A plane in the sky? Jupiter? Motion is relative. If the train is on a perfectly smooth track so that it doesn't jiggle about when moving relative to the ground then alone straight horizontal lines of track the person on the train is in an inertial frame. He cannot work out his motion relative to the ground without looking out the window. His claim "I am in an inertial frame" is true and allows him to consider himself at rest. With the clocks being at rest with respect to him then their times will read the same from his point of view.

Consider the case where all the windows are blacked out so the person on the train cannot get any information outside of the train's interior. By your logic he can work out his velocity by comparing clock times, that the amount the clock times disagree by allows you to find his speed. But this is not possible as the fact he's in an inertial frame means he's 'blind' to the motion of things outside of the train. He can't tell if the train is stationary or moving relative to the Earth, provided that motion is constant. If a specific speed could be obtained then it would be associated to a preffered frame and even if that were the case there's absolutely no reason to think it would be the ground's rest frame (particularly given the Earth is not in inertial motion, it rotates).

You're wrong, as he's in an inertial frame. If you put two sync'd clocks at opposite corners of the room you're in right now and then sat in the middle you'd see them reading the same times, right? Right. After all you and they are stationary, the ground isn't moving underneath you, right? Wrong. They read the same times because you are stationary relative to them and in that frame you're at the midpoint between them. The motion of the Earth under your feet (or not) is irrelevant. After all, why pick Earth? What about the Moon, you're not stationary relative to the Moon, should the clocks tell you the relative motion between you and the Moon, as you claim it does between the train passenger and the Earth? Its the same setup, 2 clocks, an observer and some container within which they set which is moving relative to something else.

Unless you believe the universe has a prefered frame and that frame is some point of ground on the Earth's surface your argument falls apart, independent of whether or not it passes experimental tests.

You're describing the system from the point of view o an observer at rest with respect to the ground and made the mistake of thinking that the person on the train sees exactly the same sequence of events in the same time frames, which isn't the case. This is the counter-intuitive nature of special relativity, you've gotten stuck on the first big hurdle I'm afraid.

Sure, but I'd request you actually take the time to read his writings on the matter and then to learn the basics of special relativity because presently I'd say you lack the mathematical tools and conceptual understandings required to get your head around the counter-intuitive nature of the non-Euclidean geometry at play in relativity.

As I said previously, look at threads started by the poster Jack_ (if you aren't he) and you'll find this setup has been beaten to death, full Lorentz transform examinations have been done, including diagrams, all in an effort to explain to Jack what we're now going to end up explaining to you.

You really do need to spend some time finding out what SR is about, how its constructed and how it describes such systems, as you're currently displaying considerable naivety about it. If you don't know what SR actually says and why it says it it's a little unwise to try to argue that its wrong in what it says, particularly when those people you're trying to discuss SR with actually know some SR.

Given that comment we're now moving away from somewhat uninformed naivety to wilful ignorance and arrogance. You have simply utterly failed to understand relativity, either conceptually and quantitatively and given the pseudo-rhetorical question I quoted you saying just above suggests you haven't really made any effort to understand. Am I wrong in this evaluation, have you put in a fair chunk of time in order to try to understand SR? If not then its both silly and arrogant to say "Its obviously wrong, its impossible!" when you know full well you've not got a perfect understanding of the subject matter.

And you know this how? Have you done your own experiments to test this premise? If so, what was it and when did you do it? If not then explain why you think you're justified in making what seems to be an unjustified claim. Those people who have done experiments in relation to relativity have found relativity to be vindicated, the issue of simultaneity and motion altering clocks is fundamental in the design of the GPS network, as they are basically clocks which shine light (ie radio signals) at observers from various places in the sky.

So please explain why you think you have insight into the universe when you have not actually examined the universe yourself, you're ignoring those people who have and you haven't got much, if any, understanding of the models of said people.

I'll suggest to you what I suggested to Jack (among other things), go learn how to construct and understand space-time diagrams in 1+1 dimensional special relativity. The choice in frames then can be expressed as different time slices of a worldline/light cone diagram. It'll make your mistake a bit clearer.

13. ### Jack_BannedBanned

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You are correct, but your proof skills are lacking.

14. ### Jack_BannedBanned

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1,383
You're wrong, as he's in an inertial frame. If you put two sync'd clocks at opposite corners of the room you're in right now and then sat in the middle you'd see them reading the same times, right? Right. After all you and they are stationary, the ground isn't moving underneath you, right? Wrong. They read the same times because you are stationary relative to them and in that frame you're at the midpoint between them. The motion of the Earth under your feet (or not) is irrelevant. After all, why pick Earth? What about the Moon, you're not stationary relative to the Moon, should the clocks tell you the relative motion between you and the Moon, as you claim it does between the train passenger and the Earth? Its the same setup, 2 clocks, an observer and some container within which they set which is moving relative to something else.

This is your best post yet to validate the OP's argument.

The train observer is sitting with clocks at the flash points and they read the same time since they are in the frame.

The embankment observer is sitting with co-located clocks at the flash points and they read the same time.

Light is c in all frames and will take the same time to travel equidistances.

So, each observer will see the strikes simultaneously as the OP contends and you just proved.

Good Job!!!
:bravo:

15. ### Janus58Valued Senior Member

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2,285
Nope, because in neither frame do the clocks at the end of the train read the same time when each lightning strikes them. The clocks at the ends of the train are synchronized in the train frame, but not in the embankment frame .

16. ### Jack_BannedBanned

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1,383
Seems you have having a problem taking a frame as stationary.

Let's keep it simple.

Both frames sync all their clocks.

Do you understand this?

We assume that this definition of synchronism is free from contradictions, and possible for any number of points;
http://www.fourmilab.ch/etexts/einstein/specrel/www/

Now, the clocks in the train frame as stationary are all synched. The lightning flashes at equidistant points.

Exercise:

When will the light strike M'? Oh, Let's assume the distance to these synched clocks at light emission are a distance d.

17. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,167
Experiments say otherwise, MD - this is a variation on Michelson Morley.
You're arguing with reality, dude.
You're also arguing with the postulate of light invariance, which you clearly don't understand. Go to youtube and search for "relativity".

Consider how you measure the speed of a real train. Do you need to consider the Earth's rotation or orbit? Why not?
The invariance of light speed means that you measure the same speed without adjusting for you own movement, just like you can measure the speed of a train while standing on the ground without adjusting for the movement of the ground as the Earth turns and orbits.

Last edited: May 13, 2010
18. ### PeteIt's not rocket surgeryRegistered Senior Member

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*sigh*

Ferrous Cranus
Ferrous Cranus is utterly impervious to reason, persuasion and new ideas, and when engaged in battle he will not yield an inch in his position regardless of its hopelessness. Though his thrusts are decisively repulsed, his arguments crushed in every detail and his defenses demolished beyond repair he will remount the same attack again and again with only the slightest variation in tactics. Sometimes out of pure frustration Philosopher will try to explain to him the failed logistics of his situation, or Therapist will attempt to penetrate the psychological origins of his obduracy, but, ever unfathomable, Ferrous Cranus cannot be moved.

Last edited: May 13, 2010
19. ### Motor DaddyValued Senior Member

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5,425
The points in the example coincide with each other as Einstein clearly states. Are you saying Einstein didn't mean to say the points coincide? They do, the diagram shows that fact, and that is what Einstein states. What you are saying with the front of the train's point lining up, and the rear not yet lined up means that the observer either isn't in the middle of the length of the train, or that he is in the middle of that length, and that his midpoint doesn't line up with the embankment observers midpoint, as is also clearly stated in the diagram and text. Read chapter 9 and look at that diagram. Einstein makes perfectly clear the diagram and the wording.

20. ### Motor DaddyValued Senior Member

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If I put two synchronized clocks in a room and sit an equal distance apart from each, the only way I will see them as synchronized is if the room has an absolute zero velocity. You don't seem to understand the significance of light traveling independently of frames. Light travels a specific speed, regardless of the motion of any object it is traveling towards or away from. Light's speed is not dependent on another object, or even its own source. Light travels at c, period. If the room the clocks are in has an absolute velocity, even though the clocks are an equal distance from me, as measured in my frame, the light travels different distance to impact me, as I was "hastening" towards one light and moving away from the other light. The light from each clock will hit me at different times unless I have a true zero velocity.

Last edited: May 13, 2010
21. ### PeteIt's not rocket surgeryRegistered Senior Member

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MotorDaddy, the constancy of light says exactly the opposite of what you think it does, and experiments agree.
The Michelson Morley experiment showed that the speed of light, measured on Earth without allowing for Earth's rotation or orbit, does not change over the course of a year.

You're arguing with reality.

22. ### PeteIt's not rocket surgeryRegistered Senior Member

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Like I said earlier in the thread, that book by Einstein really isn't the best way to learn this stuff. It's a hundred years old, the language is archaic, and there have been a thousand better explanations written since then. It's not surprising that people find it hard to follow.

In this particular case, the diagram illustrates a snapshot of the scenario "considered with reference to the railway embankment". It's not supposed to be an absolute picture of reality - that's really the whole point. In Einstein's words, it's "reality (considered with reference to the railway embankment)".

So, Einstein said that the points coincide "As judged from the embankment." Not as judged from the train. Einstein could have done better by adding a diagram showing the train's reference frame. There are many better explanations that spell this out. Try here, for example: Relativity. Or here: Discovering the Relativity of Simultaneity: