Today Special Relativity dies

[Alkatran]

Ram are you just ignoring what people have been trying to tell you?! It's been said time and time again that if something is "at the same time" in one frame, it doesn't mean it is in another! (most likely no others!)

This is where your flaw in thinking is: Just because your clock are undergoing the same speed changes doesn't mean they are in synch! They are only in synch in the "clock" frame ("train" frame)

If you are moving, your reality is distorted (see my first post, which you seem to have conveniently ignored) through time, so things that are NOW for me are LATER for you, or maybe BEFORE. This is how we can both be moving slow as seen by the other person!

 

[jcsd]

Straight line non-accelerated relative motion can’t slow down the rate of any clock. As Lorentz pointed out, a “force” has to be placed on a clock timing mechanism, or removed from it, to change the clock’s rates. “Kinematics” without force changes no clock rates.

The clocks don't 'slow down' there is no mystery force, they simply run at different rates.

 

[Janus]

no one's yet given me a reason to believe they'd emit photons non-simultaneously in EITHER frame, moving or not.

does the phrase "relative to each other" actually have a meaning in this case? i mean i just threw it out there without thinking of consequences because i cannot fathom the difference it would make.

so if you can, tell me what the difference would be if they emitted photons simultaneously in the "picture frame" and in the "relative to each other frame)

thanks in advance

Go back to the animations I gave earlier. In that example, if both observers do not agree that both flashes struck each other at the same time, we would have a paradox.

For instance: Suppose that there is an explosive device at the point where they pass each other that has to be triggered by both. Each observer will only trigger the device if he sees the flashes arrive simultaneously. It is obvious that in the frame of the embankment observer that the flashes arrive at the same instant as the observer on the railway car. Thus both should trigger the device and set off the explosive. If the Railway car observer did not see the flash arrive at the same instant, he would not trigger the device from his side and the device would not go off. This would set up a paradox where for one observer the track is destroyed,and for the other it isn't.

Now we include the postulates of Relativity.

1. The laws of physics remain the the same for all observers in all frames regardlesss of their relative motion.

I don't think we have any argument there because if this weren't true then we'd really have a case of different realities in different frames.


2. The speed of light(in a vacuum) is invarient for all observers regardless of their relative moton.

IOW, the speed of light is a constant when measured relative to an observer by that observer.

Now this is a sticking point for some. In fact, earlier you said that is was just an assumption. But it is much more than that. It in itself is a consequence of the first postulate.

The speed of light is determined by two universal constants : permeability and permitivity. These two constants also are what determine the strengths of the magnetic and electrostatic fields. Thus in order for the speed of light to have different values relative to an observer as measured by that observer, these constants would have to have different values for this observer also(the strength of magnets would change, etc.). The laws of physics would have to change for every observer depending on his relative motion to others.

It is actually worse than that. For if two beams of light were directed at the observer from two sources each moving at different velocities from the observer, in order for that observer to see the light from each be dependant on the velocity of each source (Galileian addition of velocities), these two constants would each have to have two different values each at the same time.

In fact, it is possible to show that if the speed of light wasn't invarient for every observer, light produced in one frame could not even be detected by a frame that was moving relative to that frame.

Add to this the fact that the invarience of the speed of light has been confirmed by multiple experiments.

The animations I provided show what happens according to the frames of both observers in order to uphold the principles above. As a result, simultanity is found to fail at distance between frames.

But this is a small price to pay, since at least this doesn't lead to any real paradoxes.

 

[russ_watters]

Straight line non-accelerated relative motion can’t slow down the rate of any clock. As Lorentz pointed out, a “force” has to be placed on a clock timing mechanism, or removed from it, to change the clock’s rates. “Kinematics” without force changes no clock rates.[emphasis added]

Its sloppy grammar to say clocks slow down in SR, though I admit I do it too. In actuality (and of course, you already know this, as you've been part of this same discussion many, many times), time itself passes at different rates for clocks in different frames, while the clocks themselves are, to a local observer, unaffected.

 

[russ_watters]

Ram, just to hammer this in a little more, please please remember that most of the simultenaity issues we're dealing with here are the same for Galilean Relativity as they are for Einstein's. You seem to be trying to argue against SR, but in actuality, you're arguing against the original basis of physics.

One more little thought experiment:

-Suppose you have three identical clocks next to each other and synchronized.
-Move two of the clocks 300,000km apart and keep the third (ignore SR effects).
-At a pre-determined time (call it T0), both clocks emit a signal.
-If you are 200,000km from one clock and 100,000 from the other, at what time according to your clock do you receive each signal (T0+X, T0+Y)?
-Are the clocks synchronized in your frame?

 

[ram1024]

of course they are. I know the speed of light and i know the distance from the emitters. they won't hit me simultaneously, but i can easily deduce the time of emission from distance and arrival times.

 

[ram1024]

http://home.teleport.com/~parvey/train1.gif

http://home.teleport.com/~parvey/train2.gif

i'll have you look at your own animations.

look at the first animation and then look at the second one.

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right. in order to satisfy "constant relative to all viewers". SOMEONE made up length contraction. but wait. if we contract the distance to the right of the train it just leads to the light getting to the train even FASTER. we need to contract the distance BEHIND the train so that light arriving from that direction will arrive simultaneously. OH NO! protect our precious light speed!

 

[ram1024]

Ram are you just ignoring what people have been trying to tell you?! It's been said time and time again that if something is "at the same time" in one frame, it doesn't mean it is in another! (most likely no others!)

This is where your flaw in thinking is: Just because your clock are undergoing the same speed changes doesn't mean they are in synch! They are only in synch in the "clock" frame ("train" frame)

that's why i made the point in case #6 that we're determining "simultaneous synchro" by the setup in case #5.

give me your analysis for case #6 if you will be so kind :D

 

[ArmoSkater87]

no comment, for the most part...WOOSHHHHHHHHHHHH :P

 

[Doc Al]

case #5

Why I'm jumping in I have no idea. Masochism?

ONE EMITTER.

TWO OBSERVERS.

Case #5

[u](o)                    <-)|(->                    (o)[/u]

One emitter simultaneously shoots 2 photons towards two ovservers equal distance from the center (where the emitter is). the observers carry synchronized clocks to time their photon receptions

SR predicts that since this is a "inertial frame" light will hit both observers at the same time. (True / False) ?

I presume that the observer's clocks are synchronized in their own frame. If so, then the answer is: When the two observers detect the light, their clocks will read the same time. (Of course a third observer in relative motion to these two guys will disagree that the clocks were ever synchronized according to his frame.) So what?

 

[jdavel]

I have a suggestion.

Restate this problem using spacetime diagrams. ram's snapshot diagrams may seem unambiguous to him, but it's obvious that they don't seem that way to everyone else.

There's a reason that physicists discuss and explain relativity in terms of spacetime: it works!

And nothing has to be conceded on either side of the argument; spacetime is just as valid a concept in Galilean relativity as it is in SR.

 

[ram1024]

I presume that the observer's clocks are synchronized in their own frame. If so, then the answer is: When the two observers detect the light, their clocks will read the same time. (Of course a third observer in relative motion to these two guys will disagree that the clocks were ever synchronized according to his frame.) So what?

don't jump in and do one case and expect a revelation :D the "paradox" only comes after you realize the issue from multiple vantage points.

 

[russ_watters]

of course they are. I know the speed of light and i know the distance from the emitters. they won't hit me simultaneously, but i can easily deduce the time of emission from distance and arrival times.

Well herein lies the basic problem you are having with these thought experiments: you don't know what we're talking about when we say "simultaneous." You essentially answered 'yes, they are simultaneous in my frame' and then provided an explanation that says 'no, they are not simultaneous in my frame, but you can calculate that they are simultaneous to an outside observer.'

Frame, frame, frame, frame, frame, frame, frame!

In your frame, the events are not simultaneous - and that's the question I asked. That's what we mean when we say two events are not simultaneous according to a specific observer. You need to get on board with that concept. Its the root of the misunderstanding here.

 

[russ_watters]

in order to satisfy "constant relative to all viewers". SOMEONE made up length contraction. but wait. if we contract the distance to the right of the train it just leads to the light getting to the train even FASTER. we need to contract the distance BEHIND the train so that light arriving from that direction will arrive simultaneously. OH NO! protect our precious light speed!

While it may be easy enough to ignore length contraction for a train moving at 100km/hr, it is not easy to ignore for a train moving at a significant fraction of the speed of light as shown in the animations.

I mentioned before that all these thought experiments may be counterproductive: since thought experiments exist only in your head, you may start thinking the data exists only in your head too. It doesn't. The data has been collected from real experiments and demonstrates that C is constant. Maybe we should start looking at real experiments instead of thought experiments.

 

[ram1024]

see that's the thing, if simultaneity can be real at a single point, then simultaneity MUST be able to be real at a distance. not "according to an observer" but according to "reality".

to say it doesn't happen is like saying "no two things in the universe EVER happen at the same time"

whether or not they happen "at the same time to you" is merely a matter of perception and is NOT reality

 

[ram1024]

if you have any real experimental data handy you can share it, but i unfortunately do not have access to any

 

[wespe]

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right. in order to

No, it doesn't look much faster to me, it looks very close. In fact, they should be the same, if the animation is accurate. Maybe you are fixing your eyes on the red dots? Remember this is the train's perspective. The speeds must be the same wrt the train.

 

[wespe]

whether or not they happen "at the same time to you" is merely a matter of perception and is NOT reality

Why can't that perception be your reality? Nothing can interact faster than light. And aren't you one of those observers? Will you prefer someone else's perception as your reality?

 

[Doc Al]

I mentioned before that all these thought experiments may be counterproductive: since thought experiments exist only in your head, you may start thinking the data exists only in your head too. It doesn't. The data has been collected from real experiments and demonstrates that C is constant. Maybe we should start looking at real experiments instead of thought experiments.

Russ makes an excellent point. While these thought experiments may serve to illustrate what relativity says in various situations and to show that it's perfectly self-consistent, they cannot prove that relativity is in fact how the world really works. Only experiment can do that.

 

[Hurkyl]

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right.

I had a look at them again; they look the same speed to me.

The motion of the track kind of plays an optical illusion, though; if you still think the left bubble expands faster than the one on the right, crop the image so you can't see the track (maybe by shrinking your browser window and scrolling the image partially off the window) and see if you still think one expands faster than the other.

 

[Alkatran]

http://home.teleport.com/~parvey/train1.gif

http://home.teleport.com/~parvey/train2.gif

i'll have you look at your own animations.

look at the first animation and then look at the second one.

what looks fishy about the second animation?

that's right, the speed of the bubble expansion from the left emitter is MUCH faster than the speed of the bubble on the right. in order to satisfy "constant relative to all viewers". SOMEONE made up length contraction. but wait. if we contract the distance to the right of the train it just leads to the light getting to the train even FASTER. we need to contract the distance BEHIND the train so that light arriving from that direction will arrive simultaneously. OH NO! protect our precious light speed!

They're expanding at the same speed. The train track gives the illusion that one is moving more quickly than the other (remember that this is lgiht speed relative to the train and NOT the track!)

Watch the bubbles (the left sides of them) at each frame they move approximately one "track" (little brown line). They are moving at the same speed.

 

[Hurkyl]

no one's yet given me a reason to believe they'd emit photons non-simultaneously in EITHER frame, moving or not.

Take case two. Suppose each clock is reset to zero when the photons are emitted simultaneously in the picture frame.

Now, note the times at which each clock receives the others photon; they will be different. (as can easily be shown in the picture frame)


If the photons were emitted simultaneously in the clock frame, then the clocks would be synchronized in the clock frame. Furthermore, it takes the same time for the photon to get from A to B as it does from B to A. (Remember that the clocks are stationary in their rest frame!) Thus, the clocks must read the same time when they receive the other's photon.

Since the clocks, in fact, do not read the same time when they receive the other's photon, we conclude that the photons were not emitted simultaneously in the clock frame.

 

[jcsd]

Or just imagine a device to sychronize them:

at the half way point we have a device that emits a photon to each clock, when that photon arives at the clocks they tick. This synchronises the clocks in the staionary frame, but in a moving frame one photon will have further to travel than the other so they CANNOT tick at the same time in the moving frame. As our photon device sychronises the clocks perfectly in the rest frame this must hold true for all clocks that are synchronised in their rest frame whether we use this device or not.

 

[ram1024]

They're expanding at the same speed. The train track gives the illusion that one is moving more quickly than the other (remember that this is lgiht speed relative to the train and NOT the track!)

Watch the bubbles (the left sides of them) at each frame they move approximately one "track" (little brown line). They are moving at the same speed.

wondering if you guys have eyes...

look at example 2 yet again:

http://home.teleport.com/~parvey/train2.gif

now halfway through the animation, the light from emitter(L) starts. it covers a FULL distance in that time while light from emitter(R) travels half the remaining distance.

add that to the fact that the picture is skewed (look at the bubbles in the last frame they're not even centered to the sources anymore)

the light on the left is traveling about 4x faster than the light on the right.

 

[Hurkyl]

now halfway through the animation, the light from emitter(L) starts. it covers a FULL distance in that time while light from emitter(R) travels half the remaining distance.

So? The left emitter was much closer to the meeting point when it fires than the right emitter was.

add that to the fact that the picture is skewed (look at the bubbles in the last frame they're not even centered to the sources anymore)

The sources moved. What else would you expect?

 

[wespe]

wondering if you guys have eyes...

no wonder we are having communication problems.
we can't even agree on something as simple as this.

 

[ram1024]

so speed is distance over time

left light travels 1 full distance in 1/2 the time the right light.

left light is comparatively twice as fast.

what happened to relatively constant?

 

[Doc Al]

The two light "spheres" expand at the same rate. You are "measuring" distance (I suppose) by counting the moving rails: incorrect. The speed of the light is invariant with respect to the observer.

 

[Hurkyl]

so speed is distance over time

Pull out your ruler and measure the distance on your screen between the point where the right emitter fires and the point where the bubbles meet.

Pull out your stopwatch and measure how long it takes.

Divide.

Repeat for the left emitter.

Compare.


My measurements:
7.5 cm distance for the right emitter.
I average 1.83 seconds over three trials.
Velocity is 4.10 cm/sec


3.75 cm for the left emitter.
0.83 seconds
Velocity is 4.52 cm/sec

That's a 10% relative error; well within my confidence in my time measurements.


Certainly nowhere near twice as fast.

 

[ram1024]

is that picture based on experimental data?

because it's way way off.

 

[Hurkyl]

What's off about it?

Light travels at the same speed in all directions from the point of emission.

What else should happen?

 

[ram1024]

well in the second picture nothing's moving.

picture 1. the train is making relative progress towards the emitter (source) of the photon to the right.

picture 2. the train is making NO relative progress towards the emitter (source)

poke a pen on the center of the emissions in picture 1. movement is measured by the train towards emitter to the right and away from emitter to the left.

poke a pen on the center of emissions in picture 2. no relative motion is made towards the locations of the sources. indeed the locations of the sources seem to be traveling down the tracks. what's with that?

 

[Hurkyl]

poke a pen on the center of emissions in picture 2. no relative motion is made towards the locations of the sources. indeed the locations of the sources seem to be traveling down the tracks. what's with that?

In picture 2, the sources are the red dots, the same as in picture one, and they are very clearly drifting leftwards with the tracks.

And because the sources are moving, they cannot occupy the center of emissions for the entire animation!

 

[ram1024]

i'm not talking about the red dots. ignore the red dots completely.

poke your pen in the centers of the expanding spheres.

picture 1 = movement towards expanding sphere to the right
picture 2 = no movement towards expanding sphere to the right.

so what's going on?

 

[Hurkyl]

i'm not talking about the red dots. ignore the red dots completely.

poke your pen in the centers of the expanding spheres.

picture 1 = movement towards expanding sphere to the right
picture 2 = no movement towards expanding sphere to the right.

so what's going on?

The centers of the expanding spheres should not be moving, do you agree? If one thing goes left at c and one thing goes right at c, then their midpoint should be stationary, right?


Why is this not a contradiction? Because "The center of the sphere of light" is not an object; it is a geometric calculation.

The two animations demonstrate how an object can satisfy this geometric description in one frame and not the other. The emitters, which are stationary in the first animation, remain in the center of the sphere. The mitters, which are moving in the second animation, leave the center of the sphere.

But in any given frame, the center of a sphere of light cannot move; the light going left has the same speed as the light going right, so the point midway between them must remain stationary.