Relativity of simultaneity objections

[mirrormirror]

You always have to have synchronization to measure simultaneity.

I have two clocks, struck by lightning, stopped when they were struck. Both clocks read 12:00:05, but by itself that doesn't tell you that the lightning strikes were simultaneous. Supplse that one was on Eastern time and the other was on Central time. Then the strikes occurred an hour apart. It is only if the clocks are synchronized that you can use their readings to determine simultaneity.

This goes back to the original problem. You must synchronize the clocks to determine simultaneity, so how do you synchronize the clocks? If you use Einstein's convention then clocks which are synchronized in one reference frame will not be synchronized in other reference frames. I.e. simultaneity is relative.

yes but we had already established that the two clocks on the train reference (one front door one back door) were already synchronised. we are not talking about two clocks one in the train one in the platforms

 

[Nugatory]

But the most unexpected ( for me ) is that they measured it like you said and it's always 2*L/c even when the box is moving. that's weird! I don't understand why that happens.

The very flippant response would be that the universe is under no obligation to act the way you think it ought to.

A somewhat less flippant response is to point out that the speed of light is so high that none of us have any natural experience observing things moving at relativistic velocities, so we have to be a bit careful about trusting our intuition here. It's worth noting that the equations of special relativity reduce to those of classical mechanics if you assume that all the speeds involved are small compared to the speed of light.

An even less flippant answer is that light waves in a vacuum are fundamentally different than (for example) water waves in water (which do behave as you're expecting). The difference is that the water waves are moving at a constant speed relative to the water, and when I'm measuring my speed relative to them I can look down at the water, see if I'm moving relative to the water or are "really" at rest. You can't do that in a vacuum - there's just you and the light wave.

My favorite argument (other than the experimental results, which pretty much trump all the arguing of course) is that the speed of light in a vacuum can be calculated from the laws of electricity and magnetism, which do not care how fast you're moving. So if there's a light wave in my vicinity, I expect that I'll measure its velocity to be c - but someone moving past me had better get the same result too, because he's supposed to be subject to the same laws of electricity and magnetism.

The history here is interesting. Maxwell discovered these laws in the 1860s, and for the next half-century the single greatest unsolved problem in physics was how to reconcile these laws with our intuition based on the way that water waves work with water and sound waves work with air, and so forth. Special relativity was that resolution, and MM-style experiments confirmed that it's a good one.

In historical context it's not at all surprising that the title of Einstein's classic 1905 paper on special relativity was "On the electrodynamics of moving bodies". You can find copies on line; it's not exactly a gentle tutorial introduction but the math is surprisingly undemanding and it's a good read.

 

[Dale]

yes but we had already established that the two clocks on the train reference (one front door one back door) were already synchronised.

Again. How are they synchronized? You never specified that. You simply tried to wave your hand and gloss over it.

This thread has not progrressed anywhere since post 2.

 

[Dale]

in one way it's c+v in the other it's c-v.

That doesn't give you 2L/c. That gives you 2L/(c-v²/c).

Here are a list of experiments showing that the speed of light does not depend on the motion of the source:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#moving-source_tests

 

[mirrormirror]

The very flippant response would be that the universe is under no obligation to act the way you think it ought to.

A somewhat less flippant response is to point out that the speed of light is so high that none of us have any natural experience observing things moving at relativistic velocities, so we have to be a bit careful about trusting our intuition here. It's worth noting that the equations of special relativity reduce to those of classical mechanics if you assume that all the speeds involved are small compared to the speed of light.

An even less flippant answer is that light waves in a vacuum are fundamentally different than (for example) water waves in water (which do behave as you're expecting). The difference is that the water waves are moving at a constant speed relative to the water, and when I'm measuring my speed relative to them I can look down at the water, see if I'm moving relative to the water or are "really" at rest. You can't do that in a vacuum - there's just you and the light wave.

My favorite argument (other than the experimental results, which pretty much trump all the arguing of course) is that the speed of light in a vacuum can be calculated from the laws of electricity and magnetism, which do not care how fast you're moving. So if there's a light wave in my vicinity, I expect that I'll measure its velocity to be c - but someone moving past me had better get the same result too, because he's supposed to be subject to the same laws of electricity and magnetism.

The history here is interesting. Maxwell discovered these laws in the 1860s, and for the next half-century the single greatest unsolved problem in physics was how to reconcile these laws with our intuition based on the way that water waves work with water and sound waves work with air, and so forth. Special relativity was that resolution, and MM-style experiments confirmed that it's a good one.

In historical context it's not at all surprising that the title of Einstein's classic 1905 paper on special relativity was "On the electrodynamics of moving bodies". You can find copies on line; it's not exactly a gentle tutorial introduction but the math is surprisingly undemanding and it's a good read.

I can understand why C is constant and I accept that. What I don't understand is why the time is always 2*L/c. for the front door, since the door is moving away from the light (despite the fact that we don't know it yet), it makes sense that the light beam would need more time to reach it ( from the center ) than the other light beam going from the center to the back door ( clock ) which is moving towards the light.

 

[Doc Al]

I can understand why C is constant and I accept that.

It's not clear that you do.

What I don't understand is why the time is always 2*L/c. for the front door, since the door is moving away from the light (despite the fact that we don't know it yet), it makes sense that the light beam would need more time to reach it ( from the center ) than the other light beam going from the center to the back door ( clock ) which is moving towards the light.

From the viewpoint of observers moving within the box, the doors are stationary.

Viewed from some other frame, one in which the box (and its doors) are moving, you could say that the doors are moving away from or towards the beam of light. But not from the frame of the box itself.

 

[Nugatory]

I can understand why C is constant and I accept that. What I don't understand is why the time is always 2*L/c. for the front door, since the door is moving away from the light (despite the fact that we don't know it yet), it makes sense that the light beam would need more time to reach it ( from the center ) than the other light beam going from the center to the back door ( clock ) which is moving towards the light.

Go back to the train again. You, standing in the middle of the train, send a flash of light out in both directions.

I, standing on the platform say the train is moving forwards at speed S, the front of the train is moving away from the light, and the back of the train is moving towards the light.

You on the train say that you're at rest, while I, the platform, and the ground are moving backwards at speed S. You also say that the distance between the you and the front of the train is not changing and the front of the train is not moving away from the light; and likewise the back of the train is at rest and not moving towards the light.

We both have to be able to describe this situation consistently, such that the distance (as we see it) traveled by the light, divided by the travel time (as measured with our synchronized clocks at the various points along the light's path) comes out to be c. It can be done, but only if we accept that clocks moving relative to one another cannot stay in sync so will eventually disagree about which events happen at the same time. And that's where relativity of simultaneity (which started this thread) comes from.

 

[darkhorror]

Sounds to me like it isn't just simultaneity messing you up but the very basics of relativity.

Lets say there are two people one standing on earth, and another in a spaceship they are moving away from each other at .5c. How fast are they moving and with respect to what? How fast will each see light moving away from it?

 

[mirrormirror]

It's not clear that you do.


From the viewpoint of observers moving within the box, the doors are stationary.

Viewed from some other frame, one in which the box (and its doors) are moving, you could say that the doors are moving away from or towards the beam of light. But not from the frame of the box itself.

yes, the observer within the box cannot fathom whether or not the box is moving (but it's still moving even if he doesn't know it). That's why I thought that by installing two clocks one at the front and one at the back, it would be possible to actually have evidence on whether it is moving or not. Because if indeed the box is moving, light will reach later the front door than the back door thus he can determine if it's moving. But Janus said that they made the experiment and there was no time difference. THAT is that part which i don't understand HOW it happens.

 

[WannabeNewton]

Because if indeed the box is moving, light will reach later the front door than the back door thus he can determine if it's moving.

Nono. In his frame the box is at rest with respect to him. As far as he's concerned the two light rays reach the respective ends of the box at the same time.

 

[mirrormirror]

Sounds to me like it isn't just simultaneity messing you up but the very basics of relativity.

Lets say there are two people one standing on earth, and another in a spaceship they are moving away from each other at .5c. How fast are they moving and with respect to what? How fast will each see light moving away from it?

hm, if one is moving at 0.5c from point M and the other at -0.5c from point M ( minus stands for opposite direction ) then they each will "see" the other moving at -c from them.

 

[mirrormirror]

Nono. In his frame the box is at rest with respect to him. As far as he's concerned the two light rays reach the respective ends of the box at the same time.

I guess I'm getting confused because most probably i have in my mind a notion of "absolute frame of reference" where no-matter what each observer "thinks" (whether he is stationary or moving ), there is some absolute movement.

 

[mirrormirror]

Go back to the train again. You, standing in the middle of the train, send a flash of light out in both directions.

I, standing on the platform say the train is moving forwards at speed S, the front of the train is moving away from the light, and the back of the train is moving towards the light.

You on the train say that you're at rest, while I, the platform, and the ground are moving backwards at speed S. You also say that the distance between the you and the front of the train is not changing and the front of the train is not moving away from the light; and likewise the back of the train is at rest and not moving towards the light.

We both have to be able to describe this situation consistently, such that the distance (as we see it) traveled by the light, divided by the travel time (as measured with our synchronized clocks at the various points along the light's path) comes out to be c. It can be done, but only if we accept that clocks moving relative to one another cannot stay in sync so will eventually disagree about which events happen at the same time. And that's where relativity of simultaneity (which started this thread) comes from.

hm, but the fact that I ( who am on the train ) think that I'm at rest, doesn't really mean that I'm at rest! The fact that it's difficult for me to tell who's moving, doesn't mean that I'm not moving and that the other guy is. There "must" be some - even theoretical - frame of reference.

 

[WannabeNewton]

I guess I'm getting confused because most probably i have in my mind a notion of "absolute frame of reference" where no-matter what each observer "thinks" (whether he is stationary or moving ), there is some absolute movement.

Ah ok. Now we're getting somewhere. If we are talking about uniform motion (constant velocities) then there exists no such absolute frame of reference (this isn't special to special relativity by the way this is also true in Newtonian mechanics as described ages past by Galileo; in fact that aspect of Newtonian mechanics is called Galilean relativity). Uniform motion is relative.

 

[Doc Al]

Because if indeed the box is moving, light will reach later the front door than the back door thus he can determine if it's moving.

Why do you think that? Within the box the doors are stationary, yet light moves at speed c. So it takes the same amount of time for light to reach any door, as long as the distance is the same.

Do you understand why, given the fact that the speed of light is invariant, that someone moving with the box cannot determine that the box is moving?

Of course, observers in a different frame, who see the box and its doors as moving, will see the light reach the doors at different times. But not the observers moving with the box.

 

[mirrormirror]

Why do you think that? Within the box the doors are stationary, yet light moves at speed c. So it takes the same amount of time for light to reach any door, as long as the distance is the same.

Do you understand why, given the fact that the speed of light is invariant, that someone moving with the box cannot determine that the box is moving?

Of course, observers in a different frame, who see the box and its doors as moving, will see the light reach the doors at different times. But not the observers moving with the box.

that's the part that i don't understand. the doors are not in reality stationary, they are moving forwards, they just "look" stationary to the inside observer, so in order to make sure he puts two clocks, one in the front one in the back, so because of the fact that the doors indeed are moving ( along with the box ), the front clock will be reached later than the back door ?

Light doesn't care what the frame of reference is or what the inside observer believes, neither do the laws of nature. So if the box is moving, the front door clock will be reached later. That's what I think at least.

so i guess what I'm thinking is: exactly because the speed of light is invariant, that gives us a tool ( by installing two clocks ) to be able to tell if the box is moving. If it wasn't invariant we wouldn't be able.

 

[WannabeNewton]

Let me ask you this my friend: why should the frame of the observer who sees the box moving uniformly with respect to him/her be any more correct/privileged than the frame of the observer who sees the box at rest with respect to him/her? As you said, Nature doesn't play favorites when it comes to inertial reference frames.

 

[mirrormirror]

Let me ask you this my friend: why should the frame of the observer who sees the box moving uniformly with respect to him/her be any more correct/privileged than the frame of the observer who sees the box at rest with respect to him/her? As you said, Nature doesn't play favorites when it comes to inertial reference frames.

that's a good question. i think the answer is this:

they both can conduct the experiment with the two clocks in the front and the back of their respective box. Whoever finds a difference in the measured time can deduce that they are moving. if they don't find a difference, they aren't.

 

[WannabeNewton]

Ah but if observer 1 is enclosed in a cubical box that is moving uniformly along with him then he will see both rays of light emitted by him, towards the front and back of his box, reach their respective ends at the same time and ditto for observer 2 within his own cubical box.

EDIT: crud half my post disappeared when editing :{

 

[mirrormirror]

Ah but if observer 1 is enclosed in a cubical box that is moving uniformly along with him then he will see both rays of light emitted by him, towards the front and back of the box, reach their respective ends at the same time and ditto for observer 2 within his own cubical box. If the boxes are transparent and observer 2 looks at the light rays emitted in observer 1's box, he will see a discrepancy in the time because observer 1 is moving with some uniform velocity relative to him. If we now have observer 1 look at the light rays emitted in observer 2's box, he will now see a discrepancy in the time because observer 2 is moving with some uniform velocity relative to him. Do you see how the relative nature of uniform motion and the constancy of the speed of light in inertial reference frames leads to a kind of symmetry amongst the inertial observers?

I see what you mean. What I don't understand is WHY each observer will see the light reaching the doors of his own box, at the SAME TIME. Despite the fact that he DOESN'T know if he's moving, light "knows", because he is actually moving despite the fact that he doesn't know it.

Imagine if we made a computer simulation of this: the simulation KNOWS that the box is moving (because it's a parameter you set, it's in the initial data), thus the light beam will reach it later ( in the front ) than in the back.

So for example if in the computer simulation light moves at an invariant speed of 300pixel / second and the box at 10 pixel / second, the light beam will reach the front door later than the back.

 

[WannabeNewton]

The thing is that light travels at the same speed in all inertial reference frames. This may be what is bugging you (and don't feel bad about it because it is definitely counter intuitive). It is not like water waves and such, which must move at some speed relative to a medium, where one can actually make the distinction you are speaking of.

 

[Nugatory]

hm, but the fact that I ( who am on the train ) think that I'm at rest, doesn't really mean that I'm at rest! The fact that it's difficult for me to tell who's moving, doesn't mean that I'm not moving and that the other guy is. There "must" be some - even theoretical - frame of reference.

But why are you so quick to think that you on the train are not "really" at rest, and that I on the platform might be "really" at rest?

You may remember that far back in this thread I advised you to consider the situation from the point of view a hypothetical third observer watching through a telescope from Mars - as far as he's concerned you and I are both moving at many miles a second and finds this discussion between the two of us to be utterly absurd. You may argue that just proves that he's not "really" at rest either because he's going around the sun, and it will all make sense if we just recognize that the sun is at rest with the planets orbiting around it... But an astronomer a few stars over, on a planet orbiting his sun, will have a lot of trouble seeing it that way.

This might be a good time to mention Einstein's first postulate (I already mentioned the second postulate, speed of light not affected by motion of emitter or detector): The laws of physics are unaffected by constant motion; for example the laws of physics don't change with the seasons even though the Earth is moving in different directions in March and September.

It follows that all motion is relative. It never makes sense to say that something is moving without saying what it's moving relative to, and that no experiment can detect such constant motion.

We on Earth tend to get sloppy about this because it's so natural to do everything relative to the ground under our feet. When the police officer hands me a speeding ticket for driving 60 in a 45 mph zone, he means that I was driving at 60 mph relative to the road surface, and the judge is not going to be impressed by my argument that I was going at 15 mph relative to the car I was passing. Nonetheless, that sign that said "Speed Limit 45" really meant "Speed Limit 45 relative to the ground".

 

[mirrormirror]

The thing is that light travels at the same speed in all inertial reference frames. This may be what is bugging you. It is not like water waves and such, where one can actually make the distinction you are speaking of.

no i don't think that this is bugging me. I think the computer simulation example was pretty good. So in the computer simulation of this we have a box which moves in the screen at 10pixel/second to the right. inside the box there is a light emitter ( which also moves at 10 pixel/second ), at some point the emitter emits light in both directions which travels at 300 pixel / second in EACH direction. So the box itself doesn't "know" if it's moving or not, BUT the light beams will reach the front of the box later than it will reach the rear one

 

[mirrormirror]

But why are you so quick to think that you on the train are not "really" at rest, and that I on the platform might be "really" at rest?

You may remember that far back in this thread I advised you to consider the situation from the point of view a hypothetical third observer watching through a telescope from Mars - as far as he's concerned you and I are both moving at many miles a second and finds this discussion between the two of us to be utterly absurd. You may argue that just proves that he's not "really" at rest either because he's going around the sun, and it will all make sense if we just recognize that the sun is at rest with the planets orbiting around it... But an astronomer a few stars over, on a planet orbiting his sun, will have a lot of trouble seeing it that way.

This might be a good time to mention Einstein's first postulate (I already mentioned the second postulate, speed of light not affected by motion of emitter or detector): The laws of physics are unaffected by constant motion; for example the laws of physics don't change with the seasons even though the Earth is moving in different directions in March and September.

It follows that all motion is relative. It never makes sense to say that something is moving without saying what it's moving relative to, and that no experiment can detect such constant motion.

We on Earth tend to get sloppy about this because it's so natural to do everything relative to the ground under our feet. When the police officer hands me a speeding ticket for driving 60 in a 45 mph zone, he means that I was driving at 60 mph relative to the road surface, and the judge is not going to be impressed by my argument that I was going at 15 mph relative to the car I was passing. Nonetheless, that sign that said "Speed Limit 45" really meant "Speed Limit 45 relative to the ground".

all of those observers are inherently wrong, either they are on the train, on the platform, on mars, on another solar system or on andromeda. They can see only their relative movements, but that doesn't mean that there is no TOTAL DEFINITE ABSOLUTE movement of each of them in the universe (though i think that according to my rationale they can find it out). Someone "outside" of the universe could see their absolute movements ( nomatter how complicated they would be ). Please take a look at my computer simulation version of this, posted just above

 

[WannabeNewton]

It seems like looking at things from the perspective of the simulator amounts to being in some external reference frame that acts as an absolute reference frame with regards to uniform motion. This is exactly what we don't have in nature. Your reference to someone "outside the universe" hopefully agrees with how I've interpreting your statement; we don't have external observers like that. There is no "outside the universe".

 

[DrGreg]

...because he is actually moving despite the fact that he doesn't know it.

No is isn't "actually moving". There's no such thing as "actually moving". The only sort of motion is one thing relative to something else. He's fully entitled to consider himself at "actually at rest". The laws of physics don't care whether his box is moving relative to something else or not. The person always measures the speed of light to be c, no matter how fast the box is moving relative to something else (as long as the motion is uniform, i.e. not accelerating).

It sounds difficult to believe, but it's the assumption on which all of relativity is based. So if you find something else that seems to contradict this assumption, it must be the "something else" that is wrong, if you accept relativity.

 

[Nugatory]

no i don't think that this is bugging me. I think the computer simulation example was pretty good. So in the computer simulation of this we have a box which moves in the screen at 10pixel/second to the right. inside the box there is a light emitter ( which also moves at 10 pixel/second ), at some point the emitter emits light in both directions which travels at 300 pixel / second in EACH direction. So the box itself doesn't "know" if it's moving or not, BUT the light beams will reach the front of the box later than it will reach the rear one

Take a look at my post #37 and the comment about water waves. You're using the surface of the screen to say that the light only travels at speed c relative to the screen surface, not everything else. That's a good analogy for water waves moving through the water at a constant speed relative to the water - but it doesn't work for light in a vacuum, where there's nothing but you and the light and no computer screen or water surface or other background to check your speed against.

 

[Nugatory]

someone "outside" of the universe could see their absolute movements (no matter how complicated they would be )

Maybe, but that's a question for the philosophers who can talk about unmeasurable unprovable things such as what "outside the universe" might mean. Physics is the science of what we can, at least in principle, measure and observe.

With tongue slightly in cheek, I'm going to suggest that your position can be paraphrased as "If I believe that if there were such a thing as 'outside the universe', and if there were an observer there, and if he the saw the world in a particular way, then I would find it very hard to sensibly explain all the observations that we living in the universe have made". That's true enough, but the easy way out of that dilemma is to not make those assumptions about things "outside the universe".

That's essentially what Einstein's first postulate suggests that we do, although the notion is far older.

One caution: Because physics is a science of what we can in principle measure, threads that wander too far off into philosophical questions about things that cannot in principle be measured tend to get locked on the grounds that they're no longer about physics. (They also tend to become intellectually sterile well before that point).

 

[mirrormirror]

Take a look at my post #37 and the comment about water waves. You're using the surface of the screen to say that the light only travels at speed c relative to the screen surface, not everything else. That's a good analogy for water waves moving through the water at a constant speed relative to the water - but it doesn't work for light in a vacuum, where there's nothing but you and the light and no computer screen or water surface or other background to check your speed against.

Yes i understand what you mean, that the computer screen acts like sort of an "ether" but i think the analogy is not valid. The simulation doesn't happen in the screen, it happens in computer memory and in the cpu ( so it's just mathematics ). The screen is just the visual representation medium of the universe. So write off that 300 pixels/second speed. Let's say that light in the simulation moves at 300.000.000 m /second ( just like in real life ) and that for practical reasons we represent 1.000.000 to 1 pixel in the screen or else we would need a huge screen!

 

[Nugatory]

I understand what you mean, that the computer screen acts like sort of an "ether" but i think the analogy is not valid. The simulation doesn't happen in the screen, it happens in computer memory and in the cpu so it's just mathematics)

Yes, it's just mathematics, but it would be mathematics that doesn't accurately simulate the behavior of the real world. If I were going to make it accurate, I'd have to add another option to the menu bar: "Show how it all looks for an observer represented by a pixel moving at speed X relative to the top left corner of the screen" and there would be absolutely nothing special about the view that I get when I choose X=0.

There are some pretty decent relativistically accurate simulations out there.
Here's one that you might try: http://gamelab.mit.edu/games/a-slower-speed-of-light/

 

[mirrormirror]

i think that what i fail to grasp is WHY ( that is, for what practical reasons ) the light beams will reach the MOVING box doors at the same time, like all of you said. The way i think of it, exactly due to the fact that c is constant and invariant, it will take it more time to reach the front clock than the rear one, because the front one is moving away from it and the back moving towards it. The observer inside the box doesn't know this, but he will notice it when he he sees the front clock stopped at 12:00:11 and the back clock stopped at 12:00:10.

Janus said ( in post 27 ), experiments showed they both will stop at 12:00:10, it's that I can't see how that can happen practically.

Anyway i will sleep over it and think it again. Thank you all for your help and your CONSTANT AND INVARIANT patience :-)

 

[WannabeNewton]

There is no universal coordinate time in SR as there is in Newtonian mechanics. Maybe this is what is confusing you? It is an alien concept at first for sure.

 

[darkhorror]

There is no absolute frame of reference. This is a fact. the speed of light is constant for all inertial frames of reference, this is also a fact. These are tested and observed.

If you would like to learn about relativity and how the universe works you need to get rid of your preconceived notions of how you think the universe works.

 

[darkhorror]

Think of it like this let's say I am standing on Earth and there is a spaceship moving away from me at .9c. In Earth's frame of reference light is moving .1c away from the ship in the same direction of it's motion. While in the ships frame of reference that light is moving away from it at c, and in the ships frame of reference it's speed is 0, and the Earth is moving away from it at .9c. Also in the ships frame of reference light is moving away from the Earth at .1c in the direction of motion. While in Earth's frame of reference that light is moving away from the Earth at c.

 

[phinds]

... doesn't mean that there is no TOTAL DEFINITE ABSOLUTE movement of each of them in the universe (though i think that according to my rationale they can find it out). Someone "outside" of the universe could see their absolute movements

This is nonsense. You can repeat it to yourself as often as you like --- make it your mantra in fact --- and it will still be nonsense.