I Simultaneity: Train and Lightning Thought Experiment

[Janus]

May you tell me why not? Did they have any logic counter-arguments or thought experiments that were against Einstein's train?

They didn't have any problem with the validity of the argument, but did with its soundness.

For example, the following is a perfectly valid logical argument.

All swans are white.
My pet bird is a swan
Therefore, my pet bird is white.
The conclusion directly follows from the premises.

However, it not a sound logical argument, because one of the premises is false. Not all swans are white.

Many physicists of the time felt that there must have been a problem with Einstein's postulates. You have to keep in mind that Newtonian Physics had been the undisputed champion for a long time, and Einstein was saying that it was wrong. This was a hard pill to swallow. Until a definitive real-life observation or experiment could be performed to test Einstein's theory, it was an untested theory and could be ultimately wrong.

 

[Herman Trivilino]

May you tell me why not?

The experimental evidence available at the time wasn't enough. There are always competing explanations, and many times all of them make perfect sense. But only those explanations that match what we observe are the ones we deem "correct".

Did they have any logic counter-arguments or thought experiments that were against Einstein's train?

Lorentz ether theory. But it maintains that there is a special reference frame. When the experimental evidence showed that this special frame (in which this thing called the ether was at rest) was undetectable, it became difficult to believe it existed. And while this was going on the experimental evidence in support of Einstein's explanation kept growing. A century later there is just no reasonable way to believe in any explanation other than Einstein's.

 

[Peter Martin]

I am puzzled over einstein's thought experiment on simultaneity.

In this experiment, a man is standing on a train platform. A woman is sitted in the middle of a moving train traveling towards the man. When the train is half past the man, lightning strikes at the same instant at both ends of the train. The man sees the lightning at the same time. This makes sense since the distance traveled by light from both ends is the same.

However, the thought experiment propose that the woman sees the light from the front of the train first. This is because she 'runs' into light from the front since she is moving forward with the train.She then conclude that lightning struck the front first since she is equidistant from the front and back of the train.

I am confused. Since the train is not accelerating, it can be treated as an inertial frame of reference. Speed of light c should be constant in the woman's frame of reference. Thus when only regarding the woman's frame, shdn't she see both lightning at the same time? Since both light travels at c and have to cover half of the train's length.

The fact that the woman runs into the front lightning is the observation from the man's frame of reference. So when we are talking about the woman's observations, why are we trying to use the man's frame of reference to predict the results? Shdn't we be isolating the woman's frame of reference and analysing that independently?

I hope my words are clear! Thanks for reading and help me out if you can[emoji1]

I've pondered this thought experiment for years and think I've arrived at the simplest explanation. The reader is tricked by the description of the two strikes as simultaneous ("at the same time" as you put it). But that's only from the man's point of view. Let's re-describe the situation.

The woman on the train sees the two strikes as simultaneous. For this to be the case, because the woman is moving toward the location of the front strike and that flash has a shorter distance to travel to reach her than does the rear strike, the man would have to see the rear strike first, followed by the front strike.

You may be tempted to say that the rear strike really does occur first in this case; but, as the story proves, simultaneity and sequentiality are perceptions, not reality!

 

[Peter Martin]

The reader is misled when the strikes are described as "simultaneous". They are simultaneous only from the man's point of view. Because she is moving forward, for her to see them as simultaneous, the rear strike would have to occur first because it has a longer distance to cover before reaching her.

Actually, saying the rear strike would have to occur first is inaccurate. It would have to occur first in the man's reference frame. As the story proves, simultaneity is in the mind of the beholder, not in reality!

 

[vanhees71]

The readers are mislead by the refusal to use the adequate language, which is to use four-vectors for the "events" (strikes) in Minkowski space. Then you can calculate, what any observer observes without making 1000 confusing words.

 

[Herman Trivilino]

The reader is tricked by the description of the two strikes as simultaneous.

In that sense, the reader is also tricked by the description of a pitcher throwing a baseball at a speed of 98 mi/h. The ball doesn't really move that fast, it's just that from the pitcher's perspective the speed is 98 mi/h.

You may be tempted to say that the rear strike really does occur first in this case; but, as the story proves, simultaneity and sequentiality are perceptions, not reality!

Baseball pitchers are paid a lot of money for doing something that's not real?

Things that are relative, like simultaneity and speed, are things described by physics. Whether they're real or not is a matter of subjective perception, not objective reality.

On the other hand, the sequential ordering of events with timelike separation is preserved. For example, Queen Elizabeth II assumed the duties of the monarchy after her father, King George VI died. The order of those two events is not relative, all observers will agree on it. And also that every effect was preceded by its cause.

 

[JulianM]

The reader is misled when the strikes are described as "simultaneous". They are simultaneous only from the man's point of view. Because she is moving forward, for her to see them as simultaneous, the rear strike would have to occur first because it has a longer distance to cover before reaching her.

Actually, saying the rear strike would have to occur first is inaccurate. It would have to occur first in the man's reference frame. As the story proves, simultaneity is in the mind of the beholder, not in reality!

For this to happen the closing speed between the woman and the flash needs to be greater than c.

A better way to look at this is by considering observable events.

 

[Peter Martin]

I am puzzled over einstein's thought experiment on simultaneity.

In this experiment, a man is standing on a train platform. A woman is sitted in the middle of a moving train traveling towards the man. When the train is half past the man, lightning strikes at the same instant at both ends of the train. The man sees the lightning at the same time. This makes sense since the distance traveled by light from both ends is the same.

However, the thought experiment propose that the woman sees the light from the front of the train first. This is because she 'runs' into light from the front since she is moving forward with the train.She then conclude that lightning struck the front first since she is equidistant from the front and back of the train.

I am confused. Since the train is not accelerating, it can be treated as an inertial frame of reference. Speed of light c should be constant in the woman's frame of reference. Thus when only regarding the woman's frame, shdn't she see both lightning at the same time? Since both light travels at c and have to cover half of the train's length.

The fact that the woman runs into the front lightning is the observation from the man's frame of reference. So when we are talking about the woman's observations, why are we trying to use the man's frame of reference to predict the results? Shdn't we be isolating the woman's frame of reference and analysing that independently?

I hope my words are clear! Thanks for reading and help me out if you can[emoji1]

I agree. Here's my thought on the subject of inertial frames of reference and the principles describing the propagation of light.

Does Einstein’s train-lightening thought experiment violate SR?

Let’s propose a scenario which doesn’t differ significantly from Einsetin’s.

A high-speed bullet train runs on a straight portion of track. A woman sits in an isle seat at the train’s midpoint. The doors between the cars are open, allowing the woman to see all the way to the front and rear of the train, where at each location a strobe light is mounted. The two lights are wired to a switch at the woman’s seat.

As the train plummets ahead, she throws the switch. Let’s assume Einstein’s conclusion that she sees the forward flash first.

Now, the train is an inertial reference frame. SR states that all the laws of physics — including those pertaining to light — are the same for all inertial reference frames — that there exist no “preferred” reference frames. Any inertial reference frame has an equal claim to being “at rest” relative to other frames.

But the train is a preferred reference frame. (Or more accurately, a “non-preferred” reference frame.) By analyzing data entirely from within the train, she can conclude that the train is moving. Were the train at rest on the tracks, there is no doubt she would see the strobe flashes as simultaneous. Knowing the train’s length and having an atomic clock, she could even calculate the train’s speed based on the interval between the arrival of the two flashes.

I’d appreciate a clear explanation of what’s wrong with this picture.

 

[Orodruin]

By analyzing data entirely from within the train, she can conclude that the train is moving

Wrong. She can conclude that the train is moving relative to the ground, but for all relevant purposes this is described as the ground moving in the train's rest frame. All that she will be able to conclude is what the relative velocity between the ground and the train is, but any observer can do this based on their own rest frame.

 

[Janus]

I agree. Here's my thought on the subject of inertial frames of reference and the principles describing the propagation of light.

Does Einstein’s train-lightening thought experiment violate SR?

Let’s propose a scenario which doesn’t differ significantly from Einsetin’s.

A high-speed bullet train runs on a straight portion of track. A woman sits in an isle seat at the train’s midpoint. The doors between the cars are open, allowing the woman to see all the way to the front and rear of the train, where at each location a strobe light is mounted. The two lights are wired to a switch at the woman’s seat.

As the train plummets ahead, she throws the switch. Let’s assume Einstein’s conclusion that she sees the forward flash first.

Now, the train is an inertial reference frame. SR states that all the laws of physics — including those pertaining to light — are the same for all inertial reference frames — that there exist no “preferred” reference frames. Any inertial reference frame has an equal claim to being “at rest” relative to other frames.

But the train is a preferred reference frame. (Or more accurately, a “non-preferred” reference frame.) By analyzing data entirely from within the train, she can conclude that the train is moving. Were the train at rest on the tracks, there is no doubt she would see the strobe flashes as simultaneous. Knowing the train’s length and having an atomic clock, she could even calculate the train’s speed based on the interval between the arrival of the two flashes.

I’d appreciate a clear explanation of what’s wrong with this picture.

In the scenario you give, Einstein conclusion would not be that she would see the forward flash first, but that she would see both flashes at the same time She would also conclude that both strobes were triggered simultaneously. An observer on the embankment would agree that the light from the flashes reach her at the same time, but would not agree that the strobes were triggered simultaneously. If she timed the throwing of her switch such that, according to her the signals traveling along the wires reached the strobes at the same moment as she was passing the embankment observer, you have basically the same thing as Einstein's train experiment where you have just changed the frame in which the flashes are deemed to occur simultaneously. (even in the original set up, our train observer would agree that the light flashes hit the embankment observer at the same moment.)

The only thing you have added is that she initiates the strobes by throwing a switch connected by wires to the strobes. But there will be a propagation delay between the throwing of the switch and the strobes firing. For her this delay will be equal for both strobes. For the embankment observer, it will not be. For him, you would have to apply the relativistic addition of velocities to the signals.
Electric signals travel a bit slower than c. Lets' use 0.95c as an example, and assume that the train is moving at 0.99 c relative to the tracks.

Then for the embankment observer, the signal traveling in the direction of the train's motion would be moving at
(0.99c+0.95c)/(1+0.99c(0.95c)) = 0.9997c, relative to himself and 0.0497c with respect to the train.

The signal traveling in the other direction would be moving at
(0.99c-0.95c)/(1-0.99c(0.95c)) = 0.6723c relative to him and 0.2777c with respect to the train.

Since switch is at the midpoint between the strobes on the train, this signal will reach its strobe first triggering its flash before the other according to the embankment frame.

Introducing the switch and signals traveling along them the wires just adds another complicating factor to the scenario. One which includes more than just the relativity of simultaneity which the scenario is meant to illustrate.

 

[Ibix]

By analyzing data entirely from within the train, she can conclude that the train is moving.

How? By noting that she saw the flashes non-simultaneously? That just tells her that the lights weren't synched properly.

If, for some reason, she knows that the lights are synched correctly in the ground frame then she can deduce that she is moving in that frame. But that's all.

 

[itfitmewelltoo]

It may be easier to recognize that for the stationary man, he "sees" the from the front strike reach the women before the rear strike. For him, the relative velocity of the light from the front is c-v. The rear strike light reaches her at a speed of c+v. v being the velocity of the train. The man agrees that the woman does not see simultaneous events. They are separated by 2v.

 

[Peter Martin]

Wrong. She can conclude that the train is moving relative to the ground, but for all relevant purposes this is described as the ground moving in the train's rest frame. All that she will be able to conclude is what the relative velocity between the ground and the train is, but any observer can do this based on their own rest frame.

I agree that she can calculate the train's speed only relative to the ground. But considering the train "at rest" and the ground "speeding past", tell me how the ground's speed can affect how light behaves in the in the train. If you were in a basement laboratory and did the experiment with the two strobes you would be shocked if the flashes arrived at the center location sequentially. So (to repeat) how does the "moving" ground affect how light behaves in the "stationary" train?

 

[Orodruin]

But considering the train "at rest" and the ground "speeding past", tell me how the ground's speed can affect how light behaves in the in the train.

It doesn't. Hence you cannot conclude that the train is "moving". What matters is in which frame the flashes are supposed to be simultaneous. This is not something universal. If they are simultaneous in the train frame they will not be in the ground frame and vice versa. The situation is completely symmetric.

 

[Janus]

I agree that she can calculate the train's speed only relative to the ground. But considering the train "at rest" and the ground "speeding past", tell me how the ground's speed can affect how light behaves in the in the train. If you were in a basement laboratory and did the experiment with the two strobes you would be shocked if the flashes arrived at the center location sequentially. So (to repeat) how does the "moving" ground affect how light behaves in the "stationary" train?

To repeat what Orodruin has already stated, it doesn't. The flashes produced by strobes which are activated by the train observer flipping a switch are different flashes than the ones that would be produced by the lightning strikes which hit the ends of the train as described in the original version of the experiment.

Let's assume that both set of flashes are in play. The lightning strikes hit the ends of the trains simultaneously in the embankment frame as the two observers pass each other, and the strobes at the ends of train go off simultaneously in the train frame as the two observers pass.
The conclusion of both observers will be that there were four separate flashes. Each observer will see the light from two of the flashes simultaneously and the light from the other two arriving at different times. Each will judge a different pair of flashes as having occurred simultaneously; Lightening strikes for the embankment, strobes for the train. Neither of them will say that either strobe fired at the same moment as lightning struck that end of the train.

 

[Ibix]

But considering the train "at rest" and the ground "speeding past", tell me how the ground's speed can affect how light behaves in the in the train.

To restate what Orodruin and Janus have said - it doesn't. The point is that when you set up the experiment, you choose in which frame the flashes are simultaneous.

That frame is significant to this experiment alone. It's not significant to any physics. For example you could rig red flashlamps that fire simultaneously in the train frame and blue flashlamps that fire simultaneously in the ground frame and run both experiments (i.e., a "ground is at rest" and a "train is at rest" version) in one pass by the embankment.

 

[Orodruin]

To underscore Ibix’s point: The entire point of the example of the train and light flashes is to demonstrate that what simultaneous means depends on the frame. It is a very common misconception among laymen and physics students alike to think that it is possible to state that two events are simultaneous without understanding that the specification of the frame they are simultaneous in actually affects their physical setup.

 

[JulianM]

What must happen is that in each frame the two flashes illuminate the observer in that frame on both sides of the observer at the same time. This is a an observable event.

The passenger sees both sides of the platform observer illuminated at the same time and the platform observer sees both sides of the passenger illuminated at the same time.

They do not see rhesus events happen at the same time unless they are co-located.

 

[Ziang]

After Einstein's train left the park, it railed along a very long swimming pool.
On the ground, two ends of the pool were touched with two sticks at the same time. Water waves from the two ends met each other right at the middle point O of the pool. The observer standing at point O took a picture of the waves and sent it to the lady sitting on the train.
On the train, according to SR, two ends of the pool were not touched simultaneously, the lady wondered why the waves met each other right at the middle point O.

 

[Ibix]

the lady wondered why the waves met each other right at the middle point O.

Then she wondered whether mechanical wave speeds in a medium might be different if the waves were moving with or against the bulk motion of the medium...

 

[Janus]

After Einstein's train left the park, it railed along a very long swimming pool.
On the ground, two ends of the pool were touched with two sticks at the same time. Water waves from the two ends met each other right at the middle point O of the pool. The observer standing at point O took a picture of the waves and sent it to the lady sitting on the train.
On the train, according to SR, two ends of the pool were not touched simultaneously, the lady wondered why the waves met each other right at the middle point O.

And we are back to the addition of velocities theorem. If s is the speed of the waves through the water as measured by the pool and the observer standing at point O, and v is the velocity of the train relative to the pool, then the lady on the train will measure the waves traveling through the pool in the same direction as she is relative to the pool as being
(v-s)/(1-vs/c²)
relative to the train.
and the waves traveling in the opposite direction as moving at
(v+s)/(1+vs/c²)

Basically what happens according her is that while one set of waves leaves one end of the pool before the other set of waves leaves its end, the waves that left first are traveling slower with respect to the pool then the waves that left later, and this difference in wave speed results in them still meeting at the midpoint of the pool.
Both she and the Observer at point O agree that the waves meet at the midpoint, They just disagree as to the sequence of events that led to this end result.

 

[Ibix]

One can also make a general observation that all classical mechanical phenomena (except gravity) are applied electromagnetism, when you get right down to it. They're all about atoms interacting through their electromagnetic fields. So all mechanical phenomena are necessarily compatible with relativity, no matter how counterintuitive it may appear.

Far and away the simplest way to understand this, I think, is a Minkowski diagram of the sort I posted in #70. Then you can see clearly that nothing about the 4d object changes; it's just your interpretation of the data you receive that produces different 3d slices of it.

 

[Peter Martin]

After Einstein's train left the park, it railed along a very long swimming pool.
On the ground, two ends of the pool were touched with two sticks at the same time. Water waves from the two ends met each other right at the middle point O of the pool. The observer standing at point O took a picture of the waves and sent it to the lady sitting on the train.
On the train, according to SR, two ends of the pool were not touched simultaneously, the lady wondered why the waves met each other right at the middle point O.

Love it!

 

[JulianM]

Love it!

What Ziang is describing is the motion of a wave in a medium.

Sounds too much like Aether doesn't it?

 

[Ibix]

What Ziang is describing is the motion of a wave in a medium.

Sounds too much like Aether doesn't it?

You have that backwards. Aether was expected to behave like the water does, but the whole thing turns out not to be a good model for the propagation of light. Water still behaves like water.

 

[JulianM]

You have that backwards. Aether was expected to behave like the water does, but the whole thing turns out not to be a good model for the propagation of light. Water still behaves like water.

That's what I was saying. Ziang's description of waves in a pool is reminiscent of aether theory, which we know is incorrect and is not the way light behaves.

 

[Ziang]

... the waves that left first are traveling slower with respect to the pool then the waves that left later,...

The medium (water) does not depend on moving direction of waves. Would you tell me how water waves can travel at different speeds in different directions with respect to the pool?

 

[Ibix]

The medium (water) does not depend on moving direction of waves. Would you tell me how water waves can travel at different speeds in different directions with respect to the pool?

Because velocities don't add linearly and the pool is in motion in this frame.

 

[Janus]

The medium (water) does not depend on moving direction of waves. Would you tell me how water waves can travel at different speeds in different directions with respect to the pool?

Now we go back to what Ibix said in post #112.
The waves moving through the water depends on interaction between the molecules, which in turn relies on electromagnetism. Since the whole initial premise of the Train experiment is that the speed of light (and thus the speed of electromagnetic interaction) is invariant. This means the speed of interaction between water molecules according to the train observer will be dependent on this fact. The speed of the waves measured by them relative to themselves also depends on this. This in turn results in her measuring the speed of the wave with respect to point O to differ depending on the direction the waves are traveling.
This is how things work in a Relativistic universe. Your whole objection seems to be based on the idea that the conclusions run counter to Newtonian rules. We don't live in a Newtonian universe, so we shouldn't expect its behavior to be constrained to Newtonian rules.

 

[Ziang]

So I can say that according to SR,
If a medium is moving straight at a constant velocity, the velocity of mechanic waves in/on the medium depends on the direction of wave propagation.
For an example, if the water was touched at the center of a moving pool, then the waves look like eggs instead of circles.