1. Two planets, A and B exist few light years apart in diffrent solar systems. Their relative sped is zero.
A ship came from outside, and will pass A, and then B. Ship is travelling with a speed of 0.999c. Ship is programmed to broadcast via radio wave its current time when passing A and B. A and B are also programmed to do so, they transmit their local times (wich are synchronized, A and B has the same time, and relative speed = 0, time passes equally on them).

Observer is on ship:
- passed A, sent ship time, got A's time
- passed B, sent ship time, got B's time
after comparing times, B is only a few minutes ahead of A due to time dilatation
result in comparing internal ships time when he passed A and B it much greater, because also of time dilatation. time go slower on B and A while they are traveling relative to ship.

More time passed for ship than for A or B. Observer is older.

Also due to length contraction, if observer is on that ship, and manage to accelerate agnist an object it want to go to (B), it can shrink the distance and reach it faster than it would be seen if the observer is off the ship.

Observer is on planet B:
- passed ship, sent B's time, got ships time
- asked A to transmit A's time and ships time when ship passed A
result in A and B time diffrence tells me that ship traveled from A to B few years
ships time (broadcasted to A and B in moment of passing each of them) tells me that only few minutes passed for it.

Observer is older because more time passed for A or B than for ship.

There is no acceleration involved. How will you counter this paradox now?
Is my thinking flawed or knowledge incomplete? Or im just right, and there is no symmetry in relativity.

One last thing, was the symmetry proven by someone - or they just assumed its a fact and created certain space-time properties to explain it?
Is the symmetry proven by a solid experiment/observation, or its just an assumption?

2.

3. They're both right because you can't make such direct observations meaningful until the ship is at rest relative to the planets, in which case, they'll all agree.

Also, relativity was an idea, an assumption, but since then, there has been a mountain of evidence collected to support it over other known possibilities.

4. Originally Posted by phy_11
Two planets, A and B exist few light years apart in diffrent solar systems. Their relative sped is zero.
A ship came from outside, and will pass A, and then B. Ship is travelling with a speed of 0.999c. Ship is programmed to broadcast via radio wave its current time when passing A and B. A and B are also programmed to do so, they transmit their local times (wich are synchronized, A and B has the same time, and relative speed = 0, time passes equally on them).

Observer is on ship:
- passed A, sent ship time, got A's time
- passed B, sent ship time, got B's time
after comparing times, B is only a few minutes ahead of A due to time dilatation
result in comparing internal ships time when he passed A and B it much greater, because also of time dilatation. time go slower on B and A while they are traveling relative to ship.

More time passed for ship than for A or B. Observer is older.

Also due to length contraction, if observer is on that ship, and manage to accelerate agnist an object it want to go to (B), it can shrink the distance and reach it faster than it would be seen if the observer is off the ship.

Observer is on planet B:
- passed ship, sent B's time, got ships time
- asked A to transmit A's time and ships time when ship passed A
result in A and B time diffrence tells me that ship traveled from A to B few years
ships time (broadcasted to A and B in moment of passing each of them) tells me that only few minutes passed for it.

Observer is older because more time passed for A or B than for ship.

There is no acceleration involved. How will you counter this paradox now?
Is my thinking flawed or knowledge incomplete? Or im just right, and there is no symmetry in relativity.
You're neglecting the Relativity of Simultaneity. While it is true that for the Ship observer that time passed slower for A and B while he passed between them, it is also true that the times at A and B will not be synchronized according to him. It will be later at B than it is at A. When he gets to B and checks the time on it, he will see that it reads much later than what the time was at A when he left, and that the difference between the time at A when he left and the time he reads at B when he arrives will be greater than the time that he measured as passing on his own clock.

For example, lets assume that it takes one year for him to pass from A to B by his clock, and that time passes 1/2 as fast on B as it does for him due to time dilation, meaning that 1/2 year passes for B, according to him. Then also, according to him, it will always be 1 1/2 years later at B than it is at A, even though according to A and B, it is the same time on both planets.

Thus if he passes A when at the start of 2011 by A's clock, it will already be 6 months into 2012 by B's clock at that same moment according to him. B's clock will accumulate 6 more months during his trip and reads 2013 when he arrives, with 1 year having past on his own clock.

From B's standpoint, it is the start of 2011 when the ship leaves A, It takes two years for the ship to make the trip, making it 2013 when it arrives and time dilation will have made it so that 1 yr will have past on the ship clock.

IOW, even though the observers will disagree as to who's clock ran slower during the trip, everyone will still agree as to what the respective clock readings on the ship and B will be when the ship passes B, and there is no paradox.

5. You're neglecting the Relativity of Simultaneity.
Ok, lets change subject to this, because its more basic while it covers this paradox.

Imagine a train moving with a high speed.

One person is inside, and one on station.
Train has a computer with sensor programmed to send signal with speed of c in both directions, when signal hits backward and forward wall of train, it will make a lights to pulse. Two lights are in the back and forward of the train, with equal distance from persin in train (observer).
I think i described it in enough details. The sensor is in the middle of a train, and will fire signals when pass person on station.

If observer is the person inside train, he will see both lights reach him at the same time. He will ask later person on station what did he saw, and person from station (wich is not an observer) will have to report same thing - both lights reached person in train at the same time. He couldnt tell that forward light reached person in train first, because that person experienced something diffrent.
Its like being hit by a car. Parts of your body are all over the place, car is in blood, and with the last breath you ask witness what happend. He replies 'nothing, he missed'.
Whats the conclusion? Light traveled slower/faster than c in person_on_station point of view (not an observer!). If train would be moving with almost speed of c, person on station would report (hes not observer, just a person, observer is in train) that light from end to forward section of the train is travelling faster while this from the opposite direction is almost still.

6. He saw the two lights hit the guy at the same time, but not fire at the same time. The one from forward fired later and covered less distance from the person on the platform's point of view.

7. Originally Posted by phy_11
You're neglecting the Relativity of Simultaneity.
Ok, lets change subject to this, because its more basic while it covers this paradox.

Imagine a train moving with a high speed.

One person is inside, and one on station.
Train has a computer with sensor programmed to send signal with speed of c in both directions, when signal hits backward and forward wall of train, it will make a lights to pulse. Two lights are in the back and forward of the train, with equal distance from persin in train (observer).
I think i described it in enough details. The sensor is in the middle of a train, and will fire signals when pass person on station.

If observer is the person inside train, he will see both lights reach him at the same time. He will ask later person on station what did he saw, and person from station (wich is not an observer) will have to report same thing - both lights reached person in train at the same time. He couldnt tell that forward light reached person in train first, because that person experienced something diffrent.
Its like being hit by a car. Parts of your body are all over the place, car is in blood, and with the last breath you ask witness what happend. He replies 'nothing, he missed'.
Whats the conclusion? Light traveled slower/faster than c in person_on_station point of view (not an observer!). If train would be moving with almost speed of c, person on station would report (hes not observer, just a person, observer is in train) that light from end to forward section of the train is travelling faster while this from the opposite direction is almost still.
Let's simplify this a bit.

You have two observers, one on the embankment, and one on a train car. Two flashes occur that originate at points equally distant from the embankment observer and timed that they arrive at the embankment observer at the same time as the train observer passes him, so that both observers see the flashes simultaneously, such a shown in the following animation, where the flashes are shown by expanding circles.
This animation shows things from the rest frame of the embankment.

The following animation shows the same situation from the rest frame of the train car.

Given that light must travel at c as measured by the observer at rest in this frame (the train observer), the light must expand as circles with a center that maintains a constant distance from the observer.

Since both observers must be an equal distance from the points where the flashes were emitted when they see the flashes, this means that the train observer has to be closer to one flash origin than the other when the flashes originate.

The only way he can be an unequal distance from these points when the flashes originate, and have the light from both flashes travel at the same speed relative to himself, and see the flashes at the same time when he is equally distant from these point, the flashes have to originate at different times.

IOW, events that are simultaneous in one frame are not simultaneous in the other.

8. Originally Posted by phy_11
Originally Posted by Janus
You're neglecting the Relativity of Simultaneity....and....IOW, events that are simultaneous in one frame are not simultaneous in the other.
Ok, lets change subject to this, because its more basic while it covers this paradox.

Imagine a train moving with a high speed.

One person is inside, and one on station.
Train has a computer with sensor programmed to send signal with speed of c in both directions, when signal hits backward and forward wall of train, it will make a lights to pulse. Two lights are in the back and forward of the train, with equal distance from persin in train (observer).
I think i described it in enough details. The sensor is in the middle of a train, and will fire signals when pass person on station.

If observer is the person inside train, he will see both lights reach him at the same time. He will ask later person on station what did he see...
The Relativity of Simultaneity resolves this apparent paradox - as Janus explains.

I may be over-complicating this question, but I think the OP is considering the observer in the train to be the "at rest" observer. He sees the light pulses (or reflections, if you will) from each end of the train emanate at the same time.

The person at the side of the tracks is in the "moving" frame according to the way this experiment is described in the OP. The person standing beside the track will see the light pulses emanate from the front and back walls of the train at different times.

The net effect is the same in terms explaining this phenomenom. The only difference is whether you arrange the experiment so the observer on the train is in the rest frame or the observer beside the track is in the rest frame.

Chris

9. when trian passes person on station, a sensor send signal to both light sources.
Are you telling me, that signal will reach forward light source first?
signal is also travelling at speed of c.

sensor, and both people are in straight line when sensor fires.

10. Originally Posted by phy_11
when trian passes person on station, a sensor send signal to both light sources.
Are you telling me, that signal will reach forward light source first?
signal is also travelling at speed of c.

sensor, and both people are in straight line when sensor fires.
In the frame of reference of the observer on the train, the light signals will reach the front and back walls of the train at the same time.

As I understand it, in the frame of reference of the observer standing beside the track, the light signal will reach the back wall of the train before the light signal reaches the front wall of the train.

Chris

11. Originally Posted by phy_11
when trian passes person on station, a sensor send signal to both light sources.
Are you telling me, that signal will reach forward light source first?
signal is also travelling at speed of c.

sensor, and both people are in straight line when sensor fires.
As the last poster pointed out, this depends on who you ask.

Remember, the speed of light is invariant, meaning that everyone measures it as having the same speed relative to themselves.

For train observer, sitting in the middle of the train, the light expands outward at c with him at the center, and reaches the front and back at the same time.

For the embankment observer, the same light expands outward at c with himself at the center. In the meanwhile, the back of the train is coming towards him and is rushing towards the light headed towards it, and the front of the train is moving away from him and is running away from the light headed towards it. Ergo, the light reaches the rear of the train before it reaches the front.

12. phy_11, it might pay you to think of it like this.

Light always propagates at c, in all directions, from a point that is at rest in relation to the observer, whatever the relative motion between the light source and the observer.

If a moving object passes you and flashes a beacon, from your point of view the light propagates from the place the beacon was when it flashed, regardless of the motion of the beacon afterwards. From your point of view, the object passes you but the light propagates a spherical wavefront that is at rest in relation to you, and the object then moves through that sphere as it propagates, so the spherical wavefront does not remain centred on the source.

However, if you are riding with the object in question, and so are at rest in relation to the source, the light will propagate at c in all directions from the source - the spherical wavefront remains centred on the source.

13. For the embankment observer, the same light expands outward at c with himself at the center. In the meanwhile, the back of the train is coming towards him and is rushing towards the light headed towards it, and the front of the train is moving away from him and is running away from the light headed towards it. Ergo, the light reaches the rear of the train before it reaches the front.
Ok. So the signal will actually hit back wall first. Let it have a timer and record the time of receiving signal. Front wall the same. (Time measured in train, 2 synchronized clocks, time slowed by the same fraction bue to move relative to observer).

If there is a symmetry, person in train must in any case answer that those timers are equal. If back wall was hit first - they cant be.

14. Let's put the light in the middle of the train carriage.

From the point of view of an observer sitting under the light, in the train carriage:

From the point of view of an observer standing on the embankment as the train passes by:

Light always propagates at c in all directions, from a point at rest in relation to the observer, hence the relativity of simultaneity.

15. Thanks for tryuing to help me, but i understand it. I know the light will travel always at speed of c from observers poit of view.

I want to hear that there is NO symmetry in the universe. Your picture just ilustrated that.

light reached back first.
If there are clocks wich stop when hit by light, whe forward one would be:

- more advanced if observer is on station
- equally advnaced if observer is in train

Observer decides about that state. My point is to either understand symmetry (in wich case both clocks must ALWAYS be at same time), or hear from you that there is no such thing, wich i already know assuming im right.

Perhaps my definition of symmetry is diffrent than yours.
Lets define it:
3 people are in this train scenario. Call them A, B and C.
C is the observer, person for wich c is constant.
A is travelling in the middle of a train, B is on station.
train has 2 light emitters in the back wall and forward wall.
Back wall fire when it pass through start of station, forward wall fire when pass through end of it:

16. Originally Posted by phy_11
For the embankment observer, the same light expands outward at c with himself at the center. In the meanwhile, the back of the train is coming towards him and is rushing towards the light headed towards it, and the front of the train is moving away from him and is running away from the light headed towards it. Ergo, the light reaches the rear of the train before it reaches the front.
Ok. So the signal will actually hit back wall first. Let it have a timer and record the time of receiving signal. Front wall the same. (Time measured in train, 2 synchronized clocks, time slowed by the same fraction bue to move relative to observer).

If there is a symmetry, person in train must in any case answer that those timers are equal. If back wall was hit first - they cant be.
You still seem to think that "synchronized" has an absolute meaning. If the timers are synchronized in the trains frame, then they are not so in the embankment frame.

Imagine that your timers start when the light reaches them like this, as seen in the train's frame:

However, this is what happens in the Embankment frame with the same light:

Thus in the Train's frame the clocks are synchronized, and in the Embankment frame they are not. And this will be true no matter what scheme you use to synchronize the timers in the Train's frame.

17. Originally Posted by phy_11
Perhaps my definition of symmetry is diffrent than yours.
Lets define it:
3 people are in this train scenario. Call them A, B and C.
C is the observer, person for wich c is constant.
A is travelling in the middle of a train, B is on station.
train has 2 light emitters in the back wall and forward wall.
Back wall fire when it pass through start of station, forward wall fire when pass through end of it:
Your example is worded oddly, so I can't tell if it is wrong, or not. People are observers too. c is constant for A, B and C. We already have the two observers, A and B, so we don't need C.

If the observer is A, they will see both lights flash at the same time, but they will calculate that B sees the lights flash at different times.

If the observer is B, they will see both lights flash at different times, but they will calculate that A sees the lights flash at the same time.

You cannot observe what happens from the point of view of another frame, you can only calculate it, based on your own observations, which are made in your own frame.

There is symmetry in the train and embankment thought experiment, and it is a symmetry that proves that there is no absolute simultaneity - it shows that simultaneity is relative.

But, what is this symmetry? Well, if you put the simultaneous lights on the train, a person on the embankment will calculate, from what they see (and subtracting the time it took the lights to reach them), that the lights were not simultaneous. They are correct. Reality, to the frame of the embankment, is that the lights were not simultaneous, even if they know the delay time between the lights flash and them seeing it. But, the observer on the embankment also understands that reality, in the frame of the train, is that the lights were simultaneous.

But, if you put the simultaneous lights on the embankment instead of the train, then it is the person on the train that will calculate, from what they see, that the lights were not simultaneous. The situation is inversely symmetrical!

And if we have two sets of lights, one on the train, and another on the embankment, where each set of lights flash simultaneously in their own frame, the other frame will calculate that the lights did not flash simultaneously. There is the symmetry!

There is no absolute simultaneity. No two events, separated by space, can be claimed to be simultaneous in any absolute way.

Events (separated by space) that are simultaneous in one frame may not be simultaneous in another frame, even after taking into account the time it takes for the light to travel to that other frame, if there is motion between those frames.

18. Another way to phrase that is that the symmetry is that there is no way to say either observer is the one that's still versus moving. They both are both.

19. Ok i think i understand where your going.
I also had misdefined term 'frame of reference', thats what you get when reading texts in 3 diffrent languages.

When im in train and passing the station, while station has 2 light sources, one in the rear and one in the front, they fire while im in the middle of the station, i see the forward one first.
This is because the 2 light sources are not in rest relative to me. Relativity says that c is constant in one frame of reference. So everything in rest in equal distance to both emitters will report they flashed in one tmie. If i move (or they move, same thing) relative to those emitters, c isnt constant. If i would move at near c velocity, i would almost never detected rear light. Light will of course travel equal distance, but as i move, speeds (me and light) can be added/subtracted.

20. Originally Posted by phy_11
Ok i think i understand where your going.
I also had misdefined term 'frame of reference', thats what you get when reading texts in 3 diffrent languages.

When im in train and passing the station, while station has 2 light sources, one in the rear and one in the front, they fire while im in the middle of the station, i see the forward one first.
This is because the 2 light sources are not in rest relative to me. Relativity says that c is constant in one frame of reference. So everything in rest in equal distance to both emitters will report they flashed in one tmie. If i move (or they move, same thing) relative to those emitters, c isnt constant. If i would move at near c velocity, i would almost never detected rear light. Light will of course travel equal distance, but as i move, speeds (me and light) can be added/subtracted.
NO.

The speed of light is constant in all frames.

A problem with your example is that you do not state in which frame (train or station) the train and station are the same length. They will not be so in both frames. The train sees the station as length contracted and the station sees the train as length contracted. So if the station sees the train as being equal length, it is the contracted length of the train that is so, meaning that the "at rest" length of the train is longer than the station.

Since the "at rest" length of the train is longer than the station, and the train sees the station as length contracted, then the train will measure the station as being shorter than the train, the ends of the station and train will not line up at the same time, and the emitters will not trigger at the same time.

In this situation, events according to the station occur like this (with the red dots representing the ends of the station):

and events according to the train occur like this:

If the station and train are the same length according to the frame of the train, then the roles of train and station are reversed.

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