Simultaneity in Special Relativity: Examining the Theory

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In summary, the conversation discusses the concept of simultaneous events in special relativity and how it is affected by the velocity of an observer. An analogy is used to explain the concept, and it is clarified that the speed of light is always c in every frame of reference.
  • #1
MartinWyckmans
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Thanks for opening the millionth thread about this. My teacher has been explaining special relativity and I became confused at simultaneity.
This is my train of thought, where did I go wrong?

Say a sensor is moving at a speed of c-1 m/s to the right. Somewhere to the left of the sensor there's a wall with a small opening. 2 balls fall down behind the wall and the distance between the balls is 1 meter, and when they pass the small opening the ray of light that gets reflected by the ball gets sent through the opening. The ray of light moves to the right, towards the sensor, with a speed of c. Taking the sensor as point of view, the light travels towards the sensor with 1 m/s. If the second ray of light coming from the second ball hits the sensor 1 second later than the first sensor, then the rays of light began traveling at the same time and the balls would fall simultaneously. In other words, if [tex]\Delta t = \Delta x / (c-v) [/tex] with [tex]\Delta t [/tex] the time difference measured in the sensor, then the actions happen simultaneously.
And say the sensor is moving towards the wall/balls/ray of light, then the formula would be [tex]\Delta t = \Delta x / (c+v) [/tex]
My teacher dismissed my question almost immediately because he said (c+v) is impossible, but I'm not saying the sensor is traveling with a speed that's (c+v). Is it still wrong?

I don't know if I've made myself very clear, so maybe an analogy works better.

Say there're 3 ancient cities; Sparta, Athens and Rome. One day, a messager from Athens visits Sparta, saying the city has been sacked. Two days later, a messager from Rome visits Sparta, saying Rome has been sacked. The Spartans are fearful and wonder whether the same empire sacked both cities. They know that the distance between Sparta and Athens is 200 kilometers and the distance between Sparta and Rome is 600 kilometers. The messagers are trained to run 200 kilometers each day. The time between the two messagers was 2 days. Calculating the time difference with the formula: [tex]\Delta t = \Delta x / v [/tex] [tex]2 = 400 / 200 [/tex] and we see that the time difference is indeed 2 days.

Does this not apply in special relativity? I know that getting the right velocity v would be very difficult to really use, but is this how it theoretically works?
 
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  • #2
Welcome to PF!
MartinWyckmans said:
... the distance between the balls is 1 meter...
In which frame of reference? Distance is relative.
MartinWyckmans said:
...the light travels towards the sensor with 1 m/s.
The light travels toward the sensor at speed c, the speed of light is c in every inertial frame of reference.
 
  • #3
MartinWyckmans said:
Taking the sensor as point of view, the light travels towards the sensor with 1 m/s.

No, it doesn't. Velocities add differently in relativity; see here:

https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity

If you plug in c and c minus 1 m/sec in the formula, you will see that it gives the result c, which means that light moves at c relative to the sensor from the sensor's point of view.
 
  • #4
Thanks both of you, I'm beginning to better understand it now!
 

1. What is simultaneity in special relativity?

Simultaneity in special relativity refers to the concept that the order of events may appear different to different observers depending on their relative motion. In other words, two events may be simultaneous for one observer, but occur at different times for another observer.

2. How does special relativity explain simultaneity?

Special relativity explains simultaneity through the principles of time dilation and length contraction. According to these principles, time and space are relative to the observer's frame of reference, and can be distorted by high speeds or acceleration.

3. Can two events truly occur simultaneously in special relativity?

No, two events cannot truly occur simultaneously in special relativity. The concept of simultaneity is relative and depends on the observer's frame of reference. It is not an absolute concept.

4. How does the theory of relativity impact our understanding of time?

The theory of relativity revolutionized our understanding of time by showing that it is not absolute and can be affected by factors such as velocity and gravity. It also introduced the concept of time dilation, where time can appear to move slower for objects in motion compared to stationary objects.

5. Does the theory of relativity have any practical applications?

Yes, the theory of relativity has many practical applications in modern technology such as GPS systems, which rely on the precise timing of signals from satellites in orbit. It is also used in particle accelerators and nuclear reactors to calculate the effects of high speeds and energy. Additionally, the theory has led to advancements in our understanding of the universe and has allowed us to make more accurate predictions in fields such as astrophysics and cosmology.

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