Flash in rocket will arrive first at front or back?

[songoku]

Homework Statement

A person in a rocket traveling at 1/2 c. There is a flash originated from the middle of the rocket. From his point of view, will the flash arrive at the front or the back of the rocket first or arrive at same time?

Homework Equations

Not sure

The Attempt at a Solution

Since the speed of flash = speed of light = c and its speed is always constant for all inertial frame of reference, the flash will arrive at the same time to front and back of the rocket?

Thanks

 

[PeroK]

Homework Statement

A person in a rocket traveling at 1/2 c. There is a flash originated from the middle of the rocket. From his point of view, will the flash arrive at the front or the back of the rocket first or arrive at same time?

Homework Equations

Not sure

The Attempt at a Solution

Since the speed of flash = speed of light = c and its speed is always constant for all inertial frame of reference, the flash will arrive at the same time to front and back of the rocket?

Thanks

That's right.

 

[songoku]

The position of the person matters in this case, right? I mean if he stands closer to the front of the rocket, he will see the flash reaches the front first because shorter distance travel by the light? Or the time difference will be really small so flash arrives at same time to front and back of rocket?

How about if instead of flash, the source of light is thunder? A person standing in the middle of train moving with 1/2 c will observe the light reaching back or front of the train first?
Since the location of the light source can not be known, it can not be determined whether the light will reach which end first?

Thanks

 

[DaveC426913]

The position of the person matters in this case, right? I mean if he stands closer to the front of the rocket, he will see the flash reaches the front first because shorter distance travel by the light?

Of course. But the question doesn't mention a person. It refers to the front and back of the rocket, which - since the flash occurred in the middle - are equidistant.

Or the time difference will be really small so flash arrives at same time to front and back of rocket?

That depends entirely on how long the rocket is.

How about if instead of flash, the source of light is thunder?

Mm. Did you want to rephrase that?

Sound will reach both ends of the rocket simulataneously. Remember, as far as the rocket is concerned, it is stationary (as is the air through which the thunder passes), and the universe is moving past it.

Since the location of the light source can not be known, it can not be determined whether the light will reach which end first?

I think we're getting off track here.

 

[songoku]

Mm. Did you want to rephrase that?

Sound will reach both ends of the rocket simulataneously. Remember, as far as the rocket is concerned, it is stationary (as is the air through which the thunder passes), and the universe is moving past it.

Sorry my bad. I mean lightning so the question is still about light, not sound.

I think we're getting off track here.

Does the question make any sense if it is about lightning? I am trying to understand how about if position of the source of light is unknown then what kind of phenomena will be observed

Thanks

 

[DaveC426913]

Does the question make any sense if it is about lightning? I am trying to understand how about if position of the source of light is unknown then what kind of phenomena will be observed

Sorry, Can you elaborate? Why would the position of the flash be unknown simply because the source is lightning?

Perhaps this is overthinking things.
The fact that the rocket is moving is utterly irrelevant. It is only moving in reference to some arbitrary external reference point.
In every way, what happens inside the rocket will be experienced just as if the rocket is not moving at all.

 

[songoku]

Maybe it is overthinking things. I will ask again if I have more solid question.

Thank you very much for the help perok and dave

 

[PeroK]

The position of the person matters in this case, right? I mean if he stands closer to the front of the rocket, he will see the flash reaches the front first because shorter distance travel by the light?

Relativity deals with Reference Frames, not what one observer "sees". In the original question it asks "from his point of view". In my opinion this is imprecise and potentially misleading language. Relativity isn't about "points of view"; it's about reference frames.

If you ask when one observer "sees" an event, then that is not the time that the event took place. Relativity is not about the delay in light signals reaching an observer. Note that a lot of material on the subject gives this impression. Even Einstein's original train and lightning thought experiment is a bit misleading as it emphasises individual observers in specific locations.

So, it makes no difference where he stands. In fact, it makes no difference whether he "sees" anything.

A better way to think about the experiment is that in the rocket there are clocks, synchronised in the rocket frame, at either end. Each clock records the time the flash reaches the end of the rocket. If the source is in the middle of the rocket, then both clocks will record the same time.

The "observer" then receives signals from each clock that communicates these times. It makes no difference how long this takes. It could be the next day. The clocks could download a whole day's data of various experiments for him to analyse.

To be be precise, the two events take place at the same time (in the rocket frame) if each flash hits the end of the rocket at the same time - this is independent of when any single observer sees these events.

 

[songoku]

Relativity deals with Reference Frames, not what one observer "sees". In the original question it asks "from his point of view". In my opinion this is imprecise and potentially misleading language. Relativity isn't about "points of view"; it's about reference frames.

If you ask when one observer "sees" an event, then that is not the time that the event took place. Relativity is not about the delay in light signals reaching an observer. Note that a lot of material on the subject gives this impression. Even Einstein's original train and lightning thought experiment is a bit misleading as it emphasises individual observers in specific locations.

So, it makes no difference where he stands. In fact, it makes no difference whether he "sees" anything.

A better way to think about the experiment is that in the rocket there are clocks, synchronised in the rocket frame, at either end. Each clock records the time the flash reaches the end of the rocket. If the source is in the middle of the rocket, then both clocks will record the same time.

The "observer" then receives signals from each clock that communicates these times. It makes no difference how long this takes. It could be the next day. The clocks could download a whole day's data of various experiments for him to analyse.

To be be precise, the two events take place at the same time (in the rocket frame) if each flash hits the end of the rocket at the same time - this is independent of when any single observer sees these events.

So is it correct to say the location of the flash matters? If the flash occurs closer to the front, the front clock will record less time compared to the other clock?

Thanks

 

[DaveC426913]

So is it correct to say the location of the flash matters? If the flash occurs closer to the front, the front clock will record less time compared to the other clock?

Based on what you know, what do you think?

 

[songoku]

I think it is correct

 

[PeroK]

So is it correct to say the location of the flash matters? If the flash occurs closer to the front, the front clock will record less time compared to the other clock?

Thanks

If you have an inertial reference frame, then normal kinematics applies. Light travels at a constant $c$. If a light source is a distance $d$ from a target, then it takes time $t = d/c$ to reach the target. If the target is further away, it takes longer to reach it.

It's only when you move between reference frames that you have the Lorentz transformation (time dilation, length contraction, relativity of simultaneity, invariance of the speed of light).

 

[songoku]

If you have an inertial reference frame, then normal kinematics applies. Light travels at a constant $c$. If a light source is a distance $d$ from a target, then it takes time $t = d/c$ to reach the target. If the target is further away, it takes longer to reach it.

It's only when you move between reference frames that you have the Lorentz transformation (time dilation, length contraction, relativity of simultaneity, invariance of the speed of light).

Thank you very much