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Dear PF Forum,

There's a problem in SR that I cannot solve (or understand)

This is, I think, a well known SR problem.

The picture is telling the problem itself.

A and B and C are in the same frame (let's call it ABC frame)

D and E are in the same frame (DE)

wrt DE

D stays and E stays above D, 300 km.

ABC are approaching DE.

When A meets D, D shines a light PERPENDICULAR above it and when the light reaches E, E meets B.

E then bounces the light vertically down. When the light reaches D again, D meets C.

wrt ABC

ABC sees that DE are approaching ABC and the light shines diagonally not vertically.

D meets with C and then later, A ask C what its clock reading when C meets D. C says 2.5 seconds

Because A knows that C is 450 km away, then A will know DE speed.

I think in SR, A doesn't have to ask C to know DE speed.

Somehow the universe has arranged it? I mean like Twin paradox, the planet doesn't have to really see that the rocket has turned around. It's sufficient that the rocket worldline has crossed planet worldline then the planet experience time dilation.

I'm sorry I can't express my point clearly. I hope you understand what I mean in the paragraph above.

ABC will see the light travels diagonally, ED will see the light travels vertically. If we do the "simple" math, we'll find that the distance and time will be in Lorentz factor ##\gamma = \frac{1}{\sqrt{1-v^2}} = 1.25##

But how can ABC sees the light diagonally? What if ABC insist that they stay and it's DE who are moving?

This is permitted in SR, right. So the solution is, it's not a beam of light. Light expands in sphere. The beam of light that ABC see traveling diagonally, will be seen by DE vertically. The beam of light that ABC see traveling vertically, will be seen by DE diagonally.

It's just that particular beam that ABC sees diagonally and seen by DE vertically that makes C sees D clock is time dilated. Of course D will see ABC clocks time dilated as well. That C clock is 2.5 seconds and D clock is 2 second, it's the relative simulatenity of events as understood by D.

That I think I have already understood above.

But what if D sends a laser beam? E will agree that E receives high intensity light. Proof that the light is vertical wrt DE.

Now, B will receive a high intensity light won't he? This both party would agree.

E and B receive high intensity light.

1. How can DE see the same light as seen by ABC vertical, while that same light travels diagonally according to ABC?

2. Shouldn't the light not affected by the motion?

3. I think it's the same whether DE bring the laser transmitter or the laser transmitter is lying around on the ground, and DE and ABC fly from different direction and when DE and ABC meets, won't they see the laser hits vertically above them?

4. I think in no 3, they both will see the laser shines diagonaly

5. Does length contraction in this case play a role in the length of the torch or emitter. Where a stationary observer will see that the torch/emitter somehow tilted and causing the light/signal to travel diagonally?

Thanks you.

There's a problem in SR that I cannot solve (or understand)

This is, I think, a well known SR problem.

The picture is telling the problem itself.

A and B and C are in the same frame (let's call it ABC frame)

D and E are in the same frame (DE)

wrt DE

D stays and E stays above D, 300 km.

ABC are approaching DE.

When A meets D, D shines a light PERPENDICULAR above it and when the light reaches E, E meets B.

E then bounces the light vertically down. When the light reaches D again, D meets C.

wrt ABC

ABC sees that DE are approaching ABC and the light shines diagonally not vertically.

D meets with C and then later, A ask C what its clock reading when C meets D. C says 2.5 seconds

Because A knows that C is 450 km away, then A will know DE speed.

I think in SR, A doesn't have to ask C to know DE speed.

Somehow the universe has arranged it? I mean like Twin paradox, the planet doesn't have to really see that the rocket has turned around. It's sufficient that the rocket worldline has crossed planet worldline then the planet experience time dilation.

I'm sorry I can't express my point clearly. I hope you understand what I mean in the paragraph above.

**Here is my problem**ABC will see the light travels diagonally, ED will see the light travels vertically. If we do the "simple" math, we'll find that the distance and time will be in Lorentz factor ##\gamma = \frac{1}{\sqrt{1-v^2}} = 1.25##

But how can ABC sees the light diagonally? What if ABC insist that they stay and it's DE who are moving?

This is permitted in SR, right. So the solution is, it's not a beam of light. Light expands in sphere. The beam of light that ABC see traveling diagonally, will be seen by DE vertically. The beam of light that ABC see traveling vertically, will be seen by DE diagonally.

It's just that particular beam that ABC sees diagonally and seen by DE vertically that makes C sees D clock is time dilated. Of course D will see ABC clocks time dilated as well. That C clock is 2.5 seconds and D clock is 2 second, it's the relative simulatenity of events as understood by D.

That I think I have already understood above.

But what if D sends a laser beam? E will agree that E receives high intensity light. Proof that the light is vertical wrt DE.

Now, B will receive a high intensity light won't he? This both party would agree.

E and B receive high intensity light.

**My question**1. How can DE see the same light as seen by ABC vertical, while that same light travels diagonally according to ABC?

2. Shouldn't the light not affected by the motion?

3. I think it's the same whether DE bring the laser transmitter or the laser transmitter is lying around on the ground, and DE and ABC fly from different direction and when DE and ABC meets, won't they see the laser hits vertically above them?

4. I think in no 3, they both will see the laser shines diagonaly

5. Does length contraction in this case play a role in the length of the torch or emitter. Where a stationary observer will see that the torch/emitter somehow tilted and causing the light/signal to travel diagonally?

Thanks you.

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