# What is the twin paradox

#### ranjitnepal

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#### analyst5

Hey, I was also thinking about starting a similar thread, but since it's already started I'll ask a straight-forward question that must be regarded here, why does the traveling twin see the clocks on the earth (the twin that stayed at home) move slower, while less proper time elapses for mih. Shouldn't he see the twin that stayed at earth age faster, and not slowe?

Mentor
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#### Nugatory

Mentor
Hey, I was also thinking about starting a similar thread, but since it's already started I'll ask a straight-forward question that must be regarded here, why does the traveling twin see the clocks on the earth (the twin that stayed at home) move slower, while less proper time elapses for mih. Shouldn't he see the twin that stayed at earth age faster, and not slowe?
You'll find a good answer in previous threads on this forum. One of ghwellsjr's spacetime diagrams explaining the twin paradox in terms of relativistic Doppler is what you're looking for.

#### ghwellsjr

Gold Member
Start with the concept of an Inertial Reference Frame (IRF), as system of spatial and time coordinates, that allow you to specify when and where each twin is. Then for any twin that is moving, time progresses more slowly compared to the Coordinate Time of the IRF, the faster he moves, the slower his time progresses. So if both twins start out together at rest in the IRF but then one of them moves away and comes back, the one that moved will have elapsed less time the the one that stayed put. Simple, isn't it?

#### ghwellsjr

Gold Member
Hey, I was also thinking about starting a similar thread, but since it's already started I'll ask a straight-forward question that must be regarded here, why does the traveling twin see the clocks on the earth (the twin that stayed at home) move slower, while less proper time elapses for mih. Shouldn't he see the twin that stayed at earth age faster, and not slowe?
If you draw the motions of the twins in a diagram of an IRF, you can also draw in the light signals that depict what each one sees and your answers will be clear as a bell. I have already invited you in one of your other threads to specify a scenario and I will do all the work for you but you still have not taken me up on my offer. If it is just a simple twin scenario, you can do a search on my name for the word "diagram" and you will find lots that already depict the twin scenario and answer your questions. Please do that before asking for one that is only insignificantly different.

But to answer your question: the traveling twin will see the earth twin age more slowly than himself on the outbound portion of his trip but more quickly on the inbound portion and it is the sum of the two that ends up with the traveling twin seeing the earth twin age more for the entire trip. Haven't we gone over this before in one of your threads, including Durant's threads that I linked to?

#### analyst5

If you draw the motions of the twins in a diagram of an IRF, you can also draw in the light signals that depict what each one sees and your answers will be clear as a bell. I have already invited you in one of your other threads to specify a scenario and I will do all the work for you but you still have not taken me up on my offer. If it is just a simple twin scenario, you can do a search on my name for the word "diagram" and you will find lots that already depict the twin scenario and answer your questions. Please do that before asking for one that is only insignificantly different.

But to answer your question: the traveling twin will see the earth twin age more slowly than himself on the outbound portion of his trip but more quickly on the inbound portion and it is the sum of the two that ends up with the traveling twin seeing the earth twin age more for the entire trip. Haven't we gone over this before in one of your threads, including Durant's threads that I linked to?
Actually, I've never asked you a thing about time dilation and this is the first time. My questions on previous threads were related to other relativistic effects. But thank you indeed for the appropriate answer.

#### Dale

Mentor

ranjitnepal, please put in some effort before you ask others to do so. This topic has been discussed at length here: https://www.physicsforums.com/search.php?searchid=3774972 [Broken]

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