How Does Special Relativity Affect Perceived Distances Between Planets?

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Discussion Overview

The discussion revolves around the effects of special relativity on the perceived distances between two planets, A and B, which are 4 light years apart in Tom's rest frame. The scenario involves Mary traveling at 0.8c towards Tom and the implications of relativistic effects on distance measurements from different frames of reference.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that when Mary passes planet B, she perceives Tom to be 2.4 light years away, while Tom measures Mary to be 4 light years away in his rest frame.
  • Others argue that the concept of "now" differs between Tom and Mary due to the relativity of simultaneity, complicating their distance measurements.
  • A participant suggests that both Tom and Mary could consider themselves at rest and measure the distance to each other as 2.4 light years at certain times, depending on their relative motion.
  • Some participants emphasize that the definition of distance is not symmetric between the two frames, as it depends on the chosen reference frame.
  • One participant introduces a geometric analogy to illustrate the differences in perceived distances in spacetime, suggesting that the geometry of spacetime leads to different results than in Euclidean space.
  • There is a discussion about the implications of time dilation, with participants noting that both observers will perceive the other's clock as running slower.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of distance and simultaneity in different frames. There is no consensus on how to reconcile the differing perceptions of distance between Tom and Mary.

Contextual Notes

Participants highlight the importance of specifying the frame of reference when discussing distances and times, as well as the potential confusion arising from using planets as reference points. The discussion remains unresolved regarding the implications of simultaneity on distance measurements.

  • #91
dwspacetime said:
View attachment 337748
no matter which event happens first one of them is going to have 2.4 lys before 4 years why
This is simply wrong. This is not a proper spacetime diagram. The proper spacetime diagram was posted by me above. Please refer to that instead of making up your own faulty notions. Your diagram completely fails to account for the relativity of simultaneity. You will simply never understand relativity without a proper understanding of the relativity of simultaneity. Instead you will get lost in a quagmire of imagined paradoxes.

Note in the actual diagram how the blue and red lines do not coincide! They have different slopes. This is the relativity of simultaneity.

It is true that the event where C and D meet on M and the event where C’ and D’ meet on T have spacelike separation. This means that the order in which they will happen is going to depend on the reference frame - again relativity of simultaneity! In the frame I based the diagram on they both occur at the same time due to symmetry.

However, in T’s frame CD occurs before C’D’ but in M’s frame C’D’ occurs before CD. This must also be the case due to symmetry.
 
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  • #92
Orodruin said:
This is simply wrong. This is not a proper spacetime diagram. The proper spacetime diagram was posted by me above. Please refer to that instead of making up your own faulty notions. Your diagram completely fails to account for the relativity of simultaneity. You will simply never understand relativity without a proper understanding of the relativity of simultaneity. Instead you will get lost in a quagmire of imagined paradoxes.

Note in the actual diagram how the blue and red lines do not coincide! They have different slopes. This is the relativity of simultaneity.

It is true that the event where C and D meet on M and the event where C’ and D’ meet on T have spacelike separation. This means that the order in which they will happen is going to depend on the reference frame - again relativity of simultaneity! In the frame I based the diagram on they both occur at the same time due to symmetry.

However, in T’s frame CD occurs before C’D’ but in M’s frame C’D’ occurs before CD. This must also be the case due to symmetry.
thanks for introducing all that to me.
 
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  • #93
dwspacetime said:
that tells me that depending on which inertia frame you are in the sequence of events could be reversed.
That is correct; the time ordering of spacelike separated events is frame dependent.

dwspacetime said:
the reason for something to happen could be the result.
No, it can't, because spacelike separated events in relativity can't be causally connected. Neither can cause the other or be an effect of the other.
 
  • #94
dwspacetime said:
the universe is wrong. sorry
That would make a great title for a book:

Sorry, The Universe is Wrong (the thoughts of a science skeptic)
 
  • #95
By definition the Universe is right. Whether that conforms to one’s own prejudice or not is a different matter. The Universe is in no way obliged to make itself easy to understand.
 
  • #96
Ok, but what is the point of this thread @dwspacetime ? You want to learn something, or just simply argue with everyone knowledgeable?
 
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  • #97
weirdoguy said:
Ok, but what is the point of this thread @dwspacetime ? You want to learn something, or just simply argue with everyone knowledgeable?
I just watched something on YouTube about twin paradox which made me think. I was always curious about relativity but never had a chance to study it.. about a week ago i digged out the book I downloaded years ago "relativity for the questioning minds" but never had a chance to finish it. I'm reading the last chaptor and everything makes sense now. I might quit my job and go to school again for this as my retirement. Thank you guys.
 
  • #99
robphy said:
Dan Styer's book?
https://www2.oberlin.edu/physics/dstyer/
Yes. It is pretty thorough which answered almost all my questions for now. It seems everything is moving in a speed of light..... The faster you move in space the slower in time...
 
  • #100
dwspacetime said:
It is pretty thorough which answered almost all my questions for now.
Unfortunately, the answers it gave you do not appear to be good ones.

dwspacetime said:
It seems everything is moving in a speed of light..... The faster you move in space the slower in time...
This is unfortunately a fairly common pop science viewpoint, but it doens't work. You will not find it in any actual textbooks or peer-reviewed papers, and you should not be using it.
 
  • #101
PeterDonis said:
Unfortunately, the answers it gave you do not appear to be good ones.This is unfortunately a fairly common pop science viewpoint, but it doens't work. You will not find it in any actual textbooks or peer-reviewed papers, and you should not be using it.
Do you have any better recommendations? Everything is moving in speed of light is not in that book. It is what I think. Unless I find better explanation, inevitably I am trying to make sense myself.
 
  • #102
dwspacetime said:
Everything is moving in speed of light is not in that book. It is what I think.
Ah, ok. Then I withdraw my comment about the book; my comment only applies to what you think.

dwspacetime said:
Do you have any better recommendations?
Yes: learn SR properly from a good textbook, such as Taylor & Wheeler's Spacetime Physics.

dwspacetime said:
Unless I find better explanation
You have already been given better explanations right here in this thread, but apparently they didn't take; instead of listening to them and discarding your own wrong understanding, you are questioning them. So now we're to the point of telling you to go learn properly from a textbook. We can only do so much here; if you're not going to listen to the answers you get here to your questions, from people who know a lot more about the subject than you do, continuing the discussion is pointless.

This thread is now closed.
 
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