Question about Special Relativity: Mary from planet B flys at 0.8c to Tom on planet A...

[dwspacetime]

Define what you mean by ”when”. This is what seems to confuse you.

I let you define the "when" whatever way you want. once you answer the question. I will ask you the second question with all the wording the same as i used in the first except switch Tom with Mary. answer the 2nd question with the same definition of "when" you use for the first question then compare the two answers.

 

[Orodruin]

use whatever convention you want to use

No I won’t because it will just lead to you misunderstanding things as this thread has shown. You need to specify what you want to know.

you should get the same answer for the two questions by using whatever convention if you are consistent or you are wrong.

No, you are wrong here. Switching Tom and Mary also switches the ”according to whom”.

Say for example:
M is 4 ly away from T in T’s rest frame, the same event for M is 2.4 ly away from T in M’s rest frame.

The switching is:
T is 4 ly away from M in M’s rest frame, the same event for T is 2.4 ly away from M in T’s rest frame.

Both are true and there is no paradox. Although the statements are the same, the events involved are not. Please refer to the Euclidean analogue (which you have completely ignored so far - that’s just bad form).

 

[dwspacetime]

No I won’t because it will just lead to you misunderstanding things as this thread has shown. You need to specify what you want to know.No, you are wrong here. Switching Tom and Mary also switches the ”according to whom”.

Say for example:
M is 4 ly away from T in T’s rest frame, the same event for M is 2.4 ly away from T in M’s rest frame.

The switching is:
T is 4 ly away from M in M’s rest frame, the same event for T is 2.4 ly away from M in T’s rest frame.

Both are true and there is no paradox. Although the statements are the same, the events involved are not. Please refer to the Euclidean analogue (which you have completely ignored so far - that’s just bad form).

 

[dwspacetime]

it is a paradox. basically my two questions are the same question even I switched them. but you get me two different answers. mary moving towards tom is the same as tom moving towards mary

 

[Orodruin]

it is a paradox. basically my two questions are the same question even I switched them. but you get me two different answers.

I gave you exactly the same answer for the switched question.

Here is the Euclidean equivalent:

Basically you are confused that I could make C and C’ the same length and still have D and D’ the same length.

There is no paradox here. Just you confusing yourself with your own misconceptions about what the answer should be.

 

[dwspacetime]

I gave you exactly the same answer for the switched question.

Here is the Euclidean equivalent:
View attachment 337740
Basically you are confused that I could make C and C’ the same length and still have D and D’ the same length.

There is no paradox here. Just you confusing yourself with your own misconceptions about what the answer should be.

sir what you did is "correct" after wrongfully made my two same statements stated in two different ways two different statements. the two statements are the exactly same although I switched two names. but for the same statements you answered in M is frame T is 4 lys in one answer but in the other T is 2.4ly.

 

[dwspacetime]

I gave you exactly the same answer for the switched question.

Here is the Euclidean equivalent:
View attachment 337740
Basically you are confused that I could make C and C’ the same length and still have D and D’ the same length.

There is no paradox here. Just you confusing yourself with your own misconceptions about what the answer should be.

let me make it simple. basically I stated T and M are moving towards each other which is all i tell you. and you tell me T is 4 lys in M's frame and is also 2.4 lys in M's frame

 

[Histspec]

let's make it very simple. let me modify my original question to this. Tom and Mary in spacetime. in Tom's frame Mary is moving towards him in 0.8C and in Mary's frame Tom is moving towards her in 0.8C. when Mary is 4 lys away from Tom in Tom's frame how far is Mary from Tom? why?

Note that your statement "when Mary is 4 lys away" from Tom has to be the result of a measurement along a distance separated by two endpoints that are constantly at rest, otherwise there is no "distance" onto which we can apply the length contraction formula, and there would be nothing to discuss about.... So there is always something equivalent to your planets, if you want it or not.

It's like the relativity of motion: If a ball is at rest in Tom's frame, it is obviously in motion in Mary's frame. Do you think this "asymmetry" violates the relativity principle?

Now let's discuss your length contraction experiment along the same lines:
a) An experimentalist defines a distance of 4 lightyears in Tom's frame S, consisting of endpoints that are both constantly at rest with respect to Tom.
b) So in all other inertial frames those endpoints have a constant speed different from zero and the distance in between them is length contracted. So Tom measures a (proper) length of 4 lightyears, while Mary measures a (contracted) length of 2.4 lightyears.
c) What is the cause of this asymmetry? Well, simply YOUR (being an experimentalist) decision to measure endpoints that are at rest in S.
d) Of course, the experimentalist can decide to choose ''another set of endpoints'' constantly at rest in Mary's frame S' that are separated by 4 lightyears. In this case everything is reversed: The S' observer measures the length of 4 lightyears, while the S observer measures 2.4 lightyears. However, since this distance is defined by a new set of endpoints, it has nothing to do with the distance measured by Tom...

 

[Orodruin]

sir what you did is "correct" after wrongfully made my two same statements stated in two different ways two different statements. the two statements are the exactly same although I switched two names. but for the same statements you answered in M is frame T is 4 lys in one answer but in the other T is 2.4ly.

I did nothing of the sort. I switched M and T everywhere in the original statement, which is the only way you can claim symmetry. Please stop being disingenuous.

Here is the case of your actual setup in Minkowski space. Based on the inertial frame where both T and M have the same speed so you do not misconstrue either of them being assumed to be stationary.

C and C’ are both 4 ly and D and D’ are both 2.4 ly. Everything is symmetric. The ”weird” orthogonality is just how orthogonality works in Minkowski space.

 

[dwspacetime]

Note that your statement "when Mary is 4 lys away" from Tom has to be the result of a measurement along a distance separated by two endpoints that are constantly at rest, otherwise there is no "distance" onto which we can apply the length contraction formula, and there would be nothing to discuss about.... So there is always something equivalent to your planets, if you want it or not.

It's like the relativity of motion: If a ball is at rest in Tom's frame, it is obviously in motion in Mary's frame. Do you think this "asymmetry" violates the relativity principle?

Now let's discuss your length contraction experiment along the same lines:
a) An experimentalist defines a distance of 4 lightyears in Tom's frame S, consisting of endpoints that are both ''constantly at rest'' with respect to Tom.
b) So in all other inertial frames those endpoints have a constant speed different from zero and the distance in between them is length contracted. So Tom measures a (proper) length of 4 lightyears, while Mary measures a (contracted) length of 2.4 lightyears.
c) What is the cause of this asymmetry? Well, simply YOUR (being an experimentalist) decision to measure endpoints that are at rest in S.
d) Of course, the experimentalist can decide to choose ''another set of endpoints'' constantly at rest in Mary's frame S' that are separated by 4 lightyears. In this case everything is reversed: The S' observer measures the length of 4 lightyears, while the S observer measures 2.4 lightyears. However, since this distance is defined by a new set of endpoints, it has nothing to do with the distance measured by Tom...

dont confuse yourself too much. again T and M are moving towards each other in whatever speed if numbers confuse you. in M's frame there should be one distance from T or two different distances? why dd you tell me in M's frame T is both 4 lys and 2.4 lys.

 

[Orodruin]

and you tell me T is 4 lys in M's frame and is also 2.4 lys in M's frame

No. As you can see from the diagram - and as I also told you previously - you change the relevant events when you change the statement. You should not be surprised that T is 4 ly away at some time and 2.4 ly away at a different time.

 

[dwspacetime]

No. As you can see from the diagram - and as I also told you previously - you change the relevant events when you change the statement. You should not be surprised that T is 4 ly away at some time and 2.4 ly away at a different time.

this is interesting. but the sequence of events doesn't seem correct. that is weird

 

[Histspec]

dont confuse yourself too much. again T and M are moving towards each other in whatever speed if numbers confuse you. in M's frame there should be one distance from T or two different distances? why dd you tell me in M's frame T is both 4 lys and 2.4 lys.

a) If the experimentalist (=you) measures two endpoints at rest in Tom's frame separated by 4 light years, the only length measured by Mary is 2.4 lightyears.
b) If the experimentalist (=you) measures two endpoints at rest in Mary's frame separated by 4 light years, the only length measured by Tom is 2.4 lightyears.

You made your decision by choosing a). Is it really that hard to understand?

 

[dwspacetime]

a) If the experimentalist (=you) measures two endpoints at rest in Tom's frame separated by 4 light years, the only length measured by Mary is 2.4 lightyears.
b) If the experimentalist (=you) measures two endpoints at rest in Mary's frame separated by 4 light years, the only length measured by Tom is 2.4 lightyears.

You made your decision by choosing a). Is it really that hard to understand?

4 lys away should happen before 2.4 lys away but they seem switched somehow. if it is true the universe is wrong. sorry

 

[Orodruin]

this is interesting. but the sequence of events doesn't seem correct. that is weird

This is completely unhelpful. You cannot just state that something seems ”weird” without specifying what it is. If you do that there is no way to correct you.

(Spoiler alert: Everything in that diagram is in accordamce with what has been specified. If something seems ”weird” to you, then that is where your understanding of special relativity is lacking.)

 

[Orodruin]

4 lys away should happen before 2.4 lys away but they seem switched somehow. if it is true the universe is wrong. sorry

Look at the diagram. In both cases the 4 ly distances occur before the 2.4 ly distances. There is nothing weird going on here.

 

[dwspacetime]

This is completely unhelpful. You cannot just state that something seems ”weird” without specifying what it is. If you do that there is no way to correct you.

(Spoiler alert: Everything in that diagram is in accordamce with what has been specified. If something seems ”weird” to you, then that is where your understanding of special relativity is lacking.)

Sorry I was just talking with myself. the event M is 4 lys away in T's frame should happen before the event M is 2.4 lys away. if that is the case then the event T 2.4 lys away happens before T is 4 lys away in M' frame. is that not weird. this must be one of the weird reality of spacetime

 

[Orodruin]

if that is the case then the event T 2.4 lys away happens before T is 4 lys away in M' frame.

No, this is wrong and a misrepresentation of what the diagram shows.

T 2.4 ly away in M’s frame is the simultaneity D in the diagram. T 4 ly away in M’s frame is the simultaneity C’ in the diagram. C’ clearly lies below D.

 

[dwspacetime]

No, this is wrong and a misrepresentation of what the diagram shows.

T 2.4 ly away in M’s frame is the simultaneity D in the diagram. T 4 ly away in M’s frame is the simultaneity C’ in the diagram. C’ clearly lies below D.

no matter which event happens first one of them is going to have 2.4 lys before 4 years why

 

[dwspacetime]

No, this is wrong and a misrepresentation of what the diagram shows.

T 2.4 ly away in M’s frame is the simultaneity D in the diagram. T 4 ly away in M’s frame is the simultaneity C’ in the diagram. C’ clearly lies below D.

that tells me that depending on which inertia frame you are in the sequence of events could be reversed. the reason for something to happen could be the result.

 

[Orodruin]

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.

 

[dwspacetime]

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.

 

[PeterDonis]

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.

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.

 

[PeroK]

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)

 

[Orodruin]

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.

 

[weirdoguy]

Ok, but what is the point of this thread @dwspacetime ? You want to learn something, or just simply argue with everyone knowledgeable?

 

[dwspacetime]

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.

 

[robphy]

"relativity for the questioning minds"

Dan Styer's book?
https://www2.oberlin.edu/physics/dstyer/

 

[dwspacetime]

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...

 

[PeterDonis]

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.

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.

 

[dwspacetime]

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.

 

[PeterDonis]

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.

Do you have any better recommendations?

Yes: learn SR properly from a good textbook, such as Taylor & Wheeler's Spacetime Physics.

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.