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

Orodruin said:

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

dwspacetime said:

use whatever convention you want to use

dwspacetime said:

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

Orodruin said:

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 said:

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

Orodruin said:

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.

Orodruin said:

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.

dwspacetime said:

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?

dwspacetime said:

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.

Histspec said:

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...

dwspacetime said:

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

Orodruin said:

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 said:

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.

Histspec said:

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 said:

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

dwspacetime said:

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 said:

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.)

dwspacetime said:

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

Orodruin said:

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.

Orodruin said:

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 said:

View attachment 337748
no matter which event happens first one of them is going to have 2.4 lys before 4 years why

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.

dwspacetime said:

that tells me that depending on which inertia frame you are in the sequence of events could be reversed.

dwspacetime said:

the reason for something to happen could be the result.

dwspacetime said:

the universe is wrong. sorry

weirdoguy said:

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

dwspacetime said:

"relativity for the questioning minds"

robphy said:

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

dwspacetime said:

It is pretty thorough which answered almost all my questions for now.

dwspacetime said:

It seems everything is moving in a speed of light..... The faster you move in space the slower in time...

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.

dwspacetime said:

Everything is moving in speed of light is not in that book. It is what I think.

dwspacetime said:

Do you have any better recommendations?

dwspacetime said:

Unless I find better explanation