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For any event that has a space-like separation from the event at the origin, it is possible to choose a time axis that will make the two events occur at the same time, so that the two events are simultaneous in some frame of reference. Therefore, which of the events with space-like separation comes before the other in time also depends on the frame of reference of the observer. Since space-like separations can be traversed only by exceeding the speed of light; this violation of which event can cause the other provides another argument for why particles cannot travel faster than the speed of light, as well as potential material for science fiction about time travel. Similarly for any event with time-like separation from the event at the origin, a frame of reference can be found that will make the events occur at the same location. Because the relations

Δ s A C 2 = ( x A x B ) 2 + ( x A x B ) 2 + ( x A x B ) 2 ( c Δ t ) 2 > 0

and

Δ s A B 2 = ( x A x B ) 2 + ( x A x B ) 2 + ( x A x B ) 2 ( c Δ t ) 2 < 0 .

are Lorentz invariant, whether two events are time-like and can be made to occur at the same place or space-like and can be made to occur at the same time is the same for all observers. All observers in different inertial frames of reference agree on whether two events have a time-like or space-like separation.

The twin paradox seen in space-time

The twin paradox discussed earlier involves an astronaut twin traveling at near light speed to a distant star system, and returning to Earth. Because of time dilation, the space twin is predicted to age much less than the earthbound twin. This seems paradoxical because we might have expected at first glance for the relative motion to be symmetrical and naively thought it possible to also argue that the earthbound twin should age less.

To analyze this in terms of a space-time diagram, assume that the origin of the axes used is fixed in Earth. The world line of the earthbound twin is then along the time axis.

The world line of the astronaut twin, who travels to the distant star and then returns, must deviate from a straight line path in order to allow a return trip. As seen in [link] , the circumstances of the two twins are not at all symmetrical. Their paths in space-time are of manifestly different length. Specifically, the world line of the earthbound twin has length 2 c Δ t , which then gives the proper time that elapses for the earthbound twin as 2 Δ t . The distance to the distant star system is Δ x = v Δ t . The proper time that elapses for the space twin is 2 Δ τ where

c 2 Δ τ 2 = Δ s 2 = ( c Δ t ) 2 ( Δ x ) 2 .

This is considerably shorter than the proper time for the earthbound twin by the ratio

c Δ τ c Δ t = ( c Δ t ) 2 ( Δ x ) 2 ( c Δ t ) 2 = ( c Δ t ) 2 ( v Δ t ) 2 ( c Δ t ) 2 = 1 v 2 c 2 = 1 γ .

consistent with the time dilation formula. The twin paradox is therefore seen to be no paradox at all. The situation of the two twins is not symmetrical in the space-time diagram. The only surprise is perhaps that the seemingly longer path on the space-time diagram corresponds to the smaller proper time interval, because of how Δ τ and Δ s depend on Δ x and Δ t .

The space time diagram has x on the horizontal axis and c t on the vertical axis. The light cone appears as 45 degree lines coming out of the origin. The earth twin world line is a vertical line on the c t axis. The first part of the space twin world line is a line leaving the origin at an angle larger than 45 degrees but less than 90 degrees. At a point that is a vertical distance c delta t and a horizontal distance delta x from the origin, the world line of the space twin bends back toward the c t axis and hits the c t axis a vertical distance c delta t from where it changed direction.
The space twin and the earthbound twin, in the twin paradox example, follow world lines of different length through space-time.

Questions & Answers

What is the importance of de-broglie's wavelength?
Mukulika Reply
he related wave to matter
Zahid
at subatomic level wave and matter are associated. this refering to mass energy equivalence
Zahid
how those weight effect a stable motion at equilibrium
Nonso Reply
how do I differentiate this equation- A sinwt with respect to t
Evans Reply
just use the chain rule : let u =wt , the dy/dt = dy/du × du/dt : wA × cos(wt)
Jerry
I see my message got garbled , anyway use the chain rule with u= wt , etc...
Jerry
de broglie wave equation
LoNE Reply
vy beautiful equation
chandrasekhar
what is electro statics
fitsum Reply
when you consider systems consisting of fixed charges
Sherly
Diagram of the derive rotational analog equation of v= u+at
Nnamnso Reply
what is carat
Arnulfo Reply
a unit of weight for precious stones and pearls, now equivalent to 200 milligrams.
LoNE
a science that deals with the composition, structure, and properties of substances and with the transformations that they undergo.
LoNE
what is chemistry
Mrs Reply
what chemistry ?
Abakar
where are the mcq
Fred Reply
ok
Giorgi
acids and bases
Navya
How does unpolarized light have electric vector randomly oriented in all directions.
Tanishq Reply
unpolarized light refers to a wave collection which has an equal distribution of electric field orientations for all directions
pro
In a grating, the angle of diffraction for second order maximum is 30°.When light of wavelength 5*10^-10cm is used. Calculate the number of lines per cm of the grating.
Micheal Reply
OK I can solve that for you using Bragg's equation 2dsin0over lander
ossy
state the law of gravity 6
cletus Reply
what is cathodic protection
Ebe Reply
its just a technique used for the protection of a metal from corrosion by making it cathode of an electrochemical cell.
akif
what is interferometer
Sonu Reply
what is interferometer
Abakar
Show that n1Sino1=n2Sino2
javan Reply
Practice Key Terms 4

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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