# Connexions

You are here: Home » Content » Simultaneity And Time Dilation
Content endorsed by: OpenStax College

### Lenses

What is a lens?

#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

#### Endorsed by (What does "Endorsed by" mean?)

This content has been endorsed by the organizations listed. Click each link for a list of all content endorsed by the organization.
• OpenStax College

This module is included in aLens by: OpenStax CollegeAs a part of collection: "College Physics"

Click the "OpenStax College" link to see all content they endorse.

#### Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
• Featured Content

This module is included inLens: Connexions Featured Content
By: ConnexionsAs a part of collection: "College Physics"

"This introductory, algebra-based, two-semester college physics book is grounded with real-world examples, illustrations, and explanations to help students grasp key, fundamental physics concepts. […]"

Click the "Featured Content" link to see all content affiliated with them.

Click the tag icon to display tags associated with this content.

### Recently Viewed

This feature requires Javascript to be enabled.

### Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.

# Simultaneity And Time Dilation

Module by: OpenStax College. E-mail the author

Summary:

• Describe simultaneity.
• Describe time dilation.
• Calculate γ.
• Compare proper time and the observer’s measured time.

Do time intervals depend on who observes them? Intuitively, we expect the time for a process, such as the elapsed time for a foot race, to be the same for all observers. Our experience has been that disagreements over elapsed time have to do with the accuracy of measuring time. When we carefully consider just how time is measured, however, we will find that elapsed time depends on the relative motion of an observer with respect to the process being measured.

## Simultaneity

Consider how we measure elapsed time. If we use a stopwatch, for example, how do we know when to start and stop the watch? One method is to use the arrival of light from the event, such as observing a light turning green to start a drag race. The timing will be more accurate if some sort of electronic detection is used, avoiding human reaction times and other complications.

Now suppose we use this method to measure the time interval between two flashes of light produced by flash lamps. (See Figure 2.) Two flash lamps with observer A midway between them are on a rail car that moves to the right relative to observer B. The light flashes are emitted just as A passes B, so that both A and B are equidistant from the lamps when the light is emitted. Observer B measures the time interval between the arrival of the light flashes. According to postulate 2, the speed of light is not affected by the motion of the lamps relative to B. Therefore, light travels equal distances to him at equal speeds. Thus observer B measures the flashes to be simultaneous.

Now consider what observer B sees happen to observer A. She receives the light from the right first, because she has moved towards that flash lamp, lessening the distance the light must travel and reducing the time it takes to get to her. Light travels at speed cc size 12{c} {} relative to both observers, but observer B remains equidistant between the points where the flashes were emitted, while A gets closer to the emission point on the right. From observer B’s point of view, then, there is a time interval between the arrival of the flashes to observer A. Observer B measures the flashes to be simultaneous relative to him but not relative to A. Here a relative velocity between observers affects whether two events are observed to be simultaneous. Simultaneity is not absolute.

This illustrates the power of clear thinking. We might have guessed incorrectly that if light is emitted simultaneously, then two observers halfway between the sources would see the flashes simultaneously. But careful analysis shows this not to be the case. Einstein was brilliant at this type of thought experiment (in German, “Gedankenexperiment”). He very carefully considered how an observation is made and disregarded what might seem obvious. The validity of thought experiments, of course, is determined by actual observation. The genius of Einstein is evidenced by the fact that experiments have repeatedly confirmed his theory of relativity.

In summary: Two events are defined to be simultaneous if an observer measures them as occurring at the same time (such as by receiving light from the events). Two events are not necessarily simultaneous to all observers.

## Time Dilation

The consideration of the measurement of elapsed time and simultaneity leads to an important relativistic effect.

### Time dilation:

Time dilation is the phenomenon of time passing slower for an observer who is moving relative to another observer.

Suppose, for example, an astronaut measures the time it takes for light to cross her ship, bounce off a mirror, and return. (See Figure 3.) How does the elapsed time the astronaut measures compare with the elapsed time measured for the same event by a person on the Earth? Asking this question (another thought experiment) produces a profound result. We find that the elapsed time for a process depends on who is measuring it. In this case, the time measured by the astronaut is smaller than the time measured by the Earth-bound observer. The passage of time is different for the observers because the distance the light travels in the astronaut’s frame is smaller than in the Earth-bound frame. Light travels at the same speed in each frame, and so it will take longer to travel the greater distance in the Earth-bound frame.

To quantitatively verify that time depends on the observer, consider the paths followed by light as seen by each observer. (See Figure 3(c).) The astronaut sees the light travel straight across and back for a total distance of 2D2D size 12{2D} {}, twice the width of her ship. The Earth-bound observer sees the light travel a total distance 2s2s size 12{2s} {}. Since the ship is moving at speed vv size 12{v} {} to the right relative to the Earth, light moving to the right hits the mirror in this frame. Light travels at a speed cc size 12{c} {} in both frames, and because time is the distance divided by speed, the time measured by the astronaut is

Δt0=2Dc.Δt0=2Dc. size 12{Δt rSub { size 8{0} } = { {2D} over {c} } } {}
(1)

This time has a separate name to distinguish it from the time measured by the Earth-bound observer.

### Proper Time:

Proper time Δt0Δt0 size 12{Δt rSub { size 8{0} } } {} is the time measured by an observer at rest relative to the event being observed.

In the case of the astronaut observe the reflecting light, the astronaut measures proper time. The time measured by the Earth-bound observer is

Δt=2sc.Δt=2sc. size 12{Δt= { {2s} over {c} } } {}
(2)

To find the relationship between Δt0Δt0 size 12{Δt rSub { size 8{0} } } {} and ΔtΔt size 12{Δt} {}, consider the triangles formed by DD size 12{D} {} and ss size 12{s} {}. (See Figure 3(c).) The third side of these similar triangles is LL size 12{L} {}, the distance the astronaut moves as the light goes across her ship. In the frame of the Earth-bound observer,

L=vΔt2.L=vΔt2. size 12{L= { {vΔt} over {2} } } {}
(3)

Using the Pythagorean Theorem, the distance ss size 12{s} {} is found to be

s=D2+vΔt22.s=D2+vΔt22. size 12{s= sqrt {D rSup { size 8{2} } + left ( { {vΔt} over {2} } right ) rSup { size 8{2} } } } {}
(4)

Substituting ss size 12{s} {} into the expression for the time interval ΔtΔt size 12{Δt} {} gives

Δt=2sc=2D2+vΔt22c.Δt=2sc=2D2+vΔt22c. size 12{Δt= { {2s} over {c} } = { {2 sqrt {D rSup { size 8{2} } + left ( { {vΔt} over {2} } right ) rSup { size 8{2} } } } over {c} } } {}
(5)

We square this equation, which yields

(Δt)2=4D2+v2(Δt)24c2= 4 D 2 c 2 +v2c2(Δt)2.(Δt)2=4D2+v2(Δt)24c2= 4 D 2 c 2 +v2c2(Δt)2. size 12{ $$Δt$$ rSup { size 8{2} } = { {4 left [D rSup { size 8{2} } + { {v rSup { size 8{2} } $$Δt$$ rSup { size 8{2} } } over {4} } right ]} over {c rSup { size 8{2} } } } = { {4D rSup { size 8{2} } } over {c rSup { size 8{2} } } } + { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } $$Δt$$ rSup { size 8{2} } } {}
(6)

Note that if we square the first expression we had for Δt0Δt0 size 12{Δt rSub { size 8{0} } } {}, we get (Δt0)2= 4 D 2 c2(Δt0)2= 4 D 2 c2 size 12{ $$Δt rSub { size 8{0} }$$ rSup { size 8{2} } = { {4D rSup { size 8{2} } } over {c rSup { size 8{2} } } } } {}. This term appears in the preceding equation, giving us a means to relate the two time intervals. Thus,

(Δt)2=(Δt0)2+v2c2(Δt)2.(Δt)2=(Δt0)2+v2c2(Δt)2. size 12{ $$Δt$$ rSup { size 8{2} } = $$Δt rSub { size 8{0} }$$ rSup { size 8{2} } + { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } $$Δt$$ rSup { size 8{2} } } {}
(7)

Gathering terms, we solve for ΔtΔt size 12{Δt} {}:

(Δt)21v2c2=(Δt0)2.(Δt)21v2c2=(Δt0)2. size 12{ $$Δt$$ rSup { size 8{2} } left (1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } right )= $$Δt rSub { size 8{0} }$$ rSup { size 8{2} } } {}
(8)

Thus,

(Δt)2=(Δt0)21v2c2.(Δt)2=(Δt0)21v2c2. size 12{ $$Δt$$ rSup { size 8{2} } = { { $$Δt rSub { size 8{0} }$$ rSup { size 8{2} } } over {1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } {}
(9)

Taking the square root yields an important relationship between elapsed times:

Δt=Δt01 v2c2=γΔt0,Δt=Δt01 v2c2=γΔt0, size 12{Δt= { {Δt rSub { size 8{0} } } over { sqrt {1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } =γΔt rSub { size 8{0} } } {}
(10)

where

γ=11v2c2.γ=11v2c2. size 12{γ= { {1} over { sqrt {1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } } {}
(11)

This equation for ΔtΔt size 12{Δt} {} is truly remarkable. First, as contended, elapsed time is not the same for different observers moving relative to one another, even though both are in inertial frames. Proper time Δt0Δt0 size 12{Δt rSub { size 8{0} } } {} measured by an observer, like the astronaut moving with the apparatus, is smaller than time measured by other observers. Since those other observers measure a longer time ΔtΔt size 12{Δt} {}, the effect is called time dilation. The Earth-bound observer sees time dilate (get longer) for a system moving relative to the Earth. Alternatively, according to the Earth-bound observer, time slows in the moving frame, since less time passes there. All clocks moving relative to an observer, including biological clocks such as aging, are observed to run slow compared with a clock stationary relative to the observer.

Note that if the relative velocity is much less than the speed of light (v<<cv<<c size 12{v"<<"c} {}), then v2c2v2c2 is extremely small, and the elapsed times ΔtΔt and Δt0Δt0 size 12{Δt rSub { size 8{0} } } {} are nearly equal. At low velocities, modern relativity approaches classical physics—our everyday experiences have very small relativistic effects.

The equation Δt= γ Δ t0 Δt= γ Δ t0 also implies that relative velocity cannot exceed the speed of light. As vv size 12{v} {} approaches cc size 12{c} {}, ΔtΔt size 12{Δt} {} approaches infinity. This would imply that time in the astronaut’s frame stops at the speed of light. If vv size 12{v} {} exceeded cc size 12{c} {}, then we would be taking the square root of a negative number, producing an imaginary value for ΔtΔt size 12{Δt} {}.

There is considerable experimental evidence that the equation Δt=γΔt0Δt=γΔt0 is correct. One example is found in cosmic ray particles that continuously rain down on the Earth from deep space. Some collisions of these particles with nuclei in the upper atmosphere result in short-lived particles called muons. The half-life (amount of time for half of a material to decay) of a muon is 1.52μs1.52μs size 12{1 "." "52" "μs"} {} when it is at rest relative to the observer who measures the half-life. This is the proper time Δt0Δt0 size 12{Δt rSub { size 8{0} } } {}. Muons produced by cosmic ray particles have a range of velocities, with some moving near the speed of light. It has been found that the muon’s half-life as measured by an Earth-bound observer (ΔtΔt size 12{Δt} {}) varies with velocity exactly as predicted by the equation Δt=γΔt0Δt=γΔt0 size 12{Δt=γΔt rSub { size 8{0} } } {}. The faster the muon moves, the longer it lives. We on the Earth see the muon’s half-life time dilated—as viewed from our frame, the muon decays more slowly than it does when at rest relative to us.

### Example 1: Calculating ΔtΔt for a Relativistic Event: How Long Does a Speedy Muon Live?

Suppose a cosmic ray colliding with a nucleus in the Earth’s upper atmosphere produces a muon that has a velocity v=0.950cv=0.950c size 12{v=0 "." "950"c} {}. The muon then travels at constant velocity and lives 1.52μs1.52μs size 12{1 "." "52" ital "μs"} {} as measured in the muon’s frame of reference. (You can imagine this as the muon’s internal clock.) How long does the muon live as measured by an Earth-bound observer? (See Figure 4.)

Strategy

A clock moving with the system being measured observes the proper time, so the time we are given is Δt0=1.52μsΔt0=1.52μs. The Earth-bound observer measures ΔtΔt as given by the equation Δt= γΔt0 Δt= γΔt0 . Since we know the velocity, the calculation is straightforward.

Solution

1) Identify the knowns. v=0.950cv=0.950c, Δt0=1.52μsΔt0=1.52μs

2) Identify the unknown. ΔtΔt

3) Choose the appropriate equation.

Use,

Δt=γΔt0,Δt=γΔt0,
(12)

where

γ=11 v2c2 .γ=11 v2c2 .
(13)

4) Plug the knowns into the equation.

First find γγ.

γ = 1 1 v 2 c 2 = 1 1 ( 0.950 c ) 2 c 2 = 1 1 ( 0.950 ) 2 = 3.20. γ = 1 1 v 2 c 2 = 1 1 ( 0.950 c ) 2 c 2 = 1 1 ( 0.950 ) 2 = 3.20.
(14)

Use the calculated value of γγ size 12{γ} {} to determine ΔtΔt.

Δ t = γΔt 0 = ( 3.20 ) ( 1.52 μ s ) = 4.87 μ s Δ t = γΔt 0 = ( 3.20 ) ( 1.52 μ s ) = 4.87 μ s
(15)

Discussion

One implication of this example is that since γ=3.20γ=3.20 size 12{γ=3 "." "20"} {} at 95.0%95.0% size 12{"95" "." 0%} {} of the speed of light (v=0.950cv=0.950c size 12{v=0 "." "950"c} {}), the relativistic effects are significant. The two time intervals differ by this factor of 3.20, where classically they would be the same. Something moving at 0.950c0.950c size 12{0 "." "950"c} {} is said to be highly relativistic.

Another implication of the preceding example is that everything an astronaut does when moving at 95.0%95.0% size 12{"95" "." 0%} {} of the speed of light relative to the Earth takes 3.20 times longer when observed from the Earth. Does the astronaut sense this? Only if she looks outside her spaceship. All methods of measuring time in her frame will be affected by the same factor of 3.20. This includes her wristwatch, heart rate, cell metabolism rate, nerve impulse rate, and so on. She will have no way of telling, since all of her clocks will agree with one another because their relative velocities are zero. Motion is relative, not absolute. But what if she does look out the window?

### Real-World Connections:

It may seem that special relativity has little effect on your life, but it is probably more important than you realize. One of the most common effects is through the Global Positioning System (GPS). Emergency vehicles, package delivery services, electronic maps, and communications devices are just a few of the common uses of GPS, and the GPS system could not work without taking into account relativistic effects. GPS satellites rely on precise time measurements to communicate. The signals travel at relativistic speeds. Without corrections for time dilation, the satellites could not communicate, and the GPS system would fail within minutes.

An intriguing consequence of time dilation is that a space traveler moving at a high velocity relative to the Earth would age less than her Earth-bound twin. Imagine the astronaut moving at such a velocity that γ=30.0γ=30.0 size 12{γ="30" "." 0} {}, as in Figure 5. A trip that takes 2.00 years in her frame would take 60.0 years in her Earth-bound twin’s frame. Suppose the astronaut traveled 1.00 year to another star system. She briefly explored the area, and then traveled 1.00 year back. If the astronaut was 40 years old when she left, she would be 42 upon her return. Everything on the Earth, however, would have aged 60.0 years. Her twin, if still alive, would be 100 years old.

The situation would seem different to the astronaut. Because motion is relative, the spaceship would seem to be stationary and the Earth would appear to move. (This is the sensation you have when flying in a jet.) If the astronaut looks out the window of the spaceship, she will see time slow down on the Earth by a factor of γ=30.0γ=30.0 size 12{γ="30" "." 0} {}. To her, the Earth-bound sister will have aged only 2/30 (1/15) of a year, while she aged 2.00 years. The two sisters cannot both be correct.

As with all paradoxes, the premise is faulty and leads to contradictory conclusions. In fact, the astronaut’s motion is significantly different from that of the Earth-bound twin. The astronaut accelerates to a high velocity and then decelerates to view the star system. To return to the Earth, she again accelerates and decelerates. The Earth-bound twin does not experience these accelerations. So the situation is not symmetric, and it is not correct to claim that the astronaut will observe the same effects as her Earth-bound twin. If you use special relativity to examine the twin paradox, you must keep in mind that the theory is expressly based on inertial frames, which by definition are not accelerated or rotating. Einstein developed general relativity to deal with accelerated frames and with gravity, a prime source of acceleration. You can also use general relativity to address the twin paradox and, according to general relativity, the astronaut will age less. Some important conceptual aspects of general relativity are discussed in General Relativity and Quantum Gravity of this course.

In 1971, American physicists Joseph Hafele and Richard Keating verified time dilation at low relative velocities by flying extremely accurate atomic clocks around the Earth on commercial aircraft. They measured elapsed time to an accuracy of a few nanoseconds and compared it with the time measured by clocks left behind. Hafele and Keating’s results were within experimental uncertainties of the predictions of relativity. Both special and general relativity had to be taken into account, since gravity and accelerations were involved as well as relative motion.

1. What is γγ size 12{γ} {} if v=0.650cv=0.650c size 12{v=0 "." "150"c} {}?

#### Solution

γ=11v2c2=11(0.650c)2c2=1.32γ=11v2c2=11(0.650c)2c2=1.32 size 12{γ= { {1} over { sqrt {1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } = { {1} over { sqrt {1 - { { $$0 "." "650"c$$ rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } =1 "." "32"} {}

2. A particle travels at 1.90×108 m/s1.90×108 m/s size 12{1 "." "90" times "10" rSup { size 8{8} } "m/s"} {} and lives 2.10×108 s2.10×108 s size 12{2 "." "10" times "10" rSup { size 8{ - 8} } s} {} when at rest relative to an observer. How long does the particle live as viewed in the laboratory?

Δt=Δt01v2c2=2.10×108 s1(1.90×108 m/s)2(3.00×108 m/s)2=2.71×108 sΔt=Δt01v2c2=2.10×108 s1(1.90×108 m/s)2(3.00×108 m/s)2=2.71×108 s size 12{Δt= { {Δt rSub { size 8{0} } } over { sqrt {1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } = { {2 "." "10" times "10" rSup { size 8{ - 8} } " s"} over { sqrt {1 - { { $$1 "." "90" times "10" rSup { size 8{8} } " m/s"$$ rSup { size 8{2} } } over { $$3 "." "00" times "10" rSup { size 8{8} } " m/s"$$ rSup { size 8{2} } } } } } } =2 "." "71" times "10" rSup { size 8{ - 8} } " s"} {}

## Section Summary

• Two events are defined to be simultaneous if an observer measures them as occurring at the same time. They are not necessarily simultaneous to all observers—simultaneity is not absolute.
• Time dilation is the phenomenon of time passing slower for an observer who is moving relative to another observer.
• Observers moving at a relative velocity vv size 12{v} {} do not measure the same elapsed time for an event. Proper time Δt0Δt0 size 12{Δt rSub { size 8{0} } } {} is the time measured by an observer at rest relative to the event being observed. Proper time is related to the time ΔtΔt size 12{Δt} {} measured by an Earth-bound observer by the equation
Δt=Δt01v2c2=γΔt0,Δt=Δt01v2c2=γΔt0,
(16)

where

γ=11v2c2.γ=11v2c2.
(17)
• The equation relating proper time and time measured by an Earth-bound observer implies that relative velocity cannot exceed the speed of light.
• The twin paradox asks why a twin traveling at a relativistic speed away and then back towards the Earth ages less than the Earth-bound twin. The premise to the paradox is faulty because the traveling twin is accelerating. Special relativity does not apply to accelerating frames of reference.
• Time dilation is usually negligible at low relative velocities, but it does occur, and it has been verified by experiment.

## Conceptual Questions

### Exercise 1

Does motion affect the rate of a clock as measured by an observer moving with it? Does motion affect how an observer moving relative to a clock measures its rate?

### Exercise 2

To whom does the elapsed time for a process seem to be longer, an observer moving relative to the process or an observer moving with the process? Which observer measures proper time?

### Exercise 3

How could you travel far into the future without aging significantly? Could this method also allow you to travel into the past?

## Problems & Exercises

### Exercise 1

(a) What is γγ size 12{γ} {} if v=0.250cv=0.250c size 12{v=0 "." "250"c} {}? (b) If v=0.500cv=0.500c size 12{v=0 "." "500"c} {}?

(a) 1.0328

(b) 1.15

### Exercise 2

(a) What is γγ size 12{γ} {} if v=0.100cv=0.100c size 12{v=0 "." "100"c} {}? (b) If v=0.900cv=0.900c size 12{v=0 "." "900"c} {}?

### Exercise 3

Particles called ππ size 12{π} {}-mesons are produced by accelerator beams. If these particles travel at 2.70×108m/s2.70×108m/s size 12{2 "." "70" times "10" rSup { size 8{8} } `"m/s"} {} and live 2.60×108s2.60×108s when at rest relative to an observer, how long do they live as viewed in the laboratory?

#### Solution

5 . 96 × 10 8 s 5 . 96 × 10 8 s size 12{5 "." "96" times "10" rSup { size 8{ - 8} } " s"} {}

### Exercise 4

Suppose a particle called a kaon is created by cosmic radiation striking the atmosphere. It moves by you at 0.980c0.980c size 12{0 "." "980"c} {}, and it lives 1.24×108s1.24×108s when at rest relative to an observer. How long does it live as you observe it?

### Exercise 5

A neutral ππ size 12{π} {}-meson is a particle that can be created by accelerator beams. If one such particle lives 1.40×1016s1.40×1016s as measured in the laboratory, and 0.840×1016s0.840×1016s when at rest relative to an observer, what is its velocity relative to the laboratory?

0.800c0.800c

### Exercise 6

A neutron lives 900 s when at rest relative to an observer. How fast is the neutron moving relative to an observer who measures its life span to be 2065 s?

### Exercise 7

If relativistic effects are to be less than 1%, then γγ size 12{γ} {} must be less than 1.01. At what relative velocity is γ=1.01γ=1.01 size 12{γ=1 "." "01"} {}?

#### Solution

0 . 140 c 0 . 140 c size 12{0 "." "140"c} {}

### Exercise 8

If relativistic effects are to be less than 3%, then γγ size 12{γ} {} must be less than 1.03. At what relative velocity is γ=1.03γ=1.03 size 12{γ=1 "." "03"} {}?

### Exercise 9

(a) At what relative velocity is γ=1.50γ=1.50 size 12{γ=1 "." "50"} {}? (b) At what relative velocity is γ=100γ=100 size 12{γ="100"} {}?

#### Solution

(a) 0.745c0.745c size 12{0 "." "745"c} {}

(b) 0.99995c0.99995c size 12{0 "." "99995"c} {} (to five digits to show effect)

### Exercise 10

(a) At what relative velocity is γ=2.00γ=2.00 size 12{γ=2 "." "00"} {}? (b) At what relative velocity is γ=10.0γ=10.0 size 12{γ="10" "." 0} {}?

### Exercise 11

Unreasonable Results

(a) Find the value of γγ size 12{γ} {} for the following situation. An Earth-bound observer measures 23.9 h to have passed while signals from a high-velocity space probe indicate that 24.0 h24.0 h size 12{"24" "." 0" h"} {} have passed on board. (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?

#### Solution

(a) 0.996

(b) γγ size 12{γ} {} cannot be less than 1.

(c) Assumption that time is longer in moving ship is unreasonable.

## Glossary

time dilation:
the phenomenon of time passing slower to an observer who is moving relative to another observer
proper time:
Δt0Δt0. the time measured by an observer at rest relative to the event being observed: Δt=Δt01v2c2=γΔt0Δt=Δt01v2c2=γΔt0, where γ=11v2c2γ=11v2c2
this asks why a twin traveling at a relativistic speed away and then back towards the Earth ages less than the Earth-bound twin. The premise to the paradox is faulty because the traveling twin is accelerating, and special relativity does not apply to accelerating frames of reference

## Content actions

PDF | EPUB (?)

### What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks