Physics Chapter 15: Special Relativity - Space and Time

the speed of light is a

constant

What is time dilation?

Stretching of time due to motion in space?

motion in space affects

motion in time

Is it possible for a person with a 70-year life span to travel farther than light travels in 70 years?

Yes, a person moving at speeds close to c lives longer

Can you travel while remaining in one place in space?

Yes, in time

Does light travel through space? Through time? Through both?

Yes, through space; no, through time; no, through both

all motion is relative and

all frames of reference are arbitrary

the longer distance of a light flash on a high-speed ship must be divided by a correspondingly longer time interval to yield

an unvarying value for the speed of light

first postulate of special relativity

assumes our inability to detect a state of uniform motion; all the laws of nature are the same in all uniformly moving frames of reference

second postulate of special relativity

assumes the speed of light is constant; the speed of light in empty space will always have the same value regardless of the motion of the source or the motion of the observer

Explain why it is that when we look out into the universe, we see into the past

because it takes time for light to travel from one place to another, so we always see distant places as they were when the light left: in the past

from an external reference frame, a light clock in a high-speed spaceship will appear to have its flash

bounce up and down along a longer diagonal path

people aboard the spaceship do not see objects as being contracted in the direction of motion

but observers outside their reference frame do

What is the second postulate of special relativity?

c is always the same whether the source, the receiver, or both move

What is the ratio of distance to time for light waves (similar to g in free fall)?

c, the speed of light

space-time

combination of space and time, viewed in special relativity as two parts of one whole

special theory of relativity

describes how time is affected by motion in space at constant velocity, and how mass and energy are related; you travel through both space and time (space-time)

an object's speed cannot be measured relative to

empty space

both space and time

exist only within the universe

time only travels

forward

postulate

fundamental assumption

the speed of light in all reference frames

is always the same

the laws of physics are the same for (within) a fancy cruise whether

it moves uniformly or is at rest

The path of light of a light clock in a high-speed ship is seen to be longer when viewed from a stationary frame of reference; why, then, does the light not appear to be moving faster?

it takes correspondingly more time

Suppose you're shining a light while riding on a train; when you shine the light in the direction the train is moving, how would the speed of light appear to an observer standing at rest outside the train?

it would appear the same to the observer because the speed of light does not vary

How long would a meterstick appear if it were traveling at 99.5% the speed of light, but with its length perpendicular to its direction of motion?

it's length would not change, since change is only parallel to motion, not perpendicular

when an object moves at a very high-speed relative to an observer

its measured length in the direction of motion is contracted

one realm of time seems the same as any other to a person, but not to an observer

located outside the person's frame of reference

motion through space is related to

motion through time

If you were traveling in a high-speed spaceship, would metersticks on board appear contracted to you?

no because in your frame of reference, there is no length contraction (the relative speed of you and the sticks is zero)

Can you get younger by traveling at speeds near the speed of light?

no, because the effects of relativity are always attributed to the "other guy" (you can't GET younger, you age slower)

If you were in a smooth-riding train with no windows, could you sense the difference between uniform motion and rest? Between accelerated motion and rest?

no; yes

it is physically impossible for observers in different frames of reference to refer to

one and the same realm of space-time

How long would a meterstick appear if it were thrown like a spear at 99.5% the speed of light?

one-tenth as long

by standing still, a person

only travels through time

an object's speed can only be measured relative to

other objects

there is no physical experiment that can determine

our state of uniform motion

under the laws of physics (specifically time dilation) time travel towards the future is

possible

in moving through space, we change our rate of

proceeding into the future (time itself is altered)

If a person from an external frame of reference sees a ship pass and thinks its inhabitants' time is running slow, how do the people within the ship see that person's time running?

slow also, each sees the same effect in the other

What is space-time?

space and time are two parts of one whole

light travels through

space only and is timeless

the amount of contraction is related to

the amount of time dilation

What unifies space and time?

the constancy of the speed of light

for everyday speeds, the amount of contraction is too small to be measured, but in relativistic speeds

the contraction becomes noticeable

the contraction of speeding objects is

the contraction of space itself

space contracts in only one direction

the direction of motion

What is the first postulate of special relativity?

the laws of nature are the same in all uniformly moving reference frames

as relative speed gets closer and closer to the speed of light

the measured lengths of objects contract closer and closer to zero

time can be stretched depending on

the motion between the observer and the events being observed

length contraction

the observable shortening of objects moving at speeds approaching the speed of light

the formula for time dilation shows

the relation between proper time and relative time

Is stationary observers measure the shape of an emblem on a fast-moving rocket ship as exactly circular, then what is the shape according to observers on the rocket ship?

the shape is elliptical, with the long axis in the direction of motion

the length contraction equation is one of the reasons that

the speed of light is the upper limit for the speed of any object

time dilation simplified

the stretching of time; by moving through space, we alter our rate of moving into the future

time dilation

the stretching of time; it occurs very slightly for everyday speeds, but significantly for speeds approaching the speed of light

How could you detect acceleration in a smooth-riding train with no windows using a bowl of water?

the surface of water in the bowl would not be horizontal, therefore, the train is neither moving uniformly nor at rest

What are the present-day obstacles to time travel towards the future?

there are currently no means to propelling such a massive body to such speeds and there is no way to effectively shield occupants from space radiation

if someone could come close to the speed of light and were traveling alongside a light beam

they would measure the light as moving away from them at 300,000km/h

if a person were to somehow travel through space at the speed of light

they would not be travelling through time at all

What are the present-day obstacles to interstellar space travel?

Fuel energy and radiation

If you were traveling in a high-speed rocket ship, would clocks on board appear to you to be running slow?

No because in your frame there is no time dilation; the relative speed of you and the clocks is zero

People who ride on a bus are moving through space, but what else are they moving through?

time

for moving objects, space as well as

time undergoes changes

The two-mile-long linear accelerator at Stanford University is less than a meter long to the electrons that travel in it, explain

to the electrons, length contraction shortens their journey

Suppose you're playing catch with a friend in a moving train; when you toss the ball in the direction the train is moving, how does the speed of it appear to an observer standing at rest outside the train?

to the observer, the ball appears to travel faster

if the windows of a moving car were covered, you could not truly determine

whether your car was moving at constant velocity or was at rest

everyone who measures the speed of light from any frame of reference

will get same value: c

Light travels a certain distance in, say, 10,000 years; can an astronaut travel more slowly than the speed of light and still travel the same distance in a 10-year trip?

yes because of time dilation

Is it possible for a person to be biologically older than their own parents?

yes, if the person stays behind while their parents take a relativistic trip

you can see into the past, but

you cannot go into it

If you were in a high-speed spaceship traveling away from Earth at a speed close to that of light, how would you measure your pulse? How would the measurements of the pulses of your friends back on Earth compare if you monitored them from your ship?

your would measure your pulse the same, but that of your friends on Earth would appear slower