Question

A planet is 10 light years away from Earth. What speed would you need to go for a trip to the planet and back to take only 5 years, as measured by you? You can ignore the time it takes to get up to speed or stop. b. how long would your trip take as measured by someone who stayed on Earth?

Answer 1

a. speed, v = 0.97 c

b. time, t' = 20.56 years

Given:

t' = 5 years

distance of the planet from the earth, d = 10 light years = 10 c

Solution:

(a) Distance travelled in a round trip, d' = 2d = 20 c = L'

Now, using Length contraction formula of relativity theory:

`L'' = L'\sqrt{1 - (v^(2))/(c^(2))}` (1)

time taken = 5 years

We know that :

time =
`(distance)/(speed)`

5 =
`(L'')/(v)` (2)

Dividing eqn (1) by v on both the sides and substituting eqn (2) in eqn (1):

`\frac{L'\sqrt{1 - (v^(2))/(c^(2))}}{v} = 5`

`\frac{20'\sqrt{1 - (v^(2))/(c^(2))}}{v} = 5`

Squaring both the sides and Solving above eqution, we get:

v = 0.97 c

(b) Time observed from Earth:

Using time dilation:

`t'' = \frac{t'}{\sqrt{1 - (v^(2))/(c^(2))}}`

`t'' = \frac{5}{\sqrt{1 - ((0.97c)^(2))/(c^(2))}}`

Solving the above eqn:

t'' = 20.56 years