# Introductory Special Relativity book theory question

#### laocmo

According to the examples often used in introductory special relativity theory books, if a spaceship leaves the earth and travels towards a far distant galaxy at near the speed of light. Then turns around and returns to Earth, perhaps 10 years will have passed for the traveler and perhaps 1000 years for the earthlings. Now if during the traveler's entire trip a continuous movie were made of the earth from the ship, it would have to show the frantic motion of everything on Earth traveling at tremendous speeds. Even chemical reactions, etc. would have to speed up so that 10 years worth of observer time would accomplish 1000 years or earth time efforts. Where does all the energy come from to increase the speed, kinetic energy, momentum, etc. of the stationary earthlings? According to theory the earthlings would notice nothing out of the ordinary? Even though they are moving around at very large velocities to get all that 1000 years worth of activity finished in the traveler's 10 years of observation.

#### oz93666

It's not the case that Earth life speeds up , but that spaceship life slows down , because the spacecraft undergoes acceleration/deceleration.

Suppose on Earth everything was filmed and at the same time transmitted to the travelling spacecraft .What would the astronaut see??

On the outward journey speed starts low and transmissions appear normal . Under constant acceleration the astronauts speed increases until near light speed.

After 500 years of Earth time the ship has reached it's destination, the astronaut has only seen 5 years of transmissions .Now the ship decelerates stops turns round and accelerates back to earth .... During this turnaround the astronaut sees speeded transmission from Earth, ... 990 years worth of Earth news received in the turnaround period ...

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#### laocmo

"It's not the case that Earth life speeds up , but that spaceship life slows down , because the spacecraft undergoes acceleration/deceleration."

Certainly from the viewpoint of the traveler, on returning to Earth, life on Earth has sped up, else how could the earthlings have accomplished in 10 years, 1000 years worth of things involving velocity, accelerations, change in momentums, etc.? All this requires energy. Where does it come from?

"After 500 years of Earth time the ship has reached it's destination, the astronaut has only seen 5 years of transmissions."

True, but those transmissions paint a very false picture of what has gone by on the earth. The earthlings have expended 500 years worth of energy, frantically hurrying to and fro to accomplish 500 years worth of work in the 5 years of the astronaut's time. The slowness of the speed of information transmission has created an illusion that only 5 years of progress on earth has taken place. Whereas truly 500 years of things have happened.

#### laocmo

Response

"It's not the case that Earth life speeds up , but that spaceship life slows down , because the spacecraft undergoes acceleration/deceleration."

Certainly from the viewpoint of the traveler, on returning to Earth, life on Earth has sped up, else how could the earthlings have accomplished in 10 years, 1000 years worth of things involving velocity, accelerations, change in momentums, etc.? All this requires energy. Where does it come from?

"After 500 years of Earth time the ship has reached it's destination, the astronaut has only seen 5 years of transmissions."

True, but those transmissions paint a very false picture of what has gone by on the earth. The earthlings have expended 500 years worth of energy, frantically hurrying to and fro to accomplish 500 years worth of work in the 5 years of the astronaut's time. The slowness of the speed of information transmission has created an illusion that only 5 years of progress on earth has taken place. Whereas truly 500 years of things have happened.