# Relativity in Ender's Game

1. Apr 7, 2006

### byee614

Pretty basic relativity questions. Have any of you read Ender's Game by Orson Scott Card? Great book by the way. But I'm questioning if the issues with relativity in the book are accurate.

In the book, a military legend, Mazer Rackham, defeated an army of alien bugs 80 years ago. Realizing that his expertise would be needed in the future when starships are ready to attack the alien homebase, they decide to put Mazer in a spaceship, send him off at near light speed, and then have him return, just in time to train the next military genius, Ender Wiggins. Mazer says only eight years passed for him on the ship, but 80 years passed for everyone else. This does not sound right to me because on the return trip, any time that was gained for Mazer would have been lost. Am I wrong?

Second. In the book, they have a device that is capable of instantanous communication across the universe. While I don't expect that to be possible because it is way faster than the speed of light, they use this device which makes me wonder about another age question.

At the end of the book, Ender decides to leave on a ship with his sister. He leaves behind his brother on Earth. For Ender, only 2 years passes on the ship. But for his brother, 50 years passes. Ender uses the communication device to talk with his brother who is 77 years old, while Ender is still young. Is this possible? This seems wrong to me because Ender flew away from Earth, which could also be viewed as the Earth flying away from Ender. It doesn't make sense to me that Ender should stay young while his brother gets old.

I know the book is fiction and that an instantaneous communication device is probably impossible, but are these other relativity concepts flaws in his book?

2. Apr 7, 2006

### Ich

The first is correct, the one who changes his velocity on his way from one event (departure) to another (arrival) will age less compared to the one who stays at the same velocity.
The second cotradicts the relativity principle. If "instantaneous" is a relative concept, ie depending on the observer´s speed, this would lead to the possibility of time travel with all the paradoxes involved. It works if SR is invalid and there is a preferred frame and absolute time (a Lorentz-ether).
As long as the earth is moving slowly wrt this preferred frame, I´d say that all three effects are consistent.

3. Apr 7, 2006

### byee614

But he traveled away from Earth. And then returned to Earth. From Mazer's frame of reference, you could say the Earth departed away from him and then returned to him. So how would you determine who aged more?

4. Apr 7, 2006

### Ich

Nope, you can´t say that. Between departure and arrival, one of them has to change his velocity. That means acceleration (at least in flat spacetime), and that is nothing you can transform away in SR. It´s a triangle in a spacetime diagram: earth runs along one side, the spaceship along the other two sides. Like in euklidean space the two sides are always longer as the one, in non-euklidean spacetime they are actually always shorter. "Shorter" means that less time elapsed.

5. Apr 7, 2006

### byee614

Ok, I don't understand 90% of the things you just said so I'll just have to assume you're right.

6. Apr 7, 2006

### Ich

Ok that´s the correct attitude;)

7. Apr 7, 2006

### chroot

Staff Emeritus
8. Apr 7, 2006

### yogi

byee614: "This does not sound right to me because on the return trip, any time that was gained for Mazer would have been lost. Am I wrong?"

Yes - you are wrong about this - for an oputward velocity v equal to the same inward velocity, half the time is lost each way - it appears from Doppler shifts that the outgoing clock attached to the spaceship is running slower when receding and runniing faster when approaching - but there is more too it. A really excellent non-math treatise can be found in the book "Conceptual Physics." There are a lot of used copies floating around - probably pick one up on Amazon for a few bucks.

9. Feb 16, 2011

### kaylah

I am not totally sure whether I remember correctly in saying that the faster you go, the slower time gets until eventually at the speed of light, time stops. This would explain why it takes so little time on the spacecraft and so much time on Earth.

10. Feb 16, 2011

### HallsofIvy

Staff Emeritus
Yes, you are wrong. the time change depends on relative speed, not relative velocity- the direction does not matter.

The "twin paradox". There are two things acting here. First, the brother is in the earths gravitational field and Ender is not. Second, in order to go away and then come back, Ender must have accelerated which destroys the symmetry.

11. Feb 16, 2011

### JesseM

Mazer doesn't have an inertial frame of reference for the whole trip--an inertial frame is one moving at constant velocity (constant direction and speed as measured by any other inertial frame), which feels no G-forces. Mazer has to accelerate (change direction) to turn around and return to Earth after he's been moving away for awhile, he'll feel G-forces at that point which tell him he's changing velocities as seen in any inertial frame. The usual laws of SR such as time dilation based on relative velocity only apply in inertial frames, not non-inertial frames.