Time Dilation and Future Interstellar Travel

[JuggyJugs]

Hello all,

I am new to the Forum and have no formal Physics training. I have an interest in the subject mainly due to other pursuits.

I am reading Joe Haldeman's The Forever War. I would definitely recommend this book to anyone with an interest in Time Dilation or Humans vs Aliens at war.

Here's a snippet from Wikipedia

The Forever War is a 1974 science fiction novel by Joe Haldeman. It won the Nebula Award in 1975,[1] and both the Hugo and Locus Awards in 1976.[2] An action-laden and contemplative story of an interstellar war between humanity and the enigmatic Tauran species, it deals with themes like the inhumanity of war and bureaucracy, as well as with the results of time dilation space travel which may cause a soldier to return to his home after centuries have passed.

The last part gets to my questions.

If I go from Earth to planet xyz at x speed that gets me there in a reasonable time, say 6 months, will I probably experience a fair amount of time dilation, let's say 20 years? When I arrive i am told to wait for more of my people to arrive on another ship that left 3 months after I did.

I can't really wrap my head around this part, which is funny because I am creating this scenario.

Do they arrive in 3 months or in 20 years?

 

[DaveC426913]

The Forever War is an awesome book.

Your trip took 20 years from the frame of reference on Earth or on planet XYZ. From your FoR in the spaceship, the trip took 6 months.

When you arrive at XYZ, they will have been waiting for your arrival for 19 1/2 years. 3 months after you arrive, the next ship will arrive.

 

[JuggyJugs]

Thanks for the response. So many more questions come to mind now.

 

[DaveC426913]

Ask away.

 

[Nabeshin]

Forever War is pretty good on the whole matter of relativistic time dilation, although it's a bit unfortunate that there is the need to the mysterious "collapsars" anyways.

 

[curiousphoton]

If I go from Earth to planet xyz at x speed that gets me there in a reasonable time, say 6 months, will I probably experience a fair amount of time dilation, let's say 20 years? When I arrive i am told to wait for more of my people to arrive on another ship that left 3 months after I did.

I can't really wrap my head around this part, which is funny because I am creating this scenario.

Do they arrive in 3 months or in 20 years?

Most likely neither. They left on a different ship, therefore traveled at a different velocity. The next ship will arrive around 3 months later, depending on it's relative velocity to the ship you took (whether it's speed was slightly higher or lower).

 

[DaveC426913]

Most likely neither. They left on a different ship, therefore traveled at a different velocity. The next ship will arrive around 3 months later, depending on it's relative velocity to the ship you took (whether it's speed was slightly higher or lower).

How does this help the OP understand the issue?

 

[curiousphoton]

How does this help the OP understand the issue?

The answer I provided is correct and in the most simple terms I could develope.

I did not see his original question directly answered in any other responses, therefore I posted a reasonable solution that hopefully increases his understanding and curiosity of the subject matter.

Does my solution need to meet your requirements?

 

[JesseM]

The answer I provided is correct and in the most simple terms I could develope.

I did not see his original question directly answered in any other responses, therefore I posted a reasonable solution that hopefully increases his understanding and curiosity of the subject matter.

Does my solution need to meet your requirements?

I think Dave was making the point that your answer has nothing to do with understanding relativity, it would only be relevant as a practical matter if someone were to actual try to do this with physical spaceships. For the purposes of a thought-experiment it's fine to assume multiple ships travel at the same speed (and even as a practical matter, if the type of ship was the same I don't see why different ships couldn't travel at the same speed, the space shuttle probably travels into orbit at pretty much the same speed each time after all).

 

[DaveC426913]

What he said.

 

[curiousphoton]

I think Dave was making the point that your answer has nothing to do with understanding relativity, it would only be relevant as a practical matter if someone were to actual try to do this with physical spaceships. For the purposes of a thought-experiment it's fine to assume multiple ships travel at the same speed (and even as a practical matter, if the type of ship was the same I don't see why different ships couldn't travel at the same speed, the space shuttle probably travels into orbit at pretty much the same speed each time after all).

It has everything to do with understanding relativity. His original question involved understanding time dialation, correct?

Seeing that it is impossible for the two ships to travel for the 3 month trip at the exact same velocity, they will experience different time dilations. Similar but different.

I believe this should be understood. Whether you think it should does not concern me.

 

[JesseM]

Seeing that it is impossible for the two ships to travel for the 3 month trip at the exact same velocity, they will experience different time dilations. Similar but different.

What do you mean "impossible"? It's certainly not forbidden by the laws of physics, therefore whatever practical issues you are talking about have nothing to do with understanding physics (and even in practice it would be quite possible to have two ships travel so close to the same velocity that any differences would be totally negligible). All problems in SR have simplifying assumptions of some kind--for example, many problems assume that two objects can cross paths at the exact same point in spacetime, or they assume objects can move inertially at constant velocity without tiny changes in velocity caused by colliding with interstellar hydrogen atoms.

 

[curiousphoton]

What do you mean "impossible"? It's certainly not forbidden by the laws of physics, therefore whatever practical issues you are talking about have nothing to do with understanding physics (and even in practice it would be quite possible to have two ships travel so close to the same velocity that any differences would be totally negligible). All problems in SR have simplifying assumptions of some kind--for example, many problems assume that two objects can cross paths at the exact same point in spacetime, or they assume objects can move inertially at constant velocity without tiny changes in velocity caused by colliding with interstellar hydrogen atoms.

My original post simply points out that in SR objects have specific RF's. Ship 1 has a unique RF. Ship 2 has a unique RF. Saying the ships leave 3 months apart BUT arrive EXACTLY 3 months apart is incorrect because saying they will have the exact same velocity throughout the trip is incorrect.

I'll help you out. Let's just look at the ship's leaving the Earth's atmosphere. When has the temperature been the exact same, let's say to the 10th decimal place, on 2 days? When has the wind direction and velocity been exactly the same, at the exact same time, to the 10 decimal place? The answer: never.

And your point:changes in velocity caused by colliding with interstellar hydrogen atoms.

Big picture: The two ships travel at different velocties. The point: they are in separate RF's. The conclusion: Saying they arrive exactly 3 months apart is incorrect.

 

[JesseM]

My original post simply points out that in SR objects have specific RF's. Ship 1 has a unique RF. Ship 2 has a unique RF.

If by "unique" you mean that two objects cannot in principle have exactly the same rest frame in SR, this is incorrect. The laws of physics certainly allow two inertial objects to be precisely at rest relative to one another, in which case they share the same rest frame; if you look in a relativity textbook you'll see it's routine to give example problems where this is the case. If you're just saying that in practice it's very difficult to get two objects to be exactly at rest relative to one another, of course this is true, but this is more an issue of engineering than of basic physics.

I'll help you out. Let's just look at the ship's leaving the Earth's atmosphere. When has the temperature been the exact same, let's say to the 10th decimal place, on 2 days? When has the wind direction and velocity been exactly the same, at the exact same time, to the 10 decimal place? The answer: never.

Again, this is a practical matter that has nothing to do with understanding the basic laws of physics. Similarly, in practice objects traveling through deep space can never move in a perfectly inertial way because they always collide with interstellar hydrogen atoms (although the changes in velocity due to such collisions will be absolutely tiny and therefore are negligible even in a practical sense), but relativity textbooks are filled with examples involving objects moving inertially. It is often useful in physics problems to make simplifying assumptions that illuminate the basic laws of physics that the student is trying to understand, I don't see any problem with this as long as these assumptions aren't explicitly impossible according to the same basic laws. If you don't understand this then you're missing something important about how physicists routinely think about the world, and how best to teach students.

 

[DaveC426913]

Saying the ships leave 3 months apart BUT arrive EXACTLY 3 months apart ...

Saying they arrive exactly 3 months apart ...

Nobody has said this - except you.

This is not a trivial observation. You have invented a confounding factor (by introducing a degree of exactness that no one else has mentioned) and are now pretending that it is our argument point. If we remove your initial invocation of the exactness, we can then also eliminate your refutation of it.

In short, you cancel out.

I'll help you out. Let's just look at the ship's leaving the Earth's atmosphere. When has the temperature been the exact same, let's say to the 10th decimal place, on 2 days? When has the wind direction and velocity been exactly the same, at the exact same time, to the 10 decimal place? The answer: never.

And your point:changes in velocity caused by colliding with interstellar hydrogen atoms.

Big picture: The two ships travel at different velocties. The point: they are in separate RF's. The conclusion: Saying they arrive exactly 3 months apart is incorrect.

Do you think that knowing the temperature of the Earth's atmo to the tenth decimal place helps illuminate the theory of relatvity? No? Then leave it out for clarity. The same for other things that do not directly help the understanding. As the OP grasps the basics, we can add other factors in.

Whether or not you agree with this, this is the way teaching and learning occurs.

 

[curiousphoton]

You have invented a confounding factor (by introducing a degree of exactness that no one else has mentioned) and are now pretending that it is our argument point. If we remove your initial invocation of the exactness, we can then also eliminate your refutation of it.

It was never my intention to start an argument. I simply provide a solution to the original question that I deemed more educational.

You questioned my solution. You did not believe it was appropriate.

I defended my my solution. I never meant to get into the high level of exactness, but was forced to when you continued to question me.

I don't see how you can question someone and think they aren't going to rebuttle your attacks.

The fact is you provided one answer, I provided another. Both are correct to a different level of understanding. It's ok. It's not the end of the world.

 

[DaveC426913]

I don't see how you can question someone and think they aren't going to rebuttle your attacks.

Easy. They can concede to my inescapable rightness. They would have plenty of company.