Thread: Time dilation

All right, so these last days I've been reading a little about relativistic speeds and time dilation online. Most of it makes sense to me, but there's just a little thing that I've been unable to wrap my head around. When you travel at near light speed time slows down, so when you return to Earth you may have been gone for a year but two years have passed on Earth. This I've understood, but now let's imagine a spaceship travelling from Earth to the nearest star 4.37 lightyears away. Travelling at 90 percent the speed of light this journey would take 4.86 years. Due to time slowing down to the people in the spaceship, time moves roughly 2.3 times faster outside the spaceship (used some online calculator for this, don't know if it's true), so for them the journey takes 11.17 years, is this right?

If it is, then what speed is the spaceship really travelling at? For the people on the spaceship the speed is 90 percent the speed of light, but for someone outside how can it travel anywhere near that speed when it takes more than twice the time to reach the destination? I'm sure I've critically misunderstood something somewhere, but can someone please try and tell me what?

Also, try and keep it simple. I'm not stupid but I'm no scientist either, big equations scare me

Speed is measured relative to some standard, but not necessarily the standard of the frame of reference in which the objects or persons in question are at rest.

If one was driving and used as a standard the frame of reference that one associates with the driver, then the car would be going at a speed of 0. But that's hardly useful to drivers, so drivers tend to use a speedometer that uses the reference frame in which the road is at rest.

In the case of the spaceship, they can use all kinds of frames of reference. They can calculate their speed using the reference frame of the planet they left if they wish. If they use their own reference frame (so to speak), then they will be moving at a speed of 0, but everything else will be moving. They will also notice that the distance between the place they left and the place where they plan to stop has shortened dramatically.

Special relativity uses three things to compensate for the fact that the speed of light is the same in all reference frames: time dilation, spacial contraction, and different simultaneity for different frames of reference. All of these work together in order to ensure that all frames of reference agree on what events happen, even though they disagree on when and where those events happen.

Originally Posted by Ladez

All right, so these last days I've been reading a little about relativistic speeds and time dilation online. Most of it makes sense to me, but there's just a little thing that I've been unable to wrap my head around. When you travel at near light speed time slows down, so when you return to Earth you may have been gone for a year but two years have passed on Earth. This I've understood, but now let's imagine a spaceship travelling from Earth to the nearest star 4.37 lightyears away. Travelling at 90 percent the speed of light this journey would take 4.86 years. Due to time slowing down to the people in the spaceship, time moves roughly 2.3 times faster outside the spaceship (used some online calculator for this, don't know if it's true), so for them the journey takes 11.17 years, is this right?

A little confusion here. Traveling at 0.9c it would take 4.86 years Earth time for the ship to reach the star. This works out to 2.12 years ship time.

If it is, then what speed is the spaceship really travelling at? For the people on the spaceship the speed is 90 percent the speed of light, but for someone outside how can it travel anywhere near that speed when it takes more than twice the time to reach the destination? I'm sure I've critically misunderstood something somewhere, but can someone please try and tell me what?

Also, try and keep it simple. I'm not stupid but I'm no scientist either, big equations scare me

Both the Earth and ship will agree that the relative speed between them is 0.9c. However, the occupants of the ship will not measure the distance between the star and Earth as being 4.37 light years, but due to the length contraction noted in the previous post, measure it as 1.9 light years. (and it takes 2.12 years to travel 1.9 light years at 0.9c).

I see, I just hadn't discovered that there was such a thing as length contraction too

Thank you for the answers.

Length contraction indeed. And also, the whole idea of relativistic speed is that there is no such thing as an absolute viewpoint. Measurements are inescapably limited by their reference frame.

I have moved the heated discussion to:
Kenhughes' imagination of SR

Dishmaster
(Moderator)