# Relativistic speeds - what do astronauts actually see?

kiplunk
Hi! I'm trying to wrap my head around the idea of a spaceship - [let's just say it is "fusion-powered" very very efficiently ] - under constant thrust, accelerating towards a distant star.

It accelerates for the first half of the journey, then cuts its engines & turns around 180-degrees and then begins deceleration for the remaining half of the journey.

What would the astronaut(s) see out the ports?

My bit of googling tells me that relativistic effects kick in about one-tenth c, with some chromatic aberration and distortion. The surrounding starscape begins compressing in the direction of travel as angular separation shrinks. There is increasing doppler shifting ahead, while astern a kind of optical "black hole" begins forming, slowly wrapping around the sides of the vessel, until (as one gets close to c) the field of view shrinks to a dot directly ahead. At 0.99c almost all visible radiation from the universe is confined to a region 10 degrees in radius around the direction of travel.

But then, at the midpoint of travel, the engines cut off. There is no more acceleration & the astronaut(s) are in a state of freefall as the spaceship turns around.

What happens when the engines cut off here? Does the relativistic view outside the windows disappear? Or stay the same?

And what happens when the engines kick in again for decelation? Does the whole relativistic display reverse?

I hope this isn't too weird a question. It's a major part of a story I am writing (the astronaut's first-hand experience of relativistic accelerations etc).

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But then, at the midpoint of travel, the engines cut off. There is no more acceleration & the astronaut(s) are in a state of freefall as the spaceship turns around.

What happens when the engines cut off here? Does the relativistic view outside the windows disappear? Or stay the same?

And what happens when the engines kick in again for decelation? Does the whole relativistic display reverse?

I hope this isn't too weird a question. It's a major part of a story I am writing (the astronaut's first-hand experience of relativistic accelerations etc).

These effects, which I think you've got a good idea of, are dependent only on the speed of the ship relative to the stars. They are not due at all to the acceleration, except that it's the acceleration that causes the relative velocity.

As the ship decelerates, the whole process will be undone in reverse and gradually the view of the stars will return to "normal".

Nothing happens when the ship turns 180 degrees, except of course that the front and rear of the ship are reversed.

m4r35n357
It accelerates for the first half of the journey, then cuts its engines & turns around 180-degrees and then begins deceleration for the remaining half of the journey.

What would the astronaut(s) see out the ports?
I simulated a similar idea a few years back, here are the videos I made. The scenario covers first receding then approaching, with two values of constant magnitude acceleration. There are also some side views.

Oh, there are also clocks aplenty!

Read the explanatory text. It is terse, but I can answer questions on it here if you need.

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kiplunk
This is exactly what I was after, so thank you, both of you! (Yes, PeroK, I was over-thinking the "acceleration" part! Thank you for putting me straight! )

Excellent animations, m4r35n357! They raise a number of issues for me, however. Not sure if I should make another thread or...?

Well, for one, I am assuming the spaceship's "viewport" is facing backwards, towards the octahedral space station it originated from? In this case (from what I have read), shouldn't there be a growing circular "blind spot" directly astern, growing larger the faster the spaceship travels? I get the impression (from my meagre "research") that the field-of-view shrinks/compresses in the forward direction of travel, leaving (for all intents & purposes) a "black hole" forming directly astern, which creeps up around the spaceship, compressing all radiation into a point directly ahead. I imagine that if the spaceship could achieve near-c (eg 0.999...c), all the radiations of the universe would compress into a single almost infinitely-blue-shifted photon directly ahead? with everything else around this "compressed cosmic photon" basically "outside our world/light-cone"?

The relativistic effects of near-c velocities are incredibly psychedelic & confuddling! But I want to learn more!

Thank you for bearing with me!

m4r35n357
I get the impression (from my meagre "research") that the field-of-view shrinks/compresses in the forward direction of travel, leaving (for all intents & purposes) a "black hole" forming directly astern, which creeps up around the spaceship, compressing all radiation into a point directly ahead. I imagine that if the spaceship could achieve near-c (eg 0.999...c), all the radiations of the universe would compress into a single almost infinitely-blue-shifted photon directly ahead? with everything else around this "compressed cosmic photon" basically "outside our world/light-cone"?
There is no hole, black or otherwise! The rear view is "stretched" (so appears closer), dimmed and red-shifted. The front view is "funneled" (so appears further away), brightened and blue-shifted.

Without going into too many details, this image gives a correct interpretation of how the view of a scene changes from front to rear at high relative velocity.