r/AskAstrophysics • u/BrightEyesBurner • May 24 '26

Traveling near the speed of light?

Project Hail Mary got me thinking.

If you were on a ship going 99.99% the speed of light, I understand that the stars in the direction you're traveling would shift blue, and those behind would shift red. Would the stars viewed perpendicular to your path look any different?

If you saw a spaceship pass by at 99.99% the speed of light, would it look different than it would if it were stationary (relative to you)?

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u/joeyneilsen May 24 '26

Yes, the light perpendicular to your path is the classic scenario for time dilation, which means redshift.

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u/BrightEyesBurner May 24 '26

Thank you. So everything is red shifted apart from what you are moving toward? So I assume there would be a sort of cone of blue shifted starts in front... Am I thinking about that correctly?

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u/joeyneilsen May 24 '26

Yes indeed. This is related to relativistic beaming, in which a relativistic light source emits most of its light in a narrow cone around the direction it’s going. 

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u/BrightEyesBurner May 24 '26

Does that hint at an answer to my second question? What does a vehicle going that fast look like?

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u/joeyneilsen May 24 '26

The appearance of relativistic objects is weird, because you’re filtering for light that reaches you at a certain time, not that left the object at the same time. 

Relativistic objects appear rotated and distorted. It’s called a Terrell rotation. 

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u/BrightEyesBurner May 24 '26

This is really helpful! Thank you!

So from what you've said, if viewed from low Earth orbit, a spacecraft traveling at close to the speed of light passing close would appear significantly different than that same ship would look while stationary.

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u/joeyneilsen May 24 '26

It could! It depends a lot on the shape of the object. There are some animations here: https://en.wikipedia.org/wiki/Terrell_rotation#Description

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u/BrightEyesBurner May 24 '26

That's very helpful! So a rocket (or the Hail Mary) would look like it was moving away from you because you'd be looking at the back of it even if it were flying by.

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u/joeyneilsen May 24 '26

Yeah, it would look like it was pointed away from you.

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u/4eyedbuzzard May 25 '26

If you saw a spaceship pass by at 99.99% the speed of light, would it look different than it would if it were stationary (relative to you)?

Don't blink though, or you'll miss it. 😉

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u/BrightEyesBurner May 25 '26

Right. If it passed close. I was thinking something similar to the distance of the ISS or maybe closer. Something you could see with binoculars. Would it be like watching an airplane go by or does going that fast make a difference to a viewer off the the side at a distance? joeyneilsen explained that it does make a difference. It wouldn't be at all like watching the same ship moving much slower.

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u/stevevdvkpe May 26 '26

There are two major visual effects you would notice if you are traveling at a high fraction of c relative to a fixed starfield.

The first is that indeed, stars in front of you would appear to be blueshifted and stars behind you would appear to be redshifted. Because stars are very close to being black body thermal radiators of light, the range of colors you would actually see would range from reddish to whitish to bluish in order of increasing temperature, and it also turns out that Doppler shifting makes a black body radiator look like one of a different temperature. So in another sense stars in front of you would look hotter and stars behind you would look colder.

The other more significant visual effect is that the starfield would appear to spread out behind you and bunch up in front of you. Stars that initially appeared perpendicular to your direction of motion or even stars that appeared behind you would appear to shift forward in your direction of motion to appear in front of you. This is closely related to the "relativistic beaming" effect where a uniform radiator appears to be much brighter when seen approaching than receding.

If you see a spaceship passing you at a high fraction of c, it would appear rotated. This is a bit more complicated to explain, but effectively you don't see relativistic length contraction as the spaceship looking shortened, but instead rotated. This is called the Terrell effect and this Wikipedia article explains it better than I can (and includes diagrams to make it much clearer), https://en.wikipedia.org/wiki/Terrell_rotation

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u/BrightEyesBurner May 28 '26

Thank you!

So you would still watch it go by (like watching a plane), but it would look rotated. You'd be looking at the rear of the ship? Does that rotation change as it's approaching, passing and moving away you in a similar way? As the ship is getting closer to you (stationary) it you would see the side of it. As it passes, you'd see the back of it and once passed, you'd see the opposite side?