so suppose you have two vessels in Interstellar space. One is the ITV Europa while the other one listens to the name ITV Lea Sudux.

Europa is flying at 0.5c, so 149896km/s where as the Sudux chills at 0.8c which is around 239833.6km/s. Both ships fly in opposit directions so they will pass each other. Both ships are more needle shaped and are covered by an Aerodynamic Shield which glows duo to collisions.

Now the question itself is, how would that look ? My main concern here is with Lenght Contraction and the Red Shifting of Light at such speeds. From what i understand, you would only see a Redshifted Univserse with a Shadow wherever the Shield is. The other ship would then maybe just appear as an even more Redshifted spot. But i am not sure. In theory, it might as well just be white. The ships relative velocity to each other is 1.3c, which cant happen. It has appear to be below 1c. At the same time, the relative velocity would be like 99.999999999999999999999999999999999999% of c, which would imply a Maximum Redshift, which would make the other ship appear to be just an ultra bright spot.

So yeah, i am kind of stuck. I think the other ship would appear as a bright spot when it is infront of the other ship and the fade out once it is past. But i am not sure.

Thanks for the help !


1 Answer 1


Researches have found that humans can identify things that happen in as little as 13 milliseconds. So, given the speeds of the two ships, how much time is there to "see" something?

  • I'm entirely ignoring relativistic effects, which I think will only makes things worse anyway.

First, the relative speed (as if you, the observer, is standing still) is 389729.6 km/s. But, nothing goes faster than the speed of light, so the best relative speed is 299,792 km/s.

Second, you don't tell us the length of the ships, but let's assume the target ship (the ship being observed) is 1 km. That makes the math easy.

Time of observation: 1/389726.6 = 0.0033 milliseconds << 13 milliseconds.

In short, your passengers won't see anything. The target ship would need to be a whopping 3,940 kilometers long just to meet the 13 millisecond minimum for humans to notice anything passing.


  • $\begingroup$ It would make a better answer if you wrote down also the formulas involved, so that we can skip the load of questions with just different permutations of the respective speeds. And, by the way, ignoring the relativistic effects for relativistic speeds is, well, obviously inaccurate. $\endgroup$
    – L.Dutch
    Commented Oct 8, 2020 at 18:08
  • $\begingroup$ @L.Dutch-ReinstateMonica I know it's inaccurate, but it's also irrelevant. Unless the target ship is a bazzillion km long, it won't be noticed by the passengers of the reference ship. The rest of the formulas aren't even algebra - just arithmetic. (1km ship / 299792 km/s = 0.0033 mS). $\endgroup$
    – JBH
    Commented Oct 8, 2020 at 18:22
  • $\begingroup$ For reference, see my question and answer: What would the view outside my space ship traveling at the speed of light look like? $\endgroup$
    – JBH
    Commented Oct 8, 2020 at 18:23
  • $\begingroup$ I am probably bad at asking stuff, but i was more interessted in the Redshift / Blueshift effects at such speeds. As the ships go in different directions, i thought that you would quiet clearly see the other ship . Since it is coming to the other one, the Wavelenghts get shorter to the point where it may just be white. But i am not sure if you would be able to see that against the Blueshifted Background radiation as i am not sure how that would look at 0.8c. $\endgroup$
    – Erik Hall
    Commented Oct 8, 2020 at 18:39
  • $\begingroup$ @JBH, this sounds like a duplicate of that question $\endgroup$
    – L.Dutch
    Commented Oct 8, 2020 at 19:11

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