Re: how would space appear at 90% light speed?

Question:
I've heard and read conflicting accounts of how a starfield would appear at near-light speed, including descriptions of a 'starbow' which other scientists dispute. I would like to write science fiction using scientifically accurate information and found it difficult to get a definitive answer on this one.

Answer:
Hello Gary,
I understand the difficulty you are having in writing your story to be accurate, the appearance of objects at relativistic speeds is complicated. But I applaud the effort. The physical effects that you would experience by traveling near the speed of light with respect to the objects you wish to observe are these:

• Time Dilation : Clocks moving relative to you run slower than your own.
• Length Contraction : Distances along the direction of motion are shortened
• Mass Increase : Objects moving relative to you have greater mass than at rest
• Doppler Effect: Color of light shifts to longer wavelengths (redder) if moving away and shorter wavelengths (bluer) if moving toward you.
• Aberration: Light paths bend such that you can see around moving objects and can even see them behind you.
• Beaming : Objects emitting radiation evenly in all directions at rest beam most of that radiation toward you while moving.

Now, let’s consider your starfield and what it should look like if you are in a spacecraft traveling near the speed of light with respect to the stars. We’ll take each effect one at a time and then realize that all they will add together to make the final effect. The time dilation effect will have subtle effects on the stars, mostly on their perceived brightnesses. Since time is running slower for the stars as you see it, the light coming from the stars will be dimmed; there are fewer photons of light per second of your time now arriving at your space craft. Length contraction will shorten the distances between the stars and between you and the stars along the direction of motion. This will make stars in front of you appear brighter (brightness falls off with the square of the distance). Since you are too far away from the stars to see their sizes you’ll not notice any other effects from the changing of the distances. Mass increase should not affect your view of the star field. The Doppler effect will change the color of the stars, but in very complex ways. Let’s suppose that all the stars around you all had the exact same spectrum, meaning they all have the same surface temperature. Let’s assume they are all just like the Sun. Because of the way the human eye responds to light we see the Sun’s color to be white. The stars in front of our space craft are moving toward us and hence their light is blue-shifted, so the stars would appear blue while the stars behind are moving away and would appear red. This would create the starbow effect that you mention; a rainbow of stars from blue in front to red behind. The problem however, is that the stars are not all like the Sun they have different colors to start with and different spectrums. So their appearances will change in subtle ways. Stars which are already blue and are moving toward you may become red or just white as their infrared emission blue-shifts into visible light and their blue emission shifts into the invisible UV. It depends just how fast you are going and what the star’s spectrum is like. A blue-shifted blue star will also likely dim since most of its emission is already in the blue. While a blue-shifted red star may get brighter and probably appear more white than yellow (due to the physiology of the human eye). So, in reality the appearance of a starbow is pretty unlikely. The effect of beaming will make the stars moving toward you much brighter as some of the light they would have sent off in other directions will now come right at you, while stars moving away will dim since they are beaming most of their light ahead of them. The last effect is aberration and it does the weirdest thing. The light coming from behind you will bend around the ship and appear to be coming from in front of you. All of the stars around you will become visible in front of the ship.

So, now you see why I said this was complicated (and I’ve probably missed a couple of effects from these basic effects). When you add all of the effects together what do you get? Basically, you should see the star field warping so that they are all in front of you, they should in general become brighter in front and dimmer at the edges of your forward view (which is behind you). There might be a slight rainbow effect from blue in the center to red at the edges (but not very strong).

Here a few links to some folks who I believe have done a nice job of simulating all to most of the effects you would see when traveling at very high relative speeds:

• http://www.anu.edu.au/Physics/ Searle/
• http://www.fourmilab.ch/csh ip/cship.html
• http://math.ucr.edu/home/baez/physics/Relativity/SR/Spaceship/spac eship.html
• http://spider.ipac.caltech.edu/staff/jarrett/talks/LiU/feig_le ctures/ sci_fi.html

I hope this been useful.
Good Luck,
Bryan Méndez
Science Education and Public Outreach Scientist, UC Berkeley