# The beautiful sight of the interstellar traveller

1. Jun 2, 2009

### Bouh

Hi,

I'm new to SR and GR in general. In fact, I just discovered Lorentz equations a week ago. And that got me thinking too, just as it must have happened to you too.

I like maths, but I'm more the kind of artistic person. And I was wondering what would be the sight of a traveller, on board a spacecraft, that would travel near the speed of light between Earth and a planet in another far distant galaxy. We suppose here that propulsion in not a problem and he is surrealistically able to complete the trip in a mere 2 hours (I suppose it reached 99.99...% the speed of light). I'm only interested in what he would see, looking outside the glass of his HUD.

I thought of the following. Let me know if I'm right. During the acceleration phase of the trip, I believe that he would experience a "universal crush" in a direction parallel to its trajectory. All the stars in front of him, would suddenly appear to get really close to him, but at the same time, would not be spheres any more, but simply disks of light (thus appearing has ellipsoids from his view point). They would also start to freeze, as time slows down for these stars from his point of view. The colour of the stars in front of him would progressively become blueish, while the stars being him (which are also getting closer, with ellipse-like shapes) would become more redish. At the top of his speed, he would be illuminated by a ring of light perpendicular to the direction of his travel, since all universe as nearly collapsed on a plane. The funny thing is... Earth being behind him, he is still probably able to see it for the most part of it's trip (since the distances are getting really compressed), although it appears as a flat disk too. During the slow-down phase, Earth would probably vanish, as the universe would seem to regain its usual shape and all the stars would find their place in the sky again. But he would be at a very different point in the universe.

Is that also what you think this traveller would experience? I think that would be a marvellous sight. Let me know what are your views.

--
Bouh

2. Jun 3, 2009

### sylas

It's going to help to appreciate the sights if you have eyes that can see the whole spectrum of light.

Everything ahead of you will be blue shifted into hard gamma radiation. Everything behind you will be redshifted to invisibility. On top of that, the radiation from the front will have a huge factor increase in luminosity... on top of the blueshift; and the radiation from behind will be so reduced in luminosity that you'll only be getting occasional photons.

The only visible light will be a thin ring, seen by looking sideways. The view out the front would be good, but you'd have to use a gamma ray telescope of some kind; and there would be so much, and it would need to be filtered even before reaching the telescope.

Cheers -- sylas

PS. You might like to look at http://ourworld.compuserve.com/homepages/anima/rfslink.htm [Broken] (computer generated movies of travel near the speed of light). They vary in what effects they actually show.

Last edited by a moderator: May 4, 2017
3. Jun 3, 2009

### Bouh

Ah of course the shift would go beyond the visible spectrum. I don't know why I did not appreciate that. I was transported by the dreams I suppose.

Thanks for the links, I tried to find something like this.

4. Jun 3, 2009

### Frozen Light

If anything was visible at all, it would be sped up, not slowed down.

5. Jun 3, 2009

### Bouh

Oh? I thought that since the velocity difference between the stars and the traveller is so high, if the traveller sees something, shouldn't it be slowed down? (in fact, I also believed that from the point of view of the stars, the travellers also appears to be slowed down.)

What did I got wrong?

6. Jun 3, 2009

### sylas

When you are moving towards something... or it is moving towards you... it "appears" sped up. The is the Doppler effect, which takes all the effects of time dilation and the effect of moving into or away from a moving stream of photons. You can break it down if you really want.

If something is moving towards you at high velocity, then it will be running slow, by what is called the gamma factor. On the other hand... every successive pulse or tick is emitted closer to you, and that means you receive them at shorter intervals.

The time dilation factor is
$$\gamma = \frac{1}{\sqrt{1-(v/c)^2}}$$​
The frequency of "ticks" for an approaching clock is divided by this factor, because it is running slow.

On the other hand, because each tick is emitted much closer to you, the frequency at which you receive ticks is greatly increased. The factor for this is (1-v). The frequency at which ticks are received is thus divided by the combined factor
$$\gamma(1-v) = \frac{1-v}{\sqrt{(1-v)(1+v)}} = \sqrt{\frac{1-v}{1+v}}$$​

That may be recognized as the formula for calculating blueshift... which is no accident. When something is blueshifted, you are seeing it come at you with an increased frequency.

Cheers -- sylas

7. Jun 3, 2009

### A.T.

One effect often omitted, when considering fast movement relative to distant stars is http://en.wikipedia.org/wiki/Aberration_of_light" [Broken]. When moving close to c you would see their position shifted towards your traveling direction. For example: a star which is 90° off your course would appear to be only 45° off.

Last edited by a moderator: May 4, 2017
8. Jun 3, 2009

### Bouh

Thanks to everyone for the Doppler effect explanation and the aberration of light, I now understand why the programs in the link you have send render thing so differently than what I initially imagined.

I find it less gorgeous that originally thought, because as you travel, space around you becomes wrapped into your direction of travel and you find yourself in a large tunnel of nothingness with only one light at the end of the tunnel. This kind of sight would probably glorify the religious traveller.

9. Jun 3, 2009

### sylas

Beauty can come in surprising ways. Science has let us see things previously only imagined, and many others never previously conceived of.

Think of what we have been able to see in the very small with great magnification; or the very fast with high speed video, or the very faint, with long exposure times. Then there are things we have seen in places we cannot go ourselves, or visions seen in light to which our eyes are not sensitive.

If we ever get to that point, I would expect beauty in a universe wrapped around to lie compressed in your forward field of view, with a concentric rainbow of colors from the blueshift depending on angle of view... and if it was blueshifted beyond what my own eyes can see, or compressed to a small field of view, I expect that with the capacity to travel at high velocity would come also the technology to capture the view and reform it for my eyes.

And I expect it would be beautiful.

Cheers -- sylas