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Semirhage
2002-Apr-07, 10:18 PM
I'm writing a sci-fi short story, and I wanted to be astronomically correct on this point, so I thought I'd check with the sages here.

Let's say in the future we designed spaceships that could propel themselves to something relatively close to the speed of light. Anyway, if someone was looking out the window of the ship while it was traveling at these speeds, am I correct in assuming that the (visable to naked human eye) stars would still look rather stationary (i.e. they wouldn't go "rushing by" like in TV show sci-fi)?

If I'm correct about that, am I also correct in assuming that eventually at least some noticeable parallax would occur, such that it would distort the "constellations" that one would have observed at an earlier look out the window?

Basically, is it true that in a ship going at those speeds, you wouldn't see much (if any, I suppose depending on whether there're any close by stars) movement when looking out the window, but in looking out the same window later some change might be apparent? If so, how long would it take for noticeable change to accrue? Days, weeks, months or years?

Thanks a ton for any help you can provide.

Jim
2002-Apr-08, 12:00 AM
...so I thought I'd check with the sages here.

I think it's past their bedtimes, so...

..am I correct in assuming that the ... stars would still look rather stationary (i.e. they wouldn't go "rushing by" like in TV show sci-fi)?

Yes. The distance between stars is too great to see much movement as you go "rushing by". Depending on your speed and the stars' distance, there would be some movement, but not as dramatic as Star Trek shows.

...am I also correct in assuming that eventually at least some noticeable parallax would occur, such that it would distort the "constellations" that one would have observed at an earlier look out the window?

Parallax as in stars changing positions, yes. As in a dramatic change in the constallations, not really. It would be more a matter of your viewpoint. (Okay, that sounds confusing. Try the link below, then 3D Universe/Distant Worlds/go to Star Mapper; look back at the Sun from Barnard's Star and you'll see that Orion is almost unchanged. And the change isn't due to parallax.)

Basically, is it true that in a ship going at those speeds, you wouldn't see much (if any, I suppose depending on whether there're any close by stars) movement when looking out the window, but in looking out the same window later some change might be apparent? If so, how long would it take for noticeable change to accrue? Days, weeks, months or years?

Yes, for close by stars. How long it takes is a function of their distance and your speed. You could set up a trig problem knowing the distance to the star, the angle from some arbitrary point to the star (dead ahead or dead astern, for example), and the distance you'd have to travel to reduce the angle noticeably. How long to travel that distance?

There's a very neat site that could help you some:
http://www.astronexus.com/index.html

Here's the info for Alpha Centauri:
http://www.astronexus.com/3duniv/contents/dw-pages/dw-alphacen.html

Good luck with your story!


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Hale_Bopp
2002-Apr-08, 12:55 AM
When I was young, some of my favorite pictures in astronomy books were the constellations as they appeared at different times in history. One book showed Orion as it appeared in humans early history, today, and thousands of years in the future, due to the movement of the stars through the sky.

Obviously, you can't observe this naked eye in a human lifetime, but the stars do move!

Rob

David Hall
2002-Apr-08, 03:12 AM
I think that, even though the various stars in constellations are at various distances from you, they are generally on the order of hundreds or even thousands of light-years away. If your ship only changes its position a few measly light-years, you aren't going to see much shift except in the few very close stars such as Sirius and Alpha Centauri. Now after going a few tens of light-years, you might start to see some significant changes.

So, if you know what you're looking for, you might be able to notice some distortion after a few years of travel, assuming a speed just below c. Star Trek style whizzing of course couldn't even happen in Star Trek. You would have to be travelling at several light-years per second to get objects moving by that fast.

Another thing to consider though, at relativistic speeds you will see some major distortion in your perception of the universe itself. Here's a fun little site that demonstrates the kinds of effects you would actually see:

http://www.fourmilab.ch/cship/cship.html

David Hall
2002-Apr-08, 03:35 AM
BTW, that star-mapper that Jim gave you is really keen (thanks Jim). If you really want to see what happens, try looking towards Alnilam (in Orion) from various different stars.

Since Orion is such a well-known constellation, it's easy to see when it's distorted significantly. From some locations it doesn't change much at all (from Altair, for example), but from others it's all but unrecognizable (Mizar). It's not just a function of distance, but of angle.

GrapesOfWrath
2002-Apr-08, 07:53 AM
On 2002-04-07 20:00, Jim wrote:
Try the link below, then 3D Universe/Distant Worlds/go to Star Mapper; look back at the Sun from Barnard's Star and you'll see that Orion is almost unchanged. And the change isn't due to parallax.)

I tried it, but the chart loading seemed to hang up. I assume the change that you saw was a change in magnitude of one of the stars? If you can see Orion by looking back at the Sun, you haven't changed your parallax view much. The bright stars of Orion are around 200 lightyears away right? How far is Barnard's star?

Donnie B.
2002-Apr-08, 11:45 AM
If you are moving very close to the speed of light (say, above 95% of c), you'd expect things to look very different indeed... but not because of your proper motion.

As your ship approaches the speed of light, the starfield would become more and more distorted, appearing to be concentrated in front of you, and highly blueshifted. There would be a few redshifted stars behind you, but little else would be visible.

If I recall correctly, this is all due to the way you would be intercepting the light from sources at various angles as you approach c, rather than from any exotic effects such as the gravitational bending of light beams by the increased mass of you and your ship.

You'll also need good shielding, as those blueshifted stars up ahead will start to illuminate you with ultraviolet, x-rays, and gamma rays (as you continue to accelerate).

David Hall
2002-Apr-08, 12:52 PM
Donnie, check out the link I gave above. Aberration of Light is just one of 4 seperate effects experienced at those speeds (the doppler shift that turns light into gamma rays is another).

SiriMurthy
2002-Apr-08, 12:58 PM
You'll also need good shielding, as those blueshifted stars up ahead will start to illuminate you with ultraviolet, x-rays, and gamma rays (as you continue to accelerate).


Why? I don't think that you would get exposed to any more UV or X radiation than what is already there. So, why would you get illuminated with more of these radiations just because you are approaching c?

GrapesOfWrath
2002-Apr-08, 01:01 PM
Wouldn't the existing ones still be there, shifted some?

Jim
2002-Apr-08, 05:19 PM
On 2002-04-08 03:53, GrapesOfWrath wrote:


On 2002-04-07 20:00, Jim wrote:
Try the link below, then 3D Universe/Distant Worlds/go to Star Mapper; look back at the Sun from Barnard's Star and you'll see that Orion is almost unchanged. And the change isn't due to parallax.)

I tried it, but the chart loading seemed to hang up. I assume the change that you saw was a change in magnitude of one of the stars? If you can see Orion by looking back at the Sun, you haven't changed your parallax view much. The bright stars of Orion are around 200 lightyears away right? How far is Barnard's star?


Barnard's Star is about 5.9LY from us. Looking back at the Sun from there shows a very indentifiable Orion, with one addition... a rather bright star off to the left, below the line of the belt.

It's the Sun.

Donnie B.
2002-Apr-09, 08:12 PM
On 2002-04-08 08:52, David Hall wrote:
Donnie, check out the link I gave above. Aberration of Light is just one of 4 seperate effects experienced at those speeds (the doppler shift that turns light into gamma rays is another).

Nice link! As you pointed out, I was describing a couple of different effects; I was trying not to be too technical, but rather to give a quick description.

I was interested in the difference in aberration between the classical and relativistic models at high velocities. I knew this, but didn't remember that the classical case only gave 45 degrees of aberration at a velocity of c. But it makes sense.

Donnie B.
2002-Apr-09, 08:25 PM
On 2002-04-08 08:58, SiriMurthy wrote:


You'll also need good shielding, as those blueshifted stars up ahead will start to illuminate you with ultraviolet, x-rays, and gamma rays (as you continue to accelerate).


Why? I don't think that you would get exposed to any more UV or X radiation than what is already there. So, why would you get illuminated with more of these radiations just because you are approaching c?

Yes, you do; it's due to the Doppler shift.

Imagine you're heading straight for a distant star. The beam of light coming from that star carries a certain amount of energy in each unit length. If you are moving slowly toward the star, it will "warm your face"... that is, you will absorb a little energy from that source per second.

However, if you move toward that star at near-lightspeed, you will intercept each unit-length of energy in a much shorter time. What once took an hour to absorb now takes a millisecond. Your face blisters and peels off.

In a classical universe, you could think of that light as simply "hitting you faster"... like riding a motorcycle in the rain, the faster you go, the harder the drops hit you.

In the relativistic universe, that can't happen... the light still comes in at c. Instead, as you move toward the source faster and faster, the light rises in frequency (i.e. is Doppler shifted toward the blue end of the spectrum). At speeds close to c, it will all be very energetic gamma rays.

So make sure you bring that shield!

ToSeek
2002-Apr-10, 11:55 AM
Imagine you're heading straight for a distant star. The beam of light coming from that star carries a certain amount of energy in each unit length. If you are moving slowly toward the star, it will "warm your face"... that is, you will absorb a little energy from that source per second.

However, if you move toward that star at near-lightspeed, you will intercept each unit-length of energy in a much shorter time. What once took an hour to absorb now takes a millisecond. Your face blisters and peels off.


But if you do it just right, you've come up with an effective but very expensive suntanning device. /phpBB/images/smiles/icon_wink.gif

Seriously, it occurs to me that the light would also be Doppler-shifted into much nastier stuff: UV and worse.
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SiriMurthy
2002-Apr-10, 01:49 PM
On 2002-04-09 16:25, Donnie B. wrote:


On 2002-04-08 08:58, SiriMurthy wrote:


You'll also need good shielding, as those blueshifted stars up ahead will start to illuminate you with ultraviolet, x-rays, and gamma rays (as you continue to accelerate).


Why? I don't think that you would get exposed to any more UV or X radiation than what is already there. So, why would you get illuminated with more of these radiations just because you are approaching c?

Yes, you do; it's due to the Doppler shift.

Imagine you're heading straight for a distant star. The beam of light coming from that star carries a certain amount of energy in each unit length. If you are moving slowly toward the star, it will "warm your face"... that is, you will absorb a little energy from that source per second.

However, if you move toward that star at near-lightspeed, you will intercept each unit-length of energy in a much shorter time. What once took an hour to absorb now takes a millisecond. Your face blisters and peels off.

In a classical universe, you could think of that light as simply "hitting you faster"... like riding a motorcycle in the rain, the faster you go, the harder the drops hit you.

In the relativistic universe, that can't happen... the light still comes in at c. Instead, as you move toward the source faster and faster, the light rises in frequency (i.e. is Doppler shifted toward the blue end of the spectrum). At speeds close to c, it will all be very energetic gamma rays.

So make sure you bring that shield!


I guess this explanation makes perfect sense. I wonder, from relativistic point of view, what would a stationary observer would notice! This I mention because of the weird side-effects of near-c speeds or what "would" be observed at event horizon by different observers.

Thanks for the response.