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'Fast forward' vision when travelling towards a star?

  1. Jul 16, 2013 #1
    If we would ever travel to another star, say Sirius roughly 9 light years away, at 0,1 c the travel would take 90 years.
    Since the Sirius we see today from Earth is the Sirius of 9 years ago and the Sirius we would see on arrival would be the Sirius of 90 years from now, this means that in an elapsed time for us of 90 years we would see actually 99 years of change process. 99 years of visual history would be seen in 90 years time. So we would actually see it as a movie played in fast-forward motion, sped up by 10%. We would for example see the star moving 10% faster than what currently appears from Earth. Right?

    And a related question, in an hypothetical era of space travel, if we wanted to travel to a star say 1,000 light years away (forget the speed / duration problem for a moment, say we travel at 0.2 c and we live long enough to wait for the 5,000 years journey), we would need to calculate a travel path, based on where we see the destination star today from Earth (where it was actually 1,000 years ago) and with arrival coordinates where it will be 5,000 years from now (= where it will be 6,000 years from where we see it today. I guess that we can calculate where a star will be 6,000 years ahead of the point where we see it today with quite good accuracy, but for large enough distances, would this need to predict the future motion of distant stars be a sizable problem for our hypothetical space travelers? (let alone the risk of the star or planet not being there anymore by the time we arrive :-)
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  3. Jul 16, 2013 #2


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    Right. This is very similar to the Doppler effect for sound - if an ambulance is approaching you, the pitch is higher and the different sounds follow each other a bit quicker.

    Yes, for that distant star we would have to predict the position where it appears in 6000 years. This is not an issue, however - within 6000 years, the trajectory does not change significantly, and it is possible to include gravitational effects from stars nearby. And even if that goes wrong: most stars travel something like .1 light year in 1000 years (relative to our sun). You can adjust the travel direction slightly.
  4. Jul 17, 2013 #3


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    When we point our telescope at a star, we see an apparant location of the star, and this apparant location changes as the earth moves around the sun in orbit. At one time of the orbit the earth would have a sideways movement left to right wrt the star and at the opposite side of the orbit the earth movement would be right to left. That is called aberation and you can look it up on wiki or some other site to get a feel for it.

    So, due to aberation we actually do not see an actual position of a star 1000 ly away but an apparant position of that star that changes as the earth moves around in its orbit. The star seems to be "ahead" of its "actual" position from an earth perspective. If the earth is moving right to left in the orbit the telescope has to be angled a bit to the left, and angled a bit to the right at the other side of the orbit, to see the star.

    Conversely, if the star is moving sideways and you consider the earth not ( or the sun still since aberation cannot be shut off ), then an apparant location of the sideways moving star falls under light time correction ( I think that is the term ). Here though, the telescope has to be pointed "behind" the "actual" position of the star. For a star moving left to right, you do not see where the star was 1000 ly ago when the light was emmitted, but an apparant location a bit to the left of the position 1000 ly ago.

    Using a corpuscular model of light might be easier to visualize what is going on, along with an xy graph.

    For aberation, with a moving earth in the x-direction, 1000 ly ago the star emits a copuscule of light in the y direction. Now, at present, along comes the earth and the corpuscule of light enters the top of the telescope. Since the base of the telescope moves with the earth, the corpuscule can hit the eyepiece only if the telescope is at a small angle. The position of the star has shifted to an apparant position of where it was 1000 ly ago.

    For the moving star, if the corpuscule is again emitted in the y-direction, you have to give the corpuscule a relative motion sideways equal to the speed of the star, so that c ends up again as the speed of light down the length of the angled telescope.

    Hopefully that does not complicate your travelling to stars voyage.

    Terms "ahead" and "behind" are not technical, but may help, maybe not, with the visualization.
  5. Jul 18, 2013 #4
    That was interesting, thanks!
  6. Jul 25, 2013 #5
    Your ship traveling at .1c would also see colors change. Yellow objects would shift green, blue would become dim violet. A red lamp might look orange or stay red if the lamp emitted lots of infrared.
  7. Jul 26, 2013 #6
    Thanks. But actually we don't see many colours in space except for the (basically) white of the stars. What about white? I believe everything would look more blue-ish but would the stars appear clearly more blue?
    Last edited: Jul 26, 2013
  8. Jul 26, 2013 #7


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    Stars appear white as they are not bright enough to allow color vision. With a telescope or a camera, you can see the colors. If you are far away, blue stars will continue to look blue, and the other colors will shift towards the blue. If you are so close to the star (or have such a good telescope) that you can see it as a disk, all stars will get brighter (per apparent disk area), and blue stars can appear white as the radiation is so intense that color vision gets bad again.
  9. Jul 26, 2013 #8
    (My examples in the last post assume some kind of super telescope, or perhaps giant lamps or lasers aimed at the ship from the destination.)

    There are different kinds of "white" which could be affected quite differently by blue or red shifting.

    A star like the sun generates plenty of infrared and ultraviolet, so blue shifting and red shifting will make those frequencies move into the visible spectrum. As a result, the change in color would be much less pronounced than the speed might suggest. At .1c traveling away from the sun, the shift would be to beige and might not be detectable to the human eye.

    Light from a white LED would behave very differently under the doppler effect. Some white LEDs emit just two discrete frequencies - one yellow, and one blue. If this light were blue shifted, the blue would advance into the ultraviolet leaving just the yellow, which would appear GREEN!
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