Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Doppler shift at high speeds

  1. Aug 1, 2012 #1
    I recently heard about the effects from The Doppler Effects at high relative speeds (significant fractions of c).
    For example you will eventually will even get burned/die by the radiation of the CMBR being ao intensly blueshifted while you're approaching the speed of light (relative).

    So with some universe and expansion in mind another thought came up and I wonder if anyone could help answer.

    Let's say we are now in "that time" when galaxies far away have started to become so redshifted their light will never ever reach us anymore and are out of sight.
    We leave our destination in a spacecraft in a very high speed (as high as needed for this thought).*
    - Will now eventually new galaxies reappear because the light from the galaxies are getting blueshifted?*
    (Of course just some galaxies and not galaxies in all directions.) And others disappear due to some getting redshifted? This is because the relative speed between some objects decrease while some increase? Maybe an obvious answer but I'd love to hear something more about this if it's the case, and even more if it isn't.

    Yours Sincerely
    Robin Andersson
  2. jcsd
  3. Aug 1, 2012 #2
    That's an interesting question, but in order for the cosmic microwave background to be damaging, considering that gamma-rays are at about 10^18 Hz and higher, using the Doppler shift:

    f' = f \sqrt{\frac{1+\beta}{1-\beta}}

    you can find that you would need (since the CMB is about 160 GHz) to be going at about 0.999999999999999 the speed of light and even to see X-rays 0.99999999999 the speed of light.
  4. Aug 1, 2012 #3
    What is beta in this equation? f' is the "new frequency" and f is the frequency measured "stationary"? In our case 10^18?
  5. Aug 1, 2012 #4


    User Avatar
    2016 Award

    Staff: Mentor

    Beta is speed in dimensionless units (c=1), which is simply v/c.

    I would expect that you can see galaxies which will never be visible (again) for us. Unfortunately, this requires going so far away for a long time that you cannot return to earth.
  6. Aug 1, 2012 #5
    Ah, thanks mfb. Why isn't it possible to return to Earth? (at this time when galaxies are getting so intensely redshifted, is it even in the life span of Earth?)
  7. Aug 1, 2012 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I may be missing something. I think that in order to have a shielding or cooling problem you don't have to go as fast as somebody else was talking about. You dont have to shift it up to Xray or Gamma for it to be harmful.
    The CMB was emitted by hot gas at about 3000 Kelvin and it is now redshifted by a factor of 1000. It seems to me that if you go near enough c to have a doppler blueshift of 1000 then you are heading into a doppler hotspot with a temperature of 3000 K.
    All you need is a beta β such that sqrt[ (1+β)/(1-β) ] = 1000
    So you'd already have a problem if you came within a millionth of the speed of light.

    The next thing is a rather clever SciFi scenario where Cosmologists in the distant future are frustrated because they can't see all the good stuff we can because the light is too redshifted for their instruments to focus and detect

    So they mount their best telescope on a spacecraft and propel it briefly in some direction, thus enabling it to form images of galaxies in that direction and even to map a small patch of CMB sky in that direction, because since it is say 40 billion years in the future they can't see otherwise.
    The light of many galaxies, those now within say 15 billion lightyears of us, WILL STILL BE REACHING US. But it eventually be so redshifted that our telescopes cannot focus it into useful images. We will never see those galaxies now within the CEH ever pass over the CEH and go beyond the Cosmic Event Horizon. Their slowly reddening images will pile up on the CEH, like dead leaves dropped onto a Black Hole event horizon. We never see them go thru they just become harder and harder to see.

    Yes! immediately as soon as the probe spacecraft is up to speed. the light from those galaxies is already coming in, the astronomers simply cannot see it with their stationary instruments. So they doppler blueshift the light into optical visibility.

    they should use an unmanned probe to allow rapid acceleration.

    Lawrence Krauss has a paper on the preprint arxiv called THE RETURN OF THE STATIC UNIVERSE,
    which describes the plight of future astronomers who will not even be able to detect evidence that the U is expanding, and will not know that there are other galaxies outside our own (they will all be too redshifted to see, though the light from them will still be arriving at Earth.)

    You could read his article and then write to him describing your solution to the future astronomers' problem. Offer to co-author a science fiction piece, or to write one and give him credit in the acknowledgements. He's a very good writer.

    I disagree with some other poster(s). there is no need to travel long or far in order to make the invisible galaxies visible again. You just have to get the instruments up to speed. so how long it takes depends on what acceleration you can achieve and what the instruments can withstand.

    Of course, Robin, the whole idea is insanely impractical :biggrin: But rather clever--I never heard it proposed before.
    Last edited: Aug 1, 2012
  8. Aug 2, 2012 #7
    ((Just a quicky, I read about the CMBR being harmful at (see link below) - for people being curious.))

    http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

    Is this because the wavelength is too long? Is there any limit of the length of a wave? Sorry if this is a stupid question but I haven't started studying physics yet at a university level (actually I'm starting in 3 weeks :biggrin:).

    Thanks! I'll take a look at this when I come home later today.

    Yes I agree with you marcus, and another think to think of the faster you go, the more galaxies show up. So some galaxies will start to appear much earlier than other galaxies. Then the galaxies you will see just breaks down into how fast you are able to go, right? Or am I wrong?

    And yes the idea is probably very impractical as you say, but this is just the sort of ideas I have in my mind - and I need help to answer some of them, and I also think some can be very fun to discuss. Especially the impractical ones :biggrin:.

    Thanks for your reply!

    Best Regards
    Robin Andersson
    Last edited by a moderator: May 6, 2017
  9. Aug 2, 2012 #8


    User Avatar
    2016 Award

    Staff: Mentor

    With accelerated expansion, there are still galaxies which we never can observe - or where we never can observe their current state.

    @RobinSky: Just by definition: If the spacecraft can see something earth cannot ever see, and the spacecraft can return to earth, earth could know about those events (and therefore see this events, as light is quicker than the spacecraft) => contradiction.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Doppler shift at high speeds