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

To see there is easier than to be there

  1. Dec 1, 2003 #1
    Speculum Temporis - a partial solution?

    All the pictures you can see in a mirror are messages from the past.

    Photons must first travel from the object under survay to the mirror and back to the observer. Your do not think of that in the bathroom. But imagine that the mirror is located at the distance of one light year, the image you could see would be two years old.

    If all celestial bodies besides black holes are reflective, at least some of the photons we are detecting by watching the sky, must be reflections from our very own history. By tuning at the distance of N ly's, you could watch direct broadcast of what happened 2N years ago.

    This would help us to evaluate where to travel.

    I think this could possibly work for some signals in the timeframe of minutes inside our solar system. But could it be possible in the timeframe of years or hundred years for any kind of reflections? What do You think?
    Last edited: Dec 2, 2003
  2. jcsd
  3. Dec 3, 2003 #2


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Well it's hard to think of an astronomical body that is as good a reflector as a mirror - one that you could see shapes for example. Probably the best reflector in the inner solar system is Venus, and that has the kind of reflectivity of white clouds. Very bright but no detail.

    Since natural bodies are not likely to work, let's consider artificial bodies. Could we put a mirror out in space and arrange to see our past in that? Possibly, but it would have to be a very big mirror. Astronomers just recently managed to get an image of a planet of another star. All the other planets have been discovered from their gravitational effects on their stars. The image is just a blob, below the level of real resolution. But suppose future telescopes can do a few orders of magnitude better. Then maybe a planet sized mirror would work. But of course the image would be of earth as seen from space. We could maybe see how the clouds moved forty years ago but not the people.

    Sigh, more technology. Suppose the solve that problem...
  4. Dec 7, 2003 #3
    Yes, it is hard to find an ideal mirror in space.

    But let me ask the other way round:

    - What is the most distant object in space we can get a radar signal back?
  5. Dec 7, 2003 #4


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    I don't know the answer to that. That would be an excellent question to ask on the Astronomy forum.
  6. Dec 7, 2003 #5
  7. Dec 7, 2003 #6


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Good work. I'll be talking to you.
  8. Dec 28, 2003 #7
    Nereid answered at General Astronomy & Cosmology -forum:

    " AFAIK, Saturn, its rings and moon Titan hold the record:
    http://www.aas.org/publications/baa...dps2002/109.htm [Broken]
    http://www.news.cornell.edu/Chronic...ydrocarbon.html [Broken]

    The distance? >1 billion km

    So it takes 2.25 hours to get a reflected signal back from Titan. By analysing the reflected signal we can conclude (among other things) that someone was experimenting with a high energy radar down on the earth more than two hours ago. Also if there were, say 5 seconds, black out in the power production system of the sun. We could observe its propagation through the planets so that we could see the black out in Pluto about 10 hours later.

    Thus far we can say that yes 'Speculum Temporis' is observable in solar dimensions.

    But does it exist in light year range? Why not - but the signal must be extremely weak. We probably must be able to catch single photons. This would have been totally impossible 20 years ago. But today we rapidly approaching that extreme. See for example "A broadband superconducting detector suitable for use in large arrays" by Peter K. Day et al in NATURE |VOL 425 | 23 OCTOBER 2003.
    Last edited by a moderator: May 1, 2017
  9. Dec 31, 2003 #8
    To see is a progress

    AS the progress , the time is a period, sorry it isn't a time travel question,but to see the past image is truth. it is a fun question.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook