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Object from the sun at half light speed

  1. Mar 5, 2010 #1
    Hypothetical question:

    Imagine that a massive object large enough to observe with a powerful telescope was ejected from the sun and began traveling toward earth. It would take about 8.3 minutes for the light from the sun to reach earth so we could observe the initial event. Lets imagine that the force of the ejection is strong enough to cause this object to hurtle toward earth at half the speed of light (for now, lets not get caught up in potential changes in energy and mass of the object). At half light speed, it should take the object approximately 16.6 minutes to reach the earth, assuming steady velocity and no impedance. Now, the moment we first observe in our telescope the ejection of the object, we know that we are seeing what happened 8.3 minutes earlier, but at this moment of observation, the object has already traveled half the distance from the sun to the earth. This means that the object should hit the earth approximately 8.3 minutes after we first observe it leaving the sun.

    The question is, what will we see in our telescope if we attempt to observe the objects entire journey from the sun to the earth? Will we see it leave the sun, and then hit the earth 8.3 minutes later, as if it were traveling at the speed of light? It is really only traveling at half the speed of light, which means that it took 16.6 minutes to travel the entire distance, but it is 8.3 minutes after we first observe it leaving the sun that we are hit. Any Ideas?
  2. jcsd
  3. Mar 5, 2010 #2


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    We see it moving at an apparent velocity of the speed of light. As it gets faster and faster, there's no limit to to this apparent velocity.

    You may be interested in a well known phenomenon of high speed jets of matter traveling at a small angle from directly towards us. Such jets can have an apparent transverse velocity much greater than the speed of light. See Apparent Superluminal Velocity of Galaxies at the USENET physics FAQ, which explains it in some detail. It's the same effect, essentially.

    Cheers -- sylas
  4. Mar 5, 2010 #3
    Yes, this effect is frequently observed in astronomy and goes by the names of superluminal motion. It is in some sense an optical illusion. The particle travels near the speed of light

    An example where such an effect is observable is in the jets of quasars, which is an accretion system. An accretion disk is a disk-like object with a massive stellar object in the center (a black hole, a supermassive black hole, or a neutron star). The matter is in some orbit around the heavy object, and will eventually fall into it. A jet is a thightly bundled stream of particles that is emitted from this disk, perpendicular to the plane in which the disk lies (see this wiki article: http://en.wikipedia.org/wiki/Relativistic_jet ). The particles travel near the speed of light, and if the jet is 'aimed' at us, then it looks as if these particles traveled much faster than the speed of light -- precisely in the matter you explained.

    So it's hardly hypothetical at all!

    EDIT: sylas beat me to it
  5. Mar 5, 2010 #4
    Our observations are probably the least eligible method of measurement for such velocities. If it was travelling at 99% the speed of light it would appear to us, as if it beamed here like instantly. (if we could really observe such a high velocity object).

    What we see is not reality because at those speeds the time it takes information to reach us outweights the information.

    Example: What would a super-luminal particle look like: (src wiki: tachyon)

    http://upload.wikimedia.org/wikipedia/en/6/64/Tachyon04s.gif [Broken]
    Last edited by a moderator: May 4, 2017
  6. Mar 5, 2010 #5
    It's already been said, but yes you are going to get some weird effect.

    The object will be blueshifted (which should tell us how fast it's travelling). On top of this you are going to see the image travel 1 au to 0au in 8.3 mins. As you've cleverly measured it's blueshift already, you won't be confused by the barrage of images.
  7. Mar 6, 2010 #6


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    This is a great thought provoking post. One very important thing to keep in mind is that as you observe the object, you are not only viewing the object moving through space, but you are also observing it moving through time at twice the normal rate. For example, if we look at the sun continuously, we are observing it passing through time at the same rate as us, but offset by 8 minutes into the past. Not so with the ejected object. Since it is getting closer to us by the moment, the 8 minute offset is no longer constant but moves from 8 minutes in the past, to 0 minutes in the past when it finally hits. In other words, we are watching this object move through time as if we are fast forwarding through time, effectively making the object seem to move twice as fast as it's real speed. Another way to visualize this is to imaging watching a movie of the sun at 24 fps (a movie that was shot at 24 fps). Now speed up the movie to 48 fps and we will see the sun burning off gas at twice the speed we would expect it to because we are watching it move through time at twice the normal rate of time.
  8. Mar 6, 2010 #7
    We would not see the particle until it was alongside us and then we would see two copies of the particle. One copy would appear normal going forward and the other copy would appear to be going backwards in space and time back to where it came from until we eventually saw the event that emitted the particle. Of course no such particles have actually been observed.
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