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Thread: Blueshift question

  1. #1 Blueshift question 
    Forum Senior bill alsept's Avatar
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    I have a question about blueshift caused by relativistic doppler effect. I understand that an object moving toward an observer would be blueshifted and I assume that the faster this relative motion is the more blueshift there will be. What I'm wondering is what would happen to the blue shift if the object was moving toward the observer but constantly slowing down. Is less blue shift the same as more redshift?


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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by bill alsept View Post
    I have a question about blueshift caused by relativistic doppler effect. I understand that an object moving toward an observer would be blueshifted and I assume that the faster this relative motion is the more blueshift there will be. What I'm wondering is what would happen to the blue shift if the object was moving toward the observer but constantly slowing down. Is less blue shift the same as more redshift?
    All motion is relative, but the basic idea is simply that if the velocity changes over time then so does the blue shift. Technically you could indeed say that a decreasing blue shift is equivalent to an increasing red shift.


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    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by bill alsept View Post
    I have a question about blueshift caused by relativistic doppler effect. I understand that an object moving toward an observer would be blueshifted and I assume that the faster this relative motion is the more blueshift there will be. What I'm wondering is what would happen to the blue shift if the object was moving toward the observer but constantly slowing down. Is less blue shift the same as more redshift?
    All motion is relative, but the basic idea is simply that if the velocity changes over time then so does the blue shift. Technically you could indeed say that a decreasing blue shift is equivalent to an increasing red shift.
    Thanks that's what I thought. So if you had multiple objects moving toward you and all were slowing down at different rates the ones slowing faster would have more redshift? My real question and the reason its in the "New Hypotheses and Ideas" category is: What if the universe were contracting (or condensing) instead of expanding? Wouldn't the reverse of the balloon analogy happen? As all the galaxies came together wouldn't the farthest ones be coming in faster from our point of view? And instead of an accelerating expansion the opposite would happen and the farther an incoming object was the faster it would be slowing down. In other words the farther away an incoming object is the more its relative motion is changing (slowing). Less Blue shift would be More Red Shift.
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    Anti-Crank AlexG's Avatar
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    If the universe was contracting, we wouldn't see red-shift, we would see blue shift. If an object were moving towards us slower than another, we would see less of a blue shift, but the spectral lines used to measure shift would still be on the blue side of their normal position.
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    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by AlexG View Post
    If the universe was contracting, we wouldn't see red-shift, we would see blue shift. If an object were moving towards us slower than another, we would see less of a blue shift, but the spectral lines used to measure shift would still be on the blue side of their normal position.
    I am talking about the relativistic Doppler effect and the relative change (slowing). The farthest incoming objects would be changing speeds (slowing) faster and would show less blue shift. Like Marcus said "Technically you could indeed say that a decreasing blue shift is equivalent to an increasing red shift."
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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by AlexG View Post
    If the universe was contracting, we wouldn't see red-shift, we would see blue shift. If an object were moving towards us slower than another, we would see less of a blue shift, but the spectral lines used to measure shift would still be on the blue side of their normal position.
    I am talking about the relativistic Doppler effect and the relative change (slowing). The farthest incoming objects would be changing speeds (slowing) faster and would show less blue shift. Like Marcus said "Technically you could indeed say that a decreasing blue shift is equivalent to an increasing red shift."
    Red-shift and Blue-shift are measured with respect to the baseline of no shift at all. When we say that light from a source is red shifted, we mean as compared to light from a source that is not moving at all with respect to us. So while you technically can say that decreasing blue shift is equivalent to an increasing red shift, There is still an absoluteness to red and blue shift.

    It like the following example, If you are on a ladder with people above and below you, with the people below climb towards you and the ones above climbing away, the decreasing distance between you and the ones below are equivalent to the increasing distance between you and the one above in the sense that both are getting higher as time goes on, but that does not change the fact that the one below you are below you and the one above you are above you. You would never confuse one for the other.
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Janus View Post

    Red-shift and Blue-shift are measured with respect to the baseline of no shift at all. When we say that light from a source is red shifted, we mean as compared to light from a source that is not moving at all with respect to us. So while you technically can say that decreasing blue shift is equivalent to an increasing red shift, There is still an absoluteness to red and blue shift.

    It like the following example, If you are on a ladder with people above and below you, with the people below climb towards you and the ones above climbing away, the decreasing distance between you and the ones below are equivalent to the increasing distance between you and the one above in the sense that both are getting higher as time goes on, but that does not change the fact that the one below you are below you and the one above you are above you. You would never confuse one for the other.
    Yes, precisely. That is why I specifically stated that motion is relative, hence so is energy shift.
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    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by AlexG View Post
    If the universe was contracting, we wouldn't see red-shift, we would see blue shift. If an object were moving towards us slower than another, we would see less of a blue shift, but the spectral lines used to measure shift would still be on the blue side of their normal position.
    I am talking about the relativistic Doppler effect and the relative change (slowing). The farthest incoming objects would be changing speeds (slowing) faster and would show less blue shift. Like Marcus said "Technically you could indeed say that a decreasing blue shift is equivalent to an increasing red shift."
    Red-shift and Blue-shift are measured with respect to the baseline of no shift at all. When we say that light from a source is red shifted, we mean as compared to light from a source that is not moving at all with respect to us. So while you technically can say that decreasing blue shift is equivalent to an increasing red shift, There is still an absoluteness to red and blue shift. It like the following example, If you are on a ladder with people above and below you, with the people below climb towards you and the ones above climbing away, the decreasing distance between you and the ones below are equivalent to the increasing distance between you and the one above in the sense that both are getting higher as time goes on, but that does not change the fact that the one below you are below you and the one above you are above you. You would never confuse one for the other.
    Thanks guys, I get now. I think now I will do a little reading on how, when and where the base line is measured.
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    Quote Originally Posted by bill alsept View Post
    So if you had multiple objects moving toward you and all were slowing down at different rates the ones slowing faster would have more redshift?
    No. Redshift (or blueshift) is about relative velocity. Acceleration has nothing to do with it. However, if one is in an accelerated frame of reference, then there is a redshift or blueshift based on the relative location of the object (as well as the relative velocity of the object).
    There are no paradoxes in relativity, just people's misunderstandings of it.
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