Speed of light isn't constant relative to all observers.

Discussion in 'Pseudoscience' started by Zeno, Oct 27, 2017.

  1. Zeno Registered Senior Member

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    http://calgary.rasc.ca/algol_minima.htm

    The eclipses come farther apart in time when the Earth is moving away from Algol. Therefore, the light is passing the Earth more slowly. Imagine you are standing still and that you have a line of people running past you at a constant speed and they are equally spaced apart from each other. If you are standing still and then start walking in the same direction that they're running, there will be more time in between each person reaching you and they are passing you more slowly. If you believe that the speed of light is constant relative to all observers then you will need to explain why the above reasoning doesn't apply to light.
     
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  3. Kittamaru Now nearly 40 pounds lighter. Staff Member

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    The speed of light in a vacuum is a universal constant - that is it moves at a set rate unless otherwise acted upon. It's apparent speed relative to someone else in motion is, largely, irrelevant.
     
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  5. Janus58 Valued Senior Member

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    From the Earth's perspective, it is Algol that is moving away from the Earth, thus the distance between Earth has increased between each Eclipse and the extra time it takes the light to travel that distance at c between eclipses means we see the eclipses spaced further apart in time. Doppler shift is only dependent on the relative speed between the source and observer and not on which one we consider as moving.
     
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  7. Zeno Registered Senior Member

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    It takes a longer time between eclipses because the light, relative to us, is no longer moving at c. The speed of the Earth around the Sun is 66,629 mi/hr. We are not moving directly towards and away from Algol, so our speed in that direction is actually 66,629 mi/hr * cos(17.516339) = 63,539.49055 mi/hr. So the speed of light relative to us when we are moving away is 186,264.3501 mi/sec. So the time taken increases by (186,282/186,264.3503) = 1.000094757. So 2.867321 days * 1.000094757 = 2.867592699 days. That's very close to 2.8675875347 days when the Earth is moving away from Algol at maximum speed.
     
    Last edited: Oct 27, 2017
  8. Janus58 Valued Senior Member

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    No, the light is still moving at c relative to the Earth., it just has to travel a longer distance.

    This demonstrates how Doppler shift work for light:
    If you are the source, the light expands outward from you at c equally in all directions. The two observers (the red and blue dots) here are stationary to the source and receive the waves at the same frequency as the source emits. like this:

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    If the dots were moving from right to left at the relative speed with respect to source, then tthe red dot would be running away form the expanding waves and the blue dot rushing to meet them,

    However, if you are the red or blue dots, and there is a relative motion between you and dots, the following is happening: The first part of a wave is emitted and expands outward at c from the point of emission equally in all directions. The source is moving, so is in a different spot when the next part of the wave is emitted, and this new spot is the center of that part of the wave that expands outward at c. The next part is emitted from another point and is the center of the light emitted at that moment and so on. Like this:

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    No part of the waves are ever moving at anything but c relative to the red or blue dots, but the red dot sees longer waves at a lower frequency and the blue dot sees shorter waves at higher frequency.

    The point is that having the dots moving from right to left or having the source moving from left to right are just two different views of the same thing, but if you are the source the light is seen as traveling at c with respect to yourself, and if you are the dot you measure it as traveling at c with respect to you.
     
    Last edited: Oct 27, 2017
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  9. Neddy Bate Valued Senior Member

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    Nice animations, but viewing the second one, Zeno could say that surely the moving (yellow) dot should be able to determine that the light leaving its rear end travels a greater distance per unit time than the light leaving its front end. The whole explanation is quite a bit more complicated to explain, and involves relativity of simultaneity (ROS), which most newbies to relativity find difficult to understand. Einstein's train car thought experiment (chapter 9 from the 1920 book Relativity) demonstrates ROS well, but even that should be taken in the context of the previous chapters. It took me a really long time (years) to understand SR, but I'm glad I stayed at it until it finally made sense.
     
    Last edited: Oct 31, 2017
  10. Neddy Bate Valued Senior Member

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    I just thought I should clarify that the "newbie to relativity" I was referring to was Zeno, not Janus. I know Janus understands, and is virtually an expert on SR.
     

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