Timeline for Does the Doppler effect happen instantly?
Current License: CC BY-SA 4.0
12 events
| when toggle format | what | by | license | comment | |
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| Jun 20, 2019 at 18:08 | comment | added | Azzinoth | You don't need to consider any frame where the star is at rest. You don't need to know your relative velocity to the star. | |
| Jun 20, 2019 at 18:06 | comment | added | Azzinoth | @Paradoxy Initially your antenna is at rest in inertial frame A. Your antenna measured light with frequency $f_A$. It can also measure the direction where the light is coming from. Then it accelerates parallel (or antiparallel) to the light direction. After the acceleration your antenna is at rest in frame B moving with speed v relative to A. The antenna will now measure the frequency $f_B=f_{\mathrm{A}} \sqrt{\frac{c+v}{c-v}}$. The velocity v in this formula is the relative velocity between the two frames A and B, in other words, v is the total velocity by which your antenna has accelerated. | |
| Jun 20, 2019 at 10:22 | comment | added | Paradoxy | @HarryJohnston , well we have no problem in computing effects and such that much, after all we do have all required formulas. However what we write imo should also make sense. If there is indeed only two observers in universe, antenna can't find it's velocity relative to star. There has to be another observer who is at the rest to star and close to antenna so that when antenna moves, it finds its motion by considering this new observer which somewhat seems abstract and artificial, not a natural way of analysing. I should think about this problem a little more | |
| Jun 20, 2019 at 7:36 | comment | added | Harry Johnston | You can use any frame that is convenient, it doesn't have to correspond to an actual observer. The most convenient frames to consider in this scenario, I think, are the frames corresponding to the antenna's initial velocity and its final velocity. The source really doesn't matter. | |
| Jun 20, 2019 at 5:52 | comment | added | Paradoxy | @HarryJohnston whether you are to describe this electromagnetic field by 4-momentum or by electromagnetic field tensor itself, for Lorentz transformation you need a relative speed between two observers, you can't assume any observer who is at the rest with electromagnetic field, so the only shot you have is describing this electromagnetic field in source's frame and then transform it to antenna frame which is what i have assumed/asked so far | |
| Jun 19, 2019 at 23:38 | comment | added | Harry Johnston | @Paradoxy, there's no ether, but there is an electromagnetic field, which is affected by a Lorentz transformation in just the same way that matter is. | |
| Jun 19, 2019 at 16:19 | comment | added | Paradoxy | @Azzinoth Ok let's drop the source. How are you going to define length contraction in empty space which only contains light, and time dilation (relative to what)? If there was indeed an ether everywhere, your method was absolutely correct, but there is nothing like that in SR, you need the source of light to assign a velocity to it, to assign a Lorentz contraction between antenna and star. | |
| Jun 19, 2019 at 15:53 | comment | added | Azzinoth | @Paradoxy There are electromagnetic wave peaks and valleys travelling towards you though space. When you accelerate towards them they appear to hit you faster. Calculate the frequency at which wave peaks hit your observer (considering time dilation and length contraction correctly) and you get your formula. The source where these waves originally came from doesn't matter anymore at this point. | |
| Jun 19, 2019 at 15:53 | comment | added | Azzinoth | @Paradoxy I think you mean the relative velocity to the light is always c and doesn't change? Thats true but instead the frequency changes. The point I was trying to make is, that there is no delay for angle or velocity change for the raindrops when the receiver accelerates. In the same way there is no delay for frequency change of light when the receiver accelerates. You said you find that strange, but the explanation in both cases is that the raindrops/light change their angle/frequency immediately because they are already at you location when you accelerate. | |
| Jun 19, 2019 at 15:47 | comment | added | Eric Towers | @Paradoxy : If there is only a transmitter and receiver in the world, there is not an electromagentic field, already filled with a long history of light emitted from the star. You aren't receiving light freshly emitted when you accelerate, you are receiving the light that was already in the space around you. Your receiver is a boat traveling over waves emitted long ago; their (apparent) frequency is a result of their wavelength (crest and trough spacing) and your motion through them. | |
| Jun 19, 2019 at 15:30 | comment | added | Paradoxy | Unlike raindrops which you can assign your velocity to them(and then to their clouds too),it's not possible to assign a relative velocity to the light.Assume that there is only a transmitter and receiver in the world,when receiver start its movement and become inertia,there is no way for him to understand that the source of light is moving now,because light speed is c,nevertheless,only its frequency is shifted.when antenna tries to explain why this frequency is shifted, he has to say it's the source who is moving,but with what evidence? He can't use the doppler shifted light it'd be a loophole | |
| Jun 19, 2019 at 14:24 | history | answered | Azzinoth | CC BY-SA 4.0 |