How to calculate the Doppler Shift Frequency

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Discussion Overview

The discussion revolves around calculating the Doppler shifted frequency of signals received from a spaceship traveling at relativistic speeds, specifically focusing on the scenario of a spacecraft moving away from Earth at 85% the speed of light and the implications of traveling towards stars at 99% the speed of light. Participants seek clarification on the formula and its application, as well as the effects of time dilation on observed frequencies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant requests the formula for calculating the Doppler shifted frequency for a spacecraft moving at 85% the speed of light and asks for an example using a signal frequency of 100 MHz.
  • Another participant suggests searching for "relativistic Doppler shift" as a potential resource.
  • A participant expresses a desire to see the formula applied with real numbers and seeks to understand how time dilation affects the observed frequency when traveling at 99% the speed of light towards stars.
  • A formula is provided: f_o = f_s √((1+v)/(1-v)), with explanations of the variables involved, including how to interpret the sign of the speed.
  • One participant expresses gratitude for the provided information and indicates plans to experiment with the formula in a program.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the application of the formula, as the discussion includes varying levels of understanding and requests for clarification. Some participants are focused on specific examples while others are seeking broader explanations.

Contextual Notes

The discussion does not resolve the complexities of time dilation or the exact calculations for different scenarios, leaving some assumptions and mathematical steps unaddressed.

Who May Find This Useful

Individuals interested in astrophysics, relativistic physics, or those seeking to understand the Doppler effect in the context of high-speed travel may find this discussion relevant.

mpolo
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I would like to know the formula for calculating the Doppler shifted frequency received by someone on Earth that is received from a spaceship traveling at 85 percent the speed of light away from Earth. Please show example of the formula being used. Let's say the spacecraft sent a radio signal from the ship at 100 MHZ what would the received frequency of that signal be on earth?
 
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Have you tried googling for "relativistic Doppler shift"?
 
PeroK said:
Have you tried googling for "relativistic Doppler shift"?
Yes I have looked but I would like to see someone actually do it with real numbers and a bit of explanation I am curious as to how the time dilation works in the scheme of things. In other words what would it look like if I were in a spaceship traveling at 99% the speed of light towards a group of stars. Would the light of the stars be blue shifted? If so, by how much?
 
Last edited:
mpolo said:
Yes I have looked but I would like to see someone actually do it with real numbers and a bit of explanation I am curious as to how the time dilation works in the scheme of things. In other words what would it look like if I were in a spaceship traveling at 99% the speed of light towards a group of stars. Would the light of the stars be blue shifted? If so, by how much?

The equation is:

##f_o = f_s \sqrt{\frac{1+v}{1-v}}##

Where ##f_o=## is the observed frequency, ##f_s## the frequency of the source and ##v## is the speed expressed as a fraction of the speed of light. E.g. ##v = -0.85## or ##v = 0.99## in the two examples you wanted. If ##v## is positive then motion of the source is towards the observer and if ##v## is negative then motion of the source is away from the observer.

You could put that formula into a calculator or spreadsheet and plug away!
 
Oh Man, Thanks this is perfect thanks so much! This is exactly what I was looking for! Going to have fun putting this is to a program and trying different values.
 

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