Spaceship moving away from light source

In summary, the conversation discusses using the Doppler shift equation to find the velocity of a spaceship moving away from Earth and the time it takes for sodium street lamps to become invisible to the human eye of the astronauts. The correct equation for the red shift is mentioned and the importance of understanding the principle of relativity is emphasized.
  • #1
nick227
36
0

Homework Statement



A spaceship moves radially away from the Earth with an acceleration of 20m/c2. How long does it take the sodium street lamps ([tex]\lambda[/tex]=589 nm) on Earth to be invisible (with a powerful telescope) to the human eye of the astronauts? The visible spectrum is 400-700nm.

Homework Equations



fobs = ((1+(v/c))1/2/(1-(v/c))1/2) fsource

The Attempt at a Solution



I can use the doppler shift equation, to solve for velocity, but what should I use for fobs? Should I use 400nm or 700nm. I'm thinking 400 nm because of length contraction...

After I find a velocity, I can use the acceleration given and solve for time.
 
Physics news on Phys.org
  • #2
You're on the right track, but you are solving it backwards. You know the frequency of the light source as observed in the light source's rest frame, and you know what happens to the frequency. Hint: Does the moving away from the Earth make the street lamp redshift or blueshift?
 
  • #3
Well seeing as how I am moving away from the source wouldn't it be a red shift, so 700nm. But everything i read about red shift is if the source is moving away from the observer. now assuming that doesn't matter, am i right?
finding v=c(f_o^2 - f_s^2)/(f_s^2 + f_o^2)
f_o = f of observer
f_s = f of source

so our time is v/a?
 
  • #4
The observer is moving away from the source when the source is moving away from the observer. There is no such thing as absolute position.
 
  • #5
But the latter half of the question is right, right? solving for v then saying t=v/a would determine how long it would take to observe the light shift into the non-visible spectrum?
 
  • #6
You wrote: "I'm thinking 400 nm because of length contraction..."

This is not about length contraction, but about the Doppler shift. Check your textbook for the difference between these concepts!

You wrote: "But everything i read about red shift is if the source is moving away from the observer". You should read and reread what the "principle of relativity" says!
 
  • #7
borgwal said:
You wrote: "I'm thinking 400 nm because of length contraction..."

This is not about length contraction, but about the Doppler shift. Check your textbook for the difference between these concepts!

You wrote: "But everything i read about red shift is if the source is moving away from the observer". You should read and reread what the "principle of relativity" says!

OK...but we're past that part now and we get it, just curious if utilizing the equations like that seem to be correct
 
  • #8
Do you think you have the relativistically correct (rather than the classical) equation for the red shift? If so, then of course that equation applies.
 

What is the phenomenon behind a spaceship appearing to move away from a light source?

The phenomenon behind a spaceship appearing to move away from a light source is known as the Doppler effect. This is when the frequency of light waves appears to decrease as the source of the light moves away from the observer.

How does the speed of the spaceship affect the appearance of it moving away from a light source?

The speed of the spaceship does not directly affect the appearance of it moving away from a light source. However, the faster the spaceship is moving, the greater the Doppler effect will be, making the light appear to be more redshifted.

Why does the light from the spaceship appear to be redshifted when moving away from the light source?

The light from the spaceship appears to be redshifted because as the spaceship moves away from the light source, the wavelength of the light waves appears to increase, shifting the light towards the red end of the visible spectrum.

Does the distance between the spaceship and the light source affect the appearance of the spaceship moving away?

Yes, the distance between the spaceship and the light source does affect the appearance of the spaceship moving away. The farther the spaceship is from the light source, the greater the redshift will be, making it appear to be moving away at a higher speed.

How does the angle between the spaceship's direction of travel and the direction of the light source affect its appearance?

The angle between the spaceship's direction of travel and the direction of the light source does not directly affect its appearance. However, if the spaceship is moving at an angle away from the light source, the Doppler effect will be less noticeable compared to if it were moving directly away from the light source.

Similar threads

  • Introductory Physics Homework Help
Replies
6
Views
736
  • Advanced Physics Homework Help
Replies
1
Views
2K
  • Special and General Relativity
2
Replies
36
Views
2K
  • Sci-Fi Writing and World Building
Replies
10
Views
2K
  • Special and General Relativity
Replies
6
Views
1K
Replies
15
Views
3K
  • Advanced Physics Homework Help
Replies
1
Views
4K
  • Special and General Relativity
Replies
3
Views
438
  • Introductory Physics Homework Help
Replies
8
Views
6K
  • Special and General Relativity
2
Replies
36
Views
3K
Back
Top