What Is a Redshift?

'Ask an Astronomer' uses an astronomer, a flying saucer, and a cow to help explain this difficult astronomical concept in simple terms.

Category:

Education

Tags:

• science
• astronomy
• ask
• astronomer
• Tom
• Jarrett
• redshift
• doppler
• shift
• animation
• cow
• alien
• abduction
• physics
• light

[mackenzie235]

...Say a jet fighter travelling at 500mph, fires a rocket forward [and the speed of the rocket is 300mph], then to me on the ground the rocket is travelling at 800mph. I ask this because if at an airport and you watch people standing still on the moving walkway and someone hurries past them,

he's definately going much faster!! So, do waves not have this feature? Hope this makes sense. thanks i appreciate your time.

[SpitzerJim]

In this case, the wavelength (frequency) is the dimension that changes, not the amplitude. The speed of sound depends on the medium it's traveling through, so it's not correct to say it's "fixed," but, yes, unlike when you throw a ball from a moving car and a bystander with a different reference frame sees the ball moving faster than you do, the sound waves only propagate at the speed the air will allow. That's what causes sonic booms, BTW -- and aircraft running over its own sound waves.

[mackenzie235]

i don't get it. how can you compare sound waves, which are longitudinal waves, to light waves, which are transversal waves. Into the bargin, didn't Einstien say that the speed of light never varies, ever!

[SpitzerJim]

You are correct that we're oversimplifying in order to make the phenomenon easier to understand, but the sound waves aren't changing speed, either. The speed of sound in the air to the right of you is the same as the speed of sound in the air to your left. What changes is the actual size of the wave, not how fast it propagates.

[GameMusic4u]

Thank you for the help.

[jakeavakian]

GREAT explination, yet proves proof og big bang

[UnderManiac]

Light travels as a photon in the vacuum of space. It does only travel at one speed, the speed of light.

The red shift is not because light has slowed down, rather the photons of light reaching us at a longer period (spaced out between photons). That period is the frequency at which the light reaches us, not the speed. That's what causes the red shift, a lower frequency.

Radio waves travel exactly like this. That's why we can have channels on the radio, diff freq's, arriving at light speed.

[UnderManiac]

So, to put light and sound on the same level, let's consider the electro-magnetic nature of sound and light as photons. When we receive the light we see the photons arriving at specific periods giving us the frequency (colour). When those photons arrive at elongated periods (or further apart), we see it as a lower frequency (or red shifted). If we were to receive light at a faster period (closer together) we'd perceive it as blue shifted.

[UnderManiac]

In order for light to travel as waves it would have to have a medium to travel through, thus the fictional "Ether". Light travels as photons, but acts like waves as well, the dual nature of light. Einstein has theorized this.

Sound isn't traveling faster as an object making the noise gets closer, the sound is bunched up and the frequency of those sound waves reaching us has increased causing us to hear the sound at a higher pitch. ...to be continued...

[mackenzie235]

When you say the "size" of the wave, do you mean the amplitude? Also, if the propagation speed of the air waves is the same either side of me [stationary], does this mean that the speed of sound, like the speed of light, is

independant of the speed of the source? This is the thing i don't get. Do waves and "things", objects, act differently. Is it true for all waves? contd.....

[mackenzie235]

i ask this because if at an airport and you watch people standing still on the moving walkway and someone hurries along past them, he's definately going much faster!! So, do waves not have this feature? Hope this makes sense. thanks.

[mackenzie235]

If this is a fact, is it true for all waves?, but not things or objects? This is what i find difficult to get. Say a jet fighter travelling at 500mph fires a rocket forward, and the speed of the rocket is 300mph. Is the rocket actually travelling at 800mph.[to me on the ground] contd....