Light redshifting... why?

Question

Ok i just read this sentence. "As space expands, light waves get stretched. If the universe doubles in size during the waves' journey, their wavelengths double and their energy is halved" ok so it was two sentences. If space was not expanding and galaxys were in fact just moving away from each other would there still be a redshift? And If light travels at a constant speed. then why do the waves slow down, or get longer over a distance? Unless of course my first statement is correct... if i ahd a first statement

Answer 1

Physics. Tasty, tasty physics.

It occours for the same reason a car rushing by you seems to change sound. To a stationary observer, the frequency (pitch) of the soundwaves made by the car as it drives away will decrease, so the pitch in your ear gets lower and lower as the car gets further and further from your location.

Sounds waves and light waves work much the same way - I used the car example to help with visualization. :3 It's all the Doppler Effect in action.

So, to answer your questions...

If space itself were not expanding, but galazies were moving away from each other, yes, redshift would still occour, since it's the light sources that matter, not the size of what they're contained in. If the stars are moving away, you will experience redshift looking at them.

Light does indeed travel at a constant speed, but you've got the second part wrong. Yes, the energy, wavelength, and frequency changes. But all waves in a single medium (IE space) will have the same speed.

Take a look at the equation v = lamba (f), which in words is velocity of the wave equals the wavelength times the frequency. If the wavelength doubles, frequency will halve, and velocity remains the same.

Answer 2

You are talking about the doppler effect. This is similar to sound waves being perceived as higher pitch as the source moves toward you then dropping in frequence as it goes away. If you were watching a race car coming toward you, the sound would be a high pitch until it passed, then the pitch would lower. The guy in the car hears the same pitch since he is a part of the moving system. The car coming towards you moves a little closer before emiting the next sound wave thereby shortening the wave length = higher pitch. As the car goes away, it moves a little farther before emiting the next sound lengthening the wave length = lower pitch. Light waves behave the same way when objects are moving together or apart at very high speeds.

Answer 3

The waves don't slow down, they just get "stretched" because when they say space is "stretching", they mean all space, even space that already exists. And redshifting could be caused by galaxies moving apart, but I would think it would be negligible. Redshifting is usually caused by gravitational fields.

(Or staring out the back of a Fred ship traveling near the speed of light away from an Albert-less Earth)

Answer 4

You have to think of spacetime as a grid, like graphpaper. Each square is 1X1 units. If the grid is stretched out, each square is still 1X1 units. Therefore, the speed of light stays that way in relation to the stretching universe. The waves change to accomodate this, so technically it's all really the same....just not. However, the energy in the wavelength remains the same, so if the wavelength is doubled, the energy is halved in relation to the new wavelength.

Ok, ok, nevermind...I think I'm confused. Well, I'll post anyway.

Answer 5

Frequency & wavelength are inverse functions, right?

Redshift is the radiation analog of the Doppler audio shift you hear when a car drives by with the radio up loud...the pitch seems to shift because when it's coming towards you, the sound travels at "speed of sound" + "speed of vehicle", whereas once it goes by, you hear it at "speed of sound" - "speed of vehicle".

Light does the same thing. And since the redshift can be observed in ALL directions, this leads to only two possibilities: either we are at the center of the universe, or the universe is expanding in all directions.

Answer 6

There would still be a redshift. This is most easily demonstrated by a trainwhistle changing pith. It goes to a higher shift as it approaches, (blueshift-shorter wavelength) and a deeper sound as the train passes and gets farther away.(red shift- longerwavelenth) since the train is close enough to not be affected by the expansion of the universe, and only involves changing distances, and the shift is still present, then the extrapolation is that the same would apply in a steady state universe.

Answer 7

The red/blue shift of light happens for precisely the same reason that the pitch of a sound changes as its source approaches or recedes from you. This is the Doppler Effect. A picture is worth a thousand words, so I'd recommend this website ==> http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=38

As for galaxies moving away from each other without space expanding, you just can't do that. Remember that everything is embedded IN space. An object, like a galaxy, can't move independantly of the space that contains it.

Answer 8

1. The speed of light is constant- in a vacuum. But it can be slowed down by the material it moves through. Ex: why it looks like a straw bends at a weird angle in a glass of water
2. Red shifting is just the appearance of the wavelength of light (what color we see) that appears to shift. Its in a combination of effects called the Doppler Effect. (Ex: when sirens are louder and higher pitched when coming towards you, and quieter and lower pitch when going away from you) Scientists know what the color pattern should be from stars b/c of the elements they burn, these patterns will be shifted to the left or to the right, towards the blue end or towards the red end of the color spectrum. Objects that are moving away appear to shift to the red, and most objects (stars, glaxies) appear to be redshifting. So if everything is moving away... Universe is expanding

Answer 9

If light didn't travel, you could not see far away objects. Since it does and so do we, the speed that light gets to us changes and the "shift" occurs in the visible light spectrum we see. Light is made up of many colors mixed together in a spectrum (rainbow if you prefer) that either spreads out or compacts depending upon which direction you travel from it. An interesting sudo-example is this: You ride your bicycle away from a cloud of bugs heading towards you and the number or them fade and thin to be hardly noticeable. Turn your bike around and ride into the cloud, head on, and you get a mouth and nose full of bugs. So it is with light, which is made of particles. Remember, the shift is what you see, not how you are pelted by by the light so this is what causes the change in color.

Answer 10

Your first statement - wherever you read it - is wrong.

The red shift of light is due to the Doppler effect. A receding galaxy will show a red shift whether or not that universe is expanding.

The fact that all observed galaxies show a red shift leads to the conclusion that the universe is expanding.