The Blog of Desperation

sciencenote:

The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of an actual change in the frequency of the source. Using the example above, the bug is still producing disturbances at a rate of 2 disturbances per second; it just appears to the observer whom the bug is approaching that the disturbances are being produced at a frequency greater than 2 disturbances/second. The effect is only observed because the distance between observer B and the bug is decreasing and the distance between observer A and the bug is increasing.

The Dopplereffect can be observed for any type of wave - water wave, sound wave, light wave, etc. We are most familiar with the Dopplereffect because of our experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren’s frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Dopplereffect - an apparent shift in frequency for a sound wave produced by a moving source.

carbonstuff:

Let’s talk about the dark night. Yes this post is about Batman.

Our common assumption is that the night sky is supposed to be dark with only few dots of light.

But then, aren’t there supposed to billions upon billions of stars in the night sky emitting light. Yes, they are very far away, but, there is nothing stopping (like air or glass) the light from reaching us. So, shouldn’t all those stars make the night sky (very) bright and not dark ?

This is actually called Olbers’ Paradox. 

Let’s look at the problem in another way. We can divide the universe into a series of concentric shells, being 5 light years thick. Thus, a certain number of stars will be in the shell 1,00,000 to 1,00,005 light years away. If the universe is homogeneous at a large scale (i.e., static), then there would be four times as many stars in a second shell between 2,00,000 to 2,00,005 light years away.

But, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell (intensity is inversely proportional to the square of distance). Thus the total light received from the second shell is the same as the total light received from the first shell.

Thus, the argument is that if the universe were static and filled infinitely with stars, the night sky should be much brighter than it is now.

I think you guessed the loop hole here. I said if the universe were static, which it clearly isn’t.

The Big Bang explains this paradox by saying that the universe started at a point, and expanded from that point. Thus, it is not static.

We know that the expansion is accelerating. So, two things happen. 

One is that, those stars in the night sky are moving away from us and the distance between them and us increases. This increase the time for to see them and eventually it takes millions of years for the light from those stars to reach us.

Second, which is the more important reason, is that these starts get redshifted away. Redshifting is when the wavelength of an object moving away from us goes towards the red side of the spectrum and eventually, it goes into the infra red, which we cannot see. It is like we listen to a honking truck passing by at great speed. As it moves away from us, the the sound becomes softer and softer and eventually it is inaudible.

So, because of these reasons, we never get to experience the real night sky light. But, it may be a good thing, as otherwise our eyes would be blinded by the light !

Image via Wikimedia Commons

awkwardedmilibandmoments:

Ed sends some quality time with his estranged son, just don’t tell the CSA