Next up on our tour of the properties of light and sometimes sound, is an effect first explained by an Austrian mathematician and physicist, Christian Doppler in 1842.
빛을 광자라는 '파동'의 관점에서본 특성으로 도플러 효과가 있다.
In a paper entitled, 'On the Colored Light of Binary Stars and Some other Stars of the Heavens', Doppler presented his theory that observed frequency of wave depends on both the emitted light and on the relative speed of the source and the observer.
But what does this mean?
[도플러 효과;Doppler Effect]
This raising and lowering of pitch is known as the Doppler effect and as we hinted at earlier, it also applies to light.
음파(sound wave)든 전자기 파(EM wave)든 모든 '파동'에 이 효과는 적용된다.
As light is emitted from a source, the waves being emitted can be squashed or stretched along the direction of motion. So, if a star is moving towards us, the light waves moving ahead of the star can appear to be increased in frequency or decreased in wavelength. This translates as a shift towards the blue end of the electromagnetic spectrum. Conversely, as the star moves away from us, the waves appear to be stretched resulting in longer wavelengths and a redder look to the star. For light, the shifts caused by the Doppler effect are known as blueshift and redshift.
접근하는 별의 고유색의 주파수는 청색(짧을파장)으로 편이, 멀어지는 별의 고유색이 적색(긴 파장)으로 편이 된다. 편이되는 양으로 별의 시선방향 속도를 알 수 있다. [그나저나 별의 고유색은 어찌 아나?]
Although we should note that this is not confined to the visible band of the electromagnetic spectrum. Both X-rays and radiowaves can also be blue and red shifted. The colors just speak to the direction of the shift.
편이 방향을 색깔로 표시하는데 '적색편이' 혹은 '청색편이' 이는 '긴 파장' 방향으로 혹은 '짧은 파장'으로 라는 의미이며 굳이 관습적으로 색깔을 차용한 표현이다. 도플러 편이는 가시광선 뿐만 아니라 X-선, 전파 (심지어 음파까지) 모든 파동에 적용된다.
Many stars that are observed in the night sky are actually in binary star systems. In such systems, we see stars in orbit around one another. If we view a pair of stars from the side, it will appear as though one star is moving towards us, while the other star is moving away from us. This relative motion is detected as blueshifts and redshifts that we detect from these stars.
이중성 관측에 매우 적극적으로 활용된다. 두 별이 서로 공전하면서 각각 다가올 때와 멀어질 때를 편이 특성으로 알수 있다. [이중성의 상호 공전주기로부터 별의 질량을 알아낼 수도 있다. 별의 색으로부터 온도를 알아내며, 색과 질량으로부터 별의 수명도 알 수있다.]
The same is true of galaxies. Spiral galaxies are found spinning and spiraling in space. In fact, our own galaxy, the Milky Way, takes about 240 million years to complete one full rotation.
은하의 특성을 측정할때 이용된다. 은하의 가장자리 에 위치한 별의 편이량을 계산하면 은하의 회전속도를 알 수 있다. 우리은하가 한번 회전할때 걸리는 시간은 2억4천만년이 걸린다. [태양계는 우리은하는 중심에서 2만5천 광년 떨어져있다. 회전 선속도는 초속 약 220 킬로미터다. 이 측정은 모두 도플러 편이를 측정한 결과다.]
When we look at other galaxies, we can measure the light emitted at various points across the disc to deduce the galaxy speed of rotation. When measuring light from stars, we see a shift in light towards shorter wavelengths from the side of the galaxy approaching us, it's blueshifted. If we look at the other side of the disc, it's moving away from us, we can see a shift of light towards longer wavelengths. So it's redshifted.
우리은하 외의 다른 은하에 대해서도 도플러 편이를 이용하여 회전을 계산할 수 있다.
It's interesting to note that while one of our closest neighbors M31, or the Andromeda galaxy is rotating, we also see an overall blueshift of the whole galaxy, implying that M31 is moving towards us.
우리 은하와 가까운 안드로메다 은하(M31)은 은하 전체적으로 청색 편이를 보인다. 먼 시간내에 우리은하와 안드로메다 은하가 충돌 할 것으로 예상한다.
While there are many other examples of the use of redshift and blueshift in astronomy, the most well known Doppler shift in astronomy, triggered the idea of the expanding universe.
도플러 편이는 팽창하는 우주 이론을 정립하는데 중요하게 활용됐다.
In the early 1900's, the prevailing theory was that the Milky Way, our own galaxy, was pretty much the extent of the universe.
1900년대 초만 해도 우리은하가 우주의 전부라고 생각했다.
In the early 1920's, an astronomer named Edwin Hubble, was working at the Mount Wilson observatory in the USA. He was making measurements of the distances of various Nebula, only to find that some, including what was then known as the Andromeda Nebula were far too distant to be part of our own galaxy.
1920년경 에드윈 허블은 성운을 관측했는데 그때까지만 해도 안드로메다 성운이라고 불리던 천체가 실은 외계은하라는 것을 알게됐다.
Instead these objects must be galaxies in their own right. This meant that the Andromeda Nebula became known as the Andromeda galaxy, the one just mentioned a few moments ago. Although the idea of multiple galaxies had been proposed as early as the mid 1700's, it is strange to think that the concept of galaxies is so new. This idea was only conclusively proven about 100 years ago.
In 1929, Hubble continued these observations and found a relationship between the distance and the redshift of galaxies. Hubble found that galaxies that are outside of our local group of galaxies have light that is red shifted. Not only is the redshift is the light redshifted, he also saw that the further away the galaxy is from us, the larger the change in wavelength or the larger the redshift.
허블은 외계 은하들이 모두 적색편이를 보이는 것으로 보아 우리로부터 멀어진다는 것도 알아냈다. [아주 근접한 은하들끼리 출동하기도 하지만 보편적으로 서로 멀어진다.]
If the change in wavelength is interpreted as a Doppler shift due to the motion, then Hubble's observations suggest that galaxies appear to move away from us. Galaxies that are further away from us are moving away from us at a faster speed. The modern interpretation of Hubble's observations is that the universe is expanding, which it makes it look like galaxies are moving away from us.
허블은 관측은 결국 팽창하는 우주론을 탄생시켰다. 멀리있는 은하 일수록 팽창하는 속도가 빠르다.
The description of the expansion of the universe is called The Big Bang Theory. The Big Bang explains the evolution of the universe after its beginning. Thirteen point eight two billion years later, we find ourselves here on the earth learning about Black Holes.
팽창하는 우주론은 결국 '빅뱅' 이론을 뒷바침 하게 됐다. 빅뱅 이론은 우주의 최초 모습을 설명한다. 138억년이 지난 지금 우리는 지구상에 앉아 블랙홀에 대해 배우기에 이르렀다.
Speaking of which, if we are wanting to understand Black Holes, we need to understand something about gravity. For that, let's start with the simplest version, Newtonian gravity.
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