Cosmology and astronomy
- Big bang introduction
- Radius of observable universe
- Radius of observable universe (correction)
- Red shift
- Cosmic background radiation
- Cosmic background radiation 2
- Hubble's law
- A universe smaller than the observable
- How can the universe be infinite if it started expanding 13.8 billion years ago?
Created by Sal Khan.
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- when astronomers observe far away stars and they notice the light waves are "reder", how do they know that this is not the original wave emitted? how do we know its been red shifted?(44 votes)
- According to me, astronomers observe light or others forms of wave from certain sources and then if they observe a change in it's emissions, they can say that there was a redshift or any other phenomenon ; suppose there was a galaxy X and here on earth, astronomers observed certain color of light coming from it and then after probably 20 years they observed a certain different stretched electromagnetic wave coming from our galaxy X . So, this can make them conclude that light has been redshifted.(6 votes)
- I don't understand how anything could be blue-shifted, or violet-shifted as he put it, if the universe is constantly expanding and nothing can even come close to the speed at which the universe expands?(13 votes)
- The expansion of the universe is actually quite small, it only seems large when you look at it at very large distances. The expansion of the universe will cause points in space 1 meter apart to move away from each other at about 2 * 10^-18 meters per second. To give you an idea of how small this is the diameter of an atom is is about 100 million times larger.(41 votes)
- Sal says that Red Shift results in a difference in the "perceived" light. As a question, does Red Shift, or any shift for that matter, affect the actual nature of the light, such as the energy transferred?(10 votes)
- Yes, if it were green photons as the source and they red shifted to let's say yellowish, then the energy of those photons wouldn't be energy of green photons, they would be energy of yellow. No laws are violated here because on the other side, it would be blue shifting and those photons would have slightly more energy than green.(10 votes)
- Is the Red Shift one of the main reasons that people believe the Big Bang Theory?(11 votes)
- Yes indeed. The fact that all other objects are moving away from us (and all other objects) shows that they were all together at the start.(4 votes)
- The Sun turns red during sunset. Does this have anything to do with Red Shift?(2 votes)
- No, red sunsets are a result of light being scattered by Earth's atmosphere. When the sun is setting, the light rays must travel through more atmosphere as they come in at a much shallower angle, which means that they get scattered a lot more, separating out the colors. The red still makes the most direct path while blue gets scattered the most, so sunsets and sunrises look redder towards the sun.(11 votes)
- Okay, just making sure I get this concept. If I had a flashlight and ran away from a friend while shining it in his face, would the light seem to be redder to him? Or does this concept only work on large scales, like light speed and stars?(8 votes)
- This may or may not help, but if you were driving away quickly from your friend, and instead of shining a light, you were hollering, he would indeed notice a pitch change. This is only noticeable because sound travels at slower speeds than light. If we go back to the flashlight example, the frequency change of the light would be too small to notice.(2 votes)
- If the universe is expanding and all the galaxies are moving away from each other at faster rates, then how come the Milky Way and Andromeda galaxies are expected to collide eventually?(2 votes)
- Not each and every galaxy is necessarily moving away from each other due to the expansion of the universe.
The Andromeda Galaxy and Milky Way are moving towards each other due to the strong gravitational attraction between them because of their proximity to each other.(10 votes)
- What happens if red shift applies to an object that is already red? Would it be unaffected? Would it turn a darker or more pale red? If I were to take a red plastic ball and launch it out into space to test, what would happen to the image I perceive?(3 votes)
This image shows the electromagnetic spectrum. So this answers a part of your question.
I have learned that there is no theoretical limit to the electromagnetic spectrum. Therefore objects should continue to get even more and more red.
The formula for calculating a frequency is (f = c/(lambda)) Where c is the speed of light and (lambda) is the wavelength.
And the formula for calculating how much energy a photon have is (W = h*f) Where h is plancs constant.
If we put those formulas togheter we get this (W = (hc)/(lambda) ) and this formula tells us that as the wavelengths get greater and greater the lower the photon energy will become. And those long wavelengths will be hard to observe because of the low amount of energy.
This is as i have understood it. If anyone sees an error here please correct me because i am no expert!
Hope this maybe clarified some for you :)(1 vote)
- Is there such a thing as blue shift?(3 votes)
- Yes, anything traveling towards you would have a blue shift.(4 votes)
- If the wavelengths of the photons we're getting are redder than the original wavelengths of when emitted, are our pictures/drawings on stars, solar systems, and galaxies in their original color or the one we've collected?(1 vote)
- Most astronomical pictures are not true color. Many are even taken in invisible wavelengths (Like radio waves, xrays, infrared, etc) and those are then rendered in visible colors to show the actual structures..(6 votes)
Let's say I'm over here. I'm going to do two scenarios. So I'm an observer over here. This is me. And then maybe even better, I should just draw my eyeball because we're going to be observing light. So I'm just going to draw my eyeball. So this is me in the first scenario or this is one of my eyeballs. And then this is one of my eyeballs in the second scenario. Now in the first scenario-- so let me draw it-- so in both scenarios, we're going to have an object. We're going to have some type of source of light. But in the first scenario, relative to me, the source of light will not be moving. While in the second scenario, the source of light-- just for the sake of discussion, just for fun-- will be moving at half the speed of light. Unimaginably fast speed, but let's just assuming that it is. So it's moving at-- it has a velocity of 1/2 the speed of light, 1/2 light speed away from me who is the observer. Now let's just imagine what would happen. They're both emitting light. So and they're both going to start emitting light at the exact same time. And when they start emitting light, they're both at the exact same distance from my eye. The only difference is is that this is stationary, relative to me, while this is moving away from me at half the speed of light. So let's say that after some period of time, that the light wave from this source reaches my eye. And then it looks something like this. I'll try my best to draw it. So let's say I have-- I want to draw a couple of wavelengths here. So let's say that's half a wavelength. That's a full wavelength. That's another half, a full wavelength, another half, full wavelength, and then a half, and then a full wavelength. So let me see if I can draw that. So it would look like full wavelength, full wavelength, full wave length. This is not easy to do. And then you got another full wavelength. So it would look something like that, the actual waveform. And so the front of the wave form is just getting to my eye. And then as the wave forms keep going past my eye, my eye will perceive some type of a wave length or frequency and perceive it to be some type of color, assuming that we're in the visible part of the electromagnetic spectrum. Now let's think about what's going to happen with this source. So the first thing is is that the front of the wave form is going to reach me at the exact same time. One of those neat and amazing things about light travelling in general, or especially in a vacuum, it doesn't matter that this is moving away from me at half the speed of light. The light will still move towards me at the speed of light. It's absolute. It doesn't matter if this is going away at 0.9 the speed of light. The light will still travel to me at the speed of light. And it's very unintuitive. Because in our everyday sense, if I'm moving away from you at half the speed of a bullet and I shoot a bullet, the bullet will only move towards you at-- it'll kind of-- that half of its velocity will be subtracted. And it'll only move towards me at half of its normal velocity, relative to whether it was stationary, but not the case with the light. So with that out of the way, let's think about what the wave form would look like. So by the time the light reached here, we need to think of-- let me actually redraw this over here. Let me redraw this eyeball right over here. So this is me again. So by the time the light reaches my eye-- so they both started emitting the light at the exact same time-- this guy has traveled half this distance. If it took light a certain amount of time to get this far, this guy will get half as far in that same amount of time. So by the time the light reaches my eye, this guy will have traveled about half that distance. So he would have traveled about that far. They started emitting the light at the same time. So that very first photon, if you view light as a particle, will reach my eye at the very same time as the very first photon from this guy. So the wave form is going to essentially be stretched. So instead of having-- so we're still going to have one, two, three, four full wavelengths, but they'll now be stretched. Let me see if I can draw four full wavelengths. So let me cut this in half over here. And let me cut each of those in half. So each of these are going to be a full wavelength. And then they're going to have a half wavelength in between. And so the wave form is going to look like this. Let me try my best to draw it. This is the hardest part, drawing the stretched out wave form. And there you go. It's going to look like this. And so when it gets to my eye, my eye is going to perceive it as having a longer wavelength, even though from the perspective of each of these objects, if you're traveling with each of them, the frequency and the wavelength of the light emitted is the same. The only difference is this guy's moving away from me-- or I'm moving away from it, depending on how you want to view it-- while I am stationary, while in this first case, the observer and the source are both stationary. Now, in this situation, what's my eye going to say? Well, my eye will get each of these successive pulses, or each of these successive wave trains. And it's going to say, hey, there's a longer wavelength here, a perceived longer wavelength-- let me write that-- perceived longer wavelength here, and also, a perceived lower frequency. So what would that do to the perception of the light? Let's say that this is green light. So if you are stationary with the observer, it would be green light. So let's look at the electromagnetic spectrum. I got this off of Wikipedia. So if I were stationary towards-- with the observer, we'd be in the green light part of the spectrum, so a 500 nanometer wavelength. But if all of a sudden, because the object is moving away from me at this huge velocity, the perceived wavelength becomes wider. So from my perception, it is going to have a wider wavelength. And you can see what's happening. It will look redder. It will move towards the red part of the spectrum. And this phenomenon is called redshift. And I've done a bunch of videos of the physics playlist on the Doppler effect. And over there, I talk about sound waves and the perceived frequency of sound-- if something travels towards you verses away from you-- as the exact same idea. This is the Doppler effect applied to light. And the reason why the Doppler effect works for light traveling through space and for sound traveling through air is because the sound wave in air, regardless of whether the source is moving away or towards you, the sound wave is going to move at the speed of sound in air at a certain pressure and all of that. And light is the same thing. But in a vacuum, It will always, regardless of the source, regardless of what the source is doing, the actual light wave itself will always travel at the same velocity. The only difference is is that its perceived frequency and wavelength will change. And now the whole reason why I'm talking about this is you can use this property of light, that it gets redshift, to see whether things are traveling away or towards you. And people talk about redshift because, frankly, most things are traveling away from us. And that's one of the reasons why we tend to believe in the Big Bang. The opposite, if something is traveling towards me at super high velocities, then we would have something called-- you don't hear the word-- it would be violetshift. The frequency would increase. So it would look bluer or more purple. Now the other thing I want to highlight is this redshift phenomenon, this idea, it doesn't apply only to visible light. So it could even apply the things that we can't even see. So it would only-- it would become redder. But it's not like you can even see. It could even be applied to things that are even more red than red. So maybe it's a microwave that is being emitted. But because the source is moving away from us so fast, it could be perceived as an actual radio wave. And actually, I should have talked about this in the video on the microwave background radiation is that we're perceiving it as microwaves. But these sources were moving away from us. They were being redshift. So they were not actually emitting microwave radiation. It's just what we observe-- and this is actually what would be predicted based on the Big Bang-- is actually microwave radiation. So anyway, hopefully, that gives you a sense of what redshift is. And now we can use this tool to explain why we think many, many things are moving away from us. And now let me just actually make sure you get that idea. If I have two objects, let's say that these are suns. Let's say that these are both suns, or both galaxies, either way. And because of other properties-- and I won't talk about them right now-- we know that they are probably emitting light of the same color. They're probably emitting light of the same color because we know other properties of that star or of that galaxy. Now, if what we actually perceive is that this one looks redder to us than this one, then we know that it is traveling away from us. And the redder it looks, the more its wavelength is spread out relative to this other star, the faster we know that this is moving away from us.