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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 2

Lesson 1: Physical quantities and their measurement

# Scale of the large

Attempting to comprehend the scale of the large. Created by Sal Khan.

## Want to join the conversation?

• At Sal wrote 1 Au underneath the tiny sun and earth. What does Au mean?
• AU stands for 1 astronomical unit which is the distance between the Sun and the Earth. The distance of other planets from the Sun can be measured in AU which gives a helpful comparison between distance of two objects compared to the distance between the Sun and Earth.
• Why does Jupiter have a great big red spot and Neptune have a great dark spot and how did they get there?
• If we're 200,000 distances from Earth to the Sun to the nearest star, what state are the objects between? Are there planets? Are they absurdly cold?
• There ARE planets zooming around in interstellar space, ejected from their home star systems. They are called "rogue planets".
• At Sal says that Earth would be destroyed if it was too close to Jupiter, but how would that happen?
• The gravity of Jupiter is so intense, that if our planet was right next to Jupiter we'd be smashed into Jupiter. The same would happen if our planet was right up next to any large body. If the earth did smash into Jupiter like that, it might not be completely destroyed, though: probably the earth's crust would break off, but the core might bounce off and become a moon of Jupiter. That's a theory I once read of where the moon came from, that a smaller planet collided with earth (billions of years ago), that planet lost it's crust into proto-earth, and the remaining core began orbiting the earth as our moon.
• Is there a black hole in the center of every galaxy?
• We can safely predict that there are supermassive black holes at the center of say, spiral galaxies but we have no proof to believe that every single spiral galaxy has a supermassive black hole at its center. There are just too many galaxies to survey.
• i really cant understand why the universe is 13.7 billion ly old
can somebody help me?
• I don't know if there is an answer to why the universe is 13.7 billion years old, nor will there ever be. This is similar to asking "Why did the Big Bang occur?"

If you mean "how we know", it's based primarily on cosmic microwave background (CMB) radiation that originated with the Big Bang. This radiation is still around today (but it's low energy radiation, so it's not dangerous) and is measurable.

This may help:
• Hey, i got a question: In theory, nothing can pass infinite. Applying this rule to the universe, there must be an end to the universe's expansion right?
• Infinity is not a specific value. There are different sizes of infinity. For example there are countable infinities like the number of integers or rational numbers are the same size infinities since there is a way to map one of them 1 to 1 onto the other. There are uncountable infinities like the irrational numbers.

Also if the universe is infinite and expanding there is nothing that says it can't expand infinitely. For example lets say that there is a hotel with an infinite number of rooms that is full so that there is someone in every room.

Now someone comes to the full infinite hotel and is looking for a room. How are they able to get a room? Just have the person in room 1 move to room 2 and the person in room 2 move to room 3 and so on so the person in room N moves to N+1. This frees up the 1 room needed.

Now a bus with a million people arrives, you can still accommodate them by having the person in room N move to room N + 1,000,000. This will free up the million rooms needed.

What if an infinite buss of people shows up? You just need to have the person in room N move to room 2 * N. This puts the people in the current rooms into all of the even rooms and not the infinite people in the bus can go into the infinite odd numbered rooms.

You can do this over and over and still not run out of rooms. This is the same as "expanding" the universe over and over.
• why the planets are round ?
• planets are spheres because the gravitational force is from the centre of the planet so the distance from the surface to the core should be equal a sphere is the only shape that allows this to happen that is why most celestial bodies eventually turn to spheres.