Hi everyone!
We had a great time reading all of your questions! We at GW SPS have answered a couple of them-- Check them out below and stay tuned for more answers in the coming weeks!
The universe is infinite. The universe is always expanding. How can it be both? (GW, grade 7) (Answered by Oliver Berroteran and Jacob Maibach)
To understand how something infinite can expand, we need a clear idea of what we mean when we say it expands. To do this, we start with something more familiar than the entire universe; we start with a rubber band.
Imagine a long rubber band, say one that is a whole foot long. Say we try and pull at both ends to stretch it out: we somehow get it to be double the size (it is really, really stretchy). What has changed?
We already described the first change: it is twice as long. But more is going on. Instead of focusing on the whole thing, let us focus on a small part of it, somewhere in the middle. This small section of it has probably increased in size as well. Another thing has changed, too. The rubber band is now thinner than it was before, because we spread the rubber out.
So where does that leave us then? We can now describe the stretching of the rubber band in three ways, only one of which actually requires looking at the whole rubber band. Even better, knowing that any of these had happened tells us that the others did. If we know that all the parts of the rubber band increased in size, then we know that the whole thing did. So just by looking at each part of the rubber band, we can tell that it expanded.
And that’s the trick: when thinking about something infinite, we don’t want to think about the whole thing. That is hard to understand and hard to reason about. Instead we look at what happens in each small section of it. This is called thinking locally versus thinking globally. If we think locally, it is a lot easier to see what we mean when we say that the universe is expanding.
When the universe expands, each little section of it gets bigger and spreads out, but the whole thing does not get bigger. This might seem like it contradicts what we said earlier: we said that knowing that each section of the rubber band increased in size tells us that the whole thing increased in size. The difference was that the rubber band is finite, and not infinite.
Infinity does not work like we would expect. It goes against our intuition at every turn. It’s bizarre properties have tripped up countless mathematicians and scientists over the years. When thinking about something as strange as infinity, you have to change your intuition to understand it.
Can we photograph a black hole? How? (Stefan, gr. 6) combined with
What does a black hole look like? Can it be seen? (GW, gr. 7) (Answered by Brian Alden)
It may be a bit philosophical to say, but what does it actually mean to photograph something? In a camera, the only way you can generate an image is if photons (tiny particles of light) from an object, either reflected or emitted, make their way into the camera's lens, and then onto film or a digital sensor. If no photons from the object make it to the camera, that object will not be in the photo. Black holes are called black holes because their gravitational pull is so strong, even photons traveling at the speed of light cannot escape. As no photons make it out of the black hole, and therefore not able to get to your camera, that would lead you to believe that you cannot take a picture of a black hole, right? But what if you know something is supposed to be there, you take a picture, and there is a big black hole in the photo covering up an object (star or galaxy) you know should be in the photo. What if it only covers part of it and then you see a chunk missing just like the bite out of the apple on the apple logo. That would give you a clue as to the fact that there is something in fact there. Further, the gravity of a black hole is so strong, it doesn't just cover up objects you are trying to look at, it bends light around itself and distorts images. This is called gravitational lensing and it is another way we can see the effect a black hole (or a large body of matter) has on its surroundings. So while we are not capturing photons in our camera of a black hole, we are getting images that have missing parts, and parts that are distorted in a way that we would expect if a black hole was present. Just because you can't directly see an object, doesn't mean you can't see the effects it has on its surroundings.
Regarding the effects a black hole has on its surroundings, you can also see when a black hole is pulling a large object into it. Let’s say a star is getting sucked into a black hole. It looks very much like water going down a drain in a bathtub. You can see the water swirl around the drain and eventually go into the drain. Imagine if you saw this happening and saw the water swirling around and then disappearing. This is what happens with the black hole. Pieces of the star get pulled off and begin swirling around the black hole. Eventually the pieces disappear as we can’t see the black hole. But again, just because we can’t see it directly, we can see black holes and take pictures of black holes indirectly.
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