After a short break, we tackled the Big Bang. I asked if we needed to
extend the timeline even further back than the origin of the Earth.
They were clear on the need to do so, since the Earth and the solar
system formed from a pre-existing cloud of gas. I showed them some
pictures of galaxies and then some of a fly-through movie of galaxies from
the Sloan Digital Sky Survey. I just wanted to roughly establish that
galaxies are like neighborhoods: we have ours, and we can see where
some others are too. It was clear later that they have no real idea
that galaxies are much bigger than the solar system (even though I
said it); they kept mixing up planets and galaxies. But I didn't
dwell on that; I figure there's only so much I can do in one morning,
and it was more important to establish that "things are moving apart"
than to work on a sense of scale.
Next, I took a long loose spring (almost like the helical telephone
cord that used to be on all landline phones) to which I had attached
galaxies (each galaxy had the name of a kid). Starting with the
spring scrunched up, I extended the spring 12 feet or so and got the
galaxies far apart. I did this a few times so they could see how all
galaxies moved away from each other. This was actually the first time
I had done this demo, and now I'm sure I will do it with my college
kids. I always use a balloon, and I still will, but there's something
nice about also doing the one-dimensional case. It just makes
everything a lot more visible, especially in a big room. With the
spring stretched I asked how we could figure out how long it has been
since everything was together. One kid figured it out right away. He
explained that knowing how fast they are moving and how far apart they
are, we can calculate the time it took. He used the example of a
speed of one inch per year. In that case, the number of inches apart
is the number of years it took to get that far apart. This was an
amazingly good answer; it was almost as if I had rehearsed it with him
beforehand and planted him in the audience (I swear I didn't).
Next we went outside and practiced doing the same thing with our
bodies. With Teacher Pa as the Milky Way, we all ran away from her.
Teacher Ethan ran the fastest and became the most distant galaxy by
the time I said stop. I wanted to make clear that the most distant
galaxy is further away not because it has been traveling for a longer
time, and that there was a time when all galaxies were together. We
went back inside and I drew a diagram of us as someone above the field
would have seen us. If that person came upon that scene and saw us
moving very slowly, would he guess that we had started a long time
ago, or a short time ago? (Long.) If that person came upon that
scene and saw us moving very quickly, would he guess that we had
started a long time ago, or a short time ago? (Short.) Using the same
logic, astronomers have found that all galaxies were together and
started moving apart (the Big Bang) 13.7 billion years ago. I made a
big show of extending the timeline into the hallway and out of the
school to emphasize that that is a long time.
Now, the kids may have gotten the wrong impression that we are at the
center because everything is expanding away from us. To combat that,
I had prepared two transparencies. One has a smattering of galaxies, each
with a different shape so that it's recognizable. I had prepared this by putting
graph paper behind the transparency and drawing galaxies at random coordinates.
I prepared the second one by drawing the same galaxies (now in red instead
of black) at the same coordinates multiplied by 1.5. You can pick one galaxy
in the middle to represent the Milky Way and show the initial (black) transparency
to show where the other galaxies are around it. Then overlay the red one, matching
up the Milky Way's position, to see how everything moved away from us. Here's the
cool part: now you pretend you are an alien in another galaxy, match up that galaxy
across the two transparencies, and you ALSO see that everything is expanding away
from the alien! This blew everyone's mind, including the teachers. We spent a fair
amount of time with the kids picking a galaxy, and me matching that galaxy and
showing that everything moved away from it. Bottom line: just because we
observe everything moving away from us doesn't mean we're at the center. When
there's more space everywhere, EVERY galaxy can observe this. We're not special.
People often ask, where did the Big Bang happen? It happened everywhere,
including here! All the places that all the aliens in the universe could call "here"
all overlapped , in the distant past, with what WE call "here"! I reinforced this with
the traditional balloon demo of the expanding universe: draw some dots on a partially inflated balloon, then fully inflate it and show how each dot is further from each other dot, but none is in a special or central position. If the balloon could really be completely collapsed, then all the dots would be in the same starting position without ever really moving away from its position on the balloon.
But, should we believe that we can extrapolate that far back in time?
We should look for evidence! I made a lame show of demonstrating how
things are hot when compressed (I brandished a bike pump and asked
them to notice how the valve gets hot next time they pump up a tire),
so that the whole universe would have been red hot at some point in
the past when it was highly compressed. We actually see that light:
it's called the cosmic microwave background. This is fossil evidence
of a hot early universe.
We were running out of time but the kids voted to do the marshmallow
activity rather than just stop early. I had brought white and yellow
mini marshmallows and toothpicks. These represent the building blocks
of atoms (technically, protons and neutrons, but I didn't use those
words). These building blocks can be stuck together only at very,
very high temperatures which the universe experienced only in the
first three minutes, when it was even hotter than red hot. (Kind of
like marshmallows will stick together if heated.) We had talked about
solids, liquids, and gases in previous weeks, so I sketched out
hydrogen (just one proton) and helium (two protons and two neutrons),
which they knew were gases. I had set up cups half full of a mix of
protons and neutrons, in a 7:1 ratio. I gave them the cups and told
them they had only three minutes to build as much helium (ie stick two
white and two yellows on a toothpick) as they could. Because of the
paucity of neutrons, they were typically able to build only 3 helium
atoms, with about 36 hydrogen atoms left over. This is actually the
ratio we observe! So the atoms themselves are additional fossil
evidence of a very hot early universe. [Parents: if you're curious
where the 7:1 ratio came from, that came from the even hotter
conditions in the first fraction of a second, and the observed ratio
agrees with the Big Bang model, thus providing even further fossil
evidence. If you want to read more, search "Big Bang nucleosynthesis."]
Overall, it was a VERY successful day. The kids had many additional
questions about planets and orbits which I didn't take time to answer,
and this could form the basis of an activity for my next visit, and I
do think they gained an appreciation of the basic idea that we can
tell the age of the universe from how fast it's expanding. How well
they'll remember that or be able to answer questions on an assessment,
I'm not confident. But the basic idea is not beyond the grasp of 1st
and second graders. The movie we saw before the break was great, the
spring demo was great, and the marshmallow activity was good. (We
didn't have enough time to do it really well, but it's a very
promising idea and I will develop it further. Given more time, I
would have the kids make mini posters with "raw ingredients" "elements
after cooking in the heat of the Big Bang.") The radioisotope dating with
dominoes (see previous post) came up a bit lame, but I think it's a good idea
that just needs more refinement.
As the kids went to lunch, I worked on the science section of Teacher
Pa's poster comparing different religions' creation stories. Because
I wanted to emphasize the LACK of parallelism as discussed in my
previous post, I ripped off the science column and posted it on the
wall next to the window where the religion part of the poster was. I
also did not carry over the formatting of the rows in the religion
column. I hope to post a picture here rather than describe it in
words, but I think I achieved the right balance in emphasizing how
science is different from religion while respecting both.
Update: if you look at this picture full size, you will be able to read the poster.
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