

Measure the gravitational constant like Cavendish did in 1798 - rw
http://funcall.blogspot.com/2009/04/lets-do-twist.html

======
randomwalker
The first attempt to measure the value of the Gravitational Constant (as well
as the Earth's mass; each would lead to the other given what was known at the
time) was done by measuring the gravitational pull of a mountain. A site
somewhere in Scotland was chosen, if I recall correctly from the book "A short
history of nearly everything." It took years to map out the geometry of the
mountain, and the answer was still way off because no one knew how to measure
the density of the mountain, and basically had to just guess it.

Cavendish's torsion idea was the key, and it made it possible to dramatically
reduce the masses required. The value derived from his experiments is within
1% of the modern value (rather amazingly.)

Cavendish was a very interesting character. Actually, most people who did
science in the 18th century seem to have been eccentrics, come to think of it.

------
justinsb
This reminds me of the physics "practical experiment" we had to do as part of
the Cambridge physics tripos (at the Cavendish lab). It was also about
measuring gravity, using a pendulum though I forget the exact details, but we
then iterated the experimental set up based on the initial results, in theory
to get much higher final accuracy than our clock could give.

I thought it was completely dishonest, because if your initial reading was
off, then the final answer came out ludicrously wrong. The distribution of the
outcome was no longer normally distributed around g, but now had a long tail
of catastrophically wrong values. Presumably a good experimenter would simply
have thrown away such an 'obviously wrong' value.

I'm amused to think that a large number of those same physicists that happily
sat through that experiment then went on to work as quants, where they
essentially happily rebalanced probability distributions in exactly the same
way, though with a lot more at stake this time around when they eventually got
their catastrophically wrong value.

------
cool-RR
Amazing. I never knew one could measure the gravitational force between two
non-astronomically sized objects. This is really really amazing. I can't
believe I never heard about it before.

------
pmjordan
They should recreate this experiment at school - it's one thing learning about
all the theory, but all the practical experience you get involves the earth's
gravitational field. This is way cooler, more instructive, and definitely more
impressive.

I never bothered to look up how big the weights were that cavendish used (I
assumed they were two orders of magnitude larger than the author used), nor
did I bother to estimate whether it would be feasible to measure gravity
between household objects. I'm now wishing I had. Maybe I'll try it one of
these days. (I took extra physics at school and studied physics at university
and haven't touched the subject since graduating, so I'm extremely bummed that
we never did fun experiments like this)

~~~
nkurz
We did this experiment for a college freshman physics lab. Indeed exciting to
see gravity in action! It was a lab based course, and we did similar
experiments for most of the other physical constants. I thought it was a great
way to learn.

~~~
mechanical_fish
This is the college lab experiment that I remember the best. Because my lab
partner and I did the measurement and determined that _G_ was 15% smaller [1]
than the value in the textbooks.

We asked the professor what might have gone wrong. Whereupon he exhibited the
key trait of a great science teacher: He refused to tell us.

So we worked. Oh, did we work. We armed ourselves with the backs of many
envelopes and brainstormed. We tried to think of every possible source of
systematic error. We tried to calculate the likely magnitude of each of those
sources. ("How much electric charge imbalance would be needed for static
electricity to cause the error?" "What if an elephant were pacing around
outside the lab in phase with our experiment?") We redid the experiment
several times, changing the orientation of the apparatus, moving ground wires,
et cetera.

Nothing helped. In the end, we wrote up a _really_ long lab report detailing
all the theories and the calculations and the systematic errors that we had
tried to correct for. Then we plotted up our results and stated that our
measurement was X, plus or minus something like 5%. We noted that every other
physicist for the last few hundred years had converged on a value 15% higher
than X, which suggested that we'd probably missed something, but that there
was nothing we could do about that because we didn't have any more time to
experiment.

As I recall, we got an A. I will never know how we managed to screw up the
experiment. After we turned in the report, the professor confessed that he had
no idea either. If he knew otherwise, he was an excellent actor. They plotted
all the class's results on the wall, as a time series, and ours was a big, big
outlier -- that damned apparatus worked for the groups before us, and it
worked for the groups after us. We got a lot of good-natured ribbing.

I'm not sure I could design a better two-week tour of the scientific
experience than that.

\---

[1] Or maybe it was larger. As if it really matters. Anyway, it was _way off_.

~~~
Herring
This story frustrates me like being unable to find a bug in code. Why would
you keep using the same instruments? Trying different ones would have told you
whether your technique was off.

~~~
mechanical_fish
_This story frustrates me like being unable to find a bug in code._

That is exactly the feeling, yes.

Here's an interesting fact about the profession of experimental scientist: You
feel this way _all the time_. Actual science is not like classroom science.
There's no money or glory in doing experiments which have known, explainable,
predictable, well-established answers. Nobody will pay you to measure the
value of _G_ again and again, on the off chance that one day it will be
different. We're all pretty well convinced that this doesn't happen.

Instead your job is to do experiments which have odd, inconsistent,
unpredictable, poorly-evidenced answers. Sometimes you do those experiments
for years and then find that the only reason your results are intriguingly odd
is that there's a bug in your technique or your theory. That's depressingly
common. Other times -- the interesting times -- you have to invent a brand-new
theory to explain your experiments. And sometimes that theory is even correct,
and you get to publish a bunch of papers on it -- perhaps you even get to name
it! But you must constantly worry that your brand-new theory is a load of crap
-- maybe you're just doing the experiments wrong.

If you can't learn to live with that nagging feeling of uncertainty you may
need to find a different profession.

 _Why would you keep using the same instruments?_

As others have pointed out: This is undergraduate physics lab. There is no
money to buy anything. More importantly, there is no _time_ to assemble
another apparatus. (This is another reason why this problem is a great model
of real-world scientific life. There is never enough money. There is never
enough time.)

Moreover, let's consider the possibilities. The instrument was working the
week before. With 20-20-hindsight, we know that it was working on the
following week. Odds are that the problem was not with the instrument. It was
almost certainly with the experimenters! We screwed it up. I just have no idea
how, and I never will. Arrrgh!

~~~
jgamman
there's a guy doing G-measurements at really small scales
here:<http://www.stanford.edu/group/kgb/Research/gravity2.html> i'm still
astounded at just how good the 1/r^2 relationship has held up for what? 300
years?

------
jerryji
Thanks to the Internet, now it takes only 24 years to publish one's physics
experiment result :P

Don't get me wrong, this is very cool -- hope I'll have all the facilities
(mainly a large, undisturbed basement) when my son grows up to do it together
with him.

------
RK
I remember doing this experiment as a physics undergrad. We had to set up the
torsion balance and leave it for 24 hours to let any vibrations damp out. The
problem was that anytime someone would close a door in the lab more vibrations
would start up. I don't remember how close our numbers came to Newton's
constant, unfortunately.

It is a nice application of lasers to measure measure fundamental forces. Sort
of the poor man's LIGO (minus the gravitaional waves):
<http://en.wikipedia.org/wiki/LIGO>

------
rgrieselhuber
"It's simple. Just change the gravitational constant of the universe."

------
Shamiq
Does this guy have anything to do with the Cavendish Banana?

~~~
justinsb
Not directly, but the same family:
[http://www.bananabook.org/discovolonte/2008/03/what-does-
kei...](http://www.bananabook.org/discovolonte/2008/03/what-does-
keira.html#more)

Banana Cavendish = 6th Duke of Devonshire

Gravity Cavendish = Grandson of 2nd Duke of Devonshire

I am strangely intrigued by a whole book devoted to the banana.

~~~
dhughes
That reminds me of a show where the host would connect what seemed like
totally unrelated events and show how they were connect, now that I think
about the show may have been called Connections.

Ah ha it is! <http://en.wikipedia.org/wiki/Connections_(TV_series)>

~~~
justinsb
Looks like a great series, but goodness - $135 for Season 1 alone on DVD (from
Amazon.) I can't imagine they're selling too many copies at that price.

~~~
rms
Never fear, you have the internet!
[http://www.youtube.com/results?search_type=&search_query...](http://www.youtube.com/results?search_type=&search_query=burke+connections&aq=f)

