6 ms·

Measure the gravitational constant like Cavendish did in 1798

35 points by rw 18y ago

jerryji 18y ago
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.
justinsb 18y ago
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.
pmjordan 18y ago
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 18y ago
  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 18y ago
    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 18y ago
cool-RR 18y ago
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.
randomwalker 18y ago
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.
Shamiq 18y ago
Does this guy have anything to do with the Cavendish Banana?
- justinsb 18y ago
  Not directly, but the same family: http://www.bananabook.org/discovolonte/2008/03/what-does-keira.html#more http://www.bananabook.org/discovolonte/2008/03/what-does-kei... 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 18y ago
    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) http://en.wikipedia.org/wiki/Connections_(TV_series)
    - justinsb 18y ago
      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 18y ago
        Never fear, you have the internet! http://www.youtube.com/results?search_type=&search_query=burke+connections&aq=f http://www.youtube.com/results?search_type=&search_query...
rgrieselhuber 18y ago
"It's simple. Just change the gravitational constant of the universe."
RK 18y ago
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 http://en.wikipedia.org/wiki/LIGO

35 points by rw 18y ago

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.

justinsb 18y ago

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.

pmjordan 18y ago

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 18y ago
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 18y ago
  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 18y ago

cool-RR 18y ago

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.

randomwalker 18y ago

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.

Shamiq 18y ago

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

justinsb 18y ago
Not directly, but the same family: http://www.bananabook.org/discovolonte/2008/03/what-does-keira.html#more http://www.bananabook.org/discovolonte/2008/03/what-does-kei... 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 18y ago
  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) http://en.wikipedia.org/wiki/Connections_(TV_series)
  - justinsb 18y ago
    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 18y ago
      Never fear, you have the internet! http://www.youtube.com/results?search_type=&search_query=burke+connections&aq=f http://www.youtube.com/results?search_type=&search_query...

rgrieselhuber 18y ago

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

RK 18y ago

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 http://en.wikipedia.org/wiki/LIGO