7 ms·
Bending Spacetime in the Basement (1997)
- deleted 2y ago[deleted]
- dannyobrien 2y agoJohn Walker, the author of this and host of Fourmilab, passed away last year: https://news.ycombinator.com/item?id=39297185 https://news.ycombinator.com/item?id=39297185
- defrost 2y agoAre his bumper stickers still available? Could be a rush on . . . Yes, right here, right now, you can obtain image files in a variety of resolutions and/or the master PostScript source code for The One, The Original, The Prophetic (July 1990), The Authentic “Evil Empires: One down, one to go…” bumper sticker, anticipating the obsolescence of railroad era continental-scale empires in the information age. https://www.fourmilab.ch/evilempire/ https://www.fourmilab.ch/evilempire/
- dylan604 2y agoWhen did the "one down" go away? It just morphed into a different thing, but it's still kicking, and winning in certain areas
- gsf_emergency_2 2y agoKelvin is an odd childhood nickname for a JWW, maybe a reference to Lord Kelvin, or, a referent of Kelvin R. Throop? https://www.fourmilab.ch/autofile/www/section2_46_1.html https://www.fourmilab.ch/autofile/www/section2_46_1.html https://www.keanw.com/2007/01/protecting_inte.html https://www.keanw.com/2007/01/protecting_inte.html
- jcims 2y agoI’m on my second attempt at this. The first worked but it was unwieldy and kind of unsatisfying. I’m going to make a second attempt using a bearing of ferrofluid, which has a damping effect, and use field alignment with the earths magnetic field as the source of torque. It’s really fun and at times frustrating trying to eek out effects of nanonewtons of force, highly recommended.
- out-of-ideas 2y ago> This is the consequence of all the forces of physics being gauge invariant: absolute values don't exist—only differences matter there's something about this quote that i really like, going to have to use it out of context on people
- 4b11b4 2y agoI've come back to this idea of "differences" from many angles... From images: information _is _ the differences between pixels From opinions: my opinion alone is worthless, but a difference of opinions? now we've got something From relationships, graphs, networks: edges are _differences_ between nodes. the edges are everything. nodes alone... are meaningless From music: this one is a little easier to take at face value, you can't have a sound without a change (difference) over time. the speaker cone can only wobble... Not sure about the absoluteness or correctness of this intuition, please criticize
- kortilla 2y agoFor images, you need a reference pixel to get the color right. You can apply differences from there but you’re effectively working with absolutes because of that so it’s a meaningless distinction. Any time an absolute reference point is assumed, all downstream calculations are effectively absolutes
- cgriswald 2y agoIs this strictly true? Suppose you have a reference pixel but don’t know its value. You don’t get the color right but there is plenty of other information there, right? Which pixel also probably limits the range of values since your highest and lowest values are probably within a certain range.
- brianpan 2y agoI have a blue/black (or possibly white/gold) dress that says it's relative. You can also ask a UI designer about "absolute" colors. One color needs to be different depending on the context it's in order to LOOK the same (it's size/thickness, background, what it's next to).
- b112 2y agoI saw this documentary, it was called 'Sliders'.
- asron 2y ago[flagged]
- cyberax 2y agoAnother interesting way to see the effects of gravity can be done with pendulums, using a Vening Meinesz apparatus. It's not as spectacular as seeing a torsion bar move, but it's more sensitive.
- genewitch 2y agoA cursory search on my phone gave a picture of the device and the biography of the person, but not a detailed explanation of how the device works or how to replicate it.
- gus_massa 2y agoI can't find a complete explanation, but there are some technical details in https://deepearthscience.blogspot.com/2016/06/the-gravimeter-of-professor-vening.html https://deepearthscience.blogspot.com/2016/06/the-gravimeter...
- genewitch 2y agohttps://nextcloud.projectftm.com/index.php/s/gravitycompare https://nextcloud.projectftm.com/index.php/s/gravitycompare that is really impressive, the 100 year old data compared to recent data. i'm still amused there's no real explanation of how it works. Like, can i do this with three X 9-DOF sensors in a box? you don't need light or film, you can use a TCO for accuracy in timing... I don't know why it keeps saying 3 pendulums, but a pair, but a "virtual third pendulum". but what this is, is an analog computer, right?
- cyberax 2y agoI learned about it at university. The idea is to measure the difference between periods of the short and the long pendulums. In particular, how many swings it takes for their swings to match (both pendulums arriving at the apex at the same time).
- genewitch 2y agooh, and if it takes a different amount of time, something interfered, I'll have to try and make a simulation using the formulas on that page, then i can see if it can be copied with 6|9DOF, MEMS vibrate and i think you can control how fast, or at least track how fast over a period of time. i think you'd need 3, but i haven't messed with the expensive bosch sensors in a while.
- rkagerer 2y agoI ran out of time to finish reading the article. He explained the importance of using a camera to observe the results due to the ways a human presence would interfere with the experiment. When he talks about how Archimedes might have reproduced his device using technologies of the time, did he address what historic alternative might take the place of the camera?
- mariushop 2y agoYes - a window.
- rcxdude 2y agotransparent glass was not really a thing in ancient Greece. That said with careful design you could make it work with just small openings into the room which are covered except for occasional observation.
- ahazred8ta 2y agoThe greeks and egyptians had transparent glass, even glass magnifying lenses. https://en.wikipedia.org/wiki/Burning_glass https://en.wikipedia.org/wiki/Burning_glass "your bailiff comes, shows me his writ - I, standing thus, d'ye mark me, in the sun's stream, measuring my distance, guide my focus to a point upon his writ, and off it goes in fumo!"
- dave9000 2y agoA caveat. I had the change to perform a similar experiment at work testing very sensitive accelerometers. Believe it or not, the biggest source of errors was the bending of the floor due to the test masses! A better setup would be to suspend the spheres exactly like the tuna cans, and test two configurations rotated by 90 degrees.
- lisper 2y agoHuh? Why would the bending of the floor matter?
- SamBam 2y agoI assume if your weights are resting on the floor, and the pivot point of the swing arm is resting on the floor, then the floor deflecting may cause the arm to swing even in the absence of an attraction to the weights.
- lisper 2y ago> the pivot point of the swing arm is resting on the floor But it isn't. It's suspended from a support by a thin filament. That's the whole point of the experiment.
- dave9000 2y agoFor the accelerometers the problem was the tiny tilting of the sensitive axis in the gravity field. In this case you're probably right that this doesn't matter - but in general it is difficult to rule out every second-order effect when the forces at play are so small with respect to g.
- erostrate 2y agoHow do I rule out the masses being attracted by electromagnetism?
- danbruc 2y agoYou can rule out electric forces by just grounding the masses to have them at equal potential, at least as long as your test masses are conductive. Magnetic forces seems harder, use a non-magnetic material does not really help, how would you know that it is not ever so slightly magnetic, maybe even just due to contamination? Maybe try different materials and find that the effect scales with mass but seems independent of the material, that could increase confidence as it seems unlikely that different test masses made of different materials would exhibit the same tiny magnetic forces. Also I would first do a calculation, I have no idea how the gravitational forces compares to say paramagnetic effects you might encounter, for all I know they could be of similar size or orders of magnitude apart.
- mrob 2y agoHow about deliberately putting a strong magnet close to the rotating test masses, then repeating the experiment with the magnet on the other side and verifying that it doesn't make a difference?
- danbruc 2y agoOf course, that might be the easiest way. Given that you can get tiny magnets that are pretty strong but only a millimeters or so in size and therefore also light, you could just stick them to your different test masses in various configurations and see if it makes any difference while leaving the mass distribution pretty much unchanged.
- gus_massa 2y agoThe metal masses over a styrofoam bar look as a serius problem because they may collect static electricity. Perhaps a thin coorer wire that connect them to the aluminium foil in the water may solve that.
- maurits 2y agoReminds me a bit of Greg Egans' Incandescence [1] [1] : https://en.wikipedia.org/wiki/Incandescence_(novel) https://en.wikipedia.org/wiki/Incandescence_(novel)
- dghughes 2y ago> The reason lies in the extraordinary weakness of the gravitational force. I recall reading Brian Greene giving the example of sitting in a chair. The molecules of the chair are stronger than gravity. You're not crashing through to the floor.
- PaulHoule 2y agoI did this one in the senior lab as an undergrad studying physics at New Mexico Tech. I got to use a whole gymnasium for a weekend and used a laser beam projected across the gym to read out the torsion balance. Pretty cool that you can measure the gravitational constant [1] so easily, if approximately. [1] https://en.wikipedia.org/wiki/Gravitational_constant https://en.wikipedia.org/wiki/Gravitational_constant
- Tyrannosaur 2y agoDo you remember how accurately you were able to measure the gravitational constant?
- unixhero 2y agoSince this is written in metric/si units, I will read it!
- ChuckMcM 2y agoI miss John, this has much of the charm of the old "Amateur Scientist" columns from Scientific American. I remind myself from time to time that so much great work has been done by scientists with simple hand tools and a thoughtful experiment.
- y33t 2y agoFun fact. Charles Mason and Jeremiah Dixon, using theodolites, plumb bobs, and spirit levels, were able to detect variations in gravity from the local environment in 1767. Upon completing the resurvey on the way back to Philadelphia, they noticed a systematic error and concluded it was because of gravitational deflection due to the Allegheny mountains (with the help of Maskelyne, Cavendish and the Royal Society). Sometimes I think about that and it just blows me away.
- plagiat0r 2y agoThe problem I have with the article is that gravity is not a force. Magnetic is a force, but gravity is not a force.