[HN Gopher] Bending Spacetime in the Basement (1997)
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Bending Spacetime in the Basement (1997)
Author : wizardforhire
Score : 226 points
Date : 2025-03-11 00:11 UTC (22 hours ago)
(HTM) web link (www.fourmilab.ch)
(TXT) w3m dump (www.fourmilab.ch)
| dannyobrien wrote:
| John Walker, the author of this and host of Fourmilab, passed
| away last year: https://news.ycombinator.com/item?id=39297185
| defrost wrote:
| Are 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/
| dylan604 wrote:
| When 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 wrote:
| Kelvin 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.keanw.com/2007/01/protecting_inte.html
| jcims wrote:
| I'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 wrote:
| > 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 wrote:
| I'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 wrote:
| For 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 wrote:
| Is 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 wrote:
| I 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).
| PaulDavisThe1st wrote:
| "Information: any difference that makes a difference" -
| Gregory Bateson
| nartho wrote:
| For music, a note is just a frequency. It is the difference
| between the notes that makes music, not the notes themselves
| insonable wrote:
| sounds like Robert California
| eggn00dles wrote:
| there most certainly are absolutes. theres an absolute maximum
| amount of distance a massive object can travel through
| spacetime. theres an absolute maximum amount of mass/energy
| that can exist within a volume of space before an event horizon
| forms. universal constants which have dimensions are more or
| less measurable absolutes. differences maybe more apparent but
| that doesnt mean absolutes dont exist.
| almostgotcaught wrote:
| > theres an absolute maximum amount of distance a massive
| object can travel through spacetime
|
| Wut. No there's not? Specifically for spacetime it's obvious
| to see that's not correct because the time axis has no upper
| bound.
| d1sxeyes wrote:
| I agree OP misspoke, but are we sure the time axis has no
| upper bound? Or that it doesn't loop back round on itself?
| Or that it even exists meaningfully as an axis?
| pixl97 wrote:
| Heh, i wonder what the mean free space between blackholes
| is?
|
| Now, given that in quadrillions of quadrillions of years
| all black holes will evaporate and potentially even protons
| decay there could be a time in the universe where
| everything is mass less radiation.
| awesome_dude wrote:
| An absolute on the upper bound of velocity that a massless
| object can travel through space at maybe?
|
| Although - I had it explained to me that the fabric of space
| was entering a black hole faster than light could travel,
| which is why light couldn't escape.
|
| Which made me wonder - is light (the massless object)
| travelling, or is it space travelling and carrying things at
| different velocities.
| awesome_dude wrote:
| It's all relative!
| xelxebar wrote:
| Or said another way, forces carry the structure of G-torsors!
| The terminology is way too pompous for such a simple concept,
| though. John Baez has a really clean writeup on them,
| accessible to anyone interested:
| https://math.ucr.edu/home/baez/torsors.html.
|
| Heck, even differences are usually non-physical; it's their
| ratio that matters. _I.e._ choosing feet or meters doesn 't
| change the physics; the same goes for energy. So we have two
| free parameters: choice of origin and choice of units.
|
| Baez only hints at this near the end of the above article, but
| we can actually fuse translations and scalings into a single
| group of elements that are combined translation + scale
| operations, an affine group. It turns out that this combined
| group is just a certain combination (semidirect product[0]) of
| the translation and scaling groups, as one would hope.
|
| And once we are thinking about affine groups, it's natural to
| consider more complicated ones. Most famous is probably the
| Poincare group[1]. That is, points in space are physically
| described by G-torsors over the Poincare group!
|
| [0]:https://ncatlab.org/nlab/show/semidirect+product+group
|
| [1]:https://en.wikipedia.org/wiki/Poincar%C3%A9_group
| Y_Y wrote:
| Excellent comment. The only thing I can add is the obligatory
| "why stop there?", Poincare true to form gives a beautiful
| description of the symmetries of flat spacetime, but we know
| that spacetime is only locally flat, the measurement
| symmetries at large scale are carried by the (unfortunately
| named) Killing fields[0].
|
| [0] https://en.wikipedia.org/wiki/Killing_vector_field
| bbarnett wrote:
| I saw this documentary, it was called 'Sliders'.
| cyberax wrote:
| Another 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 wrote:
| A 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 wrote:
| I can't find a complete explanation, but there are some
| technical details in
| https://deepearthscience.blogspot.com/2016/06/the-
| gravimeter...
| genewitch wrote:
| 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 wrote:
| I 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).
| rkagerer wrote:
| I 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 wrote:
| Yes - a window.
| rcxdude wrote:
| transparent 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.
| dave9000 wrote:
| A 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 wrote:
| Huh? Why would the bending of the floor matter?
| SamBam wrote:
| I 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 wrote:
| > 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.
| layer8 wrote:
| It's suspended from an aluminum ladder standing on the
| floor.
| lisper wrote:
| That's true, but the details of the placement of the
| ladder doesn't matter at all. The only thing that matters
| is that the ladder doesn't move or wobble around during
| the experiment.
| erostrate wrote:
| How do I rule out the masses being attracted by electromagnetism?
| danbruc wrote:
| You 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 wrote:
| How 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?
| gus_massa wrote:
| The 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 wrote:
| Reminds me a bit of Greg Egans' Incandescence [1]
|
| [1] : https://en.wikipedia.org/wiki/Incandescence_(novel)
| dghughes wrote:
| > 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 wrote:
| I 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
| unixhero wrote:
| Since this is written in metric/si units, I will read it!
| ChuckMcM wrote:
| I 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.
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