Bending Spacetime in the Basement (1997)
33 comments
·March 11, 2025dannyobrien
John Walker, the author of this and host of Fourmilab, passed away last year: https://news.ycombinator.com/item?id=39297185
gsf_emergency_2
Kelvin is an odd childhood nickname for a JWW, maybe a reference to Lord Kelvin, or, a referent of Kelvin R. Throop?
defrost
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.
PaulHoule
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.
out-of-ideas
> 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
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
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
brianpan
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).
cgriswald
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.
xelxebar
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 Poincaré group[1]. That is, points in space are physically described by G-torsors over the Poincaré group!
Y_Y
Excellent comment. The only thing I can add is the obligatory "why stop there?", Poincaré 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].
eggn00dles
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.
awesome_dude
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.
almostgotcaught
> 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
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?
awesome_dude
It's all relative!
insonable
sounds like Robert California
jcims
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.
rkagerer
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?
erostrate
How do I rule out the masses being attracted by electromagnetism?
danbruc
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
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?
dghughes
> 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.
maurits
Reminds me a bit of Greg Egans' Incandescence [1]
cyberax
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.
null
bbarnett
I saw this documentary, it was called 'Sliders'.
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.