wasser@mosaic.dec.com.UUCP (10/27/86)
> o Given the old "accellerating-elevator-in-space" experiment, > how can one within the elevator tell the whether he is > accelerating or under the influence of gravity? Two ways to tell if your elevator is just sitting on a big mass: 1) The acceleration is stronger at the bottom of the elevator because the bottom is closer to the center of gravity. 2) Two plumb lines will converge slightly because both strings point toward the center of gravity of the mass. The strings would hang parallel under acceleration in space. Warning: Experiment 2 can probably be fooled by appropriately shaping the large mass. -John A. Wasser Work address: Usenet: {adobe,acorn,allegra,amd,deccra,decvax,dual,flairvax,glacier,idi, ihnp4,imagen,ios,logitec,microsoft,mordor,nsc,purdue,pyramid, qubix,sci,sequent,Shasta,sun,turtlevax,ucbvax}! decwrl!dec-rhea!dec-{viking,rainbo,mosiac,colors}!wasser ARPAnet: WASSER%VIKING.DEC@decwrl.ARPA EASYnet: {VIKING,RAINBO,MOSIAC,COLORS}::WASSER USPS: LJO2/E4; 30 Porter Rd.; Littleton,MA ; 01460 Phone: (617)486-2505 P.S. Design of experiments to test for properties 1 and 2 are left as an exercise for the reader!
guy@slu70.UUCP (Guy M. Smith) (10/28/86)
In article <6108@decwrl.DEC.COM>, wasser@mosaic.dec.com.UUCP writes: > > 1) The acceleration is stronger at the bottom of the > elevator because the bottom is closer to the center of > gravity. > > 2) Two plumb lines will converge slightly because both strings > point toward the center of gravity of the mass. The strings > would hang parallel under acceleration in space. > > Warning: Experiment 2 can probably be fooled by appropriately > shaping the large mass. Actually, Experiment 1 can also probably be gimmicked. The gravitational acceleration within the earth doesn't change much until you get to the core-mantle boundary. This basically occurs because the core is much denser than the mantle so that the fact that you have less mass under you as you go down is compensated by the fact that you are closer to the dense core.
djg@nscpdc.UUCP (Derek J. Godfrey) (10/31/86)
> In article <6108@decwrl.DEC.COM>, wasser@mosaic.dec.com.UUCP writes: > > > > 1) The acceleration is stronger at the bottom of the > > elevator because the bottom is closer to the center of > > gravity. > Actually, Experiment 1 can also probably be gimmicked. The gravitational > acceleration within the earth doesn't change much until you get to the > core-mantle boundary. This basically occurs because the core is much denser > than the mantle so that the fact that you have less mass under you as > you go down is compensated by the fact that you are closer to the dense > core. I always understood gravity varied with r^2 between two bodies two non- coincident bodies but r when one is contained within the other.
throopw@dg_rtp.UUCP (Wayne Throop) (11/09/86)
So, from inside a small room can one tell the difference between the room being in a gravity field and being linearly accelerated, or what? No. And the "look for tides" or "look for mossbauer effect" or whatever won't help you, since you don't know the mass distribution around your room in either scenario. With a suitable surrounding mass distribution, tides (and other spherical gravity field effects) can be reduced to arbitrarily small effects in the gravity case. And even then, "finding a tidal effect proves you aren't accelerating" is incorrect, since the "tides" might be provided by cleverly placed auxiliary masses for the acceleration case. Further, you can't wait and try to observe special relativistic effects either as suggested here: > zdenek@heathcliff.columbia.edu (Zdenek Radouch) > But I think you'd find out anyway. The human body starts to act really > funny when it moves at the speed of light. The strongest effects are on > the digestive tract. because there are no such "funny effects" on the body. Physics (and chemistry, and digestion) are supposedly invariant across reference frames. (Zdenek's suggestion is incongruous, especially the reference to the digestive tract, and may therefore be a joke. If so, obviously I didn't get it.) And, for the same reason, these suggestions won't work either: > pmk@prometheus.UUCP (Paul M Koloc) > I think in one case as time went on time dilation would begin > setting in as well as increasing contraction (spatial) along the > line of acceleration. Perhaps so. But how would one measure it from inside the room? (Hint: one could not.) Certainly this attempt fails: > By > checking the "frequency" spectroscopy of sharp line emmissions > in the "horizontal" plane of the elevator using a grating spectro- > meter and comparing it with the standard (reference on a known > platform), perhaps the time dilation due to increasing velocity > would become apparent although the spacing of the grating rulings > should not be effected, unless rotated. because all measuring instruments will be similarly time-and-space "distorted", so that they will insist that all is as it was at rest. Even the slight cheat of "looking outside" (by refering to measurements taken on a "known platform") doesn't help, since (from inside the room) the room should have all the characteristics of the "known platform" (if it too was at rest relative to the measuring devices). The only (ah say ONLY!) way to tell is: > [...] we could open the door and look out! -- "Pwease, mistew ewevatow opewatow, I've twied EVEWYTHING... I'll give you aww this vawuabwe scientific equipment if you wouwd just teww me what's WEAWWY going on!" "Why SOIT'NY m'boy! It's REALLY another puzzle in sci.physics!" "AAAAAAAAUUUUUUUUGGGGGGGGHHHHHHHHH!!!!!!!!" -- Wayne Throop <the-known-world>!mcnc!rti-sel!dg_rtp!throopw