828 reputation
515
bio website henning.makholm.net
location Copenhagen, Denmark
age 42
visits member for 3 years, 2 months
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8h
comment Confusion I have regarding Einstein's 1905 derivation of LT
@J...: Not exactly. Sure enough the conclusions of Einstein's paper have stood the test of time, being reproduced and checked by generations of textbook authors and students. However, this in itself does not imply that the presentation in the original paper was necessarily flawless. It is at least conceivable that there could have been misprints or mistakes in the original deviations, which were not so grave that they prevented the readers from figuring out how to fix them. If that were the case, we still wouldn't expect much discussion of such trivialities in modern textbooks.
Sep
1
comment Is it possible to have things orbit around us?
... that constant being none other than the universal constant of gravity. Its dimension is acceleration [of a test particle] per mass [of the central body] per inverse distance squared, i.e. $\rm\frac{m/s^2}{kg\cdot m^{-2}}$, which is the same as $\rm\frac{s^{-2}}{kg/m^3}$, or frequency squared per density.
Aug
7
awarded  Popular Question
Jul
28
answered Why does a free-falling body experience no force despite accelerating?
Jun
21
awarded  Yearling
Jun
5
comment Why doesn't the motion of a car affect the frequency of radio stations?
@NiloCK: It allows for greater tolerances in how precisely the receiver needs to be able to tune to a particular frequency initially. The receiver can be built with cheaper components and simpler designs if it can deal with its natural tuning being off by some tens of kHz depending on the temperature, for example. (Conversely, this also lets it correct for the transmitter being slightly off-frequency).
Jun
5
comment Why doesn't the motion of a car affect the frequency of radio stations?
@Superbest: With FM modulation, a frequency shift would (to a first approximation) just lead to an DC offset of the demodulated signal. Even if the receiver doesn't adjust its tuning automatically, this is unlikely to survive the audio amplifier stages, and can't be heard even if it does. No audible warping.
May
27
awarded  Self-Learner
Apr
30
answered Is there a huge difference between centrifugal and centripetal forces?
Apr
7
comment Drying clothes with the sun's heat, without any air
When you say that water will freeze (and stay frozen, I presume) below 200 K, do you mean that its vapor pressure actually drops to 0 exactly. or merely that sublimation becomes unmeasurably slow even in a vacuum below the theoretical vapor pressure?
Apr
1
comment Is it possible to shoot bullets in space or would the recoil of the gun be too strong?
Even if the gun is floating alone in space when fired, it still has a lot more mass than the bullet, so most of the gas pressure will still go to accelerate the bullet even if the gun itself accelerates slightly backwards during the same time. "Much smaller" is definitely not right.
Mar
15
comment Is it possible to watch the same distant star's supernova event twice?
@Hagen: I'm not denying the facts, merely wondering what the explanation for those facts looks like in the photon-shell picture,
Mar
15
comment Is it possible to watch the same distant star's supernova event twice?
This does raise the interesting question of what shape the photon shell has after it passed the gravitaional lens. Apparently it gets "folded" such that it passes through us several times. But does it stay connected after folding? Are some of our observations because a "reversed" part of the photon shell sweeps past us, so we end up back "on the outside" of the deformed shell, topologically speaking? And how is that consistent with seeing an even number of images?
Mar
14
answered How can shadows ever be sharp?
Feb
23
comment Why is energy not an SI base unit?
Even more accurately: the CGPM decided on unit definitions; the BIPM is tasked with the practical implementation and dissipation of those decisions.
Jan
29
revised Will we ever be able to view the past?
ref Asimov
Jan
29
answered Will we ever be able to view the past?
Jan
19
comment Elevator cabin problem
Well, we can either analyze the situation in Newtonian mechanics, or in (the slow-speed limit of) GR. In the first case gravity is a real force, the lamp really accelerates, the person on the ground is an inertial frame, and the falling elevator is not an inertial frame. On the other hand, in GR all that changes: The elevator is an inertial frame, the lamp is at rest in it, and there is no force of gravity, except for a correction term that the guy on the ground uses to adjust for the fact that his frame is not inertial. Each perspective is valid under its own assumptions.
Jan
17
accepted Why do turbine engines work?
Jan
16
awarded  Nice Question