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Digital Studios. Bad ideas come and go in physics. But thereâs one bit of nonsense that is
perhaps more persistent than all others: the perpetual motion machine. No working perpetual motion machine has ever
been experiment verified. All break the laws of thermodynamics. In fact, we classify them based on WHICH law of
thermodynamics they break. We have perpetual motion machines of the first
kind - they violate energy conservation - they pump more energy out than they need to keep
running. This includes most of the historical devices. Then there are machines of the second kind
- theyâre a bit more subtle in their wrongness because break the second law of thermodynamics
- extracting energy by reversing entropy. Many modern âfree-energyâ devices fall
into this category. Now the best modern designs are by you - answers
to our recent challenge question, which weâll get to at the end. But first letâs take a look at examples
of what other people came up with - thisâll be a fun little journey through some pretty
terrible science. Designing a perpetual motion machine has a
very long history and became quite the craze from the middle ages through the renaissance. The first well-documented design for a perpetual
motion machine was from the 12th century. BhÄskara's wheel, named after the Indian
mathematician, was embedded with tubes of mercury that would flow from back and forth
as the wheel turned. Other types of over-balance wheels followed
through to the Renaissance and worked on the same principle. Basically the shift in the center of gravity of the spokes on one side should drive the wheel around. There were also designs that employed the
magical-seeming lodestones â magnets. For example, this ramp in which a ball is
pulled to the top by a magnet before falling through a hole and rolling to the bottom again. Then there are the self-pumping waterwheels
or self-blowing windmills. None of these machines actually worked. In every case you can find a subtle bit of
physics that the designers overlooked. The overbalance wheel pushes masses outwards
on one side, but that same at the same time increases the separation between those masses,
so that moment of inertia is unchanged. A magnet strong enough to pull a ball up anincline would also prevent it from falling through that hole. Losses due to friction mean that water or
wind driving a mill could never produce enough energy to fully complete the cycle. Close, but no perpetual lighting cigar. Now to be fair to the people working during
the middle ages and Renaissance, the law of conservation of energy hadnât been discovered
yet. Thus the proliferation of first-type machines
that could generate more energy than they used. But absolute proof of their non-viability, by the discovery of the first order of thermodynamics in the 18th century,
didnât end the craze. Inventors just graduated to instead breaking the second
law of thermodynamics. To review: the 2nd law states that entropy
can never decrease over time â in other words, energy will tend to spread itself out
as evenly as possible â to a state of maximum entropy. All machines work by riding this flow of energy
as it redistributes itself. Machines of the second kind claim to be able
to tap reservoirs of energy that are already evenly spread â effectively reversing the
growth of entropy. Before we get to modern perpetual motion machines,
letâs take a look at some actually interesting ideas that illustrate the limit of the second
law. First up, the Brownian Ratchet. It goes like this: a paddle wheel immersed
in a gas is connected to a cog with a latch that only lets it turn in one direction. Individual gas particles are moving around
with random â or Brownian motion. When one hits the wheel in one direction the
cog turns, but in the other direction the rotation is blocked. Sounds ⌠plausible. And it actually works. But ONLY if the chamber containing the cog
is at a lower temperature. If itâs at the same temperature then the
cog is as likely to be pushed back when the latch raises. Richard Feynman himself demonstrated this
mathematically. This is actually a specific example of a Maxwellâs
demon device, an entropy-reversing thought experiment that weâve discussed before. With no energy or temperature gradient, no
energy can be extracted. In fact thereâs an absolute limit to the
amount of energy that can be extracted based on a difference in temperature. That limit is defined by the most important
â perhaps the only important perpetual motion system ever conceived: the Carnot cycle. In the early 19th century, Sadi Carnot described
the most efficient possible engine â Itâs a sequence of expansion and contraction of
gas in a piston chamber that will provide the maximum possible energy as heat flows
between reservoirs of different temperature. Critically, the Carnot cycle is reversible. Drive the piston in reverse with exactly the
same amount of energy, and itâll transfer energy from cold to hot. In principle a Carnot engine could extract
energy from a temperature gradient and then pump it back in again, making it a candidate
perpetual motion machine. But even in this most ideal case the ratio
of energy in to energy out is exactly unity. No extra energy is extracted. The Carnot cycle is, in principle, a perpetual
motion process. But doesnât break any laws of thermodynamics,
so itâs not the first or second kind. In fact itâs the third kind. These are perpetual motion machines that do
not attempt to generate energy â they just keep themselves running forever. Sounds OK, right? After all, Newton tells us, an object in motion
tends to remain in motion unless acted on by an outside force. So remove all outside forces â remove all
friction, all energy losses of any kind, and surely any machine can tick on forever. Indeed there are ideal systems that, from
pure classical principles should run forever. Like a perfect Carnot engine, or a frictionless
wheel â with or without magnets and mercury tubes â or a planet orbiting a star. Just eliminate ANY leakage of energy. But thatâs not just an engineering challenge,
itâs fundamentally impossible. In fact, quantum mechanics forbids it. Due to the intrinsic quantum randomness of
all particles, as expressed by the Heisenberg uncertainty principle, everything moves. Internal parts always vibrate, guaranteeing
some friction and internal heat. Even if that heat is recaptured, the outer
walls of even the most carefully isolated system must radiate, slowly leaking energy. And then of course thereâs the gravitational
radiation that will slowly sap the âperpetual motionâ of celestial systems. The best-case scenario for a perpetual motion
machine of the third kind is a very, very slow wind-down to stillness. Now that we've reviewed three types of possible wrongness in perpetual motion machines, let's look at some of the modern proposals. Most look a lot like their ancient predecessors
â often lots of wheels with lots of magnets, somehow powering their own motion and producing
extra energy besides. These are often referred to as over-unity
devices â the ratio of energy out to energy in is greater than one. That name was actually coined by the US patent
office, which along with the UK, patent office and the French Academy of Science refuses
to even review devices claiming over-unity energy production. Understandably - it eliminates 50% of their
crank submissions. Now the purveyors of over-unity devices typically
do NOT claim to have created perpetual motion devices of the first kind. They have various explanations for how they
donât violate conservation of energy. However most end up shifting their device
to the second kind of perpetual motion: they violate the 2nd law of thermodynamics by extracting
energy where there is no energy gradient. One popular source of free energy is the zero-point
energy of the quantum vacuum, and itâs the most fun to debunk. Iâm going to have to refer you to our entire
playlist on the quantum vacuum for the physics, but the important point is that any energy
of the vacuum is exactly the same everywhere. It has no energy gradient, so isnât accessible
as an energy source. Now it may be possible to create an energy
gradient, for example with the Casimir effect in which plates are pulled together by lowering
the zero-point energy between them. But doing so repeatedly in an engine cycle
takes at least as much energy as it produces. At best you have a perpetual motion device
of the third kind. Most modern perpetual motion builders arenât
so technical. In fact the worst of them wouldnât have
physics or engineering chops to build a decent over-balance wheel. They rely more on pseudo-physics-speak - zero-point,
mag-grav, plasma, torus vibrations sound fancy, but are typically less good at generating
free energy than they are at directing you to their donate button or selling you a telluric
field wellness crystal. OK, enough with the nonsense . Letâs get
to the challenge question, in which I asked YOU to come up with a perpetual motion machine
based on the funky notion of negative mass. According to some interpretations, negative
mass should be attracted to positive mass, but positive mass should be repelled by negative
mass. The result is that a positive and negative
mass should accelerate indefinitely, potentially powering an infinite energy device. But how to build one? Letâs see what you came up with. Many of you hit on the most obvious and efficient
approach: attaching the apples to a dynamo so they chase each other in circles, driving
the generator. Here are some of the better designs. Nice adaption of the BhÄskara's wheel from
Creepy Magician. Alex Taylor has an especially cool dynamo
device in which the masses are contained in a magnetic field. He also does the full relativistic calculation
for power â nice one Alex. We have a rare non-dynamo device from Adrien
Romeo, who uses cogs with negative and positive mass teeth. This post-it will be framed in the Smithsonian
after the Romeo-engine solves humanityâs energy crisis. And the prize for most outlandish goes to
Epsilon Centauri, whose machine requires an entire bubble universe with closed, pac-man
borders, trapping the ever-accelerating apples. If you saw your name and device just now,
that means youâre a winner! You do get your pick from the Space Time store. Shoot us an email at pbspacetime@gmail.com
and weâll get your goodies to you. Feel free to check the store even if you didnât
win, or think you shouldâve won, or didnât get around to submitting but are pretty sure
you wouldâve won if you did. We all deserve merch. Link in the description. Iâm also going to post some of the best
solutions to this challenge on Instagram â check âem out at matt_of_earth. And I leave you with wise words from someone
who designed quite a few perpetual motion machines himself. Leonardo da Vinci. âOh ye seekers after perpetual motion, how
many vain chimeras have you pursued? Go and take your place with the alchemists. â da Vinci was ahead of his time in some
ways â he knew perpetual motion was bunk even before the laws of thermodynamics came
along. But vain chimeras? Had only da Vinci known that his own perpetual
motion machines couldâve score him such cool swag from Space Time. Thank you to CuriosityStream for supporting
PBS Digital Studios. CuriosityStream is a subscription streaming
service that offers documentaries and nonfiction titles from a variety of filmmakers, including
CuriosityStream originals. For example you can check out the Destination
Pluto, which follow the timeline of the New Horizons mission from its inception to the
date of its close encounter with everyone's favorite Kuiper Belt dwarf planet. You can learn more at curiositystream dot
com slash spacetime Hey guys, quick announcement - if youâre
in New York this week Iâm going a fun science thing this Thursday, 9pm at Caveat. Itâs called astronaut training, and it should
be hilarious. Well, my cohosts should be hilarious. Iâm going to try to not look silly. Iâm not hopeful on that. Link in the description. OK, and on to comments from our last episode
- the Secrets of the Cosmic Microwave Background! In which we deciphered the cryptic patterns
embedded in the oldest light in the universe. Francois Lacombe asks how astronomers distinguish
the very small temperature differences in the CMB from the rest of the microwave noise. This is a great question because isolating
the CMB is a huge challenge. The contaminating microwaves mostly come from
our galaxy - there's a lot from the dust in between the stars, and also from individual
electrons either bumping into other charged particles or circling in magnetic fields Fortunately
we can model that stuff pretty well. We know where the sources of this emission
are from observations at other frequencies and can do pretty well modeling and subtracting
them. All that galactic emission doesn't look anything
like the CMB in the way it fluctuations, so it's pretty clear when we've subtracted it
properly and only the CMB is left. Abe Dillon asks whether the "moment" of recombination
should depend on the local density. The answer is absolutely. People tend to talk about recombination as
having happened instantaneously and at the same time everywhere. That's not the case. Recombination happened when the universe became
cool enough nuclei capture electrons to form the first atoms. More dense regions were a little hotter, so
recombination started later in those. And in general the entire universe didn't
make this transition instantaneously. It took several tens of thousand years - which
is a lot considering the universe was less than 400,000 years old. It took long enough that the patterns of the
baryon acoustic oscillations frozen into both the CMB and in galaxy rings were blurred out
quite a bit because the universe took a moment to become fully transparent. Julio Toboso GarcĂa points out that the method
used to model the CMB fluctuations - multipole expansion - sounds a like Fourier Analysis. Good insight, Julio. In fact it's pretty much exactly that, but
for the surface of a sphere. Fourier analysis represents functions in one
dimension with a series of sine curves. On a spherical surface you instead use spherical
harmonics. John Rodriguez wished he'd've known that the
Physics skill tree is required to unlock the Wizard class. So if wizarding schools existed - and they
don't, okay, that's just silly - then they're probably more like Brakebills in The Magicians
by Lev Grossman. Less like Hogwarts. So basically like the most obnoxiously elite
ivy league grad program. You need 5 letters of reference from Nobel
Laureates, be in the top 0% on your GREs, and yeah, first authorship on a paper in theoretical
physics from that high-school science fair project.
Is the video posted today to /r/Physics on a patented process for generating energy via the Casimir effect legit, or is it bunkum?
https://reddit.com/r/Physics/comments/ay8op0/quantum_vacuum_energy_extraction/
No no this time it works!
Cosmic inflation and the expanding universe seem like perpetual motion in the human life time or until an extinction event of the species .