
Why a Grape Turns into a Fireball in the Microwave - ChuckMcM
https://www.wired.com/story/why-a-grape-turns-into-a-fireball-in-a-microwave/
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pgt
Summary of Video:

In a microwave oven, resonance between two grapes can ionize the air, creating
plasma. The wavelength of 2.45GHz microwaves = 12cm, but the grape's
refractive index of ~10 slows down the wave 10x, decreasing the wavelength to
about 1.2cm...about the size of a grape. This causes resonance at the center
of each grape, just like you can hear "boomy" bass in certain parts of a room.

When two grapes touch, this creates a mode (or standing wave) between them,
which builds up enough energy to ionize the air, allowing electrons to flow.
If you look at the plasma spectrum, you can see spikes for the sodium (Na) and
potassium (K) ions in the grapes.

SCIENCE, BIATCH!

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msravi
The EM wave should just bounce around inside the grape because of total
internal reflection and the high refractive index, right? Is this what is
being termed as "resonance"?

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chicob
Let's just call it raisinance.

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boardwaalk
Veritasium video also from today: [https://www.youtube.com/watch?v=wCrtk-
pyP0I](https://www.youtube.com/watch?v=wCrtk-pyP0I)

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peterwwillis
I think this is a better explanation than the linked article (and some
pictures in the video help a lot)

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ravenstine
Add a drinking glass:
[https://www.youtube.com/watch?v=sYusDh0Sw3U](https://www.youtube.com/watch?v=sYusDh0Sw3U)

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aidos
We used to do this with incandescent bulbs. I was never quite sure if it was
safe (is it?!), but the microwave itself always seemed to survive ok.

Generally the bulb would explode, we presumed due to the heat causing internal
pressure. Though at one point a bulb got really hot and melted a sort of
bubble on the side. It seemed to provide enough space for the heated air so it
never exploded and we could use it again and again.

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knolan
You can make plasma with a partially evacuated sealed bottle, it’s basially
how plasma chambers work and it’s a handy way to get plasma if you’re stuck.

You can use different gases in the bottle for different plasmas.

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SmellyGeekBoy
> it’s a handy way to get plasma if you’re stuck.

I'm finding it hard to imagine a situation where I desperately need to make
plasma but I'll file this one away just in case. ;)

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knolan
I’ve seen it used to treat cured PDMS elastomer and glass slides for bonding
microfluidic devices (aligns OH groups to the surface to achieve a covalent
bond between the PDMS and glass). Our plasma chamber was destroyed by a
student so I used a corona discharge tool, or a cattle prod as it’s better
known. Was tempted to buy a cheap microwave tho...

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Scoundreller
Is this bad for my microwave?

More importantly, would this be bad for... someone else's microwave at a
party?

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lostlogin
It can be added to the list of things to do with other people microwaves.

1) Lead pencil drawing on paper (ideally a circle) in the microwave.

2) Incandescent light bulb in the microwave.

3) CD in the microwave.

4) Grape!

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rootbear
Peppermint Patties are cool in a microwave. Only nuke them for a few seconds.
The inside expands and ruptures through the top of the patty. My sister
learned of this from her third graders (of course) who called them "alien
eggs". And the hot peppermint and chocolate result is quite tasty!

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ChuckMcM
I love the physics in this article. Who knew a lowly grape is an epic plasma
generator!

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ransom1538
You are in a car going 60mph with a balloon in the car. You slam on the gas,
which way does the balloon go? (No really, get a balloon and do it, it will
freak out everyone in the car).

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Pharmakon
Assuming a helium balloon, it moves back toward the rear.

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Pfhreak
It does the opposite, in fact.

When you accelerate, the gas in your car behaves like a fluid, piling up in
the rear of the vehicle. Helium balloons climb up the density gradient of air,
essentially seeking the lowest density point.

That area of low density is at the front of the car as you accelerate. So, the
balloon moves forward as you accelerate.

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chairmanwow
Is the gas density gradient really so significant that it allows a balloon to
move towards the front of the car? Any way we can add some calculations to
this?

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bzbarsky
The density gradient is not really that important. You could do this with a
car filled with water and a cork instead of a balloon, and the effect would be
the same, even though the density gradient in water would be pretty darn close
to zero.

What matters is that the direction of the buoyant force is precisely opposite
the direction of the overall force acting on the fluid (because that's what
keeps the hypothetical fluid that your balloon displaces from moving due to
said overall force). Importantly, this overall force needs to be measured in
the reference frame of the fluid, which in this case is accelerating.

OK, so let's draw out a force diagram on the air inside the car. There's a
force upward from the floor, exactly balanced by the force of gravity down.
There's similarly a force from the back of the car , which is what's
accelerating the air, from the point of view of a non-accelerating observer.
From the point of view of an observer in the car, however, the air is _not_
accelerating, but the force from the back of the car is still there. So there
must be a force backwards on the air, to balance the force of the back of the
car. You can call it "inertial force" or "gravity" (in the general-
relativistic sense) but the upshot is the same: the overall force on the air
is down and backwards, so the buoyant force is up and forward, along the same
line. For a helium balloon the buoyant force is stronger than "gravity", so it
goes up and forward.

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OisinMoran
Wherever I saw this idea originally (I think it may have been the TV show
Braniac) cut one grape in half so there was just a little sliver left
connecting the halves which gives a really nice arc between them.

I later reckoned a cherry tomato would probably do the same thing and it
definitely does. I remember at the time thinking the grape gave a whitish-blue
arc where the tomato gave an orangey-red one but not sure whether I was
imagining that.

Probably two of the safest microwave tricks to do, as it is just food after
all, but the CD one is definitely worth the temporary smell of burnt plastic.

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angry_octet
Possibly more potassium in the tomato? Potassium having a purple flame and
plama.
[http://www.webmineral.com/help/FlameTest.shtml](http://www.webmineral.com/help/FlameTest.shtml)

However, your eye/brain is seeing energy in a bunch of spectral bands, and
making a decision as to what colour to call it.

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intrasight
I read an article last year about ball lightning which suggested that it is
caused by spherical resonance microwave cavity. Something analogous to the
grape?

[https://en.wikipedia.org/wiki/Ball_lightning#Microwave_cavit...](https://en.wikipedia.org/wiki/Ball_lightning#Microwave_cavity_hypothesis)

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skilled
Outline:

[https://outline.com/eHJzua](https://outline.com/eHJzua)

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mabbo
> They could conceivably shrink these rules to a smaller scale, to create
> similar hotspots in nanoparticles, for example. Scientists use heated
> nanoparticles to make very precise sensors or to facilitate chemical
> reactions, says Bianucci.

This might have applications for really bad-ass cancer treatments. Build nano-
particles that have this strange property, that will be consumed by cancer
cells but not by healthy cells (note: this is the hard part). Then explode the
cancer cells via microwaves!

I'm certain there are an incredible number of flaws in this plan.

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iflp
Anyone know how to access the paper? My university has subscription for PNAS,
but I can't open the pnas.org links found on Google.

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mikestew
_Maybe they could try raisins._

Aww, man, the dude _just_ said that is has to be “watery enough”. Pay
attention to what you’re typing, Wired.

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yellowapple
Maybe a prune? Those are approximately grape sized, and usually more wet than
a raisin.

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recrof
it has to have good amount of potassium and sodium, if that's the case, it
might work.

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mithr
The paper seems like a perfect candidate for the Ig Nobel award
([https://www.improbable.com/ig/](https://www.improbable.com/ig/))

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sprokolopolis
I had just brought this effect up in conversation at a party two nights ago
and suddenly it is all over the internet again. It is nice read about the
physics this time around.

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amelius
I'm wondering how they will update the microwave manuals.

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tomatotomato37
I still believe the microwave oven was invented way ahead of its time. We were
superheating food with intense electromagnetic radiation before we figured out
Velcro.

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pbhjpbhj
I tried the whole grape thing. Very sticky. Very little "plasma" evident. All
I got was the thin bridge of skin acting like a filament in a lamp (briefly).

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arbitrage
If you don't want to wade through five paragraphs of senseless drivel to get
to the point:

Their conclusions: The grape is less like an antenna and more like a trombone,
though for microwaves instead of sound. When you play a trombone, you push
vibrating air into it. The trombone will only sustain vibrations of a
particular wavelength—the musical note you hear—depending on where you’ve
positioned the slide. Only certain wavelengths, known as standing waves, fit
perfectly inside the trombone. As vibrating air of various wavelengths enter
the trombone, the standing waves add constructively, while other wavelengths
cancel each other out. In other words, the trombone amplifies the standing
waves and mutes all others.

The grape, incidentally, is the perfect size for amplifying the microwaves
that your kitchen machine radiates. The appliance pushes microwaves into the
two grape halves, where the waves bounce around and add constructively to
focus the energy to a spot on the skin. Both grape halves happen to focus the
energy to the same tiny point. That intense energy jostles the atoms and
molecules at that spot, heating them up so much that they can no longer hold
onto their electrons, which turns them into a plasma—and boom, fireball.

tl;dr: constructive EM wave interference from the grape seed.

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erd0s
I’m gonna go put some grapes in the microwave, THEN read this article

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api
So can we make fusion with high powered microwaves and grapes? Heh.

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Pxtl
Glad I'm not the only one who was giggling wondering if this could be
applicable to fusion research. Put the deuterium into little standing-wave-
sized pellets?

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metromews
I thought the same. I am really glad of founding your comments for some
reason.

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geggam
in the things i didnt know but now i have to try category...

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monochromatic
I’m not sure I understand the distinction the article tries to draw between an
antenna and a resonating trombone. Anybody have a link to the actual paper?

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rdiddly
I found that annoying, since the resonance of EM waves in an antenna is not
only a perfectly fine analogy to sound waves resonating in the trombone, it's
_literally what 's happening_ in the grape, unless I'm missing something.

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ape4
Coming soon to a Macgyver reboot near you.

