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Why a Grape Turns into a Fireball in the Microwave (wired.com)
307 points by ChuckMcM 64 days ago | hide | past | web | favorite | 85 comments

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.


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"?

Let's just call it raisinance.

It's a grape MASER. ;-)

It wasn't exactly a huge mystery what was happening there, but it's always nice when someone makes the longform explanation.

A resonance cascade?

I would hesitate to speculate. Only Dr. Freeman has the first hand experience to weigh-in definitively on complex quantum events like a resonance cascade.

Veritasium video also from today: https://www.youtube.com/watch?v=wCrtk-pyP0I

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

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.

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.

> 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. ;)

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...

Solid glass is transparent to microwaves, but molten glass absorbs them pretty well. So, if you can get a piece of glass hot enough, the microwaves will be able to keep it hot.

Incandescent bulbs are vacuumed inside (that's why they make a popping sound when they shatter), so it was probably the plasma melting the glass.

What in the world... that's awesome.

I know what I'm doing tomorrow with our old microwave.

If you have any extra CDs around, those are entertaining (though I'd do it outside for the smoke).

Ha! Talk about your theory to practice experiments. The grape becomes a plasma, and the glass contains it - brilliant!!!

That does look like descriptions of ball lightning. I wonder if the mechanism is similar.

Hm... confined plasma... could this be used to start a fusion process?

"After decades of research and hundreds of billions of dollars spent, a breakthrough in fusion reactors is made thanks to a youtube video of a grape in a microwave."

Haha, why not? ;) At least someone more knowledgable than me could explain why this is so obviously a silly idea.

Not a physicist, I'll take a crack at it. Fusion needs two things: 1. plasma (easy part) 2. pressure (hard part) - this is normally referred to as confinement, which is misleading. What you're trying to do is squeeze the plasma into a really small volume, which is like squeezing water in your hands into a perfect compressed sphere. Try it.

Stars do it easily, because the confinement / pressure is provided by immense gravity which very conveniently works to compress a huge load of gas into a point space.

Is this bad for my microwave?

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

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!

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!

Watched some videos. Ivory soap was interesting. Steel wool was predictable, but cool.

Boiling an egg can be fun. As a teen using a friend's (probably cheap) microwave oven it blew the door open.

5) Coke bottle with a little water in the bottom and the lid screwed on tight. It may blow the door off... I'm not saying that we blew the door off the microwave at school but we were lucky no one died :D

I tried it, except for some burning smell microwave seems fine. The smell dissipates soon. It seems to work better if the turntable is not spinning.

For some reason I was not able to reproduce the effect with drinking glass, linked above, though.

Make sure there is a sane amount of grapes inside the microwave so it's not overloaded. It has problems with too little food.

If you don't want to use many grapes, put a water glass in it.

Some byproducts of the plasmagrape production are toxic though (ozone). Make sure that the vicinity of the microwave is well ventilated.

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

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).

It stays where it is, held tightly in my lap, because I know a loose balloon in a vehicle is a hazard.

I am sorry you were downvoted for that. Let me just go on record to say I got a smile out of your comment.

Nah. No big deal. Karma is irrelevant. I make a new account every few months anyways to avoid doxxing. I just say what's on my mind and people can appreciate it or not.

You've been doxxed on HN!?

No, and I'd like to keep it that way.

Slamming on the gas at 60mph probably will freak out everyone in the car, but not because of the balloon :) I'm assuming you meant slam on the brake, in which case the answer below is correct.

Actually, slamming on the break at 60mph will probably freak everyone out too, due to the jolt, albeit less so than slamming on the gas.

> albeit less so than slamming on the gas

I dunno, my turbo miata will light up the rear tires at 60mph if you drop a gear and stomp on the throttle

Cool story bro

Is the balloon filled with helium gas or air?


solution, for those busy or lazy: https://www.youtube.com/watch?v=y8mzDvpKzfY

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

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.

Tragically I seem to have read “slam” and my brain filled in “brakes”. I only recognized my error when you commented.

Not my proudest moment.

I made the same mistake! Truly suprised to go back and see that it said 'gas', would have sworn it said brakes.

It’s the brain’s autocorrect in action. At 60mph slamming the brakes makes more sense than the gas. I presume GP actually meant brakes.

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?

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.

SmarterEveryDay does a video on this — nothing like experimental data.


In either braking or accelerating, is there a chance the balloon gets caught in an eddy and spins around a bit as the air pressure redistributes itself and goes around seats/heads/etc, or would the eddys be negligible to the balloon's mass?

It's been established that it moves in the direction of acceleration, but an addendum I thought of is that the balloon's speed will at first keep getting faster as the gradient is built up as the air is squished to one side, then slow down as the air pressure reaches equilibrium again.

Lol. I did, like 20 years ago, along with CD's metal-side up and a lit match under a large container. It's nothing particularly new or original.

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.

Possibly more potassium in the tomato? Potassium having a purple flame and plama. 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.

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?

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.

Yes, that is kind of non-sensical as written. I'm going to guess that by "antenna" they are thinking of "resonant dipole" and by referring to a trombone, they are saying the grape is more of a resonant cavity. But then again, a trombone is not a resonant cavity in the RF sense -- it resonates as a tube closed at one end.

You can find more information under the term "dielectric resonator." Searching "dielectric resonator modes" yields a high concentration of promising figures.

"Ceramic antenna" is a term that will yield information on the applications of this idea to antennas.

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?


> 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.

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.

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.

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

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

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

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.

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.

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

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).

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.

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

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

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?

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

Someone has written a book about ball lightings as an energy source. Maybe grapes will do the same thing...


Grow some grapes exclusively with heavy water and then try it. See if you can make some grape neutrons.

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

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