What is the lightest thing on earth ?

I am curious .

Thing? Do you mean density?

A vacuum? Or is that cheating?

A Solid? Probably an aerogel.

A photon?

Depending upon your definition of lightest, Hydrogen, Helium, or Aerogel must be in contention.

Bah, an electron has less mass then that fatty.

A Norwegian gal in the springtime?


Photons don’t have any mass.

Photon’s don’t have any rest mass. They have mass depending on their wavelength.

Good answers so far. Of solid substances, an aerogel has the lowest density (mainly because it’s got a lot of space in it), but if you’re looking for the lightest solid at room temperature and pressure without that kind of special structure, the answer is Lithium Hydride, LiH.
I’ve worked with this stuff. If you make it without any special care, it’s a white, gray, or black powder (depending on how much dirt is in it), but if you carefully grow a crystal of it in controlled conditions, it’s a beautiful clear light blue crystal.

It reacts rapidly with air to release hydrogen, but, unlike lithium (or the other alkali metals), the reaction in this case isn’t exothermic, so it doesn’t generate heat to ignite that hydrogen. The Navy used to use it (still does, maybe) as a source of 'portable hydrogen" for lifting emergency balloons and the like.

Because it reacts so readily, you want to store it under oil. But it’s lighter than any oil you’d want to keep it in (or just about any liquid), so it floats. I had to make a special weighted “bell” to keep my sample under the oil’s surface.

Thanks for the replies .

This was the oral exam question , I was asked in 4th semester civil engineering.I answered “Helium” .

It was a long time ago . Just rememberd this after seeing a post in another thread .

If you generated aerogel in a pure hydrogen environment, would it be less dense than standard aerogel? or is it sufficiently porous that air would quickly replace the hydrogen?

I just realized that I should have said “moist air” in my response – theoretically, LiH will keep forever in dry air – it’s the water that attacks the substance. But it’s pretty reactive, so just about any air outside a hot Utah or Arizona summer day will be “moist”.

Photons, protons, they’re all the same…

If we don’t limit ourselves to solids and exclude subatomic particles, isn’t hydrogen going to be lighter than helium?

If you’'re including subatomic particles in what you mean by “lightest”, then the photon wins hands down, with a rest mass of zero. Of particles with rest mass, probably one of the neutrinos (does the hypothetical rest mass of the three neutrinos differ?) if they have rest mass (otherwise they’re tied with the photon), or else the electron, the lightest stable particle.

The problem with these is that they’re not “on” Earth in the sense a rock, an elephant, or Lake Superior are; they naturally exceed escape velocity as an integral part of their physical characteristics. They’re “on Earth” for measurable periods of time only if bound into something else or if their natural trajectories retain them here for a measurable period (like bouncing a photon between two mirrors).

Of naturally occurring substances, the answer would be hydrogen gas. In addition to accumulating in natural catchments beneath non-porous caprock, there is a portion of the exosphere that is nearly pure hydrogen. The same, of course, is true for helium, but hydrogen is significantly lighter.

I’d be interested in knowing what the lightest naturally occurring solid might be. (For ob vious reasons, lithium hydride doesn’t occur naturally.)

Reticulite is probably right up there.

Almost nothing is enough: reticulite helps measure depth of Kilauea’s caldera 500 years ago

Different gases can be permeated into or out of the aerogel, though it might be pretty slow. Better than hydrogen is vacuum. A vacuum-baked aerogel takes a while to refill itself when returned to the air, and meanwhile it can be light enough to float up to the ceiling like a balloon.

If you’re looking for naturally occurring non-spongiform solids, I’ll bet Lithium Hydroxide is the least dense. It’s what you get when you react lithium hydride with water. According to this site it does occur naturally:

Its density in the anhydrous state is 1.46 g/cm[sup]3[/sup], but the monohydrate is probably what you’d find in nature. That’s 1.53 g/cm[sup]3[/sup].
Lithium Fluoride, which occurs naturally and is a really stable crystal* has a density of 2.635 g/cm[sup]3[/sup].

*Despite its similarity to sodium chloride, and the listed values for solubility, I’ve worked with this stuff a lot. It doesn’t dissolve readily, and it takes a great polish.

A photon is definitely the lightest thing on earth. Anything else is much darker.

Quoth sweeteviljesus:

In modern usage, “mass” is synonymous with “rest mass”. The concept of mass increasing with velocity, or photons having a mass dependent on their frequency, is an awkward kludge that introduces more problems than it solves.

Quoth Polycarp:

Both the photon and the graviton are believed to have zero mass, but we can’t prove it for either, just find ridiculously tiny upper bounds for the mass. The upper bound for the graviton mass is about a million times tighter than that for the photon, though, so if anything wins hands-down, I’d say it’s the graviton.

As for the neutrino masses, that’s a more complicated question than you realize. If by “the three neutrinos”, you mean electron neutrino, mu neutrino, and tau neutrino, they don’t even have well-defined masses at all. They can be regarded as not being the fundamental particles at all, but rather as three different superpositions of states called nu[sub]1[/sub], nu[sub]2[/sub], and nu[sub]3[/sub], which do have well-defined masses. Those masses do differ from each other, and in fact it’s from measurements of the differences that we’re able to tell that neutrinos have mass in the first place (we can’t measure the masses directly). It’s unknown how massive the lightest neutrino is (it could be arbitrarily light, and still be consistent with observations), but a reasonable guess would be something in the vicinity of 1 eV.

In any event, all three varieties of neutrino are definitely lighter than the electron, and neutrinos are stable, so the electron doesn’t have any claim here.

Quoth CalMeacham:

I must be missing something here, because ice, at around .9 g/cc, is naturally-occurring, non-spongiform, and lighter than any of those.