a reply to: Jukiodone
By reading between the lines of the pop science articles I could find, it sounds like they're talking about using a Bose-Einstein condensate
interferometer to perform gravimetry. I managed to find, e.g.,
Cold-atom gravimetry with a Bose-Einstein condensate
They demonstrate the viability of using BEC interferometry for gravimetry. This isn't even the state of the art these days, I've heard people can do
BEC on a chip now.
Now, gravimetry as it's commonly used refers to measuring the gravity of the Earth, specifically its subtle variations from place to place. This is
important because it underscores my main point: you need something about as massive as the entire Earth (~6 * 10^24 kg) to start detecting its
gravitational field in a lab.
The Greenglow people are talking about using gravimetry to circumvent stealth, i.e., detect the gravitational field of an airplane in flight. (I'm
still not sure if they claimed success, or failure, or it was all on the drawing board, or what, since I'm still trying to find info on it)
It's quite easy to do a back of the envelope calculation to see what sort of precision we're talking about here.
Let's say you have a BEC of a couple thousand Rubidium atoms, since that's pretty standard.
For the target, let's say a fully-loaded 747 flying only 100 meters over your head (in real life, I hope you wouldn't need a quantum gravimetric
detector to detect this!). That should make our estimate even more conservative.
The question is, what sort of gravitational force does the 747 exert on your BEC? Well it would be on the order of 10^-31 Newtons.
Of course, the BEC isn't that massive, so maybe the effect of that small force would be big enough to be noticeable. The BEC would accelerate on the
order of 2 * 10^-9 m s^-2. It would accelerate to two nanometers per second after one second of integration time.
To my mind, maybe that's not immediately prohibitive, but it sounds quite small. The question is, how precise are BEC interferometers, typically? The
trick is to translate that acceleration into a phase shift relative to a control beam (that's another can of worms, how do you keep one beam
unaffected), and then figure out the strength of the interference fringes. That's something I'm working on calculating now. Hard to say, but it might
be on the edges of what is possible.
On the other hand, there's a very good reason why gravitational forces between objects in a laboratory have not been measured: gravity is very very
very very very weak, as we see here.
There's a much bigger problem though, in my opinion: gravity is sourced by everything. The signal (a measly 10^-31 Newtons) would be swamped by the
gravity of the changing atmospheric weather patterns, minute variations in the flow of underground rivers, trucks driving past on the highway, the
scientist in the room walking closer or farther away, etc.
If your apparatus is precise enough to detect a 747 at 100 meters, then by necessity it's going to be affected by whether you're wearing your heavier
socks today, or forgot to brush your teeth. It'll be affected by the janitor forgetting to take the trash out of the 3rd floor men's bathroom last
night. It has
to be, otherwise you won't see #!
Anyway this is very interesting, thanks for bringing it up. I'm going to keep digging. My apologies if this is all obvious to you.
9-9-2017 by wirehead because: (no reason given)