
New test of the gravitational 1/r2 law at separations down to 52μm - bookofjoe
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.101101
======
ISL
A couple of us from the Eot-Wash group are active on HN. Happy to answer
questions.

Hopefully the lead author (an HN-lurker, as far as I know) will jump in to
bask in the limelight and answer your questions :).

~~~
mirimir
This seems like the key result:

> We find that any gravitational-strength Yukawa interaction must have λ <
> 38.6 μm. This implies that the dilaton or heavy graviton mass and the radion
> unification mass must be greater than 5.1 meV and 7.1 TeV, respectively, and
> that the largest extra dimension must have a toroidal radius less than 30
> μm. These are the tightest existing lab constraints on “string inspired” new
> gravitational phenomena.

But the key words in it mean ~nothing to me :(

So do they do anything more than constrain parameters?

~~~
ISL
The first sentence above is, for a physicist, the key result. It says that,
for the most common way one could modify gravity to incorporate a new force-
mediator, the new interaction has to turn on at distances shorter than 39
microns.

At distances longer than that, these measurements show that gravity acts, and
in a way that roughly follows the inverse-square law. (at distances of ~100
microns it very-much follows the ISL, and at distances longer than that, it
_really_ follows the ISL).

The easy way to think about this result: we are on the cusp of showing that
gravity acts over distances shorter than the width of a human hair. Only the
finest human hairs are smaller in diameter.

For many years, my goal in life (see HN handle) was to push this limit below
10-microns. Turns out that is really hard to do :).

~~~
mirimir
Thanks.

> ... and at distances longer than [100 microns], it really follows the ISL

But at cosmic distances, there's also evidence that gravity doesn't follow
ISL, right?

Are there any implications of this work for hypothesized dark matter and dark
energy?

~~~
fsh
You are probably thinking of galaxy rotation curves which do not agree with
theoretical predictions that assume only visible matter exists.

Gravity not following the ISL at large scales would indeed be a possible
explanation for this. However, most alternative theories of gravity clash with
other observations, such as the motion of binary pulsars or gravitational
waves, whereas (unmodified) General Relativity is in excellent agreement. This
is one of the reasons why the existence of dark matter is considered to be a
much better explanation for the deviation of the rotation curves.

~~~
mirimir
Yes, I was.

But I know so little physics that the distinction between dark matter and
extra terms in gravitation is unclear.

~~~
NZGumboot
It's pretty simple. You can use the known laws of gravitation and the visible
distribution of matter in a galaxy to predict how fast stars at the edge of
the galaxy should move. But when we do this we find that the stars are moving
much faster than expected. So EITHER the formula for gravity is wrong OR there
is extra matter there which we can't see, i.e. dark matter. Dark matter is
currently the preferred explanation because it also explains a bunch of other,
unrelated, phonenomena, but it has as yet never been directly detected or
measured.

------
cesarb
Ars Technica article about this:
[https://arstechnica.com/science/2020/04/gravitys-inverse-
squ...](https://arstechnica.com/science/2020/04/gravitys-inverse-square-law-
tested-at-scale-of-a-human-hair-and-passes/)

------
magicalhippo
For those of us who don't have APS access, there's the preprint:
[https://arxiv.org/abs/2002.11761](https://arxiv.org/abs/2002.11761)

------
mirimir
[https://sci-hub.si/https://doi.org/10.1103/PhysRevLett.124.1...](https://sci-
hub.si/https://doi.org/10.1103/PhysRevLett.124.101101)

Also FYI: [https://sci-hub.now.sh/](https://sci-hub.now.sh/)

------
tikej
First of all - very interesting work. I am gonna use the opportunity to ask
question to authors directly. Keep in mind that I don’t know a lot about
gravity ;)

As far as I understand speed of gravity interaction is limited by speed of
light. So my understanding is that the 1/r^2 law is only approximately true
since real interactions are retarded. Is that correct? If yes then at what
level (what order of magnitude) those corrections affect your result (I
understand that at such short distances it’s very minor but I suppose it can
be an somehow is estimated)?

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diziet
Relevant to this: Quantized Inertia -
[https://www.youtube.com/watch?v=1itasiXNUPg](https://www.youtube.com/watch?v=1itasiXNUPg)

~~~
imglorp
Very interesting, in that it is falsifiable, it makes testable predictions,
and it appears to fit a number of observations.

So is this just quackery or could there be something there?

~~~
diziet
I believe the author of this theory is working with some groups to run tests.
Let's re-visit in five years?

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jug
I wonder what scales string theories would expect? As in — is this getting
uncomfortably close for string theory to manage or still orders of a magnitude
away?

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ngcc_hk
Is it the new concern not when it is within 1/3 galaxy width but exceed that
the law may change?

~~~
willis936
“New” a hundred years ago, yes. No one has been happy with our model of
gravity since Newton’s time, and people were only happy with it then because
they didn’t know any better.

[https://en.wikipedia.org/wiki/Cosmological_constant](https://en.wikipedia.org/wiki/Cosmological_constant)

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8bitsrule
Sounds like a really clever experimental setup. I hope there's a website that
shows it off.

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z3t4
I have a hard time believing how gravity could be measured at such a small
scale. Is it possible to video record the experiment? Maybe with some layman
explanation.

I would like to make my own gravity experimentation. Is this something I could
do at home?

~~~
bookofjoe
>I would like to make my own gravity experimentation. Is this something I
could do at home?

Best EVER definition of an optimist.

~~~
benibela
Many gravity experiments can be done at home

For example, drop an apple on their head

~~~
bookofjoe
Good point

