What If Gravity Isn’t Quantum? New Experiments Explore

Sign Up on Patreon to get access to the Space Time Discord!

If we discover how to connect quantum mechanics with general relativity we’ll pretty much win physics. There are multiple theories that claim to do this, but it’s notoriously difficult to test them. They seem to require absurd experiments like a particle collider the size of a galaxy. Or we could try to physics smarter, instead of physicsing harder. Let’s talk about some ideas for quantum gravity experiments that can be done on a non-galaxy-sized lab bench, and in some cases already have been done.

Check out the Space Time Merch Store

Sign up for the mailing list to get episode notifications and hear special announcements!

Hosted by Matt O'Dowd
Written by Christopher Pollack, Lorenzo Braccini, & Matt O'Dowd
Post Production by Leonardo Scholzer, Yago Ballarini & Stephanie Faria
Directed by Andrew Kornhaber
Associate Producer: Bahar Gholipour
Executive Producers: Eric Brown & Andrew Kornhaber
Executive in Charge for PBS: Maribel Lopez
Director of Programming for PBS: Gabrielle Ewing
Assistant Director of Programming for PBS: John Campbell

Spacetime is a production of Kornhaber Brown for PBS Digital Studios.
This program is produced by Kornhaber Brown, which is solely responsible for its content.
© 2024 PBS. All rights reserved.

Space Time Was Made Possible In Part By:

Big Bang
​​Wojciech Szymski
First Principles Foundation
Bryce Fort
Peter Barrett
Alexander Tamas
Morgan Hough
Juan Benet
Vinnie Falco
Mark Rosenthal

Supernova
Grace Biaelcki
Glenn Sugden
Ethan Cohen
Stephen Wilcox
The Tyacke boys
Amy Hickman
Mark Heising

Hypernova
Richard Pavlicek
Dean Galvin
Michael Tidwell
Robert DeChellis
Stephen Spidle
Chris Webb
Justin Lloyd
David Giltinan
Kenneth See
Gregory Forfa
Alex Kern
drollere
Scott Gorlick
Paul Stehr-Green
Ben Delo
Scott Gray
Антон Кочков
Robert Ilardi
John R. Slavik
Mathew
Donal Botkin
Edmund Fokschaner
chuck zegar
Daniel Muzquiz

Gamma Ray Burst
John De Witt
Kevin Vallejo
Nathaniel Bennett
Sandhya Devi
Michael Oulvey
Robin Sur
Arko Provo Mukherjee
Mike Purvis
Christopher Wade
Anthony Crossland
Grace Seraph
Stephen Saslow
Tomaz Lovsin
Anthony Leon
Lori Ferris
Koen Wilde
Nicolas Katsantonis
Richard Steenbergen
Joe Pavlovic
Chuck Lukaszewski
Cole B Combs
Andrea Galvagni
Jerry Thomas
Nikhil Sharma
John Anderson
Bradley Ulis
Craig Falls
Kane Holbrook
Ross Story
Harsh Khandhadia
Matt Quinn
Michael Lev
Rad Antonov
Terje Vold
James Trimmier
Jeremy Soller
Paul Wood
Kent Durham
jim bartosh
John H. Austin, Jr.
Faraz Khan
Almog Cohen
Daniel Jennings
Jeremy Reed
David Johnston
Michael Barton
Isaac Suttell
Bleys Goodson
Mark Delagasse
Mark Daniel Cohen
Shane Calimlim
Tybie Fitzhugh
Eric Kiebler
Craig Stonaha
Frederic Simon
Tonyface
Jim Hudson
Alex Gan
John Funai
Adrien Molyneux
Bradley Jenkins
Vlad Shipulin
Justin Waters
Thomas Dougherty
Dan Warren
Joseph Salomone
Julien Dubois

[BrianMelancon]

Congratulations on PI million subscribers

[oniondip_]

Have we ever considered if gravity is Baroque instead of Classical?

[Valdagast]

Justice for Alice and Bob! They deserve a Nobel Prize!

[John-jcty]

"We just have to figure out how to adjust Newton's law of universal gravity with the weird but small behavior of mercury and physics is done"
"We just have to figure out how to connect gravity with quantum and physics is done"

[dyershov]

Thank you for an episode on experimental physics! The extreme engineering that has to go into these kind of experiments is absolute fun.

[Eianex]

wouldn't the atoms entangle via electromagnetism if you put two stern-gerlacs near one another? how can you make two that two stern-gerlacs avoid electromagnetism and only interact via gravity?

[chrismuratore]

Couldn't help but hear a beat drop once Matt said, so rhythmically, "spin fully up, or spin fully down.".

[paulbennett]

I'm constantly amazed that you can make understandable concepts which I shouldn't be able to understand.

[SteveSiegelin]

Another weird thought that comes to mind, and I know I've had some outlandish thoughts here, what if we're misinterpreting quantum entanglement and it's actually dimensional entanglement. This could explain why they can show such a instantaneous reaction mathematically. That also makes me think with my last comments, what if we're misinterpreting the true state of the fabric of space and it really needs to be looked at dimensionally. To reiterate with the end of my last comment if space is a fabric it has to have a certain tautness. That tension has to be the same throughout the whole universe for masses to show different gravitational forces on an equal and measurable scale. This means that I don't think we'll ever truly understand gravity until we understand what that underlying fabric and the forces acting on it really consist of.

[Gizzeit]

Yesterday I've aquired strong evidence of *macroscopic objects being in superposition.*

I was maintaining my bicycle and after disassembling rear wheel bearings I've extracted 9 bearing balls from one side and 8 balls from the other. What's the heck, I thought, it should be equal, so I searched everywhere around to find that shiny metal ball of almost size of a pea - and it was nowhere. Well, I carefully inspected the casings, and couldn't really tell if they are equal in diameter or slightly different, as they have a bit unsimilar design, and I hadn't got any appropriate measuring tools to get casings' inside.
Well, I thought, maybe they are inequal, as one I've extracted less ball from is indeed located it more tight spot, thus it theoretically could be narrower.
Long story short, I wasn't sure if it should be 9-9 or 9-8 balls, and had no way to properly _measure_ needed quantity, so the missing-or-not ball stayed in this superposition until I performed all required maintenance, applied new grease to the bearing and started filling casing with balls.

And after I've put 8 balls in there were surely a gap for the 9th one, and *immediately wavefunction have collapsed* and I saw the freaking 9th ball right in front of me, completely exposed and not covered by anything happily laying and freaking shining at lights!

The only way I can explain why I haven't seen it before is superposition. No other way, really 😂😂😂

[elishmuel]

Hi PBS Space Time, love your videos. I did prefer the ones with more equations and space-time diagrams instead of the ones with cool pictures.

[ruanholtzhausen]

What if Gravity is emergent, like temperature.

[AMx]

Win physics? I feel like finally uniting the two will just open new questions and problems to solve.

[spectator]

I gave out an audible disappointed "Aahh no way" when hearing "That would mean spacetime itself is in a superposition between states".

That's a way more complicated reality than I even want to consider! 😂

[rachel_rexxx]

Woah, that QGEM experiment is pretty ingenious! I'm sure you'll let us know when it is carried out and peer-reviewed, whether it is successful or not.

[lrwerewolf]

Woot! Dr. Eyebrain's brother made a showing around 9:30!

[miklosprisznyak]

I find Rovelli's relational quantum mechanics the most convincing interpretation. It doesn't require "wave function collapse". Read his popular book "Helgoland".

[trixinreno]

Im finishing up a BS in Biochemistry soon and I understand about 85% of your content, you make information very digestible and easy. Keep being amazing!

[nblhuntr]

The classical gravity collapsing the wave function seems to make gravity into the hypothetical observer. That deeply appeals to my lay understanding of quantum physics

[modernwarrior-bfut]

what about that new paper which says we need to cool down a ton of metal to near absolute zero and then it can act as a single particle and using that we might be able to detect graviton