An Anti-Gravity Machine
Issue #62
I’d like to tell you about my latest paper with Pablo Saldanha and Chiara Marletto. Yes, as the title suggests, it’s on how to make gravity produce repulsion between two objects!
Our result is surprising for two reasons. First, we are only aware of gravity leading to attraction and there is a fundamental relativistic reason for this (I will explain shortly). Secondly, there is no classical way to make gravity behave in this bizarre way. Our idea relies on the quantum superposition principle, a.k.a. being in two places at the same time.
OK. So why does relativity stipulate that gravity must always be attractive? Well, in order to comply with Einstein’s special relativity, all quantities in physics have to be either 4-component vectors (such as 3 components of space and one time taken together into spacetime) or 4-by-4 matrices or higher by always in multiples of 4. The reason for this weird-looking rule is that the relativistic transformations (which we call Lorentz) can only consistently act on such things.
Let’s take the electromagnetic field. The key thing is that it is described by the four-component vector called (not surprisingly) the electromagnetic vector potential. The component responsible for the repulsion of the like charges (such as two electrons) always comes with the imaginary number “i” equal to the square root of -1. This again is the requirement of relativity and I have written extensively about it in the past (and in my latest book “Portals to A New Reality”). Now when we have two charges, each brings one “i” to the interaction and the product of these two is -1. This is the minus one that leads to the repulsion between two electrons!
In gravity the fundamental quantity is not a vectorial one, but a 4-by-4 matrix. Mathematically speaking this is two electromagnetisms put together. So now we have double the number of imaginary “i”s that each gravitating object brings to the table when interacting with another gravitating objects. So with two objects, this is (-1) times (-1) = 1. It is for this reason that the gravitational forces are always attractive. In other words, if gravity were not attractive, this would violate Einstein’s special relativity. So how did Pablo, Chiara and I manage to pull this off?
Enter quantum superpositions. Imagine that one of the two gravitating masses (the “source”) is in a superposition of states while the other one (the “probe”) is localised. In the branch of the superposition where the source is closer to the probe, the gravitational attraction is stronger, while in the branch in which the source is further away from the probe, the gravitational attraction is weaker. Ok, but in both branches the force of gravity is attractive, so how do we make this into repulsion?
There are three parts of every quantum experiment. Prepare a superposition, let it evolve in time, and finally measure in any other superposition. It is this last part that gives us the repulsion. But, it only does so for one of the outcomes of the final measurement. So, what is important is the post-selection: we only have anti-gravity conditionally on observing the right outcome. On average, if both outcomes are included, gravity is always attractive and this is why there is no violation of relativity.
“Oh, well, anything can happen if we post-select”, I hear you say. Yes and no. Indeed, throwing away “bad” outcomes (like the ones where the particles attract) leads us to observe what we want (i.e., repulsion), however, classically, this is not possible, no matter how much we post-select. Our experiment, if confirmed experimentally, would therefore show that gravity can act from two different places at the same time. In other words, gravity is quantum.
How difficult is this experiment to perform? Pretty difficult. It is in the ballpark of other experiments I’ve covered in my blogs (like the B-M-V one). But, luckily, my Italian buddies Marco Genovese, Fabrizio Piacentini and Ettore Bernardi are wizards in the lab and my bet is on them to do it by 2030. So watch this space!
Take care of yourselves,
Vlatko
https://substack.com/@hawkeyespeaks/note/c-216739183?r=2p93cq
Fascinating read, but calling this "anti-gravity" feels sensationalist. Especially given that the framing in terms of a "repulsive gravitational force" leans heavily on the highly controversial physical interpretation of weak values.
As the paper itself acknowledges, gravity isn't actually pushing the probe away; the force remains strictly attractive in both branches of the superposition. The "repulsion" is essentially an illusion created by destructive interference and post-selection. By mathematically throwing away the vast majority of the normal, attractive outcomes, you are left with a skewed probability distribution where the average momentum of the remaining particles has shifted into the negative.
It is an interesting mathematical trick for probing quantum interference and entanglement, but characterizing this statistical filtering as actual gravitational repulsion—let alone as creating an "anti-gravity machine"—seems highly misleading, if not disingenuous.