If you're at a party, talking to someone, and the conversation is lagging, consider saying the title of this blog post: Space may be created by the interaction of individual quanta of gravity.
One of two things could happen, the first being most likely. Either the person will look at you like you're crazy and make an excuse to leave your company, or they'll reply, "That's so interesting. Tell me more."
Having read physicist Carlo Rovelli's book, "Reality is Not What It Seems: The Journey to Quantum Gravity," I could talk some more about quantum gravity, but only a little bit. It's an esoteric subject, not to mention it isn't scientifically proven, just conjectured for some good reasons.
One reason is that something seems to be more fundamental than the two pillars of modern physics, relativity theory and quantum mechanics. Rovelli writes:
A university student attending lectures on general relativity in the morning, and others on quantum mechanics in the afternoon, might be forgiven for concluding that his professors are fools, or they haven't talked to each other for at least a century.
In the morning, the world is a curved spacetime where everything is continuous; in the afternoon, the world is a flat one where discrete quanta of energy leap and interact.
The paradox resides in the fact that both theories work remarkably well. With every experiment and every test, nature continues to say "you are right" to general relativity, and continues to say "you are right" to quantum mechanics as well, despite the seemingly opposite assumptions on which the two theories are founded.
It is clear that something still eludes us.
That something, in this way-cool chart Rovelli shares in a What Is The World Made Of? section, is covariant quantum fields -- the one ingredient he and other fans of quantum gravity believe constitutes reality.
In his book, Rovelli fills out this chart line by line, starting with Newton and ending with quantum gravity. Along the way, he discusses how science keeps refining that core question, what is the world made of?
The Big Idea of quantum gravity is that, as shown in the chart, unlike other quantum fields like electromagnetism that exist in spacetime, the covariant quantum field of quantum gravity is spacetime. Here Rovelli explains what "covariant" means.
Fields that live on themselves without the need of a spacetime to serve as a substratum, as a support, and which are capable by themselves of generating spacetime are called "covariant quantum fields."
The substance of which the world is made has been radically simplified in recent years. The world, particles, light, energy, space, and time -- all of this is nothing but the manifestation of a single type of entity: covariant quantum fields.
Don't worry about the mention of "nodes of the graph" in the passages below. Basically it just means a web of relations.
The crucial difference between photons, the quanta of the electromagnetic field, and the nodes of the graph, the "quanta of gravity," is that photons exist in space, whereas the quanta of gravity constitute space itself.
Photons are characterized by "where they are." Quanta of space have no place to be in, because they are themselves that place.
...Physical space is the fabric resulting from the ceaseless swarming of this web of relations. The lines themselves are nowhere -- they are not in a place but rather create places through their interactions. Space is created by the interaction of individual quanta of gravity.
Another cool thing about quantum gravity is that it does away with infinity. Or at least, certain types of infinity.
When we take quantum gravity into account, the infinite compression of the universe into a single infinitely small point, predicted by general relativity at the Big Bang, disappears. Quantum gravity is the discovery that no infinitely small point exists.
There is a lower limit to divisibility of space. The universe cannot be smaller than the Planck scale, because nothing exists that is smaller than the Planck scale.
CTG is one of a handfull of current theories developed to help explain astrophysical findings, such as gravitational lensing, where the same astronomical bodies can be seen in duplicate, and other anomalous observations of light deflection around very large bodies in space.
These theories are attempts to model with mathematical precision observed results post facto. And they all center on postulating what exactly, mathematically, can explain these observations that cannot be explained by Newtonian, classic Relativity and Quantum physics.
Isn't it strange that science continues to prove human theory is limited in its accuracy and reality is actually much more layered, mysterious and in many cases, invisible?
As with the above, the newer refinements in theory are efforts to explain newer astronomical findings.
The short course is that the invisible and undetectable gravitational connections between immense objects in space, separated by hundreds of millions of miles, that holds these objects in their orbits, is unknown. But we conjecture their existence on the basis of a few measurable effects on light and matter.
Somehow the measurements of empty space, time, matter and light are connected. The data proves it. But whatever is connecting all these things is invisible to detection. And the actual mechanisms causing these connections unknown.
The very forces that pace time, connect all matter and pull on entire galexies through inconceivable distances of empty space is unknown. But the systematic influence on all these is measurable and consistent.
That scientists attempt to take limited but astounding observations they consistently detect and, rather than ignore or dismiss them, find some explanation, even though that explanation requires a mountain of mathematics and the imaginary conjectures of several things, including new forces and particles, and new relationships between these, is admirable.
They went there. They could have chosen not to. They could have claimed nothing is there and never even chose to invest in measurement. They could have said there is no proof of any of these explanations for the odd things we observed and on that basis never chosen to look and investigate. Other than that they invented a few competing explanations with imaginary stuff they dreamed up to fit the data. Welcome to Astrophysics.
But rather than say one imaginary explanation is as good as another, and therefore it's all imaginary, and entirely fabricated, they said "there is an explanation. A real one. And we are going to find it."
They could have said, we are inventing anything and everything post facto to explain the data we now have. And whenever new results come in we invent new explanations. Or that mathematically we've been able to invent a more but not entirely consistent explanation.
But it is the fact of the existence of these anomalous observations, accepted only because of the immense investment in measurement apparatus that gathered up this otherwise unknown information, that has become the basis of so much effort, thought, progress, and in time what further science has shown requires further refinement. And further invention.
Observations are never dismissed out of hand by scientists, especially when they are replicated. The hard scientific data we do have is the result of hard work based on a single belief that there is a real dynamic under the initial anecdotal findings. A real dynamic in what is entirely invisible.
Explanations abound, but none are accepted as truth nor dismissed without investigation, discovery, analysis and hypothesis testing. Certainly no one of a scientific mind settles for dismissing any evidence.
Why should that rigor apply only to the anomalous behavior of the stars and not the epic events we dismiss or ignore all the time within ourselves? Or worse still, each other?
Posted by: Spence Tepper | June 03, 2022 at 03:20 AM
Thanks for this review. Made for very interesting reading.
At a cursory level, and minus the math of course, I was aware of all of this. Except for the last, this hypothesis that gravitons might "generate" spacetime. That seems to be the exact obverse of what SR and GR postulate, which is that gravity (which Newtonian mechanics incorrectly, or at least limited-ly, views as a force) is a property of space-time (those pics we were taught in elementary physics courses, about a piece of paper or a bedsheet bent in, crimpled in, all that). My understanding was that QM does not see gravity as SR/GR do, but instead as caused by quanta called gravitons that exist within spacetime, much like light and photons. Here we have the apparently radical innovation that gravitons somehow generate spacetime.
Not sure what the implications of such might be, I know too little of either QM or SR/GR to comment meaningfully on that myself. But I would imagine that this, if true, might have direct consequences on the fundamental math of SR/GR. Which leads me to ask: Any idea if this hypothesis, that gravitons generate spacetime, is limited to the physical scale of QM, that is to say the tiny end of the spectrum (as is the case with much of QM), as opposed to the macro end that cosmology as well as relativity operate at?
(I realize that's a very fundamental question, easy to ask but very difficult to answer, unless one's actually into QM. So it's fine if you don't know. I've bookmarked a google search page on "covariant quantum fields" that I intend to follow up a bit on at leisure later on. [ Well, hopefully, because, after all, intentions, intentions! My huge bookmark list is testimony to my generally lofty intentions, and the number of those bookmarks that stay unexplored testifies to how weak is (my) flesh! :--) ] Just, if it's actually mentioned somewhere later on in that book, and if you happen to come across it while reading, and without going to any additional trouble over this: then perhaps you could clear that up.)
Posted by: Appreciative Reader | June 04, 2022 at 09:24 PM