193 points | by elsewhen14 小时前
> The discussion recalls a messy, largely forgotten episode from the dawn of the quantum era. In 1905, Einstein interpreted experimental data to mean that light is “quantized,” coming in discrete particles now called photons. Others, including Niels Bohr and Max Planck, thought that the classical, wave nature of light might still be saved. [...] Most physicists presume that everything in the world is quantized, including gravity. But proving that assumption will entail a new war, one that has only just begun.
1) No, that is not a "messy, largely forgotten episode", rather, it is frequently re-told and almost a required Inshallah of every piece on quantum physics.
2) Please spare me that "war" simile, it only shows you're an American who can not write too well. War on drugs. War on poverty. War on whatever. The Browser Wars. Dude get a grip. Don't always "killer feature", "shot him dead", "waged war on the germs in her refrigerator". We have to fight a "war" to find out whether spacetime is quantum? Rilly??
It’s understandable that you don’t like the desensitization of war that comes from our over usage of the word. Perhaps it speaks to a defect in American culture but this is how we communicate in our language. I think Arabic has too much emphasis on allah related phrases. But that’s how they speak. Nothing I can do about it. I don’t think said usage implies anything about their writing abilities.
It's not a simile, is it? They aren't saying it's like a war.
My dictionary lists a bunch of definitions for war that fit this usage.
+1, normalizing "war" as a synonym of effort is pretty much orwellian.
If this was a war, the UN headquarters has been a war-zone continuously since it started, as has every parliament, congress, and senate.
Can someone who understands this please explain it to me, thanks!
String theory and loop quantum gravity fit into this picture by trying to replace the integral over something we can't handle with an integral that matches it at large scales, but turns into something more tractable at small scales. Maybe the fact that we still can't make sense of the integral is Nature's way of telling us that she does not do the integral either...
I disagree with that part. For the strong force we have the "gluons" and they are considered particles and they have a strong self-interaction. The strong self interaction makes it a huge mess and a lot of things that involve gluons are impossible to calculate.
It's more like:
fake quote> Let's pretend for 30 minutes that the strong field don't self-interact, so we have this nice particles call gluons. Now we add this interaction to the Lagrangian to make gluons interact with other gluons, and now we have a problem.
I agree that that when gravity is small enough, then gravitons give an easy to calculate aproximation. IIRC at high enough energies calculations with gluons get not impossible to calculate too.
Can you please elaborate, the first part of the sentence says graviton is for non-self-interacting gravity, the second part of the sentence says graviton is for self-interacting (if 'its' in 'its self-interaction' refers to the graviton).
I don't intend to nitpick the sentence, just trying to understand the theory and I don't even know if particle means self interaction or the opposite and can't parse it here either...
If the answer is graviton is for non-self-interacting: what is the model for the other case (where gravity does self interact) and what would cause that self interaction if not the graviton?
The point is, we know gravitation does self-interact. But our best model, the graviton, doesn't model self-interaction. So the model is probably accurate in regimes where you'd expect little self-interaction anyways.
That said, an easier question to ponder (and many have tried) might be:
Do photons self-interact? With each other? In free space?
>The model of non-self-interacting gravity is a particle we call a "graviton,"
This needs to be emphasized even more, because it has
>when the dx represents a slight change in a function
*see the discussion around sharikous’ comment below
Physicists love to be wrong! If there is an experiment that disagree with the current theory, then is like the will west and everyone can publish their own pet theory that "fix" it. It's like raining free paper for them, their graduate students and everyone. Also, it's fun!
When experiments and theory agree, they have to use imagination to get a new "interesting" tweak that can be published. In some case the the tweak may be interesting, but most of the times it's not.
I remember a talk about a 2-sigma "particle". There was a small disagreement in some experiment, so someone did a thesis about a possible fix adding a new particle. A lot of hard work and hard calculations. It was a nice talk, and someone asked what what happened then. The sad new was that later the 2-sigma disappeared, it was only a fluke :( . This kind of work is important, but it's more boring that looking for new particles.
The problem is that nobody has successfully combined these two views into a single unified theory, known as "quantum gravity". General Relativity and quantum mechanics don't naturally fit together, and that's why we don't yet fully understand gravity in a way that reconciles both the spacetime curvature and graviton perspectives.
Classically, it is. But most physicists believe that there is a quantum theory of gravity that underlies the classical theory, and that that quantum theory will include, at some level of description, a spin-2 gauge boson that mediates the quantum gravitational interaction, called the "graviton". Our classical theory of gravity, General Relativity, would then be the classical limit of that quantum theory, just as classical Maxwell electrodynamics is the classical limit of quantum electrodynamics.
Of course, our idea of how to reconcile quantum gravity with general relativity is much less developed than our understanding of electromagnetism and the nuclear forces.
At first I thought this was a great pun. But then this is perhaps also the reason the word is actually "matters"? Where "matters" is what means something? What matters is what has an observable effect?
In fact the outstanding success of the Standard Model has posed its own problems - the lack of deviations from it makes it hard for experiments to point in a useful direction for better theories to be developed along.
https://home.cern/news/news/physics/incredible-lightness-hig...
We have anomalies (deviations from standard model) in many measurements done by several experiments. This is a good summary [1] from them up until now (sorry for the pay-walled)
Not just some dumbed down Discovery show - it pushes the limits of what a layperson can understand.
The "geometry" comes from the fact that the way we measure distances (or, well, experience time) uses the metric tensor field to do it. But it is still ultimately just a value attached to every point like any other field.
So if you take the gravity curves spacetime view, then gravity is not a force and all that. But if you take the alternative view then gravity is a force. Now, I'll leave what the distortions are that gravity produces in flat spacetime for another time, or for the reader. But I'll say this: this view is both controversial (perhaps replies will show this) and not (see above -and many other- animations).
I follow a few educators/communicators in this field and I have a feeling they're using this "gravity isn't really a force" to bridge the gap between their deep understanding and us mortals that don't poses the language / understanding to get the entire meaning behind it. Is that feeling correct or am I missing something?
The core idea is that when you move a mass, its contribution to the spacetime geometry changes, but the effects of the change of the geometry doesn't apply instantaneously to all the universe but instead the change propagates at the speed of light.
So that explains why any sudden movement of a mass creates a "crest" that moves through space at the speed of light.
Furthermore, the sources of fast movement of extremely heavy mass just happen to involve an object that wiggles back and forth in a periodic way because those events involve heavy objects orbiting other heavy objects.
That's the reason we can measure a wave with multiple crests and we can talk about a wave length of the gravitational waves: the wave length of the gravitational waves matches the period of the orbit of the heavy mass.
To that extent you can build 3 fundamental forces, electro magnetic, weak (that are called together electroweak) and the strong force. You have an extra force carrier through the Higgs that allows you to give mass to everyone.
Now you need to consider gravity because you know that gravity exist and since everything under the sub is quantised, well so should gravity.
The main issue with gravity is that it is interpreted so far as a curvature of space time, it's mainly fine for big items, but the implications for quantum field theory is that you should modify the small integral element that you use (space shouldn't have the same size) except that you look locally at space that is mainly flat... And changing the integral does not lead to well behaved behaviours.
You can start to introduce new fields but doing so also causes an issue...
Funnily enough even in the standard model something is missing, everything mostly fits, but that's the trick, mostly, neutrinos have mass and this in itself is a problem because the Higgs mechanism doesn't provide mass to them ...
Long story short, people take shortcut when explaining the messy gritty part of it, which is "fine" but not really, and from a simple standpoint one would like to have a simple field from which gravity is born, which might be but so far, to my simpleton understanding, this hasn't been too successful, unless some form of string theory is realised. But the pre requisite for this is a form of supersymmetric theory existing which is currently disfavored, but could exist in the unproved energy scales from here to the plank energy scale.
Sorry this ended being a tad long and I'm not sure this is clarifying things.
It does not have to be either or. It can be both. Both models can be useful to understand the nature of gravity and make predictions about natural phenomenon.
The disconnect seems to be unresolvable (I don't understand this part at all) and so efforts are being made to quantise gravity and incorporate it into the standard model.
There is not. Or maybe there is not. At least so far there is not. We have never observed one. "Graviton" particle is just a hypothesis. Outside of some people theorizing that graviton could exist, there is no observations that it exists.
"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity"
That is true. Classically, gravity is a fictitious force, merely a result of inertia from moving in a curved space-time.
> But how can that be true if there is a force carrying "gravity" particle? Or is the word 'force' being used loosely here?
Because we _suspect_ that the classical view is not correct. And there's a quantum description that may or may not involve curved space-time.
It's not impossible that the spacetime curvature is a mathematical artifact of a deeper theory. Merely a kinematic explanation, just like epicycles.
It's also possible that the space-time _is_ really curved, and gravitons simply cause the curvature by somehow coupling with it. And then other matter experiences this, in the manner described above.
So you have a thing, that gets interpolated updated with various functions, that overlap, and those functions only get updated at lightspeed, cause caching.
Cachesize limit should show as farway gravity sources getting bundled into lower density information functions.
That's just part of the picture.
I always thought that Veritasium video did more harm than good.
Relevant paper:
https://arxiv.org/pdf/0709.3555
You can read the first two paragraphs of the Introduction and then skip to the last sentence of the Conclusion if you want to bypass all the math.
Well, if we can detect the graviton before we have a working quantum theory of gravity, it would mean that gravity is in fact quantized and that we just need to figure it out. This would be a very big deal.
> Now graviton chasers find themselves in a peculiar position. On the main facts, everyone is in agreement. One, detecting a quantum event sparked by a gravitational wave is — surprisingly — possible. And two, doing so would not explicitly prove that the gravitational wave is quantized. “Could you make a classical gravitational wave that would produce the same signal? The answer is yes,” said Carney, who along with two co-authors analyzed this type of experiment in Physical Review D(opens a new tab) in February.
[0] https://en.wikipedia.org/wiki/Photon_statistics#Sub-Poissoni...
Gravity drive?
It would be great to have an independent gravitational wave detector though.
Now I want to shoot someone.
> You need huge masses — think planets — to significantly warp space-time and generate obvious gravitational attraction. By way of comparison, a credit card-size magnet will stick to your fridge.
By way of comparison, even an Olympic pool-size balloon of hot air will float.
Military devices either use GPS, star tracking, dead reckoning, or some combination. For submarines, detecting gravity variations could also be used, but it wouldn't rely on the quantization of gravity.
In many places on land, you can use terrain landmarks.
Since most things are already either covered, or have improvements in development, I don't really see investment for your idea.
The improvements might even use "quantum" or "gravity", but I don't think the use of "quantum gravity" is very likely.
In a global position finding system, gravitational effects could be used, as could be quantum effects. Maybe even both in the same system.
It seems really doubtful to me a practical system would depend on anything graviton related.
How does it communicate the magnitude of the change? By having lots of gravitons? Or does it have something akin to a frequency?
In this case, how does the fact that a big object is still influencing space/time around it communicate that fact when it is not moving. Is that still gravitrons?
Assuming our guess about quantum gravity are correct, the normal gravitation force use gravitons too, they are virtual gravitons but the distinction between "real" and "virtual" particles is another whole can of worms.