4 ms·
(Passing comment from a phone, so forgive the brevity.) The key predictions one typically calculates using quantum field theory are scattering amplitudes. Let’
by ssivark 5y ago
(Passing comment from a phone, so forgive the brevity.)
The key predictions one typically calculates using quantum field theory are scattering amplitudes. Let’s consider both gravity and one of the other forces in the “weak”/perturbative limit — flat space time and force carriers as (quasi)particles Eg: photons, gluons, etc. Now let’s compare the algebraic expressions for the scattering amplitude in both cases (final answer after pages of calculations).
For gluons it turns out there is one piece due to how the gluons are moving when they collide (kinematic piece), multiplied by another piece for the charge they carry (aka color); surprisingly the two pieces have a somewhat similar form if you squint (called “color kinematics duality”). Also gluons are spin-1.
Now, gravitons don’t carry charge, but they’re spin-2. If you look at the algebraic expression for the amplitude of scattering gravitons (after a much more tedious calculation) — lo and behold — it looks like it just has kinematic-like piece multiplied twice! (and no color piece.)
This is what is commonly referred to as “double copy” (of kinematic term) or “gravity = gauge (force) squared”.
This is one of the seminal papers; it’s very short, and has very few equations (but they’re written in a very abstract form) — feel free to stare at them if you like: https://arxiv.org/abs/1004.0476 https://arxiv.org/abs/1004.0476
- naikrovek 5y agoso "double copy" means "squared". no wonder no one can follow any of this. they intentionally obfuscate everything.
- jiggawatts 5y agoThat's a general problem caused by "Publish or perish". Essentially, we've incentivised intellectual wankery. Contrast this to any field of engineering: intelligibility is selected for by market forces. Obscure, hard to follow stuff is slowly (but surely) weeded out in favour of the clear and simple. In the more theoretical sciences, that's just not a goal. Quite the contrary: The goal is to make the published papers look impressive. More jargon, more complex equations, fewer diagrams, all help achieve this goal.
- deleted 5y ago[deleted]
- ssivark 5y agoNo, “square” is the more crude analogy (there’s a lot more to “multiplication” than you might be familiar with in the case of numbers), and “double copy” is more natural. Don’t ask for a simpler explanation because the actual story is not yet well understood — after all it’s active research, so scientists are still groping in the dark trying to figure things out! Nobody is trying to intentionally obfuscate anything; the authors working in this area deserve tremendous credit for their sincere and sustained (decade+) effort in trying to elucidate the structure of gravitational interactions. Please don’t go around slandering them just because you don’t understand what’s going on.
- suifbwish 5y agoNo one is cast doubt on their work; we are saying their choice of terminology SUCKS.
- suifbwish 5y agoAgree. They might as well have included the phrase “super duper” in the title to sound a bit more scientific.
- mannykannot 5y agoEven if ssivark's position that 'squared' would be less appropriate than 'double copy' were debatable, would it really help you to understand what is going on here to call it squaring rather than double copying, or would it just be giving the impression that it is clearer because it is expressed with a term that you are more familiar with? All I can say is that it would not make me more enlightened.
- dmingod666 5y agoThanks for taking the time to detail this out.