
Simple Rules ‘Bootstrap’ the Laws of Physics - theafh
https://www.quantamagazine.org/how-simple-rules-bootstrap-the-laws-of-physics-20191209/
======
jetrink
The content of the article is very interesting and worth reading, but the
framing feels circular. Some laws of nature are fundamental and other laws are
inevitable consequences of those laws. If the inevitable laws were not
inevitable, then they would be fundamental. As our knowledge and explanatory
power grows, some laws that first appeared fundamental have been explained in
terms of deeper laws, but we must still ask why the deeper laws exist.

~~~
jordanpg
I agree with this. I've always found theories of everything to be unsatisfying
in this way, because they never explain how the laws of physics got the way
they are.

Assuming the existence of some abstraction called spin to derive the
fundamental forces is a great exercise in internal consistency, but hardly
meets any definition of "bootstrapping" as I understand the term.

The only theory I've ever encountered that passes this smell test to me is the
mathematical universe hypothesis[0] because it's intuitive to me that math
just "is" in some sense and does not require any upstream mechanisms or
assumptions. As far as I'm concerned, if you have to assume the existence of
anything whatsoever, it's not bootstrapping.

[0]
[https://en.wikipedia.org/wiki/Mathematical_universe_hypothes...](https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis)

~~~
discreteevent
As to how the laws got where they are Lee Smolin and Roberto Ungar say that
it's possible they were not always the way they are and that they evolved
spontaneously over time. And that the spontaneity just is the way things are
i.e. there's no explanation for it. At least that's my recollection from
reading their book.

[https://en.m.wikipedia.org/wiki/The_Singular_Universe_and_th...](https://en.m.wikipedia.org/wiki/The_Singular_Universe_and_the_Reality_of_Time)

The other explanation seems to be the multiverse. i.e. we just got a random
set of self consistent laws. Again there seems to be no explanation as to why
or how the multiverse came to be, is there?

~~~
jordanpg
> not always the way they are and that they evolved spontaneously over time

But how did they get any way at all? How and why do they evolve?

> multiverse. i.e. we just got a random set of self consistent laws.

This raises even more questions. Why are there any multiverses with any laws
at all? From where did the stuff in the multiverses come from? What initiated
this chain of multiverse creation, or why is it inevitable?

A bootstrapping theory would explain why anything is inevitable, or why there
is something rather than nothing. Otherwise it is just a low-level physics
theory.

~~~
smallnamespace
I don’t think what you’re asking for is possible, and the answer lies more in
the realm of philosophy than in physics.

You can always keep asking the question ‘why?’ At some point, either you have
just accept something as fundamental, or the answer justifies itself, or you
have an infinite regress of whys.

~~~
jordanpg
I agree with this generally, but I don't think questions like "why are the
laws of physics the way they are" are obviously questions for philosophers
only and I'm not simply asking "why?"

Only very low-level ontological questions meet this criteria, like "why is
there something rather than nothing," but even then, I am open to the
possibility that there are reasonable explanations for these things that we
just can't articulate yet.

------
tus88
> When bootstrapping, physicists determine how elementary particles with
> different amounts of “spin,” or intrinsic angular momentum, can consistently
> behave. In doing this, they rediscover the four fundamental forces that
> shape the universe.

They have derived gravity through quantum mechanics? What rock have I been
living under?

~~~
mcaruso
Not a physicist by any means, but from the article it talks about gravity in
the context of the graviton (as one plausible solution to the problem
mentioned). Which is still hypothetical, so it's not like they've proven
gravity from first principle or anything.

~~~
nsomaru
As a layman reading this, it’s funny. Because I do not need gravity proven to
me from first principles ;)

~~~
maxnoe
Proven is the wrong word. You cannot prove a theory.

Deriving a description of gravity from first principles would be more
appropriate

~~~
hprotagonist
*an empirical theory.

I can certainly prove things about, say, addition.

~~~
solarist
if you mean you can prove 2+2==4, it's actually tautology: | | + | | == | | |
|

~~~
hprotagonist
no like, "addition is closed over the integers" is something you can show to
be True (for all members of an infinite set, no less -- so it's not
empirical.)

------
knzhou
This is a good article, but I think it's a little confusing -- it doesn't
specify the scope of the bootstrap programme.

It's true that general considerations can give you very strong constraints on
how particles of various spins can interact. For example, it's been known for
decades that at low energies, massless spin 1 particles inevitably give you
Yang-Mills and massless spin 2 particles inevitably give you general
relativity. There is also a separate idea called the conformal bootstrap which
seeks to use general principles to pin down _everything_ about a theory, but
my impression is that it only works well for simple conformal field theories
in lower dimensions.

In particular, these ideas don't tell you anything about the detailed
structure of the Standard Model: why it has the gauge groups it does, why
there are 6 quarks, the values of the quark masses, how the spontaneous
symmetry breaking works, the gauge couplings, and so on. That's the way with
almost all theoretical tools: the flashier and more general in scope they are,
the less details they actually pin down. So while formal theorists may be
excited about this, experimentalists can't really use it for anything.

------
hn_throwaway_99
Wondering if anyone could recommend a layman's introduction to "spin". I've
read the Wikipedia article, I broadly understand it, but just curious as to
how it was discovered, why it is considered an "intrinsic" form of angular
momentum, and why it is so fundamental a concept in our universe.

~~~
ISL
Objects with spin can be point-like but carry angular momentum. The electron
is one such particle.

It is "intrinsic" because it has no known moving parts. If it had moving
parts, and that motion had net angular momentum, we would refer to it in many
contexts as "orbital angular momentum".

Spin should seem counterintuitive. There are no macroscopic objects that I am
aware whose spin you can feel with your hands. Our research group has an
object with 10^23 polarized spins (and negligible net magnetic moment) --
observing the angular momentum effects directly requires a sensitive modern
torsion balance.
[https://arxiv.org/abs/0808.2673](https://arxiv.org/abs/0808.2673)

~~~
criddell
> Objects with spin can be point-like but carry angular momentum.

This is where I get stuck. The particles are all excitations of a quantum
field. So what does spin mean in that context?

~~~
millstone
Excitations can carry dynamical properties. For example, jerk a rope once, and
you will see a wave travel down the rope, which carries energy and momentum.
If you give the rope a quick twist, the wave carries angular momentum.

~~~
criddell
I have the same problem with this analogy. The rope itself is just excitations
of a quantum field. It feels like there is very little underpinning
everything.

------
eprparadox
Max Tegmark was on the Sean Carroll podcast recently, talking about the
different types of multiverse we could postulate may exist[0]. I found the
quanta article fascinating, and I don't believe it's strictly speaking
'circular'. It can also be the case that multiple solutions exist mapping the
relationships between a given set of fundamental laws. Prof. Tegmark discusses
this in the '4th level' of multiverse which examines whether it's right to
talk about a multiverse of all possible mathematically consistent universes.

[0]:
[https://www.preposterousuniverse.com/podcast/2019/12/02/75-m...](https://www.preposterousuniverse.com/podcast/2019/12/02/75-max-
tegmark-on-reality-simulation-and-the-multiverse/)

------
gus_massa
If it is inevitable, why does the weak force only affect the left-handed
fermions and not the right handed? Most of the article is just a discussion of
the applications of the symmetry, not inevitability.

U(1) [aka electromagnetism] is probably inevitable, the rest is more
controversial.

------
scotty79
This brings to mind how Noether showed that conservation laws of physics must
exist if universe has some specific symmetries.

Like translation symmetry, the simple fact that the laws of universe don't
change as you move a bit to the side mathematically implies conservation of
momentum.

------
IshKebab
> And the hour hand on a clock must spin around twice before it tells the same
> time again.

Err, not on any clock I've ever seen! But points for effort.

~~~
ahuth
This example definitely assumes familiarity with a 12-hour clock, which indeed
spins around twice before getting back to the same time.

~~~
minitech
But only once before getting back to the same state, so it’s a bit confusing.

------
beaconstudios
Sounds to me like the higher order laws of physics are emergent properties of
the lower level properties. Is this surprising? Every system in existence is
the result of the emergent organisation of lower systems - why would the laws
of physics be different?

~~~
mannykannot
It is one thing to say that something is plausible, and something quite a bit
more to be able to show that not only is it plausible, but also that it is the
way things actually are, and in a very specific way.

~~~
beaconstudios
yes, but the reporting is as if the fact that emergence exists is itself
surprising. I'm sure the actual science is a lot more sober and focused on the
genuine revelations, around how such emergence plays out.

------
Vysero
"That solution is the graviton: a spin-2 particle that couples to itself and
all other particles with equal strength."

So, has this been discovered recently? I was under the impression that the
graviton was theoretical?

~~~
qubex
What that means is that gravity, at the quantum level, must be conveyed by a
spin-2 particle, known as a ‘graviton’. It has not been detected, and probably
never will be (due to technical limitations). We can broadly pin down its
properties, but we do not yet have a full theory of how it interacts.

------
Iwan-Zotow
"As the Nobel Prize winner Steven Weinberg showed in 1964, the existence of a
spin-2 particle leads inevitably to general relativity"

Huh?!?

spin-2 particle leading to GR was the prime essence of Feynman lectures on
Gravitation, 1962

~~~
bonzini
There is a link pointing to
[https://journals.aps.org/pr/abstract/10.1103/PhysRev.135.B10...](https://journals.aps.org/pr/abstract/10.1103/PhysRev.135.B1049).
Some quick Googling shows quantum gravity being referred to as Feynman-
Weinberg gravity, so both must have contributed parts of the theory.

------
tabtab
I thought this was going to be about Twitter Bootstrap's UI library, and was
thinking, "Oh good, I can read about simple consistent rules so that it's not
so organic and unpredictable anymore!" It was not to be. Back to the UI trial-
and-error slog...

------
jejones3141
"...bootstrappers find that many different spin-0 particles are possible."

Is that a typo, or is it really possible to have spin -0?

~~~
yters
I think with some equations there is a discontinuity at zero, like tan, but it
still makes sense to talk about what happens asymptotically approaching zero
and then it matters from which side you approach zero, hence positive and
negative zero.

~~~
mjcohen
tan does not have a discontinuity at zero. |x|/x does, as does 1/x. tan's is
at pi/2 in particular and (n+1/2)pi in general.

~~~
duelingjello
Although tan has a set of discontinuities when it jumps between +∞ and -∞
through complex infinity (∞~).

tan(π(x + 0.5)) = ∞~, x ∃ ℤ

(π on my screen doesn't look much like greek pi.)

------
anigbrowl
N-gate theory offers the best hope of cutting through conflicting theories to
get at the fundamentals.

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LoSboccacc
this is the opposite of how science works. until grand unification is proven,
the four forces simmetry is a nice and convenient hypothesis

~~~
zamalek
> this is the opposite of how science works

Yep, this is dogma. People seem to be hard-wired to believe in something, for
some it's a god, for others it's a theory or hypothesis. As for the grand-
unification surrounding the the four forces, aren't we currently questioning
the existence of a fifth? That's a pretty short-lived god.

~~~
LoSboccacc
yeah there's the x17 thing too, but it's still quite to early for that.

