Why do we perceive a 4-dimensional universe as 3?

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Mon Oct 21, 2013 8:59 pm UTC

ahammel wrote:
lgw wrote:Matter is obviously tiny balls of stuff, says naive intuition.
So you just look at, say, cheese and you intuition goes "oh yeah, that's obviously made of quarks", does it?


My intuition says "that's obviously made of stuff". My intuition does not see it as an anti-node in the cheese field. Admittedly, Wallace might see it differently.

doogly wrote:But it is wrong. There is no difference between the electric field and the electron field. It's all fields.

Like, what precisely are you proposing? A particle based alternative to QFT that is just as accurate?


Yup, that exactly. There's no substantive mathematical difference between viewing an electron as a "cloud of probability" (the best explanation I ever got in high school), and as a "cloud of quasi-particles", if you choose the nature of those quasi-particles carefully. (Thanks for the term PM 2ring!)

Tchebu wrote:Moreover, as soon as you know anything at all about waves it becomes pretty obvious that things that look like cohesive lumps that move as one don't necessarily have to consist of smaller pieces moving together, so it really doesn't take much to blow any intuition about it being made of tiny balls out of the water.


Solids mostly do consist of smaller pieces moving together, is the thing. It's easy to explain the solidity of matter as a fence: the electrons are the fenceposts, the electric field is the fencing, and the nucleus (well, and the electrons not participating the process) are the post holes.

Sure, there are some things harder to explain, but that just means more work is needed to link them to intuition.

PM 2Ring wrote:Why are you trying to get away from "there's just this universal "field" that describes the probability that there's an electron at any given place"?

Personally, I like the cellular automaton approach: an electron is like a glider in Conway's Game of Life. It appears to be an oscillating entity that moves in a certain way, but really it's just an evolving pattern of the underlying substrate (the electron field).


I also like the cellular automaton approach, and serious work is being done there, but it's devilishly hard to reconcile with relativity. Really, it's velocity, not position, that's seems to interact locally, and that's not easy to make sense of.

I'm trying to get away from things that make little intuitive sense. a "field that describes the probability that there's an electron at any given place" seems rather circular and non-informative an an answer to "what is an electron?" Sure, it works, no arguments there. But it's unsatisfying.

And that's really the problem with the Standard Model in a nutshell, isn't it? It works great, but it's messy and inelegant and non-intuitive and all that sort of subjective stuff. Finding better answers to "what's really happening" that don't change the math would be, well, better!

I want intuitive explanations that are not lies to children because the math remains sound, that can be taught qualitatively at the high school level, and produce real understanding, instead of a sneaking suspicion that "those quantum guys are just running a long con on the funding groups".
"In no set of physics laws do you get two cats." - doogly

Tchebu
Posts: 564
Joined: Thu Feb 22, 2007 12:42 am UTC
Location: Montreal

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Tchebu » Mon Oct 21, 2013 9:36 pm UTC

My intuition says "that's obviously made of stuff". My intuition does not see it as an anti-node in the cheese field. Admittedly, Wallace might see it differently.


It's really not that much of an intuitive stretch to begin to view it as such. Waves in the ocean also look like a single thing moving, but it's trivial to expose its true nature as a propagating excitation in the "water level" field.

Solids mostly do consist of smaller pieces moving together, is the thing. It's easy to explain the solidity of matter as a fence: the electrons are the fenceposts, the electric field is the fencing, and the nucleus (well, and the electrons not participating the process) are the post holes.


Sure, but there's nothing wrong with first pointing out that localized lumps of excitation can exist, then point out that the way such lumps interact is pretty much the same as that of particles in many cirucumstances and then going on to say that fields are the real fundamental objects, but more often than not, we deal with their excitations, which for most intents and purposes can be talked about as particles.

This also saves you some of the trouble of having to explain wave-particle duality later because you just have to go "remember, these were really waves all along, so they're also subject to wavy phenomena".

(Also, slightly besides the point, I suppose your explanation of metals is that the fenceposts can freely slide along the fencing, which magically floats in midair? :twisted: )
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Mon Oct 21, 2013 9:43 pm UTC

Atoms as little balls are incredibly unintuitive from the get-go, and that only seems not to be the case because you've been brought up to believe that's actually what everything is made of. The notion that normal clouds are collections of individual particles is also unintuitive, really. So why should explaining electrons as clouds of (quasi, whatever that really means in this case) particles magically make them intuitive?

You find that easier to understand because you had the not-quite-true little-billiard-balls model of chemistry drilled into you. I strongly doubt that a truly naive person, whose worldview was never contaminated with less accurate historical models of the atom, would find it any less intuitive to think of electrons as continuous probability clouds than as clouds of discrete quasi-billiard-balls.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Frenetic Pony
Posts: 177
Joined: Fri Jan 27, 2012 6:31 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Frenetic Pony » Mon Oct 21, 2013 11:06 pm UTC

gmalivuk wrote:Atoms as little balls are incredibly unintuitive from the get-go, and that only seems not to be the case because you've been brought up to believe that's actually what everything is made of. The notion that normal clouds are collections of individual particles is also unintuitive, really. So why should explaining electrons as clouds of (quasi, whatever that really means in this case) particles magically make them intuitive?

You find that easier to understand because you had the not-quite-true little-billiard-balls model of chemistry drilled into you. I strongly doubt that a truly naive person, whose worldview was never contaminated with less accurate historical models of the atom, would find it any less intuitive to think of electrons as continuous probability clouds than as clouds of discrete quasi-billiard-balls.


I still don't, enjoy, which is the wrong word, the idea of "probability clouds" as an actual physical function. I totally understand the need to describe as such mathematically, that's the best way we know how at the moment. But if you think of them as "physical probability clouds" rather than something like a wave function who's locations we only know how to model as a spread of probability, well that's what gets you into MWI stuff. "The electrons is all there, all possible universes are represented and you collapse us into a definite one upon interaction." Or some such.

Then again Doogly's point also brings up MWI. "Which past is REAL? Are all possible pasts real?" Maybe I'm just missing something of course, but it begins to remind me of Keanu Reeves in The Matrix "whoa" :|

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Mon Oct 21, 2013 11:46 pm UTC

lgw:
Well, there isn't a particle based alternative to quantum field theory. If you'd like to develop one, good on you, but chapter one of most qft books is the a complete and formal proof that a field perspective is necessary. You're welcome to check for holes.

If your complaint is just that the "watered down for popular consumption" version of qft sounds weird and unintuitive, there are a few responses:
- of course it is. The crucible for a scientific theory is experiment, not your intuition.
- think deeper even on what your intuition means, and where it comes from. Where did the expectation that it would be any good outside of the regime in which your brain evolved come from?
- the process of watering down is perilous, but it is the only way. You can't take your intuition and un-water it to get a full theory that works.
-actually, is classical mechanics even entirely intuitive to you yet? There are surprises all over.

For the first year of physics, maybe, all you are doing is quantifying intuition. Of course the block falls, but precisely how fast? Of course the heavier end of the pulley is the one that goes down, but what precisely is the acceleration? And so on. But very soon, long before you have to deal with quantum field theory, you find your intuition fails you. The phenomena themselves are startling.

I mean, lightning and lodestone would be startling if they weren't all around of us. If I just gave you Newton and Coulomb and then a compass and lightning rod, you'd have questions.

Frenetic Pony:
There's nothing of MWI here. The lesson is that none of the individual classical paths are real. The reality is quantum, and the reality is a path integral.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

tooyoo
Posts: 100
Joined: Sat Jan 22, 2011 5:39 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby tooyoo » Tue Oct 22, 2013 1:33 am UTC

lgw wrote:I'm trying to get away from things that make little intuitive sense. a "field that describes the probability that there's an electron at any given place" seems rather circular and non-informative an an answer to "what is an electron?" Sure, it works, no arguments there. But it's unsatisfying.


It's actually very satisfying. In general I find that only the knowledge of both the experimental data as well as theoretical models (and the necessary mathematical background) lets you fully appreciate something. Otherwise you miss important subtleties. On the chance of digressing, let me mention spin. Which makes no intuitive sense whatsoever. The lecturer in our experimental physics lectures just said: "Well, think about the electron just spinning about an axis. The theorists will explain you things properly next term." Next term, one of the first things our lecturer in quantum theory said was: "You all know spin from your experimental lectures, so let's consider a two-state system." I hated that. This whole analogy with "think of the electron as spinning" works, but it's terribly unsatisfying because it's clearly not what's going on. I only felt that I actually understood what the hell was going on once I had studied a bit of representation theory and some quantum theory while keeping in mind the basic experimental facts (Stern-Gerlach, etc.). Took a few semesters for things to sink in. The point is that since there is no proper analogy of what's going on, studying some maths is clearly your best bet at gaining a deep understanding. (Yes. There's this thing about twisting rubber bands by 720 degrees, but you won't get any mileage out of this outside of a popular science lecture.)

lgw wrote:And that's really the problem with the Standard Model in a nutshell, isn't it? It works great, but it's messy and inelegant and non-intuitive and all that sort of subjective stuff. Finding better answers to "what's really happening" that don't change the math would be, well, better!

I don't really see where the Standard Model is that in-elegant. See above. That it's non-intuitive is not that surprising. It's the standard model. It deals with elementary particles. It took us a few thousand years to think of it. Why should it be intuitive?

lgw wrote:I want intuitive explanations that are not lies to children because the math remains sound, that can be taught qualitatively at the high school level, and produce real understanding, instead of a sneaking suspicion that "those quantum guys are just running a long con on the funding groups".


This is an important point. Without a doubt, science has to communicate its findings and make sure that people remain able to appreciate them. Yet that's happening. However, let me point out again that the formal, mathematics based understanding is actually a much more real understanding than any oh-so-well thought out analogy. Simply because it is precise, lets you ask precise questions, while offering precise answers. You might have to study hard, but if it was high-school level easy, it probably wouldn't be a research problem.

Meteoric
Posts: 333
Joined: Wed Nov 23, 2011 4:43 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Meteoric » Tue Oct 22, 2013 2:58 am UTC

gmalivuk wrote:I strongly doubt that a truly naive person, whose worldview was never contaminated with less accurate historical models of the atom, would find it any less intuitive to think of electrons as continuous probability clouds than as clouds of discrete quasi-billiard-balls.

I wonder if anyone historically tried to contextualize atoms or subatomic particles in terms of the four classical elements, rather than bite the bullet and acknowledge that tiny billiard balls are a flatly superior model?
No, even in theory, you cannot build a rocket more massive than the visible universe.

Frenetic Pony
Posts: 177
Joined: Fri Jan 27, 2012 6:31 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Frenetic Pony » Tue Oct 22, 2013 3:50 am UTC

doogly wrote:lgw:

Frenetic Pony:
There's nothing of MWI here. The lesson is that none of the individual classical paths are real. The reality is quantum, and the reality is a path integral.


Thanks

User avatar
Moose Anus
Posts: 434
Joined: Fri Oct 14, 2011 10:12 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Moose Anus » Wed Oct 23, 2013 3:44 pm UTC

I just read this thing: https://medium.com/the-physics-arxiv-blog/d5d3dc850933 (paper: http://arxiv.org/pdf/1310.4691v1.pdf)

I think they're saying that an observer outside the universe sees all of space and time at once, but an observer inside the universe only sees some of space and time.
Lemonade? ...Aww, ok.

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Fri Oct 25, 2013 9:10 pm UTC

gmalivuk wrote:Atoms as little balls are incredibly unintuitive from the get-go, and that only seems not to be the case because you've been brought up to believe that's actually what everything is made of. The notion that normal clouds are collections of individual particles is also unintuitive, really. So why should explaining electrons as clouds of (quasi, whatever that really means in this case) particles magically make them intuitive?

You find that easier to understand because you had the not-quite-true little-billiard-balls model of chemistry drilled into you. I strongly doubt that a truly naive person, whose worldview was never contaminated with less accurate historical models of the atom, would find it any less intuitive to think of electrons as continuous probability clouds than as clouds of discrete quasi-billiard-balls.

Maybe so, though I encourage you to try to teach each to younger children. I can break a block of cheese down into ever smaller pieces of cheese easily enough, and I can see that doing so changes something important, since you can't easily reassemble the block of cheese. Tiny bits of cheese bound together by some field is a very natural explanation, easily grasped.

Tchebu wrote:
My intuition says "that's obviously made of stuff". My intuition does not see it as an anti-node in the cheese field. Admittedly, Wallace might see it differently.


It's really not that much of an intuitive stretch to begin to view it as such. Waves in the ocean also look like a single thing moving, but it's trivial to expose its true nature as a propagating excitation in the "water level" field.

Solids mostly do consist of smaller pieces moving together, is the thing. It's easy to explain the solidity of matter as a fence: the electrons are the fenceposts, the electric field is the fencing, and the nucleus (well, and the electrons not participating the process) are the post holes.


Sure, but there's nothing wrong with first pointing out that localized lumps of excitation can exist, then point out that the way such lumps interact is pretty much the same as that of particles in many cirucumstances and then going on to say that fields are the real fundamental objects, but more often than not, we deal with their excitations, which for most intents and purposes can be talked about as particles.

This also saves you some of the trouble of having to explain wave-particle duality later because you just have to go "remember, these were really waves all along, so they're also subject to wavy phenomena".

(Also, slightly besides the point, I suppose your explanation of metals is that the fenceposts can freely slide along the fencing, which magically floats in midair? :twisted: )

Well, I never understood waves as "excitations in the water level", I understood them as a bunch of water molecules moving in circles. I can certainly accept that waves are just more natural to some than "bits of stuff", but there are certainly a crowd of us for whom that's not so. (And metals are like a construction fence - you can slide it along, with a bit of a shove, just don't shove too hard. :D )

doogly wrote:lgw:
Well, there isn't a particle based alternative to quantum field theory. If you'd like to develop one, good on you, but chapter one of most qft books is the a complete and formal proof that a field perspective is necessary. You're welcome to check for holes.

If your complaint is just that the "watered down for popular consumption" version of qft sounds weird and unintuitive, there are a few responses:
- of course it is. The crucible for a scientific theory is experiment, not your intuition.
- think deeper even on what your intuition means, and where it comes from. Where did the expectation that it would be any good outside of the regime in which your brain evolved come from?
- the process of watering down is perilous, but it is the only way. You can't take your intuition and un-water it to get a full theory that works.
-actually, is classical mechanics even entirely intuitive to you yet? There are surprises all over.

For the first year of physics, maybe, all you are doing is quantifying intuition. Of course the block falls, but precisely how fast? Of course the heavier end of the pulley is the one that goes down, but what precisely is the acceleration? And so on. But very soon, long before you have to deal with quantum field theory, you find your intuition fails you. The phenomena themselves are startling.

I mean, lightning and lodestone would be startling if they weren't all around of us. If I just gave you Newton and Coulomb and then a compass and lightning rod, you'd have questions.

Frenetic Pony:
There's nothing of MWI here. The lesson is that none of the individual classical paths are real. The reality is quantum, and the reality is a path integral.

Just to take the philosophical point here: if science is to be more than entertainment, is has two requirements: to lead to engineering, and to lead the everyday non-scientist to a better personal understanding of the world. There has been a real failing of the second requirement for "the physics of the small" for a while now IMO.

I'm not trying to "reason from intuition to QM", as you say you can't really do that. I'm looking for easily teachable approaches to QM (at least, at the high-school or freshman physics level) that won't need to be un-learned by those inspired to go further. There is far too much "the math is really hard, and I can't explain it to you, but trust me I'm right" going on in the physics of the small these days, and I find it infuriating. I can explain relativity well enough to anyone who can understand what a vector is, but QM? There's a bit of "I'm proud because of how hard it was for me to learn" that's not balanced by people trying to find ways to make QM easier to teach, IMO and that's no way to go on (though this forum is great as a counter-example, I have to admit, people here put real work into explanations).

Meteoric wrote:
gmalivuk wrote:I strongly doubt that a truly naive person, whose worldview was never contaminated with less accurate historical models of the atom, would find it any less intuitive to think of electrons as continuous probability clouds than as clouds of discrete quasi-billiard-balls.

I wonder if anyone historically tried to contextualize atoms or subatomic particles in terms of the four classical elements, rather than bite the bullet and acknowledge that tiny billiard balls are a flatly superior model?

Sometimes I miss the joke, but you do realize that the Western tradition of "atoms" comes from trying to explain how the four classical elements combine to make stuff, right? And this argument is at least that old, with Heraclitus arguing for everything being made of waves (or at least that's ascribed to him, though perhaps he never really said "everything flows"). Heraclitus would have found entangled quantum state pairs a comfortable idea, I think.
"In no set of physics laws do you get two cats." - doogly

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Fri Oct 25, 2013 9:16 pm UTC

lgw wrote:Well, I never understood waves as "excitations in the water level", I understood them as a bunch of water molecules moving in circles.
I genuinely do not believe that this is always how you've conceptualized waves. It is far more plausible that your memory is as shitty as everyone else's is, than that by yourself you reached an understanding that to the majority of people comes as a rather unintuitive new way of looking at waves.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
Izawwlgood
WINNING
Posts: 18686
Joined: Mon Nov 19, 2007 3:55 pm UTC
Location: There may be lovelier lovelies...

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Izawwlgood » Fri Oct 25, 2013 9:19 pm UTC

doogly wrote:For the first year of physics, maybe, all you are doing is quantifying intuition. Of course the block falls, but precisely how fast? Of course the heavier end of the pulley is the one that goes down, but what precisely is the acceleration? And so on. But very soon, long before you have to deal with quantum field theory, you find your intuition fails you. The phenomena themselves are startling.
As someone who didn't go beyond physics 101, can you tell me what the first couple of startling phenomena are? I find your description of the physics progression to be kind of fascinating.
... with gigantic melancholies and gigantic mirth, to tread the jeweled thrones of the Earth under his sandalled feet.

User avatar
davidstarlingm
Posts: 1255
Joined: Mon Jun 01, 2009 4:33 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby davidstarlingm » Fri Oct 25, 2013 9:55 pm UTC

Izawwlgood wrote:
doogly wrote:For the first year of physics, maybe, all you are doing is quantifying intuition. Of course the block falls, but precisely how fast? Of course the heavier end of the pulley is the one that goes down, but what precisely is the acceleration? And so on. But very soon, long before you have to deal with quantum field theory, you find your intuition fails you. The phenomena themselves are startling.
As someone who didn't go beyond physics 101, can you tell me what the first couple of startling phenomena are? I find your description of the physics progression to be kind of fascinating.

The first one I recalled was simply that Schrodinger's equation couldn't be derived from any observation (like we had derived F = ma and all the basic kinematic equations).

Then there was the realization that a particle could not have zero momentum; it had to have a nonzero ground state.

Intuition also failed on the other side of modern physics, in trying to figure out how length contraction can possibly work and why you can't manage to exceed the speed of light with a racecar on a bullet train on a really really fast spaceship.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Fri Oct 25, 2013 9:57 pm UTC

Rotation is wacky. Precession and nutation give you some headaches. You get them off diagonal inertia matrices and shit's funky.

The chaos with 3 point particles is a surprise. Likewise double pendulum.

Solitons, like the shallow water guys in KdV, those are wacky.

Feynman's Sprinkler can give you a great headache.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

User avatar
thoughtfully
Posts: 2253
Joined: Thu Nov 01, 2007 12:25 am UTC
Location: Minneapolis, MN
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby thoughtfully » Fri Oct 25, 2013 10:16 pm UTC

Potential energy takes a good bit of getting used to, and it's pretty early in the curriculum.
Image
Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery

User avatar
The Geoff
Posts: 144
Joined: Wed Jun 08, 2011 6:22 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby The Geoff » Fri Oct 25, 2013 11:36 pm UTC

Young's Double Slit is the big "WTF" experiment for me. Then you get into the wacky delayed erasure stuff.

As far as intuition goes, would it be fair to say we know the approximate initial state of a photon and the final state when it hits the screen, and there is no "in between" state, just a collection of possible solutions to go from one to the other?

tooyoo
Posts: 100
Joined: Sat Jan 22, 2011 5:39 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby tooyoo » Sat Oct 26, 2013 10:41 am UTC

lgw wrote:I'm not trying to "reason from intuition to QM", as you say you can't really do that. I'm looking for easily teachable approaches to QM (at least, at the high-school or freshman physics level) that won't need to be un-learned by those inspired to go further. There is far too much "the math is really hard, and I can't explain it to you, but trust me I'm right" going on in the physics of the small these days, and I find it infuriating. I can explain relativity well enough to anyone who can understand what a vector is, but QM? There's a bit of "I'm proud because of how hard it was for me to learn" that's not balanced by people trying to find ways to make QM easier to teach, IMO and that's no way to go on (though this forum is great as a counter-example, I have to admit, people here put real work into explanations).


QM is perfectly teachable at the undergraduate and even advanced high school level these days. And it does sort of happen. You need a bit of complex numbers and maybe an idea of basic linear algebra and off you go.

I assume you're talking about QFT, that's a totally different story. I sort of believe that these things trickle down over the years - 100 years ago special relativity and quantum mechanics were cutting edge, these days you teach them in high school. I guess 100 years from now they might teach QFT there. But right now, that's still clearly graduate material that most students of physics - even those with PhDs - never encounter. It simply falls squarely into the remit of modern theoretical physics. If I'm not mistaken, Wilsonian treatment of renormalisation hadn't been a textbook topic before Peskin & Schroeder. This year's nobel is awarded for one of the comparatively simpler problems related to field theory.

I sympathise with your fury. Yet you might want to look at things from the other way. The problems you seem to be interested in require a very high level of mathematical sophistication - to the extend that you cannot teach them to undergrads. We actually had a particle physics colloquium aimed at undergrads quite recently and were afterwards discussing how much they could have understood. Every time your friends or family ask you what on earth you are doing, you are immediately confronted by the fact that their mathematical training is totally insufficient - unless they study mathematics. It's not that people don't try to give easier explanations. But it's very frustrating to teach stuff if all your usual tools and methods (read: mathematics) are off limits. So you are almost forced to proceed via analogy and by simply telling people what is. If you're looking for someone who attempted to proceed differently, look at Penrose's "Road to reality".

To give you a reason why this is so: Studying theoretical physics - and mathematics - is to some extend a very linear process. You need to have mastered the high school material (newtonian mechanics, basic algebra, trigonometry, some calculus) to understand basic undergrad material (more calculus, differential equations, linear algebra, analytical mechanics, qm) which lets you understand some advanced undergraduate stuff (differential geometry, relativity, etc.). Once you arrive at a research level, you're very, very far away from the stuff you found difficult as a student. In most other fields (law, humanities) there's a lot to learn, but it's not so strictly linear.

Izawwlgood wrote:As someone who didn't go beyond physics 101, can you tell me what the first couple of startling phenomena are? I find your description of the physics progression to be kind of fascinating.

- The Stern Gerlach experiment
(which seems simple at first; but because there's so little going on it's one of the nicest experiments to look at when learning quantum mechanics)
- Bose Einstein Condensates, matter waves (since you get macroscopic, visible quantum effects)

(I edited this post to add the last paragraph.)

User avatar
eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

Re: Why do we perceive a 4-dimensional universe as 3?

Postby eSOANEM » Sat Oct 26, 2013 11:55 am UTC

thoughtfully wrote:Potential energy takes a good bit of getting used to, and it's pretty early in the curriculum.


Effective potentials take even more getting used to.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Sat Oct 26, 2013 12:17 pm UTC

And quantum mechanics (nevermind qft) is significantly more complicated than relativity, even general relativity. GR is still classical. The curved space mathematics can be a bit intricate but it's still very much a classical smooth geometry where things proceed nicely, just along some curvy geodesics. The clocks can get a bit head scratchy. It can be a tough pill to swallow but you can get it down after a semester without much problem generally.

It's conventional to put the two side by side as masterful achievements of twentieth century physics, but qm is def tricksier.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

tooyoo
Posts: 100
Joined: Sat Jan 22, 2011 5:39 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby tooyoo » Sat Oct 26, 2013 5:34 pm UTC

doogly wrote:And quantum mechanics (nevermind qft) is significantly more complicated than relativity, even general relativity. GR is still classical. The curved space mathematics can be a bit intricate but it's still very much a classical smooth geometry where things proceed nicely, just along some curvy geodesics. The clocks can get a bit head scratchy. It can be a tough pill to swallow but you can get it down after a semester without much problem generally.

It's conventional to put the two side by side as masterful achievements of twentieth century physics, but qm is def tricksier.


I guess this is a moot point, but I don't agree with this. What we generally call quantum mechanics can be taught (and is taught) in a single semester. You need some linear algebra and off you go defining states and operators. I guess if you want to get into some philosophical debate, qm might be trickier because it is indeed not classical, but I think we're talking about physics, not philosophy here. And as long as you're not looking into something crazy (e.g. any system that is a pain to quantise; or geometric quantisation) the calculations you do are fairly straightforward.

Not that GR is that much more difficult, but people do take some time to get their heads around the involved geometry. It might just be classical formalism, but you've got to learn it. The first time you calculate a curvature tensor by hand it seems daunting. And then you still have to solve a bunch of second order PDEs. And there are all sorts of questions of interpretation - who's observing what from where - one tends to forget about the physics while doing all the geometry. Add some penrose diagrams and (classical) black holes and you're busy for a while.

Not that I'm saying that GR is that much harder. You can make your life arbitrarily hard in either field. And harder if you connect them. But I really disagree with the "qm is def tricksier" bottom line. As a matter of fact, the respective (Sakurai/Wald/Weinberg-level) courses are usually taught at a similar advanced undergrad stage.

Tchebu
Posts: 564
Joined: Thu Feb 22, 2007 12:42 am UTC
Location: Montreal

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Tchebu » Sat Oct 26, 2013 6:18 pm UTC

People don't really solve Einstein's equations... they look up the solutions in a big blue book :)
Also penrose diagrams make things easier, not harder... isn't that the point? I'll grant you that showing something like the conformal invariance of the Weyl tensor is a bitch and a half though...

I would guess the intuitiveness of GR largely depends on how visual the individual person is about their differential geometry.
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

tooyoo
Posts: 100
Joined: Sat Jan 22, 2011 5:39 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby tooyoo » Sat Oct 26, 2013 7:31 pm UTC

Tchebu wrote:People don't really solve Einstein's equations... they look up the solutions in a big blue book :)


:D Nice.

Of course some solutions haven't made it into that book yet, so that some people make a decent living out of finding them.

capefeather
Posts: 98
Joined: Sun Dec 14, 2008 4:23 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby capefeather » Fri Nov 08, 2013 4:47 am UTC

I still suspect that particles are somewhat more intuitive than waves from an evolutionary standpoint.

I also kind of think that the problem with teaching QFT is less the quantum part and more the field theory part. I'm not sure there's a whole lot of motivation to adopt the field theory point of view until one applies it to quantum theory.

User avatar
ahammel
My Little Cabbage
Posts: 2135
Joined: Mon Jan 30, 2012 12:46 am UTC
Location: Vancouver BC
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby ahammel » Fri Nov 08, 2013 4:54 am UTC

capefeather wrote:I still suspect that particles are somewhat more intuitive than waves from an evolutionary standpoint.
Meaning what? Quantum field theorists have fewer children?
He/Him/His/Alex
God damn these electric sex pants!

User avatar
Copper Bezel
Posts: 2426
Joined: Wed Oct 12, 2011 6:35 am UTC
Location: Web exclusive!

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Copper Bezel » Fri Nov 08, 2013 7:01 am UTC

He means that cognitively, humans are probably wired to understand things like billiard balls more easily than things like waves, which seem like an abstraction, and that the reason is an evolutionary biology / psychology one.

Not agreeing, necessarily, just clarifying.
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

she / her / her

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Fri Nov 08, 2013 12:52 pm UTC

ahammel wrote:
capefeather wrote:I still suspect that particles are somewhat more intuitive than waves from an evolutionary standpoint.
Meaning what? Quantum field theorists have fewer children?

I certainly have no intention of spawning.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Mon Nov 11, 2013 10:14 pm UTC

capefeather wrote:I still suspect that particles are somewhat more intuitive than waves from an evolutionary standpoint.

I also kind of think that the problem with teaching QFT is less the quantum part and more the field theory part. I'm not sure there's a whole lot of motivation to adopt the field theory point of view until one applies it to quantum theory.


Well, I think waves in some medium are very easy and intuitive. It's the purely abstract waves, by which I really mean waves in purely abstract fields, that are hard to grasp.

You can intuit the elec-tric field as being made of a bunch of quasi-particles in-flight away from each charged particle, carrying that charge information. EM waves in this field are then just apparent compression/transverse/whatever waves in that medium: it's as good an explanation as any for a photon acting like both a particle and a wave, and you can explain most of high school EM easily this way.

Without that medium of quasi-particles, it's much less intuitive to explain light as a wave. Sure, there's no difference in the math whether there's this explanatory medium or not, but there's a big difference in learning curve. Of course, if anyone has a good way of explaining how you can have waves without a medium, that's just as good.

(Wow, cheesegratering elec-tric to atomic really didn't help this post make sense!)

ETA:
doogly, Dr. The Juggernaut of Touching Himself wrote:I certainly have no intention of spawning.

You don't say. :lol:
"In no set of physics laws do you get two cats." - doogly

skolnick1
Posts: 113
Joined: Sun Mar 16, 2008 9:29 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby skolnick1 » Tue Nov 12, 2013 12:59 am UTC

elasto wrote:As to why we are conscious at all rather than unthinking machines - well, that's a much deeper question.

Said the bicycle, his gears turning.

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Tue Nov 12, 2013 1:20 am UTC

skolnick1 wrote:
elasto wrote:As to why we are conscious at all rather than unthinking machines - well, that's a much deeper question.

Said the bicycle, his gears turning.


Actually, if I postulate 2 axioms:
(1) I am conscious, and
(2) Nothing supernatural is involved in that

Then we can reason a bunch from that. Where I end up is that consciousness/self-awareness is no more than the ability to make a decision by mentally modeling contrapositive hypotheticals, and deciding based on the outcome of the models. The degree to which we do this maps quite well onto intuitive notions about intelligence, consciousness, mindless actions, and so on. When we say that we are "thinking about what to do", this is what we're doing. Without a supernatural element, this modeling is what's left as "consciousness".

We do many things in life by merely following rules we have learned, and the gradient from the bicycle to a network of neurons that can make rule-based decisions and even acquire new rules seems continuous - it's not a surprise that a sufficiently complex machine can learn and follow rules. The jump seems to be to abstract mental models, but even there we'd find many increments. Merely building a 3d mental model of the world as we see it, so that we can guess the location of something that has briefly moved behind an obstacle, is a bit of a jump from a rule-based system, but is still straightforward (and we can do it now in software), and clearly has evolutionary advantage for anything that chases anything else. But moving from there to a false-but-interesting model to see how a hypothetical might play out? That seems to be the trick that matters.

It also lines up nicely with intuitions about "thoughtful" vs "thoughtless" action. ("How could you have said that? How did you think it would make her feel?" "I didn't think, I just reacted emotionally, that was my mistake." and so on.)
"In no set of physics laws do you get two cats." - doogly

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Tue Nov 12, 2013 1:43 am UTC

There is a difference; if there's a medium, then that medium has a rest frame, which you should be able to detect. And we do not.

Using quantized photons to think of as a medium for classical e&m waves is even worse. That's actually misunderstanding.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Tue Nov 12, 2013 2:07 am UTC

But it's such an intuitive misunderstanding!
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Tue Nov 12, 2013 10:40 pm UTC

doogly wrote:There is a difference; if there's a medium, then that medium has a rest frame, which you should be able to detect. And we do not.

A bunch of speed-of-light quasi-photons in motion doesn't have this problem. That's the point, really, it makes it easy to explain waves without incurring the wrath of relativity. (As an aside, I do find it humorous that the experimental apparatus for gravity wave detection is basically just a grown-up version of the Michelson-Morley apparatus.)

doogly wrote:Using quantized photons to think of as a medium for classical e&m waves is even worse. That's actually misunderstanding.

How so? I'm probably misunderstanding your wording here, as I bet you're saying something quite simple and obvious, but I don't see it.

gmalivuk wrote:But it's such an intuitive misunderstanding!

Hey now! More than anything I want to avoid "lies to children". (Who was it who said "every problem has a simple, easy to understand, wrong answer"?)
"In no set of physics laws do you get two cats." - doogly

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Wed Nov 13, 2013 1:04 am UTC

Why would a diffuse gas of photons not have a rest frame? I mean, I'm not exactly sure what you are proposing as using photons as the medium for electromagnetism. It's not really a coherent idea, because photons are the quantized excitations of the electromagnetic field. It's just sort of backwards. Photons are generally construed as mode sums over classical waves.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

lgw
Posts: 437
Joined: Mon Apr 12, 2010 10:52 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Wed Nov 27, 2013 11:41 pm UTC

doogly wrote:Why would a diffuse gas of photons not have a rest frame? I mean, I'm not exactly sure what you are proposing as using photons as the medium for electromagnetism. It's not really a coherent idea, because photons are the quantized excitations of the electromagnetic field. It's just sort of backwards. Photons are generally construed as mode sums over classical waves.


Sorry to drag on this off-topic discussion, but just wanted to clarify that I wasn't suggesting any sort of aether, but rather that an electron emits a bunch of quasi-photons, like one of those spinning lawn sprinklers, that move at c and carry the charge information (but do no work). If you shake an electron (or a hose), you'll see "apparent waves", translation or compression depending on the angle, in the pattern of the emitted particles, but much like the water coming from a shaken hose it's not "really" a wave - there's no medium which applies restorative force proportional to displacement, or however you'd describe it. It's still a "sea of quasi-photons", but they're all moving at c.

Some of those "apparent waves" will be actual work-doing photons: a set of quasi-photons all moving in the same direction with a wave-like shape (again, like the stream of water from a shaken hose if close enough that they haven't dispersed), with a wavelength, polarization, and so on.

So the common way to describe things is that everything is an excitation of the underlying fields, some "real particles", some "virtual particles" - but I just can't visualize a "field excitation". Instead I describe everything as made of "quasi particles", which sometimes exist in collections as "real particles", and which can be visualized by analogy to lawn sprinklers and hoses.
"In no set of physics laws do you get two cats." - doogly

Rhombic
Posts: 58
Joined: Wed Jan 01, 2014 11:42 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Rhombic » Sun Jan 05, 2014 9:45 pm UTC

Now, there's one theory that states that there only is ONE electron in the universe, but we perceive many as it enters and leaves many many many times our three dimensional world. Imagine a 2D world and one particle travelling instantly across a random 3D space, being each one of the infinite 2D layer it crosses rearranging. This way, there probably is no particles as we know them, being our "universe" as we know it the mere observation of an intricate particle-crossing network which appears to, but doesn't, rearrange itself in three dimensions, when it is actually impacted by perturbations in a 4D space. This way, there is an inaccessible 4D space which explains our own 3d space. In theory.

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Sun Jan 05, 2014 9:47 pm UTC

Does this "theory" make any novel testable predictions about anything?
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

SU3SU2U1
Posts: 396
Joined: Sun Nov 25, 2007 4:15 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby SU3SU2U1 » Sun Jan 05, 2014 10:09 pm UTC

ow, there's one theory that states that there only is ONE electron in the universe, but we perceive many as it enters and leaves many many many times our three dimensional world.


While elegant, it doesn't work. This theory would predict there are roughly as many positrons (electrons moving backwards in time) as electrons. This is empirically falsified.

Rhombic
Posts: 58
Joined: Wed Jan 01, 2014 11:42 pm UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Rhombic » Sun Jan 05, 2014 11:10 pm UTC

SU3SU2U1 wrote:
ow, there's one theory that states that there only is ONE electron in the universe, but we perceive many as it enters and leaves many many many times our three dimensional world.


While elegant, it doesn't work. This theory would predict there are roughly as many positrons (electrons moving backwards in time) as electrons. This is empirically falsified.


It cannot be empirically falsified because of our reduced point of view, and positrons would be electrons coming into 3D space from "below", i.e. going back in time, which would NOT end in a contradiction with the particle accelerators while, however, not being proved. Nowadays, the physicists are divided between this theory and the multiple-particles one, whose argument is that it is very difficult for the wave/particle to move in exact the same positions in our 3D world from a 4D world without the existence of a 5th dimension, and so on (6th, 7th, 8th...), which falsifies the former theory.

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Mon Jan 06, 2014 12:57 am UTC

Rhombic wrote: Nowadays, the physicists are divided

No, no they are not.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

Meteoric
Posts: 333
Joined: Wed Nov 23, 2011 4:43 am UTC

Re: Why do we perceive a 4-dimensional universe as 3?

Postby Meteoric » Mon Jan 06, 2014 8:36 am UTC

The One-Electron Universe hypothesis is famous because Richard Feynman brought it up once and because it's kind of an interesting way to think about that symmetry thing, not because it's actually viable or popular as a real explanation.

(IANAP, but would it also have problems explaining the occurrence of more than one electron-positron annihilation in the history of the universe?)
No, even in theory, you cannot build a rocket more massive than the visible universe.


Return to “Science”

Who is online

Users browsing this forum: No registered users and 9 guests