0895: "Teaching Physics"

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Maxpm
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0895: "Teaching Physics"

Postby Maxpm » Fri May 06, 2011 4:02 am UTC

Image

Title text: Space-time is like some simple and familiar system which is both intuitively understandable and precisely analogous, and if I were Richard Feynman I'd be able to come up with it.

It's funny; by trying to make things easier for us to understand, we make them more confusing.

Wayfarer247
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Re: 0895: "Teaching Physics"

Postby Wayfarer247 » Fri May 06, 2011 4:03 am UTC

Of course, that guy reminds me of people I know who just love to analyze everything. It's a simile, get over it.

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Re: 0895: "Teaching Physics"

Postby scottgoblue314 » Fri May 06, 2011 4:11 am UTC

The way my high school physics teacher explained it, imagine space-time as a 3-D grid of strings. Thread a balloon into the middle, and inflate it to represent gravitational mass. The distortion of the strings is gravity.

lly
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Re: 0895: "Teaching Physics"

Postby lly » Fri May 06, 2011 4:27 am UTC

Not quite far enough.

"Space-time is like a set of equations, which are themselves merely models..."

ianfort
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Re: 0895: "Teaching Physics"

Postby ianfort » Fri May 06, 2011 4:28 am UTC

Maybe the universe is on the surface of a rotating hypersphere, and massive objects pull the fabric outward because of the the centripetal force...

Or maybe I have no idea what I'm talking about...

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Re: 0895: "Teaching Physics"

Postby glasnt » Fri May 06, 2011 4:39 am UTC

Panel 5 : "It's being pulled by the sexual attractiveness of your mum.

Now, moving forward... "
Last edited by glasnt on Fri May 06, 2011 5:32 am UTC, edited 1 time in total.

KeithIrwin
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Re: 0895: "Teaching Physics"

Postby KeithIrwin » Fri May 06, 2011 4:52 am UTC

I had this same objection because the rubber sheet explanation is usually used to explain gravity, but the example itself depends on understanding gravity.

My alternative version of this was to imagine two parallel sheets of rubber distorted by objects place between them and then assuming that things must follow the sheets. It's still imperfect, as any metaphor will be, but it doesn't require gravity in its explanation of gravity.

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Re: 0895: "Teaching Physics"

Postby freakypumps » Fri May 06, 2011 4:54 am UTC

hey glasnt, wouldn't that be anti-gravity?

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Re: 0895: "Teaching Physics"

Postby Plasma Mongoose » Fri May 06, 2011 4:58 am UTC

Trying to think up the right analogies for string theories are why scientist find it so hard to explain to the laypeople.
A virus walks into a bar, the bartender says "We don't serve viruses in here".
The virus replaces the bartender and says "Now we do!"

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Re: 0895: "Teaching Physics"

Postby somethingoutrageous » Fri May 06, 2011 5:01 am UTC

Space-time is like a questionable sausage: nobody knows who or what made it, or where it came from, or what it's made of, and if you eat it all you'll probably die. Moral: don't eat space-time.

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Re: 0895: "Teaching Physics"

Postby ianfort » Fri May 06, 2011 5:02 am UTC

glasnt wrote:It's being pulled by the sexual attractiveness of your mum.

Now, moving forward...


There's no need to be insulting. I already acknowledged I was probably wrong.

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ysth
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Re: 0895: "Teaching Physics"

Postby ysth » Fri May 06, 2011 5:09 am UTC

No, if you were Feynman, you'd be dead.
A math joke: r = | |csc(θ)|+|sec(θ)| |-| |csc(θ)|-|sec(θ)| |

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Samik
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Re: 0895: "Teaching Physics"

Postby Samik » Fri May 06, 2011 5:26 am UTC

Wayfarer247 wrote:Of course, that guy reminds me of people I know who just love to analyze everything. It's a simile, get over it.

As a someone who uses metaphors liberally just in my basic patterns of speech, I hate that one guy who always calls you out on the imperfections in every single one, as if you were trying to leverage each and every metaphor you've ever used to pull down the wool and slip some major fallacy past everyone for your nefarious purposes.

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Re: 0895: "Teaching Physics"

Postby Samik » Fri May 06, 2011 5:31 am UTC

lly wrote:Not quite far enough.

"Space-time is like a set of equations, which are themselves merely models..."

Wait a second, isn't that circular?

"Space-time is like a set of equations, which are themselves merely models..." ...of space-time? Hence why space-time is related to them via simile?


Admission: I'm no physicist. If you had intended to end that sentence another way, my mistake.

lly
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Re: 0895: "Teaching Physics"

Postby lly » Fri May 06, 2011 5:41 am UTC

Samik wrote:
lly wrote:Not quite far enough.

"Space-time is like a set of equations, which are themselves merely models..."

Wait a second, isn't that circular?

"Space-time is like a set of equations, which are themselves merely models..." ...of space-time? Hence why space-time is related to them via simile?


Admission: I'm no physicist. If you had intended to end that sentence another way, my mistake.


"...and 'all models are wrong, but some are useful'"

Or if you prefer, "the map is not the territory."

americablanco
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Re: 0895: "Teaching Physics"

Postby americablanco » Fri May 06, 2011 5:58 am UTC

Samik wrote:As a someone who uses metaphors liberally just in my basic patterns of speech, I hate that one guy who always calls you out on the imperfections in every single one, as if you were trying to leverage each and every metaphor you've ever used to pull down the wool and slip some major fallacy past everyone for your nefarious purposes.


I'm that guy. I have a friend who is that guy, too. We get along really well, :)

More on topic (sort of): when seeing the title I hoped for a 3rd Rock from the Sun reference, possibly how Dr. Dick Solomon would not grade the class on a curve, but on a transient loop "whose last two loops are not visible because they are merely factors of time." Well... whatever.

notapplicable
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Re: 0895: "Teaching Physics"

Postby notapplicable » Fri May 06, 2011 6:09 am UTC

Although it has been said before, about how Randall channels our lives into comics as he writes them, but this is uncanny. TED just released this video:
http://www.ted.com/talks/sean_carroll_distant_time_and_the_hint_of_a_multiverse.html

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Re: 0895: "Teaching Physics"

Postby SBT_downunder » Fri May 06, 2011 6:17 am UTC

"Relativity Visualized" by Lewis Carroll Epstein does a pretty good job of describing it actually. Forget the rubber sheet, space is like a sheet of paper (notice that I said space, not spacetime, because gravity comes about from the curvature of both space and time, and the analogy works better when you consider both independently). Approximate curvature by putting paper cones on the sheet of paper (a cone being formed by cutting a "pizza slice" out of a circle of paper, and joining the edges together, or by a similar process to folding up a circular coffee filter). A straight path drawn across the flattened cone will be curved when the two edges (where the pizza slice were removed) are stuck together to create the conical shape. The path of an object through space is given by tracing straight lines, and then folding up the sections representing curvature. Notice that it doesn't matter whether the cones are "above" or "below" the flat sheet - you get the same result either way, since we're talking about drawing straight lines on a curved surface - hence addressing the objection most students pose in the real-life equivalent of panel 2. Gravity is just "too much space in too small a region" and objects don't "roll downhill" into the gravity well of a star or planet.

The curvature of time can be represented by bending the time axis over on a spacetime diagram (so that you have to travel further "up the page" one one side of the diagram, than the other side of the diagram, to reach the same time in the future).

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Re: 0895: "Teaching Physics"

Postby SciBoy » Fri May 06, 2011 6:37 am UTC

KeithIrwin wrote:I had this same objection because the rubber sheet explanation is usually used to explain gravity, but the example itself depends on understanding gravity.

Actually, it doesn't. You don't need to understand gravity to know that a ball placed on a rubber sheet will pull the sheet down. That you can observe in reality without knowing anything about gravity at all. In this case this effect is used to explain how gravity affects space/time, not what gravity is. We still don't really know what gravity is, unless I missed some recent revelation, we don't know how gravity can affect stuff so far away.

Have they found the graviton?
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Re: 0895: "Teaching Physics"

Postby Arancaytar » Fri May 06, 2011 7:09 am UTC

Title text: Space-time is like some simple and familiar system which is both intuitively understandable and precisely analogous, and if I were Richard Feynman I'd be able to come up with it.


I thought that was already Richard Feynman at the blackboard. :P
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Re: 0895: "Teaching Physics"

Postby Randomizer » Fri May 06, 2011 7:39 am UTC

Nice comic. Heh, it wasn't until not too long ago that I started to think, "Hey, wait a minute, they're using something that depends on gravity to explain how an object can warp space to create... gravity."

Then the apple left my hand and went back into the tree. The tree itself uprooted and floated into space. Earth slowly fell apart, and soon I was all alone.

But really, I feel like a kid who's come to realize Santa doesn't exist but still doesn't know how the presents get under the tree every year.
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Re: 0895: "Teaching Physics"

Postby Chrisfs » Fri May 06, 2011 7:51 am UTC

I loved the rubber sheet analogy (though it was told as a trampoline analogy to me originally).

Yes it uses gravity as a feature of the analogy, but the problem is not that people can't intuitively grasp the concept of gravity at all. People see the effects of gravity every day. It's used to explain a possible 'why' of gravity, not an existence of it.

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Re: 0895: "Teaching Physics"

Postby Chrisfs » Fri May 06, 2011 8:00 am UTC

SciBoy wrote:
KeithIrwin wrote:I had this same objection because the rubber sheet explanation is usually used to explain gravity, but the example itself depends on understanding gravity.

Actually, it doesn't. You don't need to understand gravity to know that a ball placed on a rubber sheet will pull the sheet down. That you can observe in reality without knowing anything about gravity at all. In this case this effect is used to explain how gravity affects space/time, not what gravity is. We still don't really know what gravity is, unless I missed some recent revelation, we don't know how gravity can affect stuff so far away.

Have they found the graviton?


It's under the couch. It's what causes other dropped objects to be drawn under the couch.

A bit more seriously,
I understood the concept of gravity warping space time just fine (in a layman sort of way). but the idea of gravitons simply confused me. It seems completely at odds with the bending space time thing. It also implies that it takes time for gravity to affect distance objects. And that all particles are emitting a constant stream of these things ???

Almgren
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Re: 0895: "Teaching Physics"

Postby Almgren » Fri May 06, 2011 8:09 am UTC


Blarg
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Re: 0895: "Teaching Physics"

Postby Blarg » Fri May 06, 2011 8:11 am UTC

It's a funny comic, and a welcome departure for the usual formula. For a long time it seems we've been getting comics where the guy teaching the class would be a lazy jerk and the student would be a nitpicky jerk, only this time the student is not the hero of the story. He's just being a pedant, and when the long-suffering teacher (probably having endured several previous XKCD protagonists) figures that the only escape is to humor the student for now, the student responds with dismissive derision, possibly whilst masturbating furiously under the desk.

richardr
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Re: 0895: "Teaching Physics"

Postby richardr » Fri May 06, 2011 8:32 am UTC

I'm currently reading Roger Penrose's "The Road To Reality", and I'm going to laminate this cartoon and use it as a bookmark.
For those who've not tripped over this in the local bookshop, it's about 1,000 pages long, and the first third or so is a tour through the maths required for the rest of the book where we finally get to some physics. There are almost no physical analogies - everything follows from the maths - and it uses nothing like the notation I remember from university. It's extremely hard-going if you try to use it as a textbook and work through the examples, but if you're lazy and just work on understanding the concepts then it's just hard-going. But rewarding. :D

Technical Ben
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Re: 0895: "Teaching Physics"

Postby Technical Ben » Fri May 06, 2011 8:56 am UTC

lly wrote:Not quite far enough.

"Space-time is like a set of equations, which are themselves merely models..."

This. Whenever someone tries to "prove" to me that a whale and a tea pot can appear in mid air and shout "not again!" I say their models must be wrong.
It's all physics and stamp collecting.
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Re: 0895: "Teaching Physics"

Postby collegestudent22 » Fri May 06, 2011 9:17 am UTC

SciBoy wrote:
KeithIrwin wrote:I had this same objection because the rubber sheet explanation is usually used to explain gravity, but the example itself depends on understanding gravity.

Actually, it doesn't. You don't need to understand gravity to know that a ball placed on a rubber sheet will pull the sheet down. That you can observe in reality without knowing anything about gravity at all. In this case this effect is used to explain how gravity affects space/time, not what gravity is.


I would think it is entirely plausible to extend the rubber sheet analogy, as gravity's effects on a 4D space-time with 3D matter in it is exactly analogous (minus differences in the curvature equations) to the 2D sheets response to what (in it's contact frame) amounts to a 1D mass. The difference is in the extension to more dimensions. Because what is being defined is not gravity - we still don't know (although there are some theories - quantum loop, graviton, etc.) how the force actually works, just what its effects are.

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Re: 0895: "Teaching Physics"

Postby finnbryant » Fri May 06, 2011 9:26 am UTC

Chrisfs wrote: It also implies that it takes time for gravity to affect distance objects.

exactly right. My understanding is that if the sun ceased to exist, we'd only stop orbiting about 8 mins later.

otherwise gravity would break the speed of light barrier...

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Re: 0895: "Teaching Physics"

Postby PlaidViking » Fri May 06, 2011 9:34 am UTC

As a physics teacher, I love it when students point out the flaw in the rubber sheet analogy, especially as it does only represent a 2-D space-time. The analogy I use as a follow up is tasty and accounting for 3-D. Using a bit of preparation time earlier, you make some medium sized transparent containers of gelatin (enough for each student). I recommend cherry gelatin as space-time should be red (and is on average redshifted), though blueberry can be used for observational reasons. I usually also toss some small cherries in there to represent objects in space time.

Once the gelatin has set you can bring them out to the students along with a straw. By carefully shoving the straw so that its end is inside the mold, the students can lightly blow in the straw and visualize the disturbation of space-time in 3-D and have a fun snack while you continue to discuss gravitation.

A couple of places for error that usually pop up are a student blowing too hard, which provides its own means of correction with the student and or his notebook getting bits of gelatin on him. Another error is if a student wiggles the straw around too much while inserting it, the seal between the edge of the straw and the outside won't be as tight and air might leak out dimishing the effect.

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Re: 0895: "Teaching Physics"

Postby tastelikecoke » Fri May 06, 2011 9:46 am UTC

This is so like my brainstorming problem with the rubber sheet model. Now that I thought about it, you can just say that the balls are being pulled downward by the wind so the analogy would work.

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snowyowl
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Re: 0895: "Teaching Physics"

Postby snowyowl » Fri May 06, 2011 10:37 am UTC

Chrisfs wrote:
SciBoy wrote:Have they found the graviton?


It's under the couch. It's what causes other dropped objects to be drawn under the couch.

A bit more seriously,
I understood the concept of gravity warping space time just fine (in a layman sort of way). but the idea of gravitons simply confused me. It seems completely at odds with the bending space time thing. It also implies that it takes time for gravity to affect distance objects. And that all particles are emitting a constant stream of these things ???

Well, bending space-time is predicted by General Relativity, while force-carrying particles are predicted by Quantum Mechanics.The two theories don't exactly get along together, so it's entirely possible there's no such thing as gravitons. And then again, there might be: remember, "space-time bending" is just a useful analogy which is an approximate description of the way the universe works.

Gravity does take a while to affect distant objects, though. Gravity waves travel at the speed of light. Relevant comic.
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-mr. bill
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Re: 0895: "Teaching Physics"

Postby -mr. bill » Fri May 06, 2011 11:01 am UTC

The Title Text.

Search for "Feynman 'Fun to Imagine' 4: Magnets" - from a series of BBC interviews in the 80s.

On the rubber band analogy for magnetic attraction:

"I can't explain that attraction in terms of anything else that is familiar to you."
"I'd be cheating you.... So I have cheated very badly."

I'm going to guess that he felt the same way about the rubber sheet analogy.

Just one of the great teachers.

-mr. bill

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ManaUser
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Re: 0895: "Teaching Physics"

Postby ManaUser » Fri May 06, 2011 11:49 am UTC

The trouble with the rubber sheet analogy isn't so much that the objects distort it only because of gravity, but that creating a depression in the sheet only effects other because they roll "downhill". The first I can accept because "mass distorts space-time" is kind of a given, you're not trying to prove it, just help visualize it. Okay, but for the analogy to be useful at all it should at least explain how distorting space-time would have the effect of attracting other objects without relying on "down".

hamjudo
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Re: 0895: "Teaching Physics"

Postby hamjudo » Fri May 06, 2011 12:23 pm UTC

Okay, so replace the image in your mind of the rubber sheet, with a 3d space with a vector at each location. Each vector in the space is the sum of a vector for each mass in the system. The vector points towards the mass, the length of the vector corresponds to the mass divided by the square of the distance from the mass. Each mass in the system has a direction and a velocity. You can calculate where an object will be in the next time step, by altering the velocity and direction of travel based on the little vector at that spot on the field.

The cool thing, is that motion through this field corresponds fairly well with objects in the real universe that are large enough so that quantum effects are minimal, small enough, so they don't create black holes, and are moving sufficiently below the speed of light so that we can ignore certain parts of special relativity, or are massless and moving at the speed of light. The second cool thing, is that if we collapse this down to two dimensions, it also corresponds with the famous rubber sheets.

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Re: 0895: "Teaching Physics"

Postby Mental Mouse » Fri May 06, 2011 12:37 pm UTC

This is a tricky analogy, but it's one that's worth salvaging with a bit of explanation:

The better examples of this metaphor (including IIRC the original) do not have balls rolling around on the rubber sheet, only creating gravity fields. Observers are instead represented by ants, clinging to the sheet. That is, the ants are assumed to be small enough that their weight is irrelevant to their motion, what affects them is the curvature of the sheet.

The "external gravity" around the visualized model is not part of the metaphor, it's just used to set up the prop. This would work just as well if the balls were instead pushed into the sheet by magnetic fields or simple posts. (And yes, the geometry works just fine on the other, "bumpy" side of the sheet.)

Hah, just realized why this situation feels familiar -- this metaphor is affected by the ELIZA effect: intuitions from real life that don't properly apply.

(Edited for clarity and adding the name)

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DougDean
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Re: 0895: "Teaching Physics"

Postby DougDean » Fri May 06, 2011 12:57 pm UTC

The problem with the rubber-sheet visual as presented by most popular science explanations is that they forget to include time among the things being curved.

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Re: 0895: "Teaching Physics"

Postby xnick » Fri May 06, 2011 1:17 pm UTC

Epic alt-text! :D

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Re: 0895: "Teaching Physics"

Postby Pxtl » Fri May 06, 2011 1:40 pm UTC

Good to know I'm not the only one who hates the "rubber sheet" explanation - explaining gravity *with* gravity is circular reasoning. I've always thought that Gravitons provide the best understanding of gravity... although they don't really cover the spacetime-warping aspect.

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unus vox
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Re: 0895: "Teaching Physics"

Postby unus vox » Fri May 06, 2011 2:34 pm UTC

Space-time is like an analogy: while appearing straightforward, it can be easily distorted and thus extremely difficult to understand.
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