Experiments Reveal That Deformed Rubber Sheet Is Not Like Spacetime

KentuckyFC writes "General relativity is mathematically challenging and yet widely appreciated by the public. This state of affairs is almost entirely the result of one the most famous analogies in science: that the warping of spacetime to produce gravity is like the deformation of a rubber sheet by a central mass. Now physicists have tested this idea theoretically and experimentally and say it doesn't hold water. It turns out that a marble rolling on deformed rubber sheet does not follow the same trajectory as a planet orbiting a star and that the marble's equations of motion lead to a strangely twisted version of Kepler's third law of planetary motion. And experiments with a real marble rolling on a spandex sheet show that the mass of the sheet itself creates a distortion that further complicates matters. Indeed, the physicists say that a rubber sheet deformed by a central mass can never produce the same motion of planet orbiting a star in spacetime. So the analogy is fundamentally flawed. Shame!"

Comments

Um...

[MightyMartian]

I'm not sure the analogy was ever meant to be a rigorous and exact model, but more of a kind of way of visualizing space-time. All analogies break down if you try to map them exactly to the phenomenon you're trying to explain. After all, it's an analogy, not a model.

Re:Um...

[joe_frisch]

I have never like it as an analogy either. In the classic classroom rubber-sheet demonstration the marble rolls toward the bowling ball because the EARTH's gravity causes it to roll down hill. This is nothing at all like the way general relativity works.

General relativity requires a curvature of space-time, not just space. The best analogy I've seen comes from Kip Thorne (I think); Imagine 2 ants on the surface of an orange, both walking towards the "north" pole. Walking is an analogy to moving forward in time. After a while some "force" has brought them closer together (because they are near the pole).

Re:Um...

[bloodhawk]

I'm not sure the analogy was ever meant to be a rigorous and exact model, but more of a kind of way of visualizing space-time. All analogies break down if you try to map them exactly to the phenomenon you're trying to explain. After all, it's an analogy, not a model.

^this, many analogies in science are made to give a layperson a general/basic understanding of the concepts at work. They were never meant to be or expected to be working mathematical models.

Re:Um...

[Twinbee]

Here's my problem. What does the rubber sheet model do to explain relativity any more than explain Newtonian gravity? It seems perfect to demonstrate the latter.

Re:Um...

[girlintraining]

^this, many analogies in science are made to give a layperson a general/basic understanding of the concepts at work. They were never meant to be or expected to be working mathematical models.

But how can we claim to be more smarter than the next person if we can't take a useful analogy and utterly destroy it by being overly pedantic?

The main way in which we acquire new knowledge is by relating it to old knowledge. We introduce concepts progressively, building on primitives and emerging with complex models. Geometry can be reduced to a finite set of axioms (with an optional postulate) yet results in a near-infinite number of complex interactions. When we describe how computers work, we discuss in terms of layers of abstraction, from transistors and resistors, to APIs and data flows.

Yet at every level and skill level, I can find people who scoff at those who continue to conceptualize things based on a earlier or lower level of abstraction. These people are what I call petty intellectuals: They aren't actually smart or gifted, they just read a lot of books and memorized a bunch of shit, and think this makes them "better" than others. The truly gifted will make you feel like you, too, can be gifted. This is the real lesson out of this article -- people who pick apart analogies for being "wrong" are usually simple-minded folk of average to below-average intelligence who desperately want to be "better" than you.

The rubber sheet analogy works because it gives us a way to visualize a natural phenomenon; Not everyone has an aptitude for complex math, or the patience for it. The essentials of the theory of relativity can be relayed without resorting to complex math -- ie, describing space time as a "rubber sheet". It may not be as accurate, but accuracy is not the goal: Understanding is. It is also why we talk about "strings" in string theory, despite them having not much to do with a ball of yarn. It's why Heisenburg's black cat is forever dying in internet memes. It's why quarks have some rather strange names ... owing to leading a decidely charmed existance. Communicating concepts and relationships is what analogies are good for: They build a foundation for later learning to be given context and meaning.

This is not a small problem in the scientific community either: Richard Feynman was laughed at for years for Feynman diagrams. He was told in no uncertain terms that visualizing these complex interactions couldn't be done, shouldn't be done, and was an abomination and a sin against those who practiced "proper" science. It wasn't supposed to be simple, dammit.

Today, the Feyman diagram is one of the most recognizable images in quantum physics. The pedantics lost... but it was a bitter fight.

Thought experiments

[cold fjord]

Thought experiments using analogues like the rubber sheet are often useful for visualization, organizing your thoughts, or providing a template to work on, but that doesn't mean that they necessarily provide a picture that is correct in all respects. The fact that they aren't accurate in all respects doesn't mean that they aren't useful representations.

Works for the public

[BringsApples]

Maybe it's just a general analogy for general relativity that's easy to understand, and not to be taken so so literally. Did they bother to come up with another analogy? Didn't think so. What dicks.

I don't think ....

[therealkevinkretz]

... that anyone who had a grasp of high school phsyics, and who understood the analogy - of 3D matter flattened to represent a 2D metaphor for our real 3D world, which lives in 4D spacetime - or who understood that gravity attracted mass towards mass and not towards the "down" direction perpendicular to the sheet - would think for a second that such a demonstration would create the same exact trajectory as actual interaction between 3D objects in 4D spacetime.

Re:I don't think ....

[avandesande]

I don't think the scientists did either- I think this was more of a 'fun' experiment to see what happens when you actualize the analogy.

Stupid

[Anonymous Coward]

No analogy is perfect. However relativity is sufficiently complex that I don't know if any analogy would be perfect at all. This analogy at least provides a general sense of the way it works, it may not be 100% accurate but's relatively (see what I did there) close enough to provide a general understanding.

This article is troll

[BringsApples]

From TFA:

But the truth is that this work cannot diminish the extraordinary utility of this analogy. And so the public love affair with general relativity is safe. Long may it continue!

So what's the damn point?