High School Physics of Illusions: Rotating a Spring

[rahaverhma]

Today, in the morning ,One illusion came into my observation: I rotate a spring about its axis then I saw it's helical wire was advancing in the forward direction.
I can't t understand that to describe it's motion which factors should I take ?I.e. 2 factors
1) cause: ?
2) effect: motion

 

[BvU]

Haha, the spring wire does not move forward. Your eyes go along with the thread and look at a different part of the spring every time. Put a dot mark on there and see it stays in place !

 

[A.T.]

Today, in the morning ,One illusion came into my observation: I rotate a spring about its axis then I saw it's helical wire was advancing in the forward direction.
I can't t understand that to describe it's motion which factors should I take ?I.e. 2 factors
1) cause: ?
2) effect: motion

This demonstrates the principle behind screws, propellers, drills...

 

[rahaverhma]

But my sense says I see something emanating from the end and traveling forward then what is it.?

 

[XZ923]

But my sense says I see something emanating from the end and traveling forward then what is it.?

If you try out BvU's experiment, you're seeing the dot moving...

Can you please be more specific than "your sense" and "something emanating from the end?" Also please keep in mind that optical illusions are more biology than physics, and different people may perceive different illusions in a different way (remember that stupid dress picture from a few months back?)

 

[rahaverhma]

But Richard Feynman said that chemistry describes the interaction b/w substances, biology :evolution, but the subject which has nuts to describe everything in science is physics. So even that your 'biology' can be explained with the help of physics. Isn't it?

So now can you explain this phenomenon to me.

 

[BvU]

Sure. What's the phenomenon ? Bearing in mind what XZ923 warned for...

Tell us: is a spring a helix like the thread on a bolt ?
If you clamp a nut in a vise, insert a bolt and turn it, then do you see the bolt advancing? And the thread stand still ?

 

[sophiecentaur]

But my sense says I see something emanating from the end and traveling forward then what is it.?

A lot of the Illusions we see are, in my opinion. explicable in terms of evolution and in terms of what we would have seen in prehistoric life and what would be important to us.
There aren't many naturally occurring rotating helices available for animals to see but striped beasts, moving slowly forward are what they might expect to see every day That is how our brains interpret it.. So the animal brain presents the consciousness with the most likely interpretation - it's evolutionally favourable. We 'see' a moving image when presented with a sequence of cine frames because that's the most reasonable interpretation in a world where they are no sequences of discrete images in the natural world. There are many more examples.

 

[Drakkith]

But Richard Feynman said that chemistry describes the interaction b/w substances, biology :evolution, but the subject which has nuts to describe everything in science is physics. So even that your 'biology' can be explained with the help of physics. Isn't it?

Physics certainly underlies everything in chemistry and biology, but trying to describe anything but the simplest of concepts in terms of basic physics is nearly impossible. Biological organisms are extremely complex, especially when it comes to how the nervous system takes in input information, processes it, and generates an output. It's just not possible to explain the physics of why your visual system interprets the spring as moving forward when you twist it. Even if we could, it would be such a long-winded, complicated explanation that we'd be forced to condense it, leading to chemistry and maybe to abstract models that look more like descriptions of computer systems than biology or chemistry.

The differences between physics, chemistry, and biology are simply that the rules at each level arise from more basic rules in the underlying levels that have been condensed to make things easier to model and predict. For example, the rules used in chemistry arise directly from basic electromagnetic laws that govern how charged particles interact. But trying to apply these more basic laws to 10¹⁵ particles all at once is impossible. There's just too many particles. But we know the trend or pattern when we put certain particles in certain combinations in certain amounts. These rules are much more appropriate to doing chemistry than using more basic physics rules.

The same is true for biology. Cellular biology is basically a step up from chemistry (sometimes a half-step up) and examines how cells operate. From there you not only have successive levels, but you also have different specializations that branch off at each level. Someone who studies how the visual system functions is operating on a very abstract level that may be many levels above physics and chemistry.

 

[sophiecentaur]

But it isn't a beauty contest. One Science isn't more important than another. The studies are aimed at different aspects of nature.
On PF we expect to be discussing more P than other things but is P really more important than Rugby, even? Many people could give you an argument either way.

 

[sophiecentaur]

So even that your 'biology' can be explained with the help of physics. Isn't it?

No. Knowing how a component part of a system works tells you nothing about how the arrangement of the whole will operate. Solid state Physics tells you nothing about the logic of the computer program you wrote. Luckily, humans can operate in Layers, with the internal workings of Black Boxes being ignored.
I know everyone loves Richard Feynman but it cannot be true that nothing he ever said was open to question. His opinions on many things are just as fragile as yours or mine. Respect where it is due.

 

[rahaverhma]

Knowing how a component part of a system works tells you nothing about how the arrangement of the whole will operate.

Sir, as you are actually talking about the mechanisms and structure, physics (mathematics) fulfill the needs.
Mathematicians have developed the methods like Taylor's series which tells us about the adjacent parts mechanisms of a system, you just need some physical quantities.The remaining things in biology or chemistry is just same as arts, remembering (memorising) things.

Yeah I am satisfied by sir drakkith's reply more partially.

 

[tade]

Why the hell are you guys talking about perception, biology and neurology? hahaha

this is so simple. Describe a helical shape mathematically. Perform a rotation. Then a translation. Then note that these can be equivalent.This gif is for circularly polarized light, but it'll serve to demonstrate.

If you have a rotating arrow and then you push that rotating arrow in the z-direction, you get a spring shape.

Now, you can see how a rotation is equivalent to a translation, and that's how the illusion works.

 

[sophiecentaur]

and that's how the illusion works.

Exactly, it's an illusion which a more literal brain might not perceive as such. Hence "perception, biology and neurology" are relevant and interesting to many of us.

 

[tade]

Exactly, it's an illusion which a more literal brain might not perceive as such. Hence "perception, biology and neurology" are relevant and interesting to many of us.

The trick depends on the mathematical geometry, which I think that's what the OP was looking for, a "physics" approach, and also the smooth and featureless metal surface of a spring, which makes it almost indistinguishable from a magical spring that materializes at one end and evaporates at the other.

 

[sophiecentaur]

which makes it almost indistinguishable from a magical spring

Absolutely. We cannot distinguish between them because of what our minds do with the information. There is no special Physics involved and we did not evolve with rotating spirals in our look up table of effects to decode or with maths analysis. We make the best stab at it with our minds and they tell us 'horizontal motion is the most likely explanation of this confusing data'.
When such data is sub sampled (temporally) with, say movie film, the illusion can go even further and make the movement appear in the other direction (in our perception).
I guess point 2 in the OP should read " 2. effect : perception of horizontal movement"

 

[tade]

There is no special Physics involved and we did not evolve with rotating spirals in our look up table of effects to decode or with maths analysis.

It does involve the geometry of a spring though. The spring's unique geometry allows it to create the illusion, which other objects might not be able to.

Anyway, hope the OP comes back soon.

 

[houlahound]

type of from google images, these sketches blow my mind;

https://www.google.com.au/search?q=...iw=1366&bih=620#imgrc=u0hsQl0LkDUbNM:&spf=208