Is Optical Illusion a Misnomer in Physics Discussions?

[zoobyshoe]

I think that definition is referring to drawings of things that are impossible, and wouldn't apply to an actual photo/observation. For example:

No, the definition includes all things that aren't what they seem to be. It's not limited to drawings at all. For example:

"The oasis seen a couple miles in the distance turned out to be an illusion."

"The big payoff at the end of the Panzi scheme was exposed to be an illusion."

"The glory that Bob envisioned upon the announcement of his perpetual motion machine was nothing but an illusion."

"The monster of Lake Johnson was an illusion created by otters swimming in a row."

"A ring round the moon is a visual illusion created by atmospheric conditions."Now, consider a situation of Galilean Relativity:

A man standing on a flatbed car on a moving train is bouncing a ball. To an observer sitting on the same car, the ball's path describes a straight vertical line. To an observer standing on the ground some distance away from the train, the ball's path describes a series of parabolas.

Neither of these paths is illusory. Each is real in the inertial frame of the observer. The point of Relativity is to understand there is no absolute inertial frame whereby one path becomes "real" and the other an "illusion". Motion is always relative to a given observer.

 

[bahamagreen]

No. See for example here:
http://www.spacetimetravel.org/tompkins/node1.html

Figure 2
b) The actual shape of the sphere in the frame where moves fast
c) How an observer at rest in that frame would see the sphere

Can you clarify b) and c) above? It is not clear for b) if you mean "in the frame where it moves fast" or "the frame which moves fast". The former would mean the sphere in relative motion to the observer, the later would not... And c) "that frame" would also refer to to b)...

But the Tompkins links says of c) " as a measurement of its shape would show. Looking at the moving sphere, one sees it with an exactly circular outline, but rotated" which makes me wonder if the descriptions and letter assignments are messed up... because that first part about c) looks like maybe it was meant to be for b).

I'm thinking this because Wikipedia says "A previously-popular description of special relativity's predictions, in which an observer sees a passing object to be contracted (for instance, from a sphere to a flattened ellipsoid), was wrong."

I think the Tompkins link is trying to show the same thing but messed up the descriptions between b) and c)...?

 

[Pythagorean]

Here's an example of an optical illusion in which brain processing is actually injecting a perception that is not relevant to reality (a green dot). Stare at the crosshair and you should see the green dot. This is different than geometrical arrangement - you're actually perceiving something that's not there, presumably due to opponent process theory of color perception.

http://kids.niehs.nih.gov/assets/images/p/pinkdots.gif

 

[A.T.]

Here's an example of an optical illusion in which brain processing is actually injecting a perception that is not relevant to reality (a green dot). Stare at the crosshair and you should see the green dot. This is different than geometrical arrangement - you're actually perceiving something that's not there, presumably due to opponent process theory of color perception.

http://kids.niehs.nih.gov/assets/images/p/pinkdots.gif

What's more amazing is the little red rectangle pooping up at the 10 o'clock dot!

 

[A.T.]

Can you clarify b) and c) above? It is not clear for b) if you mean "in the frame where it moves fast"

This

I think the Tompkins link is trying to show the same thing but messed up the descriptions between b) and c)...?

I think the captions are fine:

http://www.spacetimetravel.org/tompkins/node1.html

a) sphere at rest in the observer's frame
b) sphere moving, relativistically contracted (what the observer's frame would measure)
c) sphere moving, relativistically contracted with signal delay (what the observer's would would see visually)
d) sphere moving, not contracted with signal delay (what the observer's would would see visually, according to pre-SR physics)

 

[bahamagreen]

Maybe I'm missing something - why is what the observer gets as a "measurement" and what the observer would "see visually" not the same? The measurement would have to be done with light, right? If so, how is the measurement light different from the visual light (to get the shape difference)? If not, what is used to measure it?

I still think the captions are clumsy. The "b)" is the only one that does not mark the end of a section that refers to it, but appears in the middle of its section.

 

[wabbit]

I would say the difference lies in how he interprets the data. The observer doing a measurement could very well do it from those same camera frames showing a round ball plus a few auxiliary measures or data. But the equations he uses do not say that the size of the object is directly proportional to its apparent width - instead he will apply formulas that take into account both time delay and length contraction, to convert the raw measurements to results about the actual shape of the object.

This isn't a fundamental difference - our brain also constantly applies complex algorithms to infer "true shape" from the raw image it receives from the retina (as many visual illusions demonstrate, the image we see is not the image on the retina), It isn't trained to do relativistic corrections though.

 

[zoobyshoe]

Here's an example of an optical illusion in which brain processing is actually injecting a perception that is not relevant to reality (a green dot). Stare at the crosshair and you should see the green dot. This is different than geometrical arrangement - you're actually perceiving something that's not there, presumably due to opponent process theory of color perception.

That's an alarmingly strong illusion. And a circle of green dots persists in my visual field when I look away.

 

[Pythagorean]

That's an alarmingly strong illusion. And a circle of green dots persists in my visual field when I look away.

Here's an even more interesting one. The afterimage changes color when the shape rotates - and it's all in your head! What the heck!? (stare at the dot as normal)

 

[wabbit]

My visual cortex begs you to stop, sir : )

 

[zoobyshoe]

My visual cortex begs you to stop, sir : )

Being a more direct articulation of what I meant to express in my post #38.

 

[A.T.]

Maybe I'm missing something - why is what the observer gets as a "measurement" and what the observer would "see visually" not the same?

Visual impression is affected by signal delay. Measurement means that you remove such known artifacts.