Is Optical Illusion a Misnomer in Physics Discussions?

[russ_watters]

[mod note: posts re-located from here: https://www.physicsforums.com/threads/how-does-light-slide-sideways.804112 ]

Some people don't like the term "optical illusion" in a science discussion because it implies that what you are seeing isn't real. I disagree. An optical illusion is an image that is just interpreted incorrectly because of a misunderstanding or incomplete (or too much irrelevant) information.

A great many optical illusions involve a form of relativity: objects that look different from different locations, camera angles, states of motion, etc. The trick is simply making you draw a conclusion about how it looks in one frame of reference based on information provided in another.

 

[A.T.]

A great many optical illusions involve a form of relativity: objects that look different from different locations, camera angles, states of motion, etc.

This kind of talk gets people confused about Relativity, by giving them the wrong idea that relativistic effects are some kind of visual artifact.

 

[russ_watters]

How does your use/definition of the word "artifact" compare with my use/definition of the word "illusion"?

 

[A.T.]

How does your use/definition of the word "artifact" compare with my use/definition of the word "illusion"?

I mean this: "look different from different locations, camera angles". Relativistic effects have nothing to do with such visual effects, but many people wrongly believe so because such explanations.

 

[russ_watters]

I mean this: "look different from different locations, camera angles". Relativistic effects have nothing to do with such visual effects...

I don't understand: in the train animations above, in one the object appears to follow the same path over and over and viewed from a different location (that carries with it a different state of motion) it appears to zigzag. Isn't that the entire issue we are discussing?

 

[A.T.]

I don't understand: in the train animations above, in one the object appears to follow the same path over and over and viewed from a different location (that carries with it a different state of motion) it appears to zigzag.

It doesn't appear to zigzag, it is zigzaging in the ground frame.

 

[russ_watters]

It doesn't appear to zigzag, it is zigzaging in the ground frame.

Fine. And that's because of (Newtonian) relativity, right?

 

[A.T.]

And that's because of relativity, right?

What exactly?

 

[russ_watters]

What exactly?

The thing you said, that I quoted. One observer sees a straight path and the other sees a zigzag because of relativity, right?

 

[A.T.]

One observer sees a straight path and the other sees a zigzag because of relativity, right?

It's not about what some guy sees. Galilean Relativity implies that the path is different in different frames of reference.

 

[russ_watters]

It's not about what some guy sees. Galilean Relativity implies that the path is different in different frames of reference.

Fine! So why did you say previously that this has nothing to do with relativity?

[Edit] I think I at least understand what you are objecting to, just not why. You don't like the word "see" or "look" and prefer "is". To me, there is no important difference there: what the observer sees, is.

It is true that the other observer sees something different: and what he sees is happening. So what is the problem with the word choice?

 

[A.T.]

So what is the problem with the word choice?

It conflates visual effects (like perspective, signal delay) with what actually happens in a particular frame of reference.

You could have several observers at rest in the ground frame, looking at the train from different directions and thus seeing different shapes of the path. But that's not what Relativity is about. It's about the one actual shape of the path in the ground frame.

 

[wabbit]

But different observers at rest wrt each other do have different frames, differing by a translation and rotation. Why should relativity be restricted to moving frames ?

 

[A.T.]

But different observers at rest wrt each other do have different frames, differing by a translation and rotation. Why should relativity be restricted to moving frames ?

The transformations apply to such frames too, but they transform the actual paths of the object, not the visual impressions some observers in those frames have.

 

[russ_watters]

I've been in a number of physics discussions where the word "illusion" has come up (sometimes brought-up by me). I like it, but others seem not to.

Here's the definition from a google: "a thing that is or is likely to be wrongly perceived or interpreted by the senses."

My perception is that people dislike the word because they think it implies that what you are seeing isn't real. The way I read it, the definition doesn't say that. It says the interpretation is wrong.

In physics, scenarios where people see different things come up all the time when transforming reference frames and, in particular, when using rotating frames. It also happens if you consider a stream of photons (or water from a hose) as a solid object instead of individual elements that move separtely from each other.

Now, perhaps people don't like it because the definition contains the word "wrong". Different views of events from different reference frames are all equally valid, so one can't be "wrong". But that's not the way an illusion makes you think something wrong. Many optical illusions (perspective tricks) are, reference frame transformations. What makes the perception "wrong" is that you draw a wrong conclusion about what you would see from a different reference frame based on what you are seeing from the provided view. Sometimes the wrong conclusion is even the very idea that only one view of the events is valid.

So, I'd like some feedback: Does my explanation make sense? Is it a wrong/non-standard interpertation? Even if correct, does a connotation on the word make you prefer to avoid it? Or, even if correct, does it often get misinterpreted by laypeople and thus you prefer to avoid it?

That last bit is may be the key. The place where this came up recently was in a thread where we were trying to explain an issue of (Newtonian) relativity to someone. On PF, as everywhere, it is difficult to know how to explain something to a layperson in order to be accurate but not so complicated as to be confusing. This is why things get "dumbed-down" in lower level classes in school. Why you learn a different version of Conservation of Mass and Conservation of Energy from what you'd learn later on, for example. Or why no one tells you at first that Newtonian physics isn't always correct. Same issue goes for analogies. An analogy can be useful - and I like them - but if a person reads into them something that isn't intended/goes past what was intended, it can lead to a wrong understanding.

Those issues are difficult, but ultimately I think the best explanation is the one that provides the student with the correct understanding with the least amount of effort. And that can vary from one student to the next. So we shouldn't need to argue about which is better because it isn't up to us (the teachers): which is better is not always the same and is up to the student to decide. The best explanation is the one that speaks to the student best.

[mod hat: part of my reason for starting this thread was so I had a place to move a discussion to keep it from dragging a thread off-topic. The next few posts are from that thread.]

[edit] Hmm...Interesting...posts are always in chronological order. Oh well...

 

[russ_watters]

Mod note:
Posts re-located from here:
https://www.physicsforums.com/threads/how-does-light-slide-sideways.804112

 

[russ_watters]

It conflates visual effects (like perspective, [snip]) with what actually happens in a particular frame of reference.

Still not seeing a difference. What you see is what actually happens, right? In the train animations, one observer sees the object move in a zigzag because that's what actually happens, right?

You could have several observers at rest in the ground frame, looking at the train from different directions and thus seeing different shapes of the path. But that's not what Relativity is about. It's about the one actual shape of the path in the ground frame.

I disagree, but we're not talking about multiple observers on the ground in the scenario in front of us anyway, we're talking about an observer on the ground and another observer on the train. They see different things because of relativity: because they are in different frames. Right?

...signal delay...

...several observers at rest in the ground frame, looking at the train from different directions...But that's not what Relativity is about...

That's relativity of simultenaity:
http://en.wikipedia.org/wiki/Relativity_of_simultaneity

 

[wabbit]

The transformations apply to such frames too, but they transform the actual paths of the object, not the visual impressions some observers in those frames have.

But are these "actual paths" something else than "actual paths as seen by an observer"? I don't mean "seen" as in visual impression necessarily, maybe a better word is "measured" - or "observed".

If they aren't then what is the fundamental distinction?

Actually I don't understand what a path is independently of an observer.

 

[Intrastellar]

What you see is what actually happens, right?

Does relativity not tell us that one can "see" things going at any speed [Edit: meaning that they are not restricted by the speed of light] if their direction of movement is towards us ? Is seeing not different from what actually happens in this example ?

 

[A.T.]

I don't mean "seen" as in visual impression

But that's what I meant in the post you replied to.

 

[wabbit]

Is the distinction between observation with the eye and observation with other instruments, or is there more to it?

 

[A.T.]

What you see is what actually happens, right?

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

 

[zoobyshoe]

Here are some more definitions of "illusion":

"something that deceives by producing a false or misleading impression of reality."

"something that looks or seems different from what it is : something that is false or not real but that seems to be true or real"

"Something that is erroneously perceived or construed"

In Relativity no perception associated with any inertial frame is considered illusory or 'not real'.

 

[Silicon Waffle]

Can humans actually be aware of their own illusions ?

 

[russ_watters]

What you see is what actually happens, right?

No.

We were talking about the animation. Previously, you said:

...it is zigzaging in the ground frame.

I agree. I also see it zigzagging, as, I'm sure, you do. So you see it zigzagging and it is zigzagging, right? So what you see is what actually happens in that case, right?

 

[russ_watters]

...something that is false or not real but that seems to be true or real"...

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

 

[A.T.]

We were talking about the animation. So you see it zigzagging and it is zigzagging, right? So what you see is what actually happens in that case, right?

Not quite. In any ground fixed frame, the object is zigzagging at a constant angle. But that is not what every ground fixed observer will see. A guy standing on a tall bridge, looking down vertically on the train, will just see a straight line, not a zigzag. And even the guy at the side of the tracks will see the zigzag angle changing over time. So it's a matter of visual perspective what some guy sees, as opposed to what relativity says actually is in his frame.

And at greater relative speeds you get additional visual distortions from signal delay as shown here:
http://www.spacetimetravel.org/tompkins/node1.html

 

[wabbit]

Not quite. In any ground fixed frame, the object is zigzagging at a constant angle. But that is not what every ground fixed observer will see.

Excatly. The two animations of the train are each in essence a film shot by one specific observer - say, one standing on the platform and looking at the train in a direction perpendicular to the tracks, etc. They are both viewpoints and paths.
What happens here is that the frame of the observer standing on the platform, with axes aligned to the tracks etc., is a privileged frame in this particular situation, one where the movie "looks nice", and that particular observer is privileged in the same way. That doesn't mean he is more right than one looking at an angle, with his frame axes oriented differently (the frame is defined by Oxyz, i.e. O and the direction and orientation of his view, not just by O).

But as I see it, this is equally a matter of seeing and a matter of reference frame - a viewpoint is essentially a frame.

 

[A.T.]

...is a privileged frame in this particular situation...

There are no privileged inertial frames.

 

[wabbit]

It is privileged wrt the situation - among observers standing on the platform, one who looks in a direction perpendicular to the tracks gets a "better view" or at least that's the view which was preferred when drawing that animation.

But replace "privileged" by "having, among observers at rest wrt the platform, a view (or a frame) best suited for filming educational clips" if you prefer, that's all I meant.

My point was simply the equivalence of the notions of "viewpoint" and "reference frame".

 

[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)