Thought Experiments: Great Fantasies In The History Of Science (PHOTOS, VIDEO)

Thought experiments are meant to illustrate a scientific or philosophical point without having to do the dirty work of an actual experiment.

You may know Schroedinger's Cat, simultaneously alive and dead, or Maxwell's Demon, moving so fast it can interfere with individual molecules, but what about the ten less famous examples in the slideshow below?

From the blind man who regains his sight but can't match up his senses, to the cannon that can shoot a projectile into orbit, these thought experiments cover a broad swath of the sciences, and remind us how engaging it can be just to speculate about them.

Comments

[Andrew Harvey]

04:09 AM on 01/11/2012

Question #1 is a perfect example of how a people of different backgrounds could come to different conclusions.

Of course, if this question was on the SAT's the correct answer would be (i)

But anyone who's ever used a rifle would have started off with the assumption that you've properly zero'd your weapon for the bullet drop; hence a falling target would only be hit by aiming low.

Go figure.

[BDinJoburg]

03:01 PM on 01/09/2012

The hunter/monkey description would perhaps be better set in a vacuum. A feather starts falling at the same speed as a cannonball (for only an instant) but the rate at which they fall soon differs on account of drag (air resistance). Drag affects the downward speed of bullet and monkey potentially unequally and a smart monkey may have a parachute on hand just to take advantage of this for such an eventuality. This may not end up being such a smart move if the hunter is accustomed to shooting at birds (hence cannonball and feather) or stationary targets and therefore has aimed high.

Similarly at extremes the case should be set on a planar earth (no escape velocity arguments) that does not spin.

But my recommendation regarding monkeys is secure your food, put away the rifle and observe how similar human and simian behaviour is. What you learn will be more valuable.

[icstars]

06:23 PM on 01/09/2012

The hunter and the monkey is a thought experiment merely to get people to understand that gravity works the same on all things. It doesn't need to be in a vacuum because it's not the hunter and the feather (and for good reason).

Hunter and monkey works very well outside of a vacuum, and actually in classrooms physics teachers often use a large ball bearing for the 'bullet' and a stuffed animal for the 'monkey'. The force with which the ball bearing is propelled directly at the monkey is often calibrated so you can clearly see the arc that the 'bullet' takes, and as long as the 'bullet' is propelled with enough force to reach the 'tree', the 'bullet' always hits the 'monkey.'

[BDinJoburg]

07:59 PM on 01/09/2012

Icstars you are correct that gravity "works the same on all things", yet have stated nothing that shows that drag "works the same on all things" in fact it seems you simply disregard drag.

The balance of these forces - gravity (a downward force) and drag (an upward force and in the case of the bullet also a backward force) is precisely why different objects have a different terminal velocity. For that matter a skydiver (one object) has a different terminal velocity depending on how they are positioned relative to the axis of gravity.

The fact that there is an arc observed in the trajectory of the ball bearing demonstrates that orthogonal forces are at play. This is entirely irrelevant to my discussion of drag in the vertical dimension.

When it is evident that objects do not always fall at the same speed in a fluid I stand by my assertion that the hunter would of necessity aim differently for a fast falling object or a slow falling object and the case is better set in a vacuum where a feather and cannonball indeed fall at equal speed.

If you are making a case simply on first approximation on small scale then the experiment is of some limited value to explain gravity. I can not however think how this explains a falcon or a paper glider to a curious child and a good physics teacher could not assert as you seem to imply that all things fall equally.

[WorkhelpWorkhelp]

04:28 AM on 01/10/2012

Poor monkey....

[jf12]

05:58 PM on 01/06/2012

The Feynman sprinkler is the best, but as a real experiment not as a thought experiment.

[yach333]

03:10 PM on 01/06/2012

They should reword it and say that it is a target hooked to a mechanism that drops the same time I pull the trigger

[unpescador]

10:55 AM on 01/06/2012

Obviously none of you peaple HUNT! A proper hunter sights his gun for the distance of his target, allowing for the drop due to gravity. At any distance where that drop is significant enough to make a difference in the trajectory a HUNTER would allow for that drop AND any movement by the target. That is why most of you should not own guns!

[GhostOfFDR]

05:52 PM on 01/06/2012

When a physicist says that a gun is pointed directly at the monkey, he means that the barrel is pointed directly that the monkey, not that the sights are pointed at the monkey. That why most of you should not by physicists.

[czs5056]

09:47 AM on 01/06/2012

the monkey and hunter website seems slightly flawed if they are conveying that i have to aim at the monkey to hit it. i can aim at the branch just below it and still make it say "ouch"

[yach333]

11:32 PM on 01/05/2012

Ok so thinking of this one most of the day and it's flawed. Let's take it to the extreme. Let's say the monkey is five hundred miles away. First I could not never hit this but that's not the point. First to hit something that far out I could not aim at the target and hit it. I would be aming really really high. Now lets give the bullet enough velocity to travel that far. So now we have the problem with time. If o could make the shot and land the bullet where I wanted it to. If it it took a minute to get there where would the monkey be in a minute. He might be at the fence throughong poop at the tourist. So maybe it's a good thing.

[Sebastin Emmanuel Mata]

02:03 PM on 01/06/2012

Physically that's wrong. If the monkey was 500 miles away, with sufficient velocity you would hit it just by aiming at it. Aiming "really high" would make you lose distance, not gain it. I'm sure that in your mind you're imagining aiming high and giving the bullet more velocity. It is unnecessary, you just need enough velocity to travel those 500 miles so that at 250 miles horizontally the bullet is at the vertical midpoint between where the monkey was in the branch and the ground.

[Lahonda]

07:04 PM on 01/10/2012

...and you better be Matthew Quigley.

[dmnlstr]

05:37 PM on 01/05/2012

The bullet and monkey experiment seems to ignore the velocity and spin of the bullet that would effect its drop

[GhostOfFDR]

05:59 PM on 01/06/2012

You have no idea what you are talking about. It does ignore atmospheric effects and the spin of the earth, which would effect its motion. The spin of the bullet stabilizes the bullet, but in the absence of atmospheric effects isn't going to effect the flight path.

[JShankel]

03:29 AM on 01/07/2012

No matter how fast something is spinning or moving, it falls at the same rate.   A bullet fired straight from 16 feet above the ground would take one second to hit the ground no matter how fast it spins or what muzzle velocity it's at.

[icstars]

06:16 PM on 01/09/2012

Very good analogy; this may help more people understand.

[Not2Tired2Stand]

01:08 PM on 01/11/2012

Your assertion that velocity, or "how fast it's moving" doesn't matter is negated by the second slide, Newton's Cannonball.

[yach333]

02:45 PM on 01/05/2012

Question? I could see if you and the monkey where straight across from eachother. If this is true then a bulet and the monkey should hit the ground at the same time. But saying it was in a tree I asume your shooting in a upward angle. If I shot almost straight up. The monkey and bullet are not going to hit the ground at the same time.

[czs5056]

09:43 AM on 01/06/2012

we did a similar demo thing in my mechanics class (physics 211) with a cannon that shot a beach ball and a Kenny from South Park doll. we were told that because gravity acted the same on the ball and the doll that it would hit the doll when it hit the ground and it did work.

[GhostOfFDR]

06:02 PM on 01/06/2012

The barrel of the gun needs to be pointed directly at the monkey. If you are pointing straight up, the monkey would be directly above you.

[WorkhelpWorkhelp]

04:30 AM on 01/10/2012

And the monkey would poop on your head.