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Measurement Uncertainty in Physics · Acedra
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Measurement Uncertainty in Physics
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0:09
Without further ado please welcome with a warm round of applause professor Walter Lewin.
0:49
A measurement without knowledge of its uncertainty is not very meaningfull. This is so key in experimental physics and yet it is not covered in high school physics and it is not covered in any college physics books. But it is so important that I want to repeat this statement. A measurement without knowledge of its uncertainty is meaningless. And I'll give you an example. When I was a kid my grandmother said do you know when you lie down that you are taller than when you stand straight up?
1:37
It puzzeled me.
1:40
We couldn't confirm it in any way we didn't have the means to do that. But it haunted me for years until I came to MIT and I decided to bring it to a test.
1:54
I reasoned as follows:
1:58
It couldn't be that when you jump out of bed that you all of a sudden become 10 cm shorter. You would know that. You would get out of bed and you would go -klunk-. You would know that. So it couldn't be 10 cm.
2:13
I also reasoned that if my grandmother is right, it couldn't be 1 mm because you would never know.
2:20
So if she is right, it is probably, a cm, maybe a little more, maybe a little less.
2:28
So if you want to measure it to see whether it is really true. The accuracy of your measurement which physicists call uncertainty. The uncertainty of your measurement would have to be about 1 mm.
2:46
And I want to bring that to a test, today.
2:51
I have here an aluminium rod.
2:55
And I'm going to measure this aluminium rod, its length, in vertical position and in horizontal position. I think none of you would believe that the rod would become longer if it is horizontal so that is not an issue. But I want to prove to you also... ...that I can measure the length of that rod to an accuracy of about 1 mm. Which was my goal.
3:22
And so I need someone to help me. Who is capable of reading Metric System. Who can read millimeters. I don't want to hear about inches and feet, it drives me nuts.
3:35
Who can we... - Can you read millimeters? Could you please come?
3:42
I'm Walter Lewin. And what is your name? David? Adrian. Adrian If you stand on this side.
3:53
Here we have the aluminium rod...
3:57
...and we are going to measure the length of this aluminium rod, in vertical position.
4:08
Do the best you can and try to get it as accurate if you can to 1 mm.
4:18
I agree with you.
4:22
149.9 - No, don't, don't go away...
4:29
So, in vertical position, the aluminium, in vertical position, is 149.9.
4:43
Plus or minus 1 mm, maybe, could we agree on that?
4:48
Plus or minus 1 mm. So plus or minus 0.1 cm.
4:57
So now if you come on this side, we are going to measure this aluminium rod in the horizontal direction.
5:08
And I want you to do the reading, so that we are all sure that it is done in a scientific manner.
5:20
Ok. Could you read...
5:30
...148.9? Yeah? That is hard to believe. I thought you told me that you know how to read metric systems.
5:42
149.6. 149.85? Boy, you're accurate.
5:54
Don't go away.
5:57
149 did you say .85?
6:03
I still think we should allow for the uncertainty of about 1 mm.
6:10
I have convinced you now that I can indeed measure to an accuracy of roughly 1 mm. And I have also convinced you that the vertical setup and the horizontal setup are both kosher. Because they give the same result.
6:26
Now, Adrian is your name. Now we need a candidate whom we will me measuring vertically... ...and who we are going to put to sleep and measure horizontally. And I would like someone who is reasonably tall because I argue that the taller person the difference between standing and lying will probably be larger than a small person.
6:52
But the person can not be this tall because then I can't measure it.
6:58
So who would want to volunteer. Who is somewhere around 6 feet. Who is willing, for the sake of science, a little sacrifice.
7:09
Would you like to try that? I see the smile on your face. You would love to do it, right? Ok, come on.
7:18
You are a bit on the short side.
7:23
Well we will accept you anyhow.
7:26
Oh-ooh, you have to take your shoes off. Now why do you have to take your shoes off, you think? What? Well, that... You go horizontal or you go vertical. Why is it important to take your shoes off?
7:41
Aaah! You're good! Because when you stand up vertically maybe you compress them. We don't want that so take your shoes off. Very good. What is your name?
7:54
Tiffany? Ok, stand here. And you are the man who knows how to read metric systems right?
8:05
So, make sure your feet are against the back. Ja? Are you nervous? You should be.
8:23
164.8 Stay here.
8:32
What was your name? That's too difficult...
8:38
"person"
8:47
Vertical. What is the... what was the reading?
8:56
You've forgotten already?
9:12
Now you see the 1 mm uncertainty is not unreasonable. 1? What do you want? 1? 164? .8.
9:30
Ok, now. May I invite you on the table. And I'd like your feet there.
9:42
And lie down. And relax. Feet all the way against there. And relax, we don't need the aluminium bar anymore.
9:55
Adrian, make sure that you agree with the way that I do this.
10:06
Ok, so we move this in again. Adrian, can you do the reading?
10:18
.0 Just relax, stay there.
10:26
Horizontal. 166.0 plus or minus 0.1 cm.
10:43
It's a little more than 1 cm and my grandmother was right. Thank you very much you can go back to your seat.
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