# B How have we figured out the Plank length of the universe?

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1. Sep 2, 2016

### hsdrop

how have we figured out plank length of the universe ?and know its that size ?

2. Sep 2, 2016

Staff Emeritus
This makes no sense. It's like saying "have we figured out the radius of an apple for a skyscraper?".

3. Sep 2, 2016

### hsdrop

lol sorry ok i know we have a planck length i was wondering how we found it

4. Sep 2, 2016

Staff Emeritus
Like we "found" the foot or the meter. We defined it.

5. Sep 2, 2016

### hsdrop

hmmmm it always the easy questions that are the most difficult to answer,lol. i know we have a minimum length that we can mescher SA particles with i was just wondering how we came up with it or how we knew that it was the smallest light we can mescher by sorry for not being more chear with my question and thank you for replying

6. Sep 2, 2016

### Student100

Plank length was defined from several other physical constants. PL, versus lengths we can actually currently measure are several orders of magnitude apart. I think the last thing I read had the smallest length measured somewhere around 10^-18m. Which is incredibly small, but no where near PL at 10^-35m.

So PL has use in theory, but we don't have any way to currently measure lengths that small. It would also be wrong to say it is the minimum length that can be measured - we can't yet test that experimentally.

7. Sep 2, 2016

### Algr

I don't think we did.

8. Sep 3, 2016

### Student100

The quote seems like gobblygook. There is no current physical significance of PL, and no way to actually test what happens when you measure lengths that small. For all we know QG could very well require other physical constants than c, G, $\hbar$. Plank just happened to define this length a certain way.

I don't think it correct to say
Without experiments, no physics. Just half-blind speculation.

9. Sep 3, 2016

Staff Emeritus
For good reason.

Planck units have no mystical power. The Planck charge is 11.7 electron charges. The Planck resistance is 30 ohms. Nothing magic happens there. It's probably true that quantum gravity becomes important at a scale of perhaps 10 or 20 Planck lengths, but that's just a statement about the relative sizes of h, c and G.

10. Sep 5, 2016

### Algr

The Planck flavor is Blueberry Fudge. The Planck address is 1325 Maple st. But that doesn't invalidate EVERYTHING Max Planck did.

BTW, Some of the formatting was lost in that quote (Those are supposed to be exponents, not ranges.) Lets see if this works:

11. Sep 5, 2016

### hsdrop

Last edited by a moderator: May 8, 2017
12. Sep 5, 2016

### Fervent Freyja

What is that supposed to mean? Are you implying that Max Planck had a small member? That is a horrible joke (though telling).

13. Sep 5, 2016

### Staff: Mentor

14. Sep 5, 2016

### OCR

Well, now you've done it !...

You're on Max's "really, really bad" list... he's likely to whop you on the head with a big board ...[COLOR=#black].[/COLOR] lol