# Re: Planck Length

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On Sunday, January 6, 2019 at 11:39:03 PM UTC, Brent wrote:
>
>
>
> On 1/6/2019 1:56 PM, agrays...@gmail.com <javascript:> wrote:
>
>
>
> On Sunday, January 6, 2019 at 7:53:52 AM UTC, Brent wrote:
>>
>> To measure small things you need comparably short wavelengths.  If you
>> make a photon with a wavelength so short it can measure the Planck
>> length it will have so much mass-energy that it will fold spacetime
>> around it and become a black hole...so you won't be able to use it to
>> measure anything.
>>
>> Brent
>>
>
> TY. That's clear enough. But there's a related question I was unable to
> explain to a friend recently. Suppose we have a small spherical cork
> floating on a lake, and we introduce a wave disturbance. If the wave length
> is much larger than the diameter of the sphere, it will just bob up and
> down as the wave passes. But if the wave length is comparable to the
> diameter, the wave will be partially reflected. What is a good *physical*
> argument for the existence of the reflected wave, tantamount to a detection
> of the cork? I am at loss to offer a physical explanation. TIA, AG
>
>
> When the wavelength is on the order of the cork dimension or smaller the
> cork can't react to the wave as if it were just part of the water. Because
> of its extent it cannot move with the water at all points, so there are
> pressure gradients around the cork which become the source of scattered
> ripples.
>
> Brent
>```
```
Thank you, but I am unable to intuit the physicality of those pressure
gradients and their wave length dependencies. I think I need to look up how
scattering amplitudes are calculated to see the wave length dependencies
for scattering. I don't recall it being done in my classical or quantum
physics courses, a long long time ago, in a galaxy far far away. AG

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