Natural units in the Schwarzschild Metric

[PeteSampras]

Homework Statement

My Teacher says that in the Schwarzschild metric he uses natural units, where he writes
$g_{rr}=1-2M/R$
He says that for one neutron star $R=5$ corresponds to approx 13 KM.

Homework Equations

$1l_p=1,616 \cdot 10^{-35}m$

The Attempt at a Solution

Unfortunately he does not says me (perhaps with a litle of arrogance) if $R=5 l_p$ or not...

But, if evaluate ( from $1-2GM/(c^2R)$)

$c^2R=(3 \cdot 10^8)^2 \cdot 5 l_p=$

$(3 \cdot 10^8 m/s)^2 \cdot 5 \cdot 1,616 \cdot 10^{-35}m=$

$7,272 \cdot 10^{-18} m^3/s^2$

But I don't know what my teacher want to say "your R=5 is similar to 13KM". More information he does not want to give me.

Help please

 

[mfb]

This is purely relativity, there is no Planck length involved.

It would be better to say R=5M. A black hole has its event horizon at R=2M (the point where g_rr gets 0), so R=5M is well outside that.

 

[vela]

Homework Statement

My Teacher says that in the Schwarzschild metric he uses natural units, where he writes
$g_{rr}=1-2M/R$
He says that for one neutron star $R=5$ corresponds to approx 13 KM.

Homework Equations

$1l_p=1,616 \cdot 10^{-35}m$

The Attempt at a Solution

Unfortunately he does not says me (perhaps with a litle of arrogance) if $R=5 l_p$ or not...

But, if evaluate ( from $1-2GM/(c^2R)$)

$c^2R=(3 \cdot 10^8)^2 \cdot 5 l_p=$

Why is $l_p$ there and what happened to $G$?

Try plugging in the values and simplifying the units on $G/c^2$. And then remember that in natural units, you're setting $G/c^2$ equal to 1.