# Time after big bang we can observe, uncertainty principle

1. Apr 20, 2006

### Finestructure

Can someone explain why the compton wavelength and event horizon are used to determine the uncertainty in what we can see if we were to look back in time to the big bang. (We can only see back to 10^-43 seconds after the big bang.)

Here's a website that derives this time (Planck time)

http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/planck.html

2. Apr 23, 2006

### Critical_Pedagogy

"Fine"; we can't see back in time. Sorry to burst your bubble.

3. Apr 23, 2006

### matt.o

Ah, yes we can. We do it every day. In fact, right at this very instant I am studying an object as it was 3.5 billion years ago. I used a radio telescope to look back in time and see what it looked like then...

4. Apr 24, 2006

### hellfire

The Compton wavelength of the Planck mass is equal to it’s Schwarzschild radius. The Planck mass or the Planck units provide a scale for which quantum phenomena of gravitational fields should become important. As there is still no completely successful theory of quantum gravitation those phenomena remain a mistery. To see how these both quantities set this quantum gravity scale consider a black hole for which the Compton wavelength and the Schwarzschild radius are roughly the same. In that case the radius of the event horizon and the uncertainty in the position of the object are similar. This questions the existence of the singularity and therefore the validity of general relativity.

5. Apr 25, 2006

### Finestructure

Thanks, I did not know that the Compton wavelength was associated iwth the Planck mass.

6. Apr 25, 2006

Staff Emeritus

The Compton wave formula is just a function of mass; put any mass in and get a wavelength (a certain length) out. The Schwartzschild radius formula from GR is another example of a function of mass that returns a length. Set 'em equal and solve for the mass, and it comes out the Planck mass. And you know what else? The common length that comes out (equal lengths by your assumption) is the Planck length. Is that significant or just a mathematical trick? Nobody knows.

7. Apr 26, 2006

### Chronos

Not a trick IMO, SA. Just a cold, hard fact. Glad you brought that up. I see too many 'precendental' papers these days on arxiv these days that conveniently ignore this constraint [or abuse the **** out of it].