Planck time

In physics, the Planck time (t_P), is the unit of time in the system of natural units known as Planck units. It is the time it would take a photon travelling at the speed of light in a vacuum to cross a distance equal to the Planck length.^[1] The unit is named after Max Planck. It is defined^[1] as

<math>t_P = \sqrt{\frac{\hbar G}{c^5}} \approx 5.39121(40) \times 10^{-44} \mbox{ s}</math>

where:

<math>\hbar = h / 2 \pi</math> is the reduced Planck constant

<math>G</math> is the gravitational constant

<math>c</math> is the speed of light in a vacuum

<math>t_P</math> is in seconds.

The two digits between the parentheses denote the uncertainty in the last two digits of the value.

Significance

According to the Big Bang theory nothing is known about the universe at time=0, though it is presumed that all fundamental forces coexisted and that all matter, energy, and spacetime expanded outward from an extremely hot and dense singularity. One Planck time after the event is the closest that theoretical physics can get to it, and at that time it appears that gravity separated from the other fundamental forces. One second is about 1.855×10⁴³ Planck times. The estimated age of the Universe in the Big Bang theory (4.3×10¹⁷ s) would be roughly 8×10⁶⁰ Planck times. The average life expectancy of a human is approximately 3.9×10⁵² Planck times. As of 2006, the smallest unit of time that has been directly measured is on the attosecond (10⁻¹⁸ s) timescale, or around 10²⁶ Planck times. ^[2][3]

See also

• Planck epoch

References

1. ^ ^{a b} Big Bang models back to Planck time. Georgia State University (19 June 2005).
2. ^ Shortest time interval measured. BBC News (25 February 2004).
3. ^ Fastest view of molecular motion. BBC News (4 March 2006).