# Why planck scale? let me explain before kill me

• luxxio
In summary: No, this is not always true. In fact, it is only true for certain types of quantum gravitational systems.
luxxio
Why we are searching quantum gravity effect at the Planck scale?
The question seems very stupid (and may be) but let me explain.
I define a quantum object when his action is of the order of Planck constant \bar h.
namely A~E \delta t~\bar h. So if i take a small amount of energy E i can take \delta t big.
Applied to gravity i can say that if i take a small (in terms of energy) gravitational system, i can see quantum effect at macroscopic level. In the case of electromagnetism i can have photon of macroscopic length.
In my opinion this claim is true, but is very very difficult to find or to build such object, like in the case of photon is very difficult create only one photon of macroscopic length.
Is this true?

There are lots of answers to your question, I'll give one.

Just look at the units. The Newton's constant can be related to a mass scale by setting hbar = c = 1. You find

$$G_N = \frac{1}{8\pi M^2}$$.

M is the Planck mass, 10^19 GeV. This is the natural scale for gravitational interactions.

There is precedent for this sort of hand-wavy estimation. In Fermi Theory, we can calculate the scattering of electrons off of neutrinos. The coupling constant in that theory, with hbar = c = 1 is given by

$$G_F \sim \frac{1}{M_W}$$

where M_W is the mass of the W boson, about 80 GeV or so. It is well established that this is the scale where the Fermi Theory fails to be valid, and we have to use a different theory.

So we expect GR to be valid up to the Planck mass, and beyond that we need a new theory.

Of course, there could be some new theory which we don;t know about, which makes these estimations wrong. But, according to all we know, this is our best guess.

BenTheMan said:
There are lots of answers to your question, I'll give one.

Just look at the units. The Newton's constant can be related to a mass scale by setting hbar = c = 1. You find

$$G_N = \frac{1}{8\pi M^2}$$.

M is the Planck mass, 10^19 GeV. This is the natural scale for gravitational interactions.

There is precedent for this sort of hand-wavy estimation. In Fermi Theory, we can calculate the scattering of electrons off of neutrinos. The coupling constant in that theory, with hbar = c = 1 is given by

$$G_F \sim \frac{1}{M_W}$$

where M_W is the mass of the W boson, about 80 GeV or so. It is well established that this is the scale where the Fermi Theory fails to be valid, and we have to use a different theory.

So we expect GR to be valid up to the Planck mass, and beyond that we need a new theory.

Yes i konw this argumentation, but let me explain better the question.
Is always true that appling the quantum condition to a gravitational system leads to the Planck scale?

## 1. Why is the Planck scale important in physics?

The Planck scale is important in physics because it is the scale at which quantum effects become significant and the laws of classical physics break down. It is also the scale at which gravity is as strong as the other fundamental forces, making it a crucial component in theories of quantum gravity.

## 2. What is the significance of the Planck length and Planck time?

The Planck length and Planck time are the fundamental units of length and time in the Planck scale. They represent the smallest possible length and shortest possible time that can be measured, beyond which the concept of space and time become meaningless.

## 3. How is the Planck scale related to the Big Bang theory?

The Planck scale is related to the Big Bang theory as it is the scale at which the universe is believed to have originated. At the Planck time, the universe was in a state of extreme density and temperature, and the four fundamental forces were unified.

## 4. Can we observe or test phenomena at the Planck scale?

Currently, we do not have the technology to directly observe or test phenomena at the Planck scale. However, scientists use mathematical models and theories, such as string theory, to make predictions about what may occur at this scale.

## 5. Why is it challenging to study the Planck scale?

The Planck scale is challenging to study because our current understanding of physics breaks down at this scale. Additionally, the energy required to probe the Planck scale is beyond the capabilities of our current technology. This scale is also difficult to study because it is extremely tiny and occurs at the very beginning of the universe, making it hard to access or observe.

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