Can Nirvana be achieved in the human realm?
Yes!
Param Pujya Dada Bhagwan, the Enlightened One, has said,
“The human body is the greatest instrument for attaining moksha (final liberation). You cannot attain it in the celestial realm, nor in the animal realm, nor any other realm. It is only in the human avatar that all the five realms of life-forms are open.”

One’s last death in this world is termed as Nirvana. Upon achieving Nirvana, the Soul is liberated forever from the cycle of birth and death. This state of ultimate liberation is called Moksha.
Until the person does not achieve Nirvana, he continues to take birth in some or the other Gati (a life form) after death; and only when one finishes all his karma, he becomes entitled to Nirvana and goes to Moksha, then there is no further birth to take place.
There are four different gatis (life forms) in this worldly life:
Dev gati – where Celestial Beings reside
Narak gati – where Hellish Beings go
Manushya gati – where one is born as a Human Being
Tiryanch gati – comprising of all Birds, Animals, Insects and Plants
And the fifth gati is the Moksh gati or the Siddha gati (attained when the Soul is liberated).

What happens if everyone reaches nirvana?
Only those with Absolute Knowledge (Kevalis and Tirthankars) reach nirvana.
Everyone will certainly reach Nirvana, but only when one gets free from all the karmas. And then,... Param Pujya Dadashri explains, “When those with Absolute Knowledge (Kevalis and Tirthankars) reach nirvana (the final death), their Soul which is released (from this world) illuminates the entire universe; it becomes omnipresent. When the Soul goes to moksha, its full light illuminates the entire Universe. The omnipresent light spreads throughout the Universe. And thereafter, it attains the abode of the Liberated Souls (Siddha gati).

There, the constant state of absolute bliss prevails. As long as body exists, there remains a burden. This body also is cumbersome. The body through which no happiness is to be exploited, such a body is burdensome for Gnani (the Enlightened One). But that cannot be avoided! Until the body reaches its discharge limit, there is no escape!”

Could it ever happen that all the Souls go off to final liberation?
Param Pujya Dadashri asks in turn, “Can a mother and son ever be the same age? The day that a son and mother are the same age, is the day that the entire world would go to moksha! It is like this: ‘May the entire world go to moksha’ is an intention that you are entitled to. However, it will never materialize. It will only materialize when a mother and child are both the same age! One man asked me, ‘Why don’t you take the whole world to moksha?’ So I told him, ‘Let me explain to you. If all the living beings became warriors, who would they attack? If all the world’s living beings became doctors, who would they heal? If all the living beings in the world became barbers, then whose hair would they cut?’ And he says, ‘I understand, I understand, I will not ask that again!’

It is Nature’s arrangement, so we do not say that more should come together. That is its arrangement, that by way of order, they all come together. Otherwise, would we not keep sending everyone invitation letters? No, but these karmic accounts have already been arranged. So we need not worry about it. Here we just come and go.

If all the Souls went to moksha (reached Nirvana), this worldly life would no longer exist. Do you wish to destroy the worldly life?... Yes, that stage (of reaching Nirvana) does come. One does indeed enter that stage, and they are on their way to moksha. But when you say, if everyone went to moksha, you have made the intention for the destruction of the worldly life, ‘let this worldly life not be!’

This worldly life is something that helps the development of the Soul! The original Soul is already developed, the Soul itself is full and complete, but right now, your progression that has come about, is such that it is entrenched in the body complex’s beliefs. Those beliefs keep shifting, shifting, shifting towards the fundamental nature of the Self and that is when completion occurs.

Did you understand all that? This worldly life is like a stream that has been eternally flowing. Every living being in it will definitely attain final liberation, sooner or later, and that is its sequential order. The sequential order means that in a smallest fraction of time, 108 living beings are definitely going to moksha in the flow of the stream! These 108 living beings come from fifteen different locations in the Universe and they go to moksha. Just like a parade of policemen passing by in rows of four one behind another,
On this side we have a stream of 108 Souls going out from the world (vyavhar rashi - vyavahar raashi means where the living entities have a name and form now of their own) to moksha, entering the Siddha kshetra (where liberated Souls reside), and
On the other hand, the same number of Souls enter into the world (vyavhar rashi) from the dormant state (avyavahar rashi. Avyavahar raashi means those without a name, who have not yet entered worldly interaction).

Therefore, in this world, there is no increase or decrease in the number of Souls. There are infinite living entities that have not yet been named (avyavhar rashi), and that is where they all come from. As soon as few go to final liberation from here, the others are immediately admitted over here. All this is a natural law. So however many there are now, remain the same. When you count them, they are always the same in number.

The world remains as it is, and nothing can change that. So the world will remain like this forever. And if you want to understand it in a scientific way, then come to me. It is not possible to measure this world with your intellect, because although there are Souls that go to final liberation (moksha) from this world, yet the world will still remain unchanged. There is no one to wind up this world.

The Soul is what we really are; it is the real Self
Hence, your curiosity to learn more about the Soul is quite natural and justified too. And today, we are very fortunate to have amongst us Pujya Deepakbhai, the Living Gnani, to whom you can directly ask any question like what is Soul, how to awaken your Soul, etc., and get a precise answer to all your questions in a simple and lucid language that’s easy to understand and gives you satisfaction!!! Please check out https://www.dadabhagwan.org/schedule/satsang-q-a-session/

Author's Bio: 

Ambalal M. Patel was a civil contractor by profession. In June 1958, spontaneous Self-Realization occurred within Ambalal M. Patel. From this point on, Ambalal became a Gnani Purush, and the Lord that manifest within him became known as Dada Bhagwan. A Gnani Purush is One who has realized the Self and is able help others do the same. Param Pujya Dada Bhagwan used to go from town to town and country-to-country to give satsang (spiritual discourse) and impart the knowledge of the Self, as well as knowledge of harmonious worldly interactions to everyone who came to meet him. This spiritual science, known as Akram Vignan, is the step-less path to Self-realization