When I was a small boy, I was adopted. I grew up remembering nothing of my birth family and unaware that I had a biological sister, named Betsy. Many years later, I went in search of my biological family, but for Betsy it was too late: she had died.

This is the story of how I was reunited with her — in Heaven.

Before I start, I should explain that I am a scientist, who has spent a lifetime studying the workings of the brain.

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Dr Eben Alexander says he was taken 'on a voyage through a series of realms' after he went into a coma when he was diagnosed with meningitis

My adoptive father was a neurosurgeon and I followed his path, becoming an neurosurgeon myself and an academic who taught brain science at Harvard Medical School.

Although nominally a Christian, I was sceptical when patients described spiritual experiences to me.

My knowledge of the brain made me quite sure that out-of-body experiences, angelic encounters and the like were hallucinations, brought on when the brain suffered a trauma.

And then, in the most dramatic circumstances possible, I discovered proof that I was wrong. Six years ago, I woke up one morning with a searing headache. Within a few hours, I went into a coma: my neocortex, the part of the brain that handles all the thought processes making us human, had shut down completely.

Dr Eben's Alexander's 'heaven' was filled with music, animals, trees, and colours and was extremely vivid

At the time, I was working at Lynchburg General Hospital in Virginia, and I was rushed to the emergency room there. The doctors ascertained that I had contracted meningitis — a rare bacterial strain of E coli was in my spinal fluid and eating into my brain like acid. My survival chances were near zero.

I was in deep coma, a vegetative state, and all the higher functions of my brain were offline. Scans showed no conscious activity whatever — my brain was not malfunctioning, it was completely unplugged.

But my inner self still existed, in defiance of all the known laws of science.

Magical: He said he found himself as a speck of awareness on a butterfly wing, among pulsing swarms of millions of other butterflies

For seven days, as I lay in that unresponsive coma, my consciousness went on a voyage through a series of realms, each one more extraordinary than the last — a journey beyond the physical world and one that, until then, I would certainly have dismissed as impossible.

For thousands of years, ordinary people as well as shamans and mystics have described brief, wonderful glimpses of ethereal realms. I’m not the first person to have discovered that consciousness exists beyond the body.

What is unique in my case is that I am, as far as scientific records show, the only person to have travelled to this heavenly dimension with the cortex in complete shut-down, while under minute observation throughout.

There are medical records for every minute of my coma, and none of them show any indication of brain activity. In other words, as far as neuroscience can say, my journey was not something happening inside my head.

Plenty of scientists have a lot of difficulty with this statement. My experience undermines their whole belief system. But the one place I have found ready acceptance is in church, where my story often tallies with people’s expectations.

My knowledge of the brain made me quite sure that out-of-body experiences, angelic encounters and the like were hallucinations, brought on when the brain suffered a trauma

Even the deep notes of the church organ and the glorious colours of the stained glass seem to echo faintly the sights and sounds of Heaven.

Here, then, is what I experienced: my map of Heaven.

After the blinding headache, when I had slipped into the coma, I gradually became aware of being in a primitive, primordial state that felt like being buried in earth.

It was, however, not ordinary earth, for all around me I sensed, and sometimes heard and saw, other entities.

It was partly horrific, partly comforting and familiar: I felt like I had always been part of this primal murk.

I am often asked, ‘Was this hell?’ but I don’t think it was — I would expect hell to be at least a little bit interactive, and this was a completely passive experience.

I had forgotten what it was even to be human, but one important part of my personality was still hard at work: I had a sense of curiosity. I would ask, ‘Who? What? Where?’ and there was never a flicker of response.

After an expanse of time had passed, though I can’t begin to guess how long, a light came slowly down from above, throwing off marvellous filaments of living silver and golden effulgence.

It was a circular entity, emitting a beautiful, heavenly music that I called the Spinning Melody. The light opened up like a rip in the fabric of that coarse realm, and I felt myself going through the rip, up into a valley full of lush and fertile greenery, where waterfalls flowed into crystal pools.

There were clouds, like marshmallow puffs of pink and white. Behind them, the sky was a rich blue-black.

This world was not vague. It was deeply, piercingly alive, and as vivid as the aroma of fried chicken, as dazzling as the glint of sunlight off the metalwork of a car, and as startling as the impact of first love.

I know perfectly well how crazy my account sounds, and I sympathise with those who cannot accept it. Like a lot of things in life, it sounds pretty far-fetched till you experience it yourself.

Despite scans showing his brain was not functioning, Dr Alexander had a vivid experience

There were trees, fields, animals and people. There was water, too, flowing in rivers or descending as rain. Mists rose from the pulsing surfaces of these waters, and fish glided beneath them.

Like the earth, the water was deeply familiar. It was as though all the most beautiful waterscapes I ever saw on earth had been beautiful precisely because they were reminding me of this living water. My gaze wanted to travel into it, deeper and deeper.

This water seemed higher, and more pure than anything I had experienced before, as if it was somehow closer to the original source.

I had stood and admired oceans and rivers across America, from Carolina beaches to west coast streams, but suddenly they all seemed to be lesser versions, little brothers and sisters of this living water.

That’s not to denigrate the seas and lakes and thunderstorms that I’ve marvelled at throughout my life. It is simply to say that I now see all the earth’s waters in a new perspective, just as I see all natural beauties in a new way.

In Heaven, everything is more real — less dense, yet at the same time more intense.

Heaven is as vast, various and populated as earth is ... in fact, infinitely more so. But in all this vast variety, there is not that sense of otherness that characterises our world, where each thing is alone by itself and has nothing directly to do with the other things around it.

Nothing is isolated in Heaven. Nothing is alienated. Nothing is disconnected. Everything is one.

I found myself as a speck of awareness on a butterfly wing, among pulsing swarms of millions of other butterflies. I witnessed stunning blue-black velvety skies filled with swooping orbs of golden light, angelic choirs leaving sparkling trails against the billowing clouds.

Those choirs produced hymns and anthems far beyond anything I had ever encountered on earth. The sound was colossal: an echoing chant that seemed to soak me without making me wet.

All my senses had blended. Seeing and hearing were not separate functions. It was as if I could hear the grace and elegance of the airborne creatures, and see the spectacular music that burst out of them.

Even before I began to wonder who or what they were, I understood that they made the music because they could not contain it. It was the sound of sheer joy. They could no more hold it in than you could fill your lungs and never breathe out.

From then on, I was back in the old, earthly world I’d left behind before my coma struck, but as a genuinely new person.I had been reborn

Simply to experience the music was to join in with it. That was the oneness of Heaven — to hear a sound was to be part of it. Everything was connected to everything else, like the infinitely complex swirls on a Persian carpet or a butterfly’s wing. And I was flying on that carpet, riding on that wing.

Above the sky, there was a vast array of larger universes that I came to call an ‘over-sphere’, and I ascended until I reached the Core, that deepest sanctuary of the Divine — infinite inky blackness, filled to overflowing with indescribable, unconditional love.

There I encountered the infinitely powerful, all-knowing deity whom I later called Om, because of the sound that vibrated through that realm. I learned lessons there of a depth and beauty entirely beyond my capacity to explain.

During this voyage, I had a guide. She was an extraordinarily beautiful woman who first appeared as I rode, as that speck of awareness, on the wing of that butterfly.

I’d never seen this woman before. I didn’t know who she was. Yet her presence was enough to heal my heart, to make me whole in a way I’d never known was possible. Her face was unforgettable. Her eyes were deep blue, and her cheekbones were high. Her face was surrounded by a frame of honey-brown hair.

She wore a smock, like a peasant’s, woven from sheer colour — indigo, powder-blue and pastel shades of orange and peach. When she looked at me, I felt such an abundance of emotion that, if nothing good had ever happened to me before, the whole of my life would have been worth living for that expression in her eyes alone.

Dr Alexander became a neurosurgeon and an academic who taught brain science at Harvard Medical School

It was not romantic love. It was not friendship. It was far beyond all the different compartments of love we have on earth. Without actually speaking, she let me know that I was loved and cared for beyond measure and that the universe was a vaster, better, and more beautiful place than I could ever have dreamed.

I was an irreplaceable part of the whole (like all of us), and all the sadness and fear I had ever suffered was a result of my somehow having forgotten this most central of facts.

Her message went through me like a breath of wind. It’s hard to put it into words, but the essence was this: ‘You are loved and cherished, dearly, for ever. You have nothing to fear. There is nothing you can do wrong.’

It was, then, an utterly wonderful experience.

Meanwhile, back on Earth, I had been in my coma for seven days and showing no signs of improvement. The doctors were just deciding whether to continue with life support, when I suddenly regained consciousness. My eyes just popped open, and I was back. I had no memories of my earthly life, but knew full well where I had been.

I had to relearn everything: who, what, and where I was. Over days, then weeks, like a gently falling snow, my old, earthly knowledge came back.

Words and language returned within hours and days. With the love and gentle coaxing of my family and friends, other memories emerged.

By eight weeks, my prior knowledge of science, including the experiences and learning from more than two decades spent as a neurosurgeon in teaching hospitals, returned completely. That full recovery remains a miracle without any explanation from modern medicine.

But I was a different person from the one I had been. The things I had seen and experienced while gone from my body did not fade away, as dreams and hallucinations do. They stayed.

Above all, that image of the woman on the butterfly wing haunted me.

And then, four months after coming out of my coma, I received a picture in the mail.

As a result of my earlier investigations to make contact with my biological family, a relative had sent me a photograph of my sister Betsy — the sister I’d never known.

The shock of recognition was total. This was the face of the woman on the butterfly wing.

The moment I realised this, something crystallised inside me.

That photo was the confirmation that I’d needed. This was proof, beyond reproach, of the objective reality of my experience.

From then on, I was back in the old, earthly world I’d left behind before my coma struck, but as a genuinely new person.

I had been reborn.

And as I shall reveal on Monday, I am by no means the only one to have glimpsed the afterlife — and the wonders it holds.

Extracted from Map of Heaven: A Neurosurgeon Explores the Mysteries of the Afterlife & The Truth About What Lies Beyond by Dr Eben Alexander, published by Piatkus at £14.99.