When you think of the word "immortality" it is hard not to feel a tingling excitement, even if those feelings are quickly followed by a sense of something more biblical, almost God-like, and then by something darker lurking in the shadow of the word.

As Western science still has not found the immortality gene, it is perhaps not surprising that in Silicon Valley and on the outskirts of Moscow the eccentric wealthy (and it always is the eccentric wealthy) are now turning their attention – and their money – to projects that are promising to deliver a new version of the age-old fantasy (or folly) of everlasting life: digital immortality. And this time it may actually work.

For writer Stephen Cave, author of the new book Immortality, digital immortality does not refer to the "legacy" we have left on our Facebook pages. Cave's book explores the quest to live for ever and how – he believes – it has been the driving force behind civilisations, coming to a climax in modern science. "Digital immortality," he says, "is about there being a silicon you for when the physical you dies" as a kind of "Plan B if bioscience fails to deliver an actual biological immortality".

And of course, he adds, biological immortality would not stop you being run over by a bus.

"So your brain is scanned and your essence uploaded into a digital form of bits and bytes, and this whole brain emulation can be saved in a computer's memory banks ready to be brought back to life as an avatar in a virtual world like Second Life, or even in the body of an artificially intelligent robot that is a replica of who we were."

For Cave, though, this "is not true immortality" as "you physically die" and this new you, "even though its behaviour could fool your mum", is then just a copy. A copy that, he admits, could carry on growing, marrying and even having children.

Currently, however, this is still "almost science fiction", as there are "three big challenges" that stand between us and digital immortality – challenges that projects such as Carbon Copies and Russia 2045 already believe they can overcome within 40 years.

"The first is that we have to be able to read all the information that makes up who you are, and this is likely to be achieved destructively by removing the human brain from the body and then preserving, slicing and scanning in the data it contains. Then there is the challenge to store an amount of information many millions of orders of magnitude bigger than the current computer systems. And finally we need to find a way to animate it."

In the end, Cave argues, "theoretically the problems of digital immortality seem solvable, but whether the solutions are practical is another story... Although when it does happen it is simply inevitable that the rich will get there as they have the most power among us."

Others are more positive about the prospect of true digital immortality within a generation.

For Dr Stuart Armstrong, the rise of the idea of digital immortality is due to the realisation that this time – perhaps – we actually have the key to immortality in our hands. Dr Armstrong is research fellow at the Future of Humanity Institute, University of Oxford.

"Technology is now advancing faster and faster and we understand it a lot better because we built it ourselves. So the problems that digital immortality is facing are merely engineering problems – albeit complicated and difficult ones – that could be solved within the decade if we decided to set up a scheme on the scale of the Manhattan Project."

In particular, he feels that "scanning is the critical problem" and that if you "spent stupid amounts of cash then within a decade many of the limitations of scanning, such as its resolution, could be solved".

If computer power continues to double every two years, as described by "Moore's law", then in the end that will not be an issue either.

"Or it may be that at first we just have to accept a trade-off between what we can do and not do," he suggests. And for Armstrong this represents true immortality, since, rather pragmatically, "if this avatar or robot is to all intents and purposes you, then it is you."

Dr Randal A. Koene, though, is determined to take digital immortality from the pages of books like Cave's and turn it into reality. Koene is founder of the non-profit Carbon Copies Project in California, which is tasked with creating a networking community of scientists to advance digital immortality – "although I prefer to talk about substrate-independent minds, as digital immortality is too much about how long you live, not what you can do with it".

And for Koene it is very much "you", there being a "continuity of self" in the same way that "the person you are today is still the same person you were when you were age five".

"This isn't science fiction, either, this is closer to science fact," he argues. Carbon Copies "is working to create a road map to substrate independence by pulling together all the research that is going on, identify where the gaps are and then what we need to do to plug it.

"A Manhattan Project can easily have its funding removed by government, whereas in this network there are usually multiple projects going on in the same area, and only one needs to succeed."

Furthermore, he feels, the tide of science is moving his way, with India expecting to have built by 2017 a supercomputer big enough to handle the one exaflop of memory required for one brain upload, and such institutions as the Allen Institute for Brain Science spending $300 million to try to crack problems he also needs to solve, such as how the brain encodes, stores and processes information. "Ultimately we won't even be aware that we are being scanned, uploaded and replaced," he believes.

In the end, in Stephen Cave's opinion, digital immortality may well turn out to be a curse, as it always does in mythology.

"If my child died and I replaced her with a digital avatar to help me overcome the grieving, would I let her grow up or even have children of her own? Would I tell her she was a copy? I can imagine just how easy it would be to tell her in a row."

The complications have more serious and wide-ranging implications if humans cannot resist the temptation to "tweak their digital avatars", which may – as Stuart Armstrong argues – lead us closer to a world of "super-upgraded copies" and "the real game changer, multiple copies or clones".

"You could copy the best five programmers in the world a million times or the best call centre worker and these copies would simply replace the humans, who would no longer have any economic value," Armstrong says. "Humans would be left to die, face a life on welfare or live under coercive regulation to control the technology."

For Koene, human societies have faced these kinds of problems many times before. What matters more, he believes, is that digital immortality is the next stage of human evolution as it will "allow us as a species to have the flexibility to survive the process of natural selection that every species has to face", whether on this planet or another.