Until recently, Bryan Johnson was paying hundreds of thousands of dollars to infuse one litre of his teenage son’s youthful plasma into his own ageing blood stream every month. “I’ve never paid more attention to what he’s eating … because that was going into my body,” the 46-year-old American tech entrepreneur says on new podcast The Immortals. He also pumped his own plasma into his 70-year-old father’s body to help improve his declining physical and cognitive health: “It was one of the most meaningful moments in his entire life. And it was the same for me.” Johnson continues to pay $2m a year for a research team to investigate how we can live longer – and he is certainly not the only rich guy in Silicon Valley dedicated to the search for eternal life.

“It took us ages to find somebody to talk to us,” says technology reporter and psychologist Aleks Krotoski, who hosts the BBC Radio 4 series. “Strangely, people who take the blood of the young are a bit reserved … ” But Johnson had just begun using an algorithm to prevent biological ageing, which sifts through all research on longevity to create the best treatment plan and he was using his own body as a petri dish for it. Doctors have told Johnson he has the heart of a 37-year-old and the lungs of an 18-year-old and he was up for talking about this. “He was very reserved at the beginning but then there was a moment,” says Krotoski. “Suddenly I saw the geek in him: the delightfully obsessive, very clever [man]. He no longer felt ‘other’; this was just his jam. If you follow him on Twitter, he’s hilarious.”

Johnson is just one of the extraordinary people Krotoski speaks to who are trying to defy death. It is a ludicrous, fascinating and at times terrifying investigation – one she started as part of her PhD in 2003. The dawn of the internet meant “it felt like all these sci-fi dreams could be made possible”, including technological singularity – technology merging with humanity to create a “post-human existence”. Ultimately, she says, this means “we shall be immortal beings”. At the time, like everybody else, Krotoski wrote this off as “mad”. Two decades later, though, the fringe idea is entering the mainstream: “It has become entrenched in Silicon Valley, particularly because technology has become so advanced in the last five years.”

Plasma transfusions to prevent ageing became a reality in 2017 with Jesse Karmazin’s vampiric startup, Ambrosia. Hundreds of clients, with a median age of 60, would pay $8,000 (£6,200) to take part in what was essentially still a trial. However, it fell out of vogue a couple of years later when the Food and Drug Administration (FDA) said in a statement: “We’re concerned that some patients are being preyed upon by unscrupulous actors touting treatments of plasma from young donors as cures and remedies. Such treatments have no proven clinical benefits for the uses for which these clinics are advertising them and are potentially harmful.” It damaged the reputation of longevity research – but the people behind its Frankenstein origins are still known as “longevity superstars”.

Michael and Irina Conboy are professors at the Department of Bioengineering at the University of California, Berkeley. In groundbreaking research in 2005, they stitched together the bodies of old mice and young mice, like conjoined twins, and found that their combined blood “had a younger profile”. This, says Michael over Zoom, presents an opportunity in age-related illness research. “Is it like an oil change for a car – will it run a lot better?” he asks. “If you can get rid of the garbage that is floating around in the blood, does the old body restore itself to health?”

The pair are quick to add that this isn’t going to have a Benjamin Button effect, and the research “wasn’t geared to make old people young” – even if this idea is being bio-hacked around the world. “It was clear that there were improvements after a couple of procedures,” says Irina. “[But] it’s not really healthy or rejuvenating to drain somebody of 70% of their blood and replace it with something.” She warns people to wait until more research is done.

Still, they believe that in the next five years we will see huge advancements in prolonging life treatments – including taking a pill instead of getting blood, and a “fountain of middle age”. “People will be able to have this high quality, productive life where they are healthy for many more decades,” says Irina. “If people choose to, they could be in their late 30s [for much longer].”

The quest for immortality doesn’t stop there. The podcast becomes even more mind-blowing as it digs deeper, from the cryptocurrency founder who created a “longevity city” in Montenegro because he believes we have a moral responsibility to stop ageing, to AI that is already being engineered to create a “post-human being” merged with machines. Last year, Amazon founder and third richest person in the world, Jeff Bezos, reportedly invested in Altos Labs, a startup working on “cellular rejuvenation programming”. PayPal co-founder Peter Thiel, meanwhile, has invested millions in the Methuselah Foundation, a non-profit that aims to make “90 the new 50 by 2030”.

There are, of course, plenty of moral concerns being raised – especially as all this is now in the hands of billionaires. “It’s the next problem waiting to be solved,” says Krotoski. “Rather than finding something that is going to give us eternal life, they believe … Providing this gift to humanity will provide glory to those who deliver it.” There are some, she says, who predict that some groups of people will be coded out: “Some contributors were very concerned about the idea that disability and certain religious ideologies won’t be part of this future. Not intentionally, but because of what is possible. It will be narrowed by those who are delivering these solutions.” The most important thing to recognise, she adds, is that “we all have a voice in this – we are not as innocent now as we were 20 years ago”.

First, though, longevity treatments need to be legitimately recognised. Celine Halioua speaks on the podcast about her mission to get the first drug that “improves” ageing to be approved – and the hack she has used to do it. She developed a drug for dogs, which required going through the veterinary track at the FDA. By approving it, the FDA would accept that longevity medicine is legitimate and age is something that can be treated. By the end of the series, we learn that she has been successful.

At the moment, ageing and death still comes to us all. Has Krotoski’s relationship with this certainty changed while making the podcast? It was the death of her father that reignited her curiosity into such technology. But, she says, it is ultimately a story about faith and belief. Belief in a pill. In an elixir. “We developed religion because we were scared of what was going to happen after we die. This is ultimately a story about that.”

• The Immortals is out now.