Now that the secret's out about billionaire Elon Musk's Hyperloop high-speed transit system, experts are assessing whether anyone could actually build the darn thing.

On one hand, there are enthusiastic fans such as John Gardi, the self-described "tinker" who figured out much of the plan before Monday's big reveal.

"Not only did Elon Musk devise a brilliant technology that was better than any of us dreamed for, it was such a good design that it brings Hyperloop right into the realm of familiarity," Gardi says in an article for the Motherboard blog.

On the other hand, there are critical reports that seem to claim that the Hyperloop — with its elevated, low-air-pressure travel tubes, its fan-equipped passenger pods and its magnetic rail-gun propulsion system — won't work.

When you look at those reports more closely, it turns out that the critics aren't really saying it won't work — just that the technical challenges could be so high that alternatives start to look more attractive. For example, in his Forbes commentary, Tim Worstall of the Adam Smith Institute says autonomous vehicles will basically render high-speed transit obsolete.



"I’ve no doubt that the Hyperloop could be built and could be made to work," he writes. "What I don’t believe is that it will be worth doing: for the purely economic reason that time spent traveling is no longer unproductive time. Thus the value of high-speed travel has fallen."

There have been questions about some of the technical points of the plan: Can the route really be made straight enough and flat enough to sustain 700 mph-plus travel? Can the pods shed enough of the heat generated by high-speed tube travel to keep the passengers from cooking? Can the pylons stand up to the stresses of California's winds and quakes? How vulnerable would the system be to sabotage?

Costs in California

But the weightiest criticism has to do with the cost: Musk and his Hyperloop teammates figure that the system can be built for $6 billion — less than a tenth of the current estimated cost of $68 billion for California's high-speed rail project, which is just beginning to get off the ground. Musk said he was motivated to come up with the Hyperloop concept because that project seemed so expensive, slow and unimaginative.



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"Was Elon Musk's mega-announcement really just a last-ditch attempt to sabotage the California High Speed Rail project, rather than a serious proposal to revolution[ize] travel?" James Sinclair writes on the Stop and Move blog. "Something smells very fishy...."

Sinclair proceeds to list the things that Musk's plan leaves out. For instance, the mapped route doesn't quite go all the way to Los Angeles, and it assumes that there'd be a new way to get across San Francisco Bay. Musk's price estimate also allocates only $1 billion for land acquisition and rights of way, primarily because he assumes that the tubes can be placed next to Interstate 5.

"Of course, you'd still need to get Caltrans to sell its own right of way alongside or in the middle of Interstate 5, which is not going to be easy or free. ... Neighbors will not support an above-ground transportation infrastructure. And it's a lot more difficult to claim you'll just use freeway right of way, since the overpasses and other nearby structures are a much greater constraint on an aerial structure," Robert Cruickshank writes on the California High Speed Rail Blog.

Even if the Hyperloop works as advertised, it wouldn't solve California's future transportation crunch, Cruickshank says: "7.4 million people per year is fine. But the HSR [high-speed rail] system will carry as many as 117 million people per year. That’s an enormous difference. As California grows and as the price of oil soars, California needs a transportation system that can move not just a few million a year, but hundreds of millions a year. HSR can do that. The Hyperloop can’t."

The main theme of the criticism is that Musk's $6 billion estimate seems to be wildly low. Dan Richard, chairman of California's High Speed Rail Authority, told the San Francisco Chronicle that the tubes alone would probably cost $15 billion to $20 billion. Also, the political resistance to Hyperloop construction might well be on a par with the resistance to high-speed rail construction.



"While we have a lot of respect for his inventiveness, I think we could tell him a few things about the realities of building in California," Richard told the Chronicle.

Who will build it?

There's been no huge surge of support from political quarters for Musk's plan. That's only to be expected just one day after the big reveal, but it does raise questions about how far the Hyperloop will get. Sinclair's scenario suggests that the Hyperloop could end up being used as nothing more than a weapon to fight high-speed rail — and he's not the only one who feels that way.

"I worry that more fully baked transportation projects might be put on hold in hopes that Musk's still-fictional idea works out," The Atlantic's Alexis Madrigal writes. "Musk's proposal, because of who Musk is, could serve as a poison dart for California's high-speed rail, and then nothing comes of it, leaving the state with an outdated passenger rail network and no Hyperloop to make up for it."

Perhaps that's why Musk changed his mind on whether or not to make a start on the Hyperloop himself: Until Monday, he said that he would leave the execution of his idea to others. But after revealing the plan, Musk told reporters that he might set up a company to build a subscale Hyperloop prototype on private land, just to show that the technical challenges can be solved.

"I'm not trying to make a ton of money on this," he said, "but I would like to see it come to fruition. And I think it might help if I did this demonstration article. I think I probably will do that, actually."

For now, the Hyperloop is an open-source concept, available to anyone who wants to capitalize on the idea. But in the end, it may turn out that the one person who can turn Elon Musk's vision into a reality is ... Elon Musk.

More about the future of transportation:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.