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In an article posted yesterday in the Daily Mail, Stephen Hawking declared his interest in exploring time travel.

Hawking had previously been relatively quiet on the topic, afraid of “being labeled a crank.” The title of his piece, “How to Build a Time Machine” is anything but quiet, as is his stated desire to visit Marilyn Monroe in her prime.

Hawking has been dealing with some more popular topics of late, in the run up to his new documentary series, Stephen Hawking’s Universe. In yesterday’s piece he envisions futuristic spaceships that can travel up to 650 million miles per hour — so fast that we won’t know whether we’ve come or gone.

Not to worry, Stephen: you’re not the only crank out there. Time travel has been kicked around by some of the most highly-regarded physicists in the past hundred years, ever since Einstein whipped some equations into the Theory of Relativity, and showed us how it might be done. Here are some perspectives on the issue from around the pantheon of popular physics:

Carl Sagan
Sagan was one of the first to explain time travel in terms we can all wrap our little minds around. In an interview with NOVA he explains:

“First of all, it might be that you can build a time machine to go into the future, but not into the past, and we don’t know about it because we haven’t yet invented that time machine. Secondly, it might be that time travel into the past is possible, but they haven’t gotten to our time yet, they’re very far in the future and the further back in time you go, the more expensive it is. Thirdly, maybe backward time travel is possible, but only up to the moment that time travel is invented. We haven’t invented it yet, so they can’t come to us. They can come to as far back as whatever it would be, say A.D. 2300, but not further back in time.”

Neil deGrasse Tyson
Tyson acknowledges that we’ve understood time travel to the future for the past hundred years. In this interview with Ted Simons he adds:

“We don’t know yet how to go into the past. There’s some peculiar solutions of Einstein’s equations that allow past travel, but then you can’t interact with your life — you’re in another place. Because imagine if you could interact with your own life. Prevent your parents from meeting. Then you were never conceived. Then you could have never been sent back in time to prevent your parents from meeting.”

Andrew Cleland
Cleland, a physicist at University of California at Santa Barbara, isn’t comfortable with the paradoxes that come with traveling back in time. Interviewed by Discover, he says:

“I’m an experimentalist, and physics is ultimately an experimental science,” Cleland says. “Any predictions that are made based on mathematics or on philosophical or intellectual speculation have to pass the test of experiment, and I am certainly not aware of any experiment that demonstrated the possibility of traveling backward in time.” He goes on to say, “Something occurs first and the outcome of that occurrence happens afterward,” Cleland says, “and there has never to my knowledge been an experiment that came out different from that. I am not aware of any experimental tests of quantum mechanics that have shown any violation of causality, in spite of the fact that many experiments could reveal such a violation.”

Paul Davies
Davies, physicist and author of the book How to Build a Time Machine. Naturally, he is optimistic about the possibility of time travel. And also, Mr. Hawking, he probably wants wants his title back. Davies tells Discover, “The reason that the public doesn’t seem to know about it is because the amount of time travel involved is so pitifully small that it doesn’t make for a ‘Doctor Who’ style adventure.” He explains: “Both gravity and speed can give you a means of jumping ahead,” Davies says. “So in principle, if you had enough money, you could get to the year 3000 in as short a time as you like — one year, one month, whatever it takes. It is only a question of money and engineering.”

Michio Kaku
Theoretical physicist Kaku tells us that we can travel in time one day, but we’re not going to be doing it on earth. And definitely not in a hot tub. In this video he declares that there is no law of physics preventing time travel. He explains:

“One way is to have a wormhole. Take a sheet or paper, draw two dots on it, fold the sheet of paper until the two dots meet, and that’s a wormhole. Alice in Wonderland had a wormhole, the looking glass of Alice, a shortcut between two points in space and time. Now there’s a catch. Using Einstein’s equations, you can show that the energy necessary to bend space and time into a pretzel is comparable to an exploding star, a black hole. Energy far beyond anything we can harness on the planet earth. So maybe in outer space.”

John Gribbin
Now that the LHC is up and running, scientists have been cooing about a new start for physics. Will the LHC show us how to travel in time? John Gribbin thinks it might, even if the possibility is tiny. He says, “The snag is that the kind of accidental ‘time tunnel’ that could be produced by the LHC in Geneva would be a tiny wormhole far smaller than an atom, so nothing would be able to go through it. So there won’t be any visitors from the future turning up in Geneva just yet. I’d take it all with a pinch of salt, but it certainly isn’t completely crazy.”

Richard Branson
Branson, leader in air eand space transportation, has also chimed in: “Time travel is not only possible, but under development at Virgin Galactic. We are engineering time traveling rockets, where time travelers will be extremely comfortable in their journey through the wormhole, and will be able to watch It’s Complicated for only 8 dollars.”

Okay maybe Branson didn’t weigh in. But the real possibility of time travel is nothing new, and talking about it is not scientific heresy. Hawking is doing us a scientific service by stirring our collective imagination. There is no shortage of strange time travel in pop culture, from Bill and Ted to Back to the Future, to when Lost got especially weird there in the fourth or fifth season. We don’t quite know how to do it yet, but traveling in time is not as far out as we think.