Last weekend, I watched the comedy Hot Tub Time Machine (starring John Cusack, Rob Corddry, Craig Robinson and Clark Duke). In a nutshell, I thought it was pretty good.

But me being me, I wondered what it would take to actually turn a hot tub into a time machine. So when I got home, I did some calculations (no, I don’t have a social life) and worked out how much power a hot tub would need to become a time machine.

Warning: You can leave your physics at the door; the following text is the product of pure science fiction, basic math and an over-active imagination.

What’s it about?

The plot of Hot Tub is fairly straight-forward: Three old friends, Adam (Cusack), Lou (Corddry), and Nick (Robinson) take Adam’s geeky nephew Jacob (Duke) on a trip to their old 1980′s holiday spot to relive their teenage years. However, times have changed, and the once party-filled snow-covered town of Kodiak Valley has turned into a dilapidated mess, a shadow of its former self.

The focus of the movie is on an empty hot tub (plus decomposing raccoon inside) that Adam and co. find outside their flea-bitten hotel room. But looks can be deceiving; this hot tub holds a space-time secret.

One night, the foursome notice the hot tub is miraculously fixed, filled with water and glowing. Naturally, they all jump in (naked, much to the shock of Jacob) and a montage of bums, bear suits, tequila shots and a Reagan mask ensues. Like any good hangover, they all wake up the following morning, heads throbbing, 24 years in the past.

The cast of Hot Tub Time Machine (Rob McEwan/IMDB).

But how did that happen? The hot tub is a time machine, of course, and it was triggered by a spillage over the temperature control panel (which also turns out to be the time travel date selector).

Time Traveling Hot Tubs: What physical laws does sci-fi routinely violate? Jennifer Ouellette investigates.

How much energy? 1.21 gigawatts?

The whole movie is basically a Back To The Future parody that goes way too far (but in a funny way). Even Crispin Glover (George McFly) makes a hilarious cameo appearance as the hotels unlucky bell boy.

As Hot Tub follows the Back To The Future theme, the amount of (sci-fi) energy required to travel through time is very well known.

In Back To The Future, the DeLorean is the time machine of choice, a stainless steel automobile sci-fi icon. The amount of energy required to power the DeLorean through time (presumably by opening a wormhole) is 1.21 gigawatts.

“1.21 gigawatts? 1.21 gigawatts? Great Scott!” Doc Brown (played by Christopher Lloyd) exclaimed in a scene of Back To The Future when his 1955 self first meets Marty McFly (Michael J. Fox). In 1985, Doc Brown stole plutonium from a Libyan terrorist group to power the car’s “Flux Capacitor.” In 1955 however, plutonium is harder to come by, hence his plan to use a bolt of lightning to power the time machine.

So what is “1.21 gigawatts”? That’s 1.21 billion watts of power, delivered to the time machine in an instant. So let’s assume the hot tub is installed with a Flux Capacitor-type device that requires the same amount of power to function.

HowStuffWorks: Could we use a wormhole to travel through time?

DeLorean DMC-12 Head on photo with doors open (Kevin Abato/Wikipedia).

Hot tub upgrade

I did a quick search online for hot tubs, and Jacuzzi being the most familiar brand, I looked into how much power is needed to power one of their most luxurious models.

(Hot Tub doesn’t specify the brand or model of its time machine; the one used in the movie looks like an old built-in design. But the Jacuzzi tub I found has its own sound system, and that’s awesome.)

The Jacuzzi J-480 can hold 6 adults, 570 US gallons of water and has 48 jets. If I were to make a time machine out of a hot tub, this would be my first choice. To power all those jets, two 5.1 brake horsepower pumps are used. That means this hot tub needs to be delivered with 7,609 watts (or 7.6 kilowatts) of mains electricity to keep the jets bubbling and swirling away (1 horsepower for an electric motor is equivalent to 746 watts).

Assuming these jets — if given enough water-swirling power — can somehow bend the laws of physics, open a wormhole and make time travel possible, 7.6 kilowatts is obviously way too wimpy to get the cast of Hot Tub back to the year 1986. The Jacuzzi is nearly 160,000 times underpowered.

A quick calculation shows that the Jacuzzi tub requires an upgrade. In fact, for its two pumps to deliver 1.21 gigawatts combined, two 810 thousand horsepower pumps would need to be installed.

As a comparison, NASA’s Space Shuttle Solid Rocket Booster Retrieval Ships are powered by two 2,900 horsepower diesel engines, whereas a single Shuttle rocket booster delivers 30 million horsepower. Therefore, our time traveling hot tub would need to be powered by something between a ship’s engine and a rocket booster, both of which are powered by combustion, not a mains electrical socket. Suddenly, Doc Brown’s use of plutonium and a bolt of lightning to power the DeLorean don’t seem like such bad ideas.

Exotic energy?

As I obviously had a weekend with nothing better to do, I also did some research into how much energy is thought to be needed to open a wormhole.

Artist’s impression of a wormhole linking two locations in space. Wormholes may also link two moments in time, allowing time travel (HSW).

Science fiction movies throw around wormholes as if we know what they are and where we can find them. Hell, we can even fit spaceships through them, popping up in different times and different locations in the universe. While this is fun (and Einstein’s theory of general relativity predict them), we have yet to find any physical evidence for their existence.

Unfortunately, there’s even less of an idea about how to make wormholes traversable (i.e. allow anything bigger than an atom to fit through the throat of a wormhole). This time it isn’t necessarily a question of “how much” energy I’d need to open a wormhole, it’s “what kind” of energy (or matter) I’d need to create.

Some speculative theories suggest that exotic matter (or matter with negative energy) would be needed to open a wormhole and pass through it.

Fortunately, neither Back To The Future or Hot Tub Time Machine mentioned anything about exotic matter.


When I finally packed up the pen, paper and calculator and went to bed, I wondered if a hot tub would ever make for a good time traveling device. Of course, the answer is no. (Besides, sitting in a hot tub filled with water, activating two 810,000 horsepower pumps probably wouldn’t be good for your health.) But I did wonder whether it would ever be possible to slip through time if we could harness this mysterious exotic matter to force open a wormhole or two.

Although we can’t entertain that thought right now (unless the Large Hadron Collider does something spectacular with the fabric of space time, opening wormholes into alternate universes — unlikely, but fun to contemplate), I thought back to the awesome interview I did with Richard Obousy when we had a chat about the possibilities of warp drives:

It’s apparent to me that a lot of people seem to want to prove why a technology is not possible, rather than think of ingenious ways to make something possible. It’s my conviction that when someone says something is “impossible,” what they really mean is “our current level of science cannot explain this, and I don’t have the motivation to explore beyond its boundaries.” –

Like warp speed, perhaps time travel shouldn’t be considered “impossible,” but for now, sit back and enjoy the movie.