# time travel!

Hi. so, i like time travel. I like it a lot.

today’s information is on the topic of how we can use einstein’s theory of gravity to generate time machines.

this is a real thing, you guys

I used different pens this time. lets see if they’re better…

It’s probably not news, but we general relativists use much more technical terms, so that we can speak of such things with a greater deal of precision.

exterminate! exterminate! exterminate the doctor!

the word “causal” refers to chains of cause-and-effect, and “causal structure” refers to how different points in a spacetime can are related…

listen, tom baker rocks. also, have you ever tried to draw a caricature of marty mcfly? it’s way hard, guys!

“Subluminal” means “slower than the speed of light”. One way to imagine a lightcone is to imagine that at point P, we set off a flashbulb. The trajectories of the photons that emerge from the lightbulb  divide the spacetime into two parts: the parts that can be affected by what happens at point P, and the parts that cannot. Also, if you start at point P, you’ll always be stuck inside this sphere of expanding photons. So the lightcone gives you a boundary in the spacetime between places you can and cannot go.

In Einstein’s theory of gravity, the spacetime pushes the matter around according to how its curved; and the matter distribution causes the spacetime to warp and curve.  One way of picturing this curvature is by looking at how the light-cones are warped and twisted and tilted in different places throughout the spacetime.

the illustrative thing about a lightcone is that it tells you which direction “forward in time”. If you can tilt them sideways and have time flow “sideways” if you want :p

In other words, we can engineer a spacetime where the light-cones all twist around on themselves, allowing us to move backwards in time. The only consequence of this engineering procedure is that dictates the type of matter required to generate it. Sometimes you end up with some pretty crazy matter.

Lets look at a couple of FANTASTIC time-machine spacetime structures!

Sorry i’m not a good artist.

This “time machine” is called a Tipler cylinder. It’s an infinitely long rotating cylinder. the rotation of the massive cylinder causes the light-cones to tip over in the vicinity of the cylinder.

sorry i’m a crummy artist.

Okay, so another type of time machines involves wormholes, where if you pass through it, you’ll end up in the past. This time machine was popular as a sand-box for time-travel thought experiments performed by Morris, Thorne  and Yurtsever.

Ok, of all time machines, the TARDIS is easiest to draw. so it’s not that i don’t love deLoreans… It’s just that it’s very late at night here right now.

There are a lot of other fun spacetimes where time travel is possible.

the planet express delivery ship has been used as a time machine. it’s also very easy to draw.)

### 8 Responses to time travel!

1. Ryan 9 November, 2010 at 8:55 pm #

I’m convinced you made most of this up. At no point did you mention flux capacitors. They’re capacitors of flux, ben!

1.12 gigwatts * 88 mph = time travel

2. Dayna 10 November, 2010 at 10:12 am #

The wormhole thing actually answered a question I emailed you guys a few weeks ago. There’s this book called “The Light of Other Days” where they create these wormholes that let you see into the past. The wormholes are too small to actually travel through, so you can only watch the events, like it’s on History Channel or what have you. And I was wondering about the science behind that, but I think you pretty much just answered that. Thanks Ben.

Also, I <3 you even more for adding Doctor Who drawings because Doctor Who is amazing!

• Ryan 10 November, 2010 at 11:50 am #

We have the e-mail, we’re just really behind on all that because there’s so much good stuff from the Paleoposse! Don’t worry, we’re pretty careful to not let stuff fall through the cracks, we’ll get to it sooner or later. Thanks again for writing in though!

3. ben 10 November, 2010 at 11:04 am #

wow there’s a book like that? also i don’t think i got your email specifcally, or i’d have talked about it in the paleoPOW

so if you’d like to ask me a question: ben@sciencesortof.com

bn

p.s. doctor who is amazing, but tom baker is REALLY HARD TO CHARICATURE because he looks like a bad drawing .

• Dayna 10 November, 2010 at 11:13 pm #

Yes. It’s called “The Light of Other Days” by Arthur C Clark and Stephen Baxter. It gets into some fun science about wormholes, but it also covers the social aspects of being able to view things from the past: such as what was Jesus really like, what were dinosaurs really like. Then it reviews the social shockwave that ripples through society because people know that people from the future can view them anytime they want. Very good book if you have time to read.

Yeah, I just sent the email to the Paleopals account because I had a few other questions about other stuff. But next time I have a physics questions, I’ll just send it to Ben. ^-^

4. Xerophytes 10 November, 2010 at 11:10 am #

I’ll send in a voice feedback on time travelling because I’m still not convinced that time travelling is possible.

• ben 10 November, 2010 at 6:36 pm #

that would be awesome.
i mean, *is time travel possible*? who know’s man, the fact is that I”M SAVING IT FOR THE SHOW.

5. Dustin K. 12 November, 2010 at 9:17 pm #

The Tipler Cylinder. Right. Time travel will be a cinch once we figure out how to build an infinitely long cylinder! (Was this the inspiration for the time turners (wrong name?) in Terry Pratchett’s ‘Thief of Time’? Take this basic idea, and put it into a monastery with monks and everything protecting and maintaining the cylinders (different sizes for different time scales). Pretty cool all told. I think I need one last closed-parenthesis. Yes.)