(TV): Meteorologist Craig Beals here, and today is October 25th, 1985.

And it's shaping up to be a beautiful day in Hill Valley.

Sunny with highs in the 70s, so perfect day to go to the flee-market,

being held at the courthouse clock tower, the same clock tower that was struck

by lightning 30 years ago, at exactly 10:04 p.m.

And looking forward to tomorrow we see another beautiful day...

(amp buzzing)

(speaker squealing)

(explosion)

(Jake): I'm Jake, and how did I get here? Let's rewind.

Whew... I have two loves in my life: science and movies.

So, I decided to put myself in one of my favourite films, Back to the Future,

to find out if you could survive it.

Could you survive the massive amount of electricity from a lightning bolt,

time travel paradoxes, or being blown back by a giant amp pressure wave?

Could you survive Back to the Future?

(explosion)

(Back to the Future theme music)

- What we just saw is exactly what happens in Back to the Future

to Marty McFly. And then, he gets up and brushes himself off.

But what would actually happen to you in real life if you were faced with a sound wave?

In other words, a giant pressure wave that had so much force,

it could literally lift you off the ground and throw you across a room.

Well, to answer the question, we have an air mortar filled to 100 psi,

and a crash-test-dummy wired with sensors,

to measure head and neck trauma from impact.

The air mortar will recreate the effect of the giant amp pressure wave,

that Marty experiences, and the crash-test-dummy will let us know

what injuries Marty would actually sustain.

It wouldn't actually look like it does in the movie.

Instead, it would look like this...

Fire in the hole!

(explosion)

Let's watch that again in slow-motion.

In the movie, Marty is hit across his entire body with the pressure wave.

But the result would be the same if the pressure was targeted just to the chest.

Because the torso has so much more surface area and weight than the legs.

Either way, it results in exactly what we're seeing.

The legs flipping over the head as the upper body is driven into the ground.

Experiencing negative 600 Gs.

It's negative Gs because of the incredibly sudden deceleration.

As the head slams into the ground, the sensors in the dummy read an impact

of 4300 HIC. Head injury Criterion.

An HIC of 2000 is a 90% chance of life-threatening injury,

and our dummy experienced more than twice that.

Marty isn't going to be walking away from this.

Oh! One other fun fact: Using a slow-motion FLIR thermal camera,

we can see what would happen to your skin when it makes contact with the ground,

right here.

The ground and the dummy's head suddenly glow bright white

from the friction of scraping across each other.

So, to add insult to injury, Marty would have multiple layers of his skull scraped away,

before finally stopping.

So, clearly, this would be devastating.

And if for some miracle you were able to survive it,

you would still have to deal with...

... the part of your body that is most sensitive to air pressure changes.

Your eardrums. But what would happen if your ear was subjected

to as much pressure as that amp can generate? Well...

That immense amount of pressure would instantaneously rupture your eardrum, like this...

Ew...

So, a massive pressure wave would do permanent damage to your ear,

and it's not just rupturing one eardrum, it's rupturing both of your eardrums,

which means that Marty McFly would've never heard the call from Doc,

assuming he'd even survive the massive pressure wave impact in the first place.

(car tires squealing)

- Jake! Look what I made!

- Doc! You made a... DeLorean.

- No, Jake, this is a time machine.

- It looks an awful lot like a DeLorean.

It does look pretty sophisticated in here.

How does it work?

- First, you turn on the time circuits.

This tells you where you're going, this tells you where you are,

and this tells you where you were. Punch in any date you want.

You wanna see when the Declaration of Independence was signed?

Just put in the date. Wanna go into the future,

and see when PewDiePie past 1 billion subscribers on YouTube?

- I don't know what any of those words mean.

- Oh, right... It hasn't happened yet.

Ah... November 5th, 1955.

- What happened on that date?

- I had just taken my last calculus final at MIT.

And I tripped on my pocket protector when it fell out of my pocket,

and I fell and hit my head, and I came up with the idea for the flux capacitor.

This is what makes time travel possible.

All you need is a little plutonium.

- Oh. Could I take it for a test drive?

Alright. Any important information I should know

before I take this around the block?

- Nope. Can't think of anything.

(car starting) - Alright. Back this up, here...

(engine roaring)

- Let's see how fast this thing can go.

(tires screeching)

Oh boy!

- Great Scott!

Don't go over 88 miles an hour!

(explosion)

- What happened?

Where are we?

(slow rock music)

Oh, no... It's 1955.

Okay. Uh, let's go back, huh? Let's just do that, nothing happened, it's okay.

Time circuit's on...

No! Argh! There's no more plutonium. Okay!

So... First rule of time travel is don't interact with anyone in a way that could potentially

influence or change the future.

So, we gotta go find Doc and figure out how we get back... to the future.

- Hey, man! That's a sharp looking set of threads you're sporting, brother!

And... what kind of car is this? - Oh, this old thing?

Well this is a 1981 DeLorean that my friend and kooky inventor, Doc,

turned into a time-machine, and then I accidentally drove it back to 1955,

and now I'm stuck here because the power source is plutonium,

and I don't have that.

- Hmm... Sounds like an interesting idea for a moving picture, man.

You know, I think I'm gonna call my cousin, Bob.

Bob Zemeckis, and tell him about this,

'cause we can make a moving picture about the future, the car...

- Okay, that was my bad. You know what, let's just go over the basic principles,

the basic rules of time travel, and also maybe find someone who knows where Doc is.

(car horn honking)

(squeaking)

Hello. Does anyone know where Doc lives?

- Of course. 1640, Riverside Drive.

- Yes! Thank you. Thank you.

Okay, so... Back to the basics of time travel.

Let's imagine time as we normally think of it:

as a straight line.

But there are a few other ways to think about time, and travelling through it.

The first is that you have three timelines: past, present, and future.

If you were able to travel through time,

that means the past and future

exists concurrently with the present.

All of time is happening at the same time.

So when you travel from the present to the past,

you are leaving your timeline and going to another one.

The same if you were

to jump to the future.

The second theory involves a wormhole, or Einstein-Rosen bridge.

The basic idea being that a wormhole could act as a shortcut,

connecting two different points in space-time.

Even though wormholes have not been proven to exist,

this concept fits within Einstein's general theory of relativity,

whereas the other two do not.

And quick side note: In regards to relativity,

we can actually travel through time... kind of.

If an astronaut on the International Space Station had an atomic clock with them,

they'd notice that when coming back home,

their clock would've ticked off less time than those on Earth.

Because time is relative.

The elapsed time between events depends on the motion of the observer.

And you can actually experience this right now.

When you look into a mirror, the light needs to reflect off you,

onto the surface of the mirror, then off the mirror, into your eyes,

which your brain then needs to process into an image.

So when you do look in the mirror, what you are seeing looking back at you

is you from the very, very near past.

But back to our straight line. If we go from this point in time,

and jump to another point in time, we have created a loop.

We left in 1985 and traveled back in time to 1955.

That moment in 1985 has now become a fixed point,

meaning when I'm born in 1968,

I will continue forward until 1985,

when I will then go back in time.

And this will loop forever. There will always be a version of me who gets to 1985,

and travels back to 1955. It's a causal loop,

where every event causes another event,

that leads to the first event happening.

So, when we... If we get back to our present,

we will no longer be in a loop, but a version of us that is slightly in the past will be.

And then they'll get out, and another version will be, and so on, and so forth.

Now, is time travel as we normally think of it physically possible?

Well, not in real life,

because it is not compatible with the laws of physics,

as far as we know. But, we are in movie physics, so we need to find Doc

and get back to our own time.

(joyful music)

(knocking)

Doc! - Do I know you?

- It's me, Jake, from the future!

- I don't know anyone from the future,

and I definitely don't know anyone named Jake.

- Doc, you gotta believe me! Today's the day when after your calculus exam,

you slipped on your pocket protector and hit your head,

and that's how you thought of the flux capacitor!

- What are you doing? You're breaking the basic laws of time travel!

Although, in this case, the ripple in the timeline should be minimal.

Have you told anyone else about this? - No...

- Good! Because that would be a bootstrap paradox.

- The what, now? - The bootstrap paradox!

So, the bootstrap paradox

is when an object, or information, or a person

gets sent back in time, but that confuses the origin of the object.

For example, when Marty McFly goes back in time,

and he plays the song Johnny B. Goode by Chuck Berry,

and then Chuck Berry hears the song, and he's like,

"I like that song, I'm gonna make it mine." And he plays it on the radio.

If Marty had never gone back into the past,

would the song have ever been created? That's the bootstrap paradox.

- Cool! So, here's the deal. I accidentally travelled back to 1955 in your time machine,

and I don't have anything to power it to get back to 1985,

so do you have any idea about how to get 1.21 gigawatts to get me back home?

- Of course! 1.21 gigawatts, that would be enough power.

Although, the only thing powerful enough in 1955 to create that much power

would be a lightning bolt. - Great!

There's actually going to be a lightning strike at the clock tower at 10:04 p.m.

- What did I say about talking about the future?!

(thunder booming)

(eerie music)

- Huh. Everything we need is already set up.

- Whoa, whoa, hey... What are you doing?

- I was gonna ask you the same thing.

- Ah... I'm just working on some amateur weather experiment.

- Yes! That's right! Because you become a meteorologist.

He's the... - I like that. Meteorologist.

Craig Beals.

- Ugh... Why do I keep messing with the future?!

- Jake! We're running out of time!

(thunder booming, car starting)

- Alright, Doc, I think we're all set. Time circuits are... on!

Put my date in, and I guess I'll see you in a little bit.

- I'll see you in 30 years!

- You know...

Gotta say... This all worked out pretty great. (lightning striking)