Name: Computing Limit - Computerphile
Uploaded: 2020-03-27T17:01:56.000Z
Duration: 15 min 2 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

It's what I want to do is focus on the real fundamental

physical limits of course getting down to the the miniaturization

And the smallest possible structures is an important aspect

But there's also a speed aspect if you even get down to that you get your density up. How fast can you process?

The reason I'm quite keen to come back to computerphile is I spent the first two years of my physics degree going shuddered on computer

and because I'm really quite a per mathematician to put it mildly, but I'm

As a cold or at least back in the dim distant days of the past I used to be okay

I started with the zx81 got into assembly language coding did a lot of from

Zx80 ones that expect from BBC micro all that type of stuff used to love coding my mantra throughout my undergraduate degree

And I was not a great student was if I can't code this. I don't understand it

So there's a piece of physics particularly piece of mathematical physics

I'd always think about how do I call this Fourier transforms convolution correlation?

Can I reduce this to a piece of discrete mathematics and therefore can I actually code it so I've always loved exploring those links between

Programming coding computers information and physics before we start thinking about limits

physically what the computers do what do they actually do they they take information in and

They do some computation and they give information out is that a fair enough description Sean sounds good

yeah, so the interesting thing is just how you define information and

There's a guy called wheeler who was the PhD supervisor of

Feynman what wheel also had this very famous phrase which was it from bit

So the idea that the fundamental quantity in the universe is not energy. It's not matter its information and

What does he mean by that well it's it's it's intriguing. Let's play around with these. Let's say. We've got a system

Entirely what we'd call reversible right? What does that mean it means that if I drop that ball?

Let's say you're as you can see I'm slowing it down

But let's imagine that I was actually just dropping it out of my hand and it was coming right back up to the same height

That would be what's called an elastic system

This ball is coming right back up to its same height its end position is exactly the same as its starting position. It's entirely reversible

There is a link to computing a promise so

What happens if we if we add some if we take some energy out of this system and there's this what we call?

Does the ball is squishy so when it hits the floor? It's not a perfectly elastic band

It just doesn't bounce as if it's a rigid object. You know what's gonna happen. It's fairly boring

Now we've got a problem because that's our final state. That's a right pot

That's a right part of the system. Do we have any information at all about the input?

It's just there you drop it from this height you drop it from that height you drop it from that height

You've lost all information about the initial state

However if it's if there's no energy leaking out of the system, then you drop it from this height

If it was a perfectly elastic system. Let's do it like that just to pretend then it comes back up to the same height

This is already telling us that there are interesting links between information and energy

Now if we look at a standard to get back into computer files territory. There's a direct relationship between the

And the information content of the output so in the second case when the balls like that

Energy's leaked out the system isn't reversible and

We've we have no idea what the input was so we've lost

Information in that sense has leaked out into the environment the fascinating aspect of this is if you take a NAND gate

So what we have now is a is a logical connection

Connection and boolean logic to this physical problem because let's set this up as a truth table

Let's put that as output which is not how computer scientist might do it

So I'll do it this way so if we go zero zero you all know how a NAND gate works zero one zero zero

Because for three of these outputs. We've got exactly the same. We've got zero

We've got no connection between this iPod

We've lost information because we don't know what our inputs were we have a lack of reversibility and that lack of reversibility is

absolutely key in terms of the connections between the physical world and the

Information world the computing world and therefore because we've got a connection from reversibility and energy

Therefore we've got a connection with the physical world in terms of how we do computation

There are multiple ways of getting to zero is basically what we've got precisely so if we've got this we can traverse the system

To know to work out what exactly are our inputs where there's only

Okay, forgive me for saying this, but is that not just because we've got two inputs that only one output through sir

Thank you for that exceptionally perceptive question John so what was and what was that's exactly the issue

So there's a whole host of gears called fredkin gates. Which are developed which have three inputs and three outputs now

We'll go into them. You don't have to forget setup like this you can have something which is called a reversible gate and

In principle if you've got that reversible gate input in place though actually

There's a difference between the physics and the engineering

engineering one of those gates and actually changing or a computer

Architecture to move towards all type of fredkin gates is going to take an awful lot of effort

and an awful lot of cost but in principle if we could do that and if we had a

Perfect perfect Fred can get then there would be no energy cost to computing

No energy no. No energy cost because here's the fascinating thing. What costs the energy is not the

Computation itself it's a raising information that and gate there

but it doesn't show all the physical connections does it usually these gates have multiple things like Earth's and

Of course is that right and in fact exactly they have written in fact

How you clear these in many cases is you ground so if you've got a 1, which is what is a 1 well?

It's a voltage how do you set that to 0 you actually ground it? So that's that's a leaker in that sense?

you're leaking out to the environment, and that's what it's a

Really good way of thinking because that's exactly what's happening here. This is not. This is unravel

Leaking out to the environment effectively or in this case the energy is leaking out to the environment

So you must observe it to see what happens generally?

sort of threat of going down the route of an electric engineering file

How does this relate to somebody who's practicing a modern-day computer?

They saw the key thing here is because it costs a certain amount of energy to erase data and for any

Computation in the way, we've set up or to lose data particularly with these there's an energy cost to doing this because it's irreversible

that means that that is setting a fundamental limit in terms of the the

Amount of energy that it needs to power a computer and in turn you know from both

environmental and also fundamental physics reasons

You know we really need to think carefully about how we actually beat that limit if we could beat that limit

What where we really come up hard against another limit is?

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Computing Limit - Computerphile

incredibly

ultimate

aspect

multiple