[tangoblue (OP)]

So I have watched a couple of videos on 'the heat death of the universe' and it is all very formulaish so I got rather lost (I'm better with biology than maths  ).  The basic idea that I got was that in 10^100 years all life in the universe is utterly screwed because there will be no usable energy left in the universe... but why?
Can anyone explain this 'heat death' in plain English for me please?

Thanks.

[Pmb]

Can anyone explain this 'heat death' in plain English for me please?

Absolutely!

If you put a hot coffee in a cold pot of water the water and the cup will eventually end up having the same temperature. This is an example of thermodynamic equilibrium. Universal heat death means that eventually the universe will achieve a state of thermodynamic equilibrium. See http://en.wikipedia.org/wiki/Heat_death_of_the_universe

[tangoblue (OP)]

So the universe will eventually reach a thermodynamic equilibrium, but why does this mean that there will be no usable energy in the universe? 

[Pmb]

So the universe will eventually reach a thermodynamic equilibrium, but why does this mean that there will be no usable energy in the universe? 

Usable energy means that you have the ability to do work. If the entire universe was in thermodynamic equilibrium you wouldn't have the ability to decrease the entropy of a system. That means that you can't build anything, life can't exist, nothing. To understand the particulars you need to study thermodynamics. But I can tell you this - in order for life to exist you have to be able to decrease the entropy of a system because life is order and order means decreaseed entropy.

See -
http://en.wikipedia.org/wiki/Entropy_and_life
http://en.wikipedia.org/wiki/What_is_Life%3F_(Schr%C3%B6dinger)

Let me hereby warn you that thermodynamics and statistical mechanics were my worst subjects in college and I don't pretend to have gotten any better since then. So I won't have a lot to say in this thread. Don't worry though. I'll be spending some time later this year to brush up on it because I want to go back to graduate school.

[tangoblue (OP)]

I think I understand that, thanks 

[Pmb]

I think I understand that, thanks 

You're quite welcome. I'm very glad to have been able to help.

[CliffordK]

Isn't the idea that the primary energy source for the Universe is the fusion of Hydrogen and Helium, and perhaps a couple of other light elements. 

Eventually all Hydrogen and Helium will be used up.  If there is no generation of new Hydrogen, then all of the stars will eventually burn themselves out and die.

[evan_au]

There is a concept called a Carnot Heat Engine, which is a theoretical concept that includes steam engines, internal combustion engines and thermal electricity generation plants.

The idea is that you can use differences in temperature to do work: drive a train, plane, automobile or lightbulb - or keep an organism alive.

If there are no differences in temperature, there is no available potential energy, and there is no way to do work or keep organisms alive.

Similar concepts apply for other potential fields, like electrical fields, gravitational fields, nuclear forces, etc.
The idea is that over time, all of these systems tend towards the lowest possible energy state, where there is no freely available energy.