It's not a matter of technology, it's physical limitations. All mechanical work and generation of energy must be less than 100% efficient, otherwise two principles of thermodynamics are violated, and you could create a First or Second order perpetual motion machine (greater than 100% means energy from nowhere, which violates Conservation of Energy; 100% efficient conversion of work into energy violates Entropy).
Now at first this may seem confusing - "why does this imply you can't hide thermal emissions"? The answer is that objects that exist in space radiate heat in the form of IR light outwards at all time. This cannot be stopped (
without violating thermodynamics), and the higher the temperature, the more brightly it radiates. This is also the
only method of cooling in space, since there is no atmosphere with which convection or conduction may take place. You cannot simply refrigerate the front of your ship and redirect the heat to the back, because refrigeration requires energy to perform, and thanks to our friend thermodynamics, this means you are creating heat, not getting rid of it. Any heat you pump to the back quickly conducts back to the front of the ship because temperature moves along heat gradients, and you now have a bright spot on your hull where you had to fire the reactor up. And if you even think about firing up your main drive in flight, we can spot you from the other side of the solar system (or further, depending on how powerful it is).
A detailed breakdown with equations and all can be found at the ever-valuable
Atomic Rockets website.
