Now we can get many devices that can make guitar sustain longer, even sustaining it indefinitely or to be more precise, sustaining forever. For example, the EBow,(http://www.ebow.com/home.php) the Fernandes Sustainer and the Sustainiac. But I believe, even when the battery power will not getting low, the sustain will decay from times to times but without we noticing it. What's the physics behind this? Does it really sustain the sound forever?