Debug.WriteLine doesn't work on different threads

var timer = new Timer
{
    Enabled = true, 
    Interval = 500
};
Debug.WriteLine("new timer created");
timer.Elapsed += (s, e) =>
{
    Debug.WriteLine("hello from timer");
    // this doesn't get shown
    // in the output window
}

What do I need to do to be able to see my last Debug.WriteLine() in the output window above?