  Double layer two-dimensional electron systems can exhibit a fascinating
collective phase believed to exhibit both quantum ferromagnetism and excitonic
superfluidity. This unusual phase has recently been found to exhibit tunneling
phenomena reminiscent of the Josephson effect. A key element of the theoretical
understanding of this bizarre quantum fluid is the existence of linearly
dispersing Goldstone collective modes. Using the method of tunneling
spectroscopy, we have demonstrated the existence of these modes. We find the
measured velocity to be in reasonable agreement with theoretical estimates.
