WEBVTT FILE

1
00:00:00.030 --> 00:00:04.040
300 trillion. That's about how

2
00:00:04.060 --> 00:00:08.120
many photons in each laser pulse fired by the ATLAS instrument.

3
00:00:08.140 --> 00:00:12.190
It sounds like a lot, but as the photons sail through the

4
00:00:12.210 --> 00:00:16.270
atmosphere and bounce off the Earth, they reflect back, just like any

5
00:00:16.290 --> 00:00:20.330
light in all directions. ATLAS receives only the photons that reflect straight

6
00:00:20.350 --> 00:00:24.420
back to the instrument, just about a dozen photons. That window of

7
00:00:24.440 --> 00:00:28.630
photons received must be very small in order to keep the timing error of

8
00:00:28.650 --> 00:00:32.810
those photons very small, which allows scientists to calculate elevation

9
00:00:32.830 --> 00:00:36.247
very precisely.
[music]