Physics group> Physics Time Factor

In the Physics group is an interesting control called the Physics Time Factor. This parameter enables full control over physical time. The particles can be made to move faster, slow down, completely freeze or move backwards. Better yet, the camera is fully controllable so it can move around in the Particular scene.

The Physics Time Factor control in the Physics group.

How Time Factor works

To freeze time, the Physics Time Factor parameter has to be keyframed down to 0. If it is simply set to 0, no particles will ever be born. You can keyframe from 0 to another value to make the particles move again. Higher values make the particles move faster.

Keyframe the Time Factor to 0 to stop particle movement.

Tutorial: Freezing Time with Time Factor

Create a new comp (Ctrl/Cmd-N). Make it 640*480 at 30 fps. and 5 seconds long. Name it 'Main'. Create a new comp-sized Solid (Ctrl/Cmd-Y). Name it 'Particles', select Make Comp Size, and click OK. Apply Effect> Trapcode> Particular to the Solid.
To be able to navigate easily in the scene, let's create a composition camera. Select Layer> New> Camera and use the 28mm preset. Now we will keyframe the camera. Make sure that you have the Camera layer selected in the Timeline. Twirl down the Position layer property by hitting 'P'.

Go to Time 2:00 (2 seconds) and set a keyframe for Position (the default: 320,240,-499).
Go to Time 3:00 (3 seconds) and set a keyframe for Position: 0,240,-380.

Try a RAM preview. You will see the view of the camera shifts the perspective of the particles.

Now let's use the Physics Time Factor control to slow things down during the camera move. Select the 'Particles' layer in the Timeline. We are going to keyframe the Time Factor parameter. Twirl down to the Physics> Physics Time Factor parameter. You can set your keyframes in the Effects palette or Timeline or both areas.

Go to Time 1:00 (1 seconds) and set a keyframe with value 1.
Go to Time 1:20 (1 seconds and 20 frames) and set a keyframe value 0.
Go to Time 3:10 (3 seconds and 10 frames) and set a keyframe value 0.
Go to Time 4:00 (4 seconds) and set a keyframe with value 1.

TIP: To set the second 0 value, you can copy/paste the first 0 value's keyframe in the Timeline.

Try a RAM preview. You will see the particles stop moving halfway through the comp. You have successfully frozen time!