# Is it possible to watch time progress faster?

1. Jan 11, 2009

### withahow

In theory, if you set up say a satellite camera pointed to a location in new york city, got in a space ship, turned on a tv with a live feed of the sim city like perspective of NYC, shot away going near the speed of light, would you start seeing time on earth progressing faster and faster?

2. Jan 11, 2009

### Steely Dan

Ignoring the part of the journey where you're accelerating to your speed, no; if your television on your ship were set up to play the video normally, it would just play the video when it received the signal from the camera, since the speed that you are traveling at doesn't affect the camera or the way it records NYC.

You might not be able to watch it in real time, in reality, because as each frame is recorded and sent by the camera, you would be farther away from Earth. In any event, this would make time appear to go slower for Earth, not faster :P

3. Jan 11, 2009

### withahow

hmm... interesting.. so if you were to watch this TV and you zoomed away from earth, youd see the signal slow down. what if you then made a 180, and started zooming back to earth. would you then see time on earth rapidly speed up on this tv?

Last edited: Jan 11, 2009
4. Jan 11, 2009

### vanesch

Staff Emeritus
The example is complicated for different reasons. The first one is that there is a "doppler effect": that is, by you receding from the earth, the successive image frames would reach you later and later (they would have to travel further and further before reaching your ship). Some people think that this is what the time dilatation is about, but it isn't. The time dilatation is what remains if you correct for this "doppler effect" in the classical way (that is, you calculate your successive distances to the earth, and, knowing the speed of the signal, which is lightspeed in this case, you correct for the "travel time"). If you would do this, indeed, earth would seem to "run slower" (even after the correction of the travel times). It would also "run slower" if you were flying towards the earth (again, after correcting the travel times, which go in the opposite direction here).

However, with an actual electronic camera, this wouldn't work in fact, because a camera encodes a datastream (analog or digital, it doesn't matter) which encodes for discrete frames every 20 milliseconds or so. As such, on the receiving side, the only thing you can do is decode these frames, and then have them displayed. If you display them using standard equipment also at 20 milliseconds (but with your spaceship's clock of course) you won't see a difference! You will see scenes which "naturally" happen every 20 milliseconds in NY, and you will see them (in your timeframe) also every 20 milliseconds. Things will seem normal to you. You simply will have a problem in the dataflow. You won't get enough frames to feed you continuously with a frame every 20 milliseconds if you are receding from earth, but that's not only relativity's fault, but also the doppler effect's fault.

5. Jan 11, 2009

### withahow

if there was a camera on earth recording 30 frames per second and you were zooming back to earth, wouldnt the video speed up?

. <- frame

(earth) . . . . . . . . . . . . . . . . . . . . (you)

The faster you travel back to earth, the faster the frames hit your receiver. if your monitor can only display 30 fps, you could discard every 29 frames and still be seeing the movie at 30 fps having time on earth speed up on your tv?

6. Jan 12, 2009

### Steely Dan

Well, the rate at which you received a frame of information would increase, definitely, as time went on and you got closer to Earth. But you wouldn't "see" a continuous stream of images until you were quite close.

7. Jan 12, 2009

### withahow

what if instead of the rocket shooting away, it was orbiting eath at an incredibly fast rate but close enough to be within signal of the video transmission?

8. Jan 12, 2009

### Steely Dan

That's a more interesting situation, I suppose. However, since SR indicates that light always travels at a constant speed c, regardless of the speed you're traveling at, then the only variation in transmission occurs when you change distance from the camera. However, because the speed of light is very fast and the Earth's diameter is not that large, comparatively speaking, you wouldn't see a very noticeable difference, I'm thinking.