# Light Speed broadcast of message back to earth

1. Jul 10, 2012

### jarroe

broadcasting a message at light speed back to earth....

if a ship was travelling at 80% speed of light (Cern proved mathmatically this speed is achievable hypothetically using anti-matter) while continually broadcasting the cockpit/crew of the ship 'real time' at light speed back to earth, would the video appear to be in slow motion to the earth viewers to accomodate the theory of relativity?

2. Jul 11, 2012

### Staff: Mentor

i would say yes. Consider the doppler effect where a person is playing a C note on a trumpet as he drives away from you it has lower pitch but he still hears it as a C note. So each second of the video feed r49m the ship, the ship has traveled further away and so the seconds are stretched out a bit more and the feed playback appears slower.

3. Jul 11, 2012

### A.T.

There are two effects here:

1) Time dialtion: Independent of movement direction. Always makes the rocket crew act slower in the Earth frame.

2) Doppler effect: Direction dependent. When the rocket flies away you will receive the information at a slower rate. If it flies towards you will receive the information at a faster rate.

The net effect is a combination of the two.

4. Jul 11, 2012

### Drakkith

Staff Emeritus
Re: broadcasting a message at light speed back to earth....

Absolutely. At 0.8 c the ship would appear to us to experience time at 60% the normal rate.

5. Jul 11, 2012

### ghwellsjr

Yes, viewers on earth would see the activity of the live broadcast running at 1/3 normal speed, as long as the ship continuously travels directly away from the earth.

6. Jul 11, 2012

### pervect

Staff Emeritus

To expand on this good answer by providing some numbers and a wiki reference, if the ship was travelling at .8c directly away from earth, you'd see the video playing at a rate of 1/3 the normal rate. If you get into the details of thinking of the video as a modulated signal, you would see both the carrier frequencies and the modulation frequencies red-shifted to 1/3 of the transmitted value.

This is calculated by the Relativistic Doppler Effect

If the ship was travelling at .8 directly towards the earth, the video would play at 3x the normal rate.

For transverse motion, see the "Transverse Doppler Shift" in the above wiki article.

7. Jul 11, 2012

### Ryan_m_b

Staff Emeritus
Re: broadcasting a message at light speed back to earth....

That would also work the other way round. If we were continually transmitting to the ship it would see us as slower.