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# Transmission time of fast rocket

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<DIV>I thought about a ship leaving the earth with a camera at the front that maintains a constant broadcast back to earth.

Hypothetically, if that ship accelerated to the speed of light quickly, what would happen? I would have thought that irrespective of the speed of the ship, the video broadcast should travel back to earth at the speed of light. But I suppose what confuses me is what happens if the camera is permanently on, does it allow you to get an image of alpha centaury faster than a ship that turns its camera on when it gets there?

For example: A ship is sent to alpha centaury at the speed of light. When it gets there, it takes a photo and sends it back to earth. So the time it takes to see the photo on Earth is the trip time of the ship plus the transmission time of the photo (8.6years). But if the camera is on from the start and constantly broadcasting to earth we won't get a video feed of the star in only 4.3 years. Because of this, does the video images on earth slow down as the ship increases its distance from earth?

Yes I know it would be impossible, but I figure that the video feed on earth should slow down. Say after 2 years something shoots by the rockets video field of view, two years of broadcast have occurred on earth but the image of what flew by will take another 2 years to reach. Therefore in the end when the ship gets to alpha centaury 4 years of transmission will take 8 years to be viewed on earth. This concept of video slowing down seems weird

My question directly relates to how the speed and distance of the rocket effects the constant video transmission on earth. The rocket only broadcasts for 4.3 years (until it reaches Alpha centaury) but because of the increasing distance from earth what the rocket sees, progressively takes longer to reach and appear on the video screen. For this to happen does the time frame of the video slow down as the ship moves away? If the video feed included a clock at the bottom of the screen would it show 4.3 years passing over 8.6 years earthtime? and if the rocket is aproaching earth from the same distance, will we get the entire video feed at once?

<DIV>If the video does slow down, does same thing happens to a star with a relative speed away from earth approaching c. Do we see that star age 2x as slow as it does from its perspective? If that star is approaching earth near c, do we see that star age faster?

And finally does the relative speed effect the period of variable stars we see from earth (approaching c towards us = faster observed period, while approaching c away from us = slower observed period)?

I am obviously not a physicist but when I think about it seems logical. But then again I could just be confused?</DIV></DIV>

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This is an interesting thought, let's see if we can simplify things a bit.

When your ship is one year into it's journey at 1 c, it sends us a signal which we receive two years into the voyage. At two light years out it sends another signal that we receive 4 years into the trip. 4 light years toward alpha centauri it sends a picture to us which we get to see 8 years after launch. If I were better with math I could tell you how a continuous signal would not vary from the above description. But I think you get what I'm trying to convey. We have members that are much more qualified to explain this, I just had to give it my best shot.

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You know I'd love to explain this, but I'll have to fnd a place to host a diagram. I'm assuming tormod killed the upload image/attach file function due to bandwidth so I won't ask he reinstate it.

Then again I could hint.

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Hypothetically, if that ship accelerated to the speed of light quickly, what would happen? ...

For example: A ship is sent to alpha centaury at the speed of light. ...

I didn't bother reading any more since it makes no sense to discuss a ship traveling at the speed of light. One can't violate the laws of physics in the setup and then wonder what the laws of physics tell us would happen: once you violate the laws of physics, things become nonsensical.

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Originally posted by: GAHD

I'm assuming tormod killed the upload image/attach file function due to bandwidth so I won't ask he reinstate it.

I'll be...it is set up for the following file types

.jpg, .gif, .png, .doc, .pdf, .xls, .jpeg, .psd, .txt

Problem is, now I can't find out where that "Allow uploads" button is! I have never switched it off.

Will check.

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Originally posted by: joe222

My question directly relates to how the speed and distance of the rocket effects the constant video transmission on earth.

I have been trying to struggle with my RF background (No not the snow we see on TV! lol). For one thing, if the ship was moving at the speed of light (ignoring all of the impossibilites of that) the RF transmission would be doppler shifted so low as to be unusable. Basically 0Hz would be my first guess. Thus no "information" (pictures) could be transmitted. Now if I try to move past that complication.

If we look at it on an instant to instant basis. A pict taken/ sent back from alpha centaury would take the same time whether it was taken by a stationary or moving ship (again ignoring doppler)

If we were to take a pict every X increment of time and send it back (again ignoring doppler) we would get the 1st image 2X later. This would be the time X at which the pict is taken and then the same time X for it's travel back to earth. The next pict taken at 2X, would take the additional 2x to return. So it would be seen 4X later. And so on. Thus the picts from each point would be twice the time of travel. 1X for the ship to get there and 1X for the signal's return trip.

What we would see from a "continuous" series of images (picts taken at regular intervals) is a logarithmic increase in time between images received.

The rocket only broadcasts for 4.3 years (until it reaches Alpha centaury) but because of the increasing distance from earth what the rocket sees, progressively takes longer to reach and appear on the video screen. For this to happen does the time frame of the video slow down as the ship moves away?

I think a lot of confusion comes from "the time frame of the video". My explanation above ignores any specific affect the movement would have on either images being picked up by the camera aim forward nor signals sent back out. Travelling at the speed of light would effectively stop either.

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Thank you for your responses

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• 2 weeks later...

indeed what Freet guesses is true information sent out by a ship travelling at the speed of light will be meaningless by the doppler shift. If the ship moves away from us, the observed frequency will be 0, so no signal can be sent out.

But lets solve all these problems and say that the ship travels at 0.9c. Then indeed compared to the ships videotapes, the timescale would be different, but compared to a normal observer on earth nothing will change; this is because while the space ship travels to alpha centauri, als the time scale on the ship changes in respect to that on alpha centauri. The net difference is exactly zero.

Bo

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