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ZunarJ5
2011-Oct-03, 10:23 PM
What would the transfer of information be like between an object moving at non relative speed (or stationary) and an object moving at near C?

This feels like fairly naive question... but I am having a hard time imagining communication between a hypothetical vessel traveling close to the speed of light and a planet.

Can anyone clear this up a little? Thank you.

Hornblower
2011-Oct-03, 10:52 PM
What would the transfer of information be like between an object moving at non relative speed (or stationary) and an object moving at near C?

This feels like fairly naive question... but I am having a hard time imagining communication between a hypothetical vessel traveling close to the speed of light and a planet.

Can anyone clear this up a little? Thank you.

If I knew why you are having a hard time with it, I might be able to offer something. Could you elaborate a bit?

Jeff Root
2011-Oct-03, 11:21 PM
Zunar,

There is a significant problem of Doppler shift as the relative
speed changes. That is handled in radio communication with
current spacecraft at lower speeds just fine. You need to have
some idea what the frequency of an incoming signal is going
to be in order to find it.

Is that what you had in mind?

-- Jeff, in Minneapolis

WayneFrancis
2011-Oct-04, 04:35 AM
What would the transfer of information be like between an object moving at non relative speed (or stationary) and an object moving at near C?

This feels like fairly naive question... but I am having a hard time imagining communication between a hypothetical vessel traveling close to the speed of light and a planet.

Can anyone clear this up a little? Thank you.

Well it is the same issue we have now. The signal gets doppler shifted and the rate of data transmission changes. We had this problem with Cassini because it some how got over looked.

Jeff Root
2011-Oct-04, 08:51 AM
The Cassini problem was with one specific signal from the Huygens
Titan atmosphere probe that could not be tracked by Cassini as the
signal Doppler shifted. As I understand it, the specific type of data
encoding used (phase-shift keying) needed to be adjusted to account
and the mission design all took the expected Doppler shifting into
account, but the data decoding system did not. So the trajectory
of the Huygens probe had to be modified. The probe was released
from Cassini a month later than originally planned in order to reduce
the Doppler shift during atmosphere entry to a range that Cassini's
decoding system could handle.

According to the Wikipedia article, two engineers at ESA who were
part of the Huygens team were worried that the Huygens / Cassini
launch, and did further testing which revealed the flaw.

http://en.wikipedia.org/wiki/Huygens_%28spacecraft%29

-- Jeff, in Minneapolis

ZunarJ5
2011-Oct-04, 04:39 PM
Sorry for the delay in responding. Life happens, lol...

What I am having difficulty with is the affect of time dilation on communication between a non relative time frame and one that is moving at close to C.

So, if Amy was flying through space at .9 c time would pass for Amy at roughly half the rate is passes for Bob on Earth. Now, if Bob called Amy on his video phone via laser powered transmitter would Amy see and hear Bob at twice the speed? Would Bob see and hear Amy at half the speed?

I understand that if A could see B out of her window she would see him moving at the same rate that B saw Amy moving... that is at a slower rate, even though B experiences time at a faster rate relative to A (due to a doppler like affect if I remember right).

How is light speed communication (via the laser transmission [radio would work too I guess]) affected?

Does the direction the vehicle is traveling in affect this? If it is traveling away, toward, or across?

Thanks :)

antoniseb
2011-Oct-04, 05:03 PM
...if Amy was flying through space at .9 c time would pass for Amy at roughly half the rate is passes for Bob on Earth. Now, if Bob called Amy on his video phone via laser powered transmitter would Amy see and hear Bob at twice the speed? Would Bob see and hear Amy at half the speed?
...

As noted by Jeff and Wayne above, the photons carrying the information from B to A would be red shifted, and Amy would get the information at a slower rate. As Bob sees it, she would get 1/10th as many bits per second, and as she sees it she'd be getting 1/5th as many bits per second. So, if the signal could be captured, she'd see a slower version of Bob, unless her system did frame buffering, in which case she'd see Bob full speed with extended periods of buffering in between.

Jeff Root
2011-Oct-04, 06:15 PM
The Doppler shift is generally (always??) much greater than the
time dilation effect, and the two have to be combined together
to get the total result. But if you ignore the time dilation and
just look at the Doppler shift, I think you will get a pretty close
approximation of what Bob and Amy will see.

Very briefly, if Amy accelerates away from Bob, then slows to
a stop and accelerates back toward Bob, both will first see the
signals from the other strongly redshifted. Amy will see signals
from Bob stop being redshifted when she stops moving away,
and the signals will become blueshifted when she starts moving
back toward Bob. Some time later, Bob sees Amy's signal stop
being redshifted and become blueshifted. Amy sees Bob's
signal blueshifted for half her trip, while Bob only sees Amy's
signal become blueshifted shortly before she returns, and all
of her signals from the second half of her trip are jammed into
a short period of time.

-- Jeff, in Minneapolis

ZunarJ5
2011-Oct-04, 07:41 PM