Re: Cosmic Background Radiation

Dear jmetolius:

"jmetolius" <jmetolius@xxxxxxxxx> wrote in message
news:1148785060.148050.285630@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
N:dlzc D:aol T:com (dlzc) wrote:
Dear jmetolius:

"jmetolius" <jmetolius@xxxxxxxxx> wrote in message
news:1148783012.353092.304710@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Thank you Scott. Would you mind if I ask a few
further questions? While I'm sure you are correct,
could you (or anyone else) offer a more detailed
explanation of why ' that motion is unimportant
once the astronaut reaches the surface of the
planet' .

Are you asking why the red/blue shift with respect to the CMB
is
unimportant?

I'm still missing something here. At what point
does the astronaut stop experiencing the
CMB being blue shifted in their direction of travel?

When they are at rest with respect to it.

I understand that, to catch up with the
receding planet, the theoretical astronaut
must accelerate to a greater speed (relative
to Earth), and then eventually to decelerate
before landing, but the deceleration will still
leave them traveling at 20,000 km/sec
relative to earth after they have landed, so
why wouldn't they be experiencing a blue
shifted CMB?

Lets get a minor detail handled. The Earth is
moving wrt the CMBR (and the average
population of observable objects) with a
speed of only about 300 km/sec. So "20,000"
is a little high for your proposed planet to be
at rest wrt the CMBR.

They will see the CMBR at the same
temperature in all directions when they are
at rest wrt to it. So even orbitting a star or a
galactic center could be a problem.

And now, why do you think it would be
important whether or not this planet is at
rest?

Please read the initial thought experiment,

I read it. I read it before I replied.

then I would be pleased if
you can offer an explanation.

You didn't answer my question.

The 20,000km/sec is just a number. It
can be any velocity you like: make it
2,000km/sec if you like.

I "like" the number 300 km/sec, since that could be at rest with
the CMBR.

It is just a thought experiment to get
a handle on the physics involved.

The physics is Doppler shift. And if someone were at rest with
respect to the CMBR, they *might* age a tiny bit faster than an
Earthman. Other than that, the physics *there* is the same as we
see here.

David A. Smith


.

Relevant Pages

  • Re: Cosmic Background Radiation
    ... explanation of why ' that motion is unimportant ... receding planet, the theoretical astronaut ... your proposed planet to be at rest wrt the CMBR. ...
    (sci.astro)
  • Re: Cosmic Background Radiation
    ... explanation of why ' that motion is unimportant ... receding planet, the theoretical astronaut ... your proposed planet to be at rest wrt the CMBR. ...
    (sci.astro)