Time dilation and expanding space

This one is a little obvious after some thought, but I haven't heard it
mentioned before.

The frequency of light is like a clock in itself - if the frequency is lower
then the clock at the source is slower as measured by an observer who also
measures that redshift. If the redshifted electromagnetic radiation was a
radio carrier wave then the frequency the observer must tune to is further
down the dial, as expected, but the sounds transmitted via that carrier wave
will also appear to be slowed down, like an audio tape running at the wrong
speed.

This is true regardless of the cause of redshift - source moving away from
observer, source near a gravitating body, or source at a very great distance
(Hubble shift).

The Hubble redshift observed on Earth must also be accompanied by time
dilation. If the frequency of light received is half, for instance, then
the clock at the emitting end of that electromagnetic transmission is
running at half the pace as the clock at the receiving end.

I assume that this time dilation is taken into consideration when observing
pulsed transmission of the rotation of galaxies (pulses will be measured as
slower than the actual rate, galaxies will appear to rotate slower then than
actually do etc.)

Anyone know more on this?

--
Kind Regards
Robert Karl Stonjek

Dear Robert Karl Stonjek:

On Feb 28, 4:45 am, "Robert Karl Stonjek"
wrote:
This one is a little obvious after some thought, but I
haven't heard it mentioned before.

The frequency of light is like a clock in itself - if the
frequency is lower then the clock at the source is
slower as measured by an observer who also
measures that redshift.

The frequency of light is not something intrinsic to light, though,
Robert. It only says something about the relationship between the
emitter and the receiver... and then only if you know something about
the emitter. (Something like characteristic stellar emissions, for
example.)

If the redshifted electromagnetic radiation was a
radio carrier wave then the frequency the observer
must tune to is further down the dial, as expected,
but the sounds transmitted via that carrier wave
will also appear to be slowed down, like an audio
tape running at the wrong speed.

Yes. The analysis of type Ia supernovae provide about four different
measures of distance that are in good agreement. Comparison of clocks
(redshift) to intensity (1/r^2), and more.

This is true regardless of the cause of redshift - source
moving away from observer, source near a gravitating
body, or source at a very great distance (Hubble shift).

The Hubble redshift observed on Earth must also be
accompanied by time dilation. If the frequency of light
received is half, for instance, then the clock at the
emitting end of that electromagnetic transmission is
running at half the pace as the clock at the receiving end.

No. Please consider that proper motion can yield a "half speed clock"
in the other frame... for both frames.

I assume that this time dilation is taken into
consideration when observing pulsed transmission of
the rotation of galaxies (pulses will be measured as
slower than the actual rate, galaxies will appear to rotate
slower then than actually do etc.)

Anyone know more on this?

David A. Smith

Robert Karl Stonjek wrote:
The frequency of light is like a clock in itself - if the frequency is lower
then the clock at the source is slower as measured by an observer who also
measures that redshift.

As you indicate, this is pretty well known. One familiar application
is that high-redshift supernovae
take longer to fade than supernovae at low redshift. This rules out
tired-light models for redshift.
There's also an extra factor of (1+z) in the equation when you
calculate a distant object's luminosity.

I assume that this time dilation is taken into consideration when observing
pulsed transmission of the rotation of galaxies...

I don't know what you are referring to here. Galaxy rotation is
measured from Doppler shifts, not
from pulses.

"dlzc" wrote in message
oups.com...
Dear Robert Karl Stonjek:

On Feb 28, 4:45 am, "Robert Karl Stonjek"
wrote:
This one is a little obvious after some thought, but I
haven't heard it mentioned before.

The frequency of light is like a clock in itself - if the
frequency is lower then the clock at the source is
slower as measured by an observer who also
measures that redshift.

The frequency of light is not something intrinsic to light, though,
Robert. It only says something about the relationship between the
emitter and the receiver... and then only if you know something about
the emitter. (Something like characteristic stellar emissions, for
example.)

If the redshifted electromagnetic radiation was a
radio carrier wave then the frequency the observer
must tune to is further down the dial, as expected,
but the sounds transmitted via that carrier wave
will also appear to be slowed down, like an audio
tape running at the wrong speed.

Yes. The analysis of type Ia supernovae provide about four different
measures of distance that are in good agreement. Comparison of clocks
(redshift) to intensity (1/r^2), and more.

This is true regardless of the cause of redshift - source
moving away from observer, source near a gravitating
body, or source at a very great distance (Hubble shift).

The Hubble redshift observed on Earth must also be
accompanied by time dilation. If the frequency of light
received is half, for instance, then the clock at the
emitting end of that electromagnetic transmission is
running at half the pace as the clock at the receiving end.

No. Please consider that proper motion can yield a "half speed clock"
in the other frame... for both frames.


There is a difference between the measured time dilation and actual time
dilation. In expanding space we expect redshift in both directions. But
time dilation is still measured at the receiver end. The frequency of light
is known for certain elements, which is how redshift is established - I
assumed this knowledge above.

Instead of a light wave, let's consider photons. The time it takes for a
photon to pass from emitter to receiver is t=d*c where d is the distance, t
is the transit interval and c is the speed of light. For two photons
transmitted 1s apart, the first photon travels distance d in dc seconds.
But space expands continually so that when the second photon is emitted d
has expanded to d' where d', the distance travelled by the second photon, is
greater than d ie d'd therefore t't

Thus a stream of photons emitted at 1s intervals arrives at a remote
receiver at intervals greater than 1s. Thus any temporal information
emitted will also be time dilated (the intervals are dilated).

But as you point out, this is true *in either direction* ie if the receiver
emits photons at 1s intervals back to the original emitter then they will be
received at intervals greater than 1s.

We know that in one's own frame, time dilation never occurs (by one's own
measure). That is not at issue. Also, when two high velocity objects pass
each other they both measure time dilation and redshift in the other.

In the case of the expansion of spacetime, redshift indicates time dilation.
Thus if the redshift halves the frequency of the emitted light, the
intervals of transmitted photons will also double by the receiver's clock -
time dilation halves the speed of the emitters clock by the receivers
measure.


--
Kind Regards
Robert Karl Stonjek

"Steve Willner" wrote in message
ps.com...
Robert Karl Stonjek wrote:
The frequency of light is like a clock in itself - if the frequency is
lower
then the clock at the source is slower as measured by an observer who
also
measures that redshift.

As you indicate, this is pretty well known. One familiar application
is that high-redshift supernovae
take longer to fade than supernovae at low redshift. This rules out
tired-light models for redshift.
There's also an extra factor of (1+z) in the equation when you
calculate a distant object's luminosity.


Amazingly, others are arguing against it. For me it was just a penny
dropping - how obvious when you think about it...

Tired light would also have to incorporate time dilation. I don't see how
one can change the frequency of an emitted source without dilating time in
some way. Tired light would necessarily have to include tired time as well.

I'm also interested to know how redshift data is effected by the concept of
a universe made up of so much dark matter. Surely dark matter contributes
to space curvature and so would have to contribute to gravitational
redshift, and with a universe made up of 90% dark matter this must
constitute an appreciable component of the redshift in the form of
gravitational redshift.

As I recall, earlier (1990s?) calculations that considered the accumulated
effect of gravitational redshift were insufficient to counter an expanding
spacetime model, but not by that much. With the large portion of dark
matter in the universe, the accumulated gravitational redshift would be more
than sufficient to account for all the observed redshift plus a bit, though
I don't have those earlier calculations to hand (doing a bit of hand waving
myself...

Thanks,
Robert

Dear Robert Karl Stonjek:

"Robert Karl Stonjek" wrote in message
...

"dlzc" wrote in message
oups.com...
Dear Robert Karl Stonjek:

On Feb 28, 4:45 am, "Robert Karl Stonjek"

wrote:
This one is a little obvious after some thought, but I
haven't heard it mentioned before.

The frequency of light is like a clock in itself - if the
frequency is lower then the clock at the source is
slower as measured by an observer who also
measures that redshift.

The frequency of light is not something intrinsic to
light, though, Robert. It only says something about
the relationship between the emitter and the receiver...
and then only if you know something about the
emitter. (Something like characteristic stellar
emissions, for example.)

If the redshifted electromagnetic radiation was a
radio carrier wave then the frequency the observer
must tune to is further down the dial, as expected,
but the sounds transmitted via that carrier wave
will also appear to be slowed down, like an audio
tape running at the wrong speed.

Yes. The analysis of type Ia supernovae provide
about four different measures of distance that are in
good agreement. Comparison of clocks (redshift)
to intensity (1/r^2), and more.

This is true regardless of the cause of redshift - source
moving away from observer, source near a gravitating
body, or source at a very great distance (Hubble shift).

The Hubble redshift observed on Earth must also be
accompanied by time dilation. If the frequency of light
received is half, for instance, then the clock at the
emitting end of that electromagnetic transmission is
running at half the pace as the clock at the receiving end.

No. Please consider that proper motion can yield a
"half speed clock" in the other frame... for both frames.

There is a difference between the measured time dilation
and actual time dilation.

If all we can do is measure, then "actual" becomes some
superfluous adjective.

In expanding space we expect redshift in both
directions. But time dilation is still measured at the
receiver end. The frequency of light is known for certain
elements, which is how redshift is established - I
assumed this knowledge above.

Understood. However you are trying to draw some artificial line
between "apparent" and "actual", and measurement will not support
such a distinction.

Instead of a light wave, let's consider photons. The
time it takes for a photon to pass from emitter to receiver
is t=d*c where d is the distance, t is the transit interval
and c is the speed of light. For two photons transmitted
1s apart, the first photon travels distance d in dc seconds.
But space expands continually so that when the second
photon is emitted d has expanded to d' where d', the
distance travelled by the second photon, is greater than
d ie d'd therefore t't

Assuming the interval is expanding...

Thus a stream of photons emitted at 1s intervals arrives
at a remote receiver at intervals greater than 1s. Thus
any temporal information emitted will also be time dilated
(the intervals are dilated).

But as you point out, this is true *in either direction* ie
if the receiver emits photons at 1s intervals back to the
original emitter then they will be received at intervals
greater than 1s.

We know that in one's own frame, time dilation never
occurs (by one's own measure). That is not at issue.
Also, when two high velocity objects pass each other
they both measure time dilation and redshift in the
other.

In the case of the expansion of spacetime, redshift
indicates time dilation. Thus if the redshift halves the
frequency of the emitted light,

It does no such thing. The emitted light is not affected by what
happens to the receiver, or the Universe the receiver is located
in. Only the receiver is affected, the emitter and light are
not.

the intervals of transmitted photons will also double
by the receiver's clock - time dilation halves the speed
of the emitters clock by the receivers measure.

I am simply trying to drive a wedge between your assumption that
anything that happens here affects what happened then/there. You
may not mean it, but that is what your words are saying.

David A. Smith

In the case of the expansion of spacetime, redshift
indicates time dilation. Thus if the redshift halves the
frequency of the emitted light,

It does no such thing. The emitted light is not affected by what
happens to the receiver, or the Universe the receiver is located
in. Only the receiver is affected, the emitter and light are
not.

the intervals of transmitted photons will also double
by the receiver's clock - time dilation halves the speed
of the emitters clock by the receivers measure.

I am simply trying to drive a wedge between your assumption that
anything that happens here affects what happened then/there. You
may not mean it, but that is what your words are saying.


I agree absolutely with you on that point. It is such a 'given' that I may
not have made it clear that I make the assumption that no effect on the
emitter is caused by anything that happens to the emitted light or the
opinion of the receiver. Time dilation is always a measured phenomena -
there is no such a thing as actual time dilation in the frame of the
observer no matter how fast that observer is travelling or how curved the
spacetime in his/her area.

When one measures redshift one is also measuring time dilation UNLESS the
redshift is caused by optical effects, scattering etc. I have been
discussing light travelling in free space.

We can consider the whole thing in a slightly different context. Let me
consider the transmission of individual photons across expanding space. The
emitter emits photons at one second intervals. But as space time is
expanding, the distance that each successive photon must travel is greater.
So the interval between each received photon is going to be greater than one
second. If the photons were indicating a clock signal then time is clearly
dilated (as measured by the receiver.). The same would be true is we now
transmit photons in the other direction- time dilation will be measured.

No wedge needed, believe me - we are in furious agreement on the issue of
remote measurement and local conditions.


--
Kind Regards
Robert Karl Stonjek

Robert Karl Stonjek wrote:
Tired light would also have to incorporate time dilation. I don't see how
one can change the frequency of an emitted source without dilating time in
some way.

I'm not sure what you mean by "would have to." The tired-light idea
is that photons magically lose energy as they travel through space.
If you are saying this doesn't agree with observation, you are of
course correct. See http://www.astro.ucla.edu/~wright/tiredlit.htm .

Surely dark matter contributes
to space curvature and so would have to contribute to gravitational
redshift, and with a universe made up of 90% dark matter this must
constitute an appreciable component of the redshift in the form of
gravitational redshift.

In standard cosmology, the redshift is not gravitational. Dark matter
affects the expansion rate and therefore the scale factor as a
function of time. See Ned's other pages for more.

Surely dark matter contributes
to space curvature and so would have to contribute to gravitational
redshift, and with a universe made up of 90% dark matter this must
constitute an appreciable component of the redshift in the form of
gravitational redshift.

In standard cosmology, the redshift is not gravitational. Dark matter
affects the expansion rate and therefore the scale factor as a
function of time. See Ned's other pages for more.


Gravitational redshift is a reality and has been measured, back in the 60s :
Effect of Gravity on Gamma Radiation
R. V. POUND and J. L. SNIDER

There must be some gravitational component in the observed cosmological
redshift, even if that component is immeasurably small. Light coming from a
planet, star, galaxy, cluster of galaxies or even larger structures is
redshifted by those structures. The greater the distance the light travels,
the more the gravitational redshift.

If the said structures have a large dark matter component then the
gravitational redshift will be greater.

But this does not mean that gravitational redshift forms a significant
portion of the measured cosmological redshift - one would have to quantify
the redshift component. Of course light received by a detector on a massive
body is blue shifted, and as we measure redshift on a planet near a star in
a galaxy that is part of a cluster of galaxies etc this may well be
sufficient to cancel out any gravitational redshift at the emitting end of
that radiation.

The question is one of whether a beam of light emitted from one massive body
and received on an identical massive body will be frequency shifted due to
gravitation.


--
Kind Regards
Robert Karl Stonjek

"Robert Karl Stonjek" wrote in message
...
Surely dark matter contributes
to space curvature and so would have to contribute to gravitational
redshift, and with a universe made up of 90% dark matter this must
constitute an appreciable component of the redshift in the form of
gravitational redshift.

In standard cosmology, the redshift is not gravitational. Dark matter
affects the expansion rate and therefore the scale factor as a
function of time. See Ned's other pages for more.


Gravitational redshift is a reality and has been measured, back in the 60s :
Effect of Gravity on Gamma Radiation
R. V. POUND and J. L. SNIDER

There must be some gravitational component in the observed cosmological
redshift, even if that component is immeasurably small. Light coming from a
planet, star, galaxy, cluster of galaxies or even larger structures is
redshifted by those structures. The greater the distance the light travels,
the more the gravitational redshift.

Light passing through a gravitational well falls into it
before climbing out again. What's the net gravitational
red shift? For gravitational wells of small extent (the
mass being bound by gravity and not subject to cosmological
expansion) the net effect is nil. For loose systems that
are not gravitationally bound, which are sublect to
cosmological expansion and for which the traversal time
of light is not (relatively) negligable, there will be a
net red shift, as the light is 'stretched' during the
traversal.


If the said structures have a large dark matter component then the
gravitational redshift will be greater.

No more so than it would be for normal matter making up
the gravitating mass.


But this does not mean that gravitational redshift forms a significant
portion of the measured cosmological redshift - one would have to quantify
the redshift component. Of course light received by a detector on a massive
body is blue shifted, and as we measure redshift on a planet near a star in
a galaxy that is part of a cluster of galaxies etc this may well be
sufficient to cancel out any gravitational redshift at the emitting end of
that radiation.

The red shift caused by the gravitational field in which
the detector resides is a constant that applies to all
received signals. Presumably the residual redshift
measured for soureces at different cosmological distances
would simply be offset by this constant amount.


The question is one of whether a beam of light emitted from one massive body
and received on an identical massive body will be frequency shifted due to
gravitation.

Simply compare the gravitational potentials of the emitter
versus the detector. The difference will lead to the
expected gravitational red shift.