I'm in a spaceship travelling at 0.866c away from Earth. AIUI, that's a
time dilation of 2, and my speedometer, which of course uses my on-board
clock and a set of mile markers laid down by a previous expedition,
indicates 2c.

Earth is beaming a laser in my direction and pulsing it once per Earth
second. Clearly these pulses are going faster than me, and I have a
handy pulse detector.

What frequency of pulse repetition will I measure with my on-board
instrumentation?

And, Captain Slog-supplemental, what if the laser is instead at my
destination planet?

--
Cheers
Clive

On 26/02/2024 10:30 am, Clive Arthur wrote:
> I'm in a spaceship travelling at 0.866c away from Earth.  AIUI, that's a
> time dilation of 2, and my speedometer, which of course uses my on-board
> clock and a set of mile markers laid down by a previous expedition,
> indicates 2c.
>
> Earth is beaming a laser in my direction and pulsing it once per Earth
> second.  Clearly these pulses are going faster than me, and I have a
> handy pulse detector.
>
> What frequency of pulse repetition will I measure with my on-board
> instrumentation?
>
> And, Captain Slog-supplemental, what if the laser is instead at my
> destination planet?

Ignoring relativity, you'd see a new pulses originating from earth at
the rate of one per second once every 7.5 seconds (Doppler effect) .
Because of time dilation you'd think that there only 3.73 second between
each new pulse.

You'd run into 7.5 pulses a second for a 1Hz pulse originating on your
destination planet, and time dilation would made you think that it was
pulsing at 14.9Hz.

So the Doppler effect is more dramatic than time dialation.

But why bother to ask the question? Darius the Dumb thinks that it is an
off-topic question - why he should feel the need to point this out when
you have marked it as off-topic in your subject line is a question worth
asking, but Darius the Dumb is remarkably dumb.

Asking an off-topic question isn't actually proof positive that you are
a troll. Most people who post here post off-topic question that they
correctly imagine will still be of interest to the people who habitually
post here, but that's a more complicated idea than Darius can get his
head around.

--
Bill Sloman, Sydney

On 26/02/2024 04:10, Bill Sloman wrote:
> On 26/02/2024 10:30 am, Clive Arthur wrote:
>> I'm in a spaceship travelling at 0.866c away from Earth.  AIUI, that's
>> a time dilation of 2, and my speedometer, which of course uses my
>> on-board clock and a set of mile markers laid down by a previous
>> expedition, indicates 2c.
>>
>> Earth is beaming a laser in my direction and pulsing it once per Earth
>> second.  Clearly these pulses are going faster than me, and I have a
>> handy pulse detector.
>>
>> What frequency of pulse repetition will I measure with my on-board
>> instrumentation?
>>
>> And, Captain Slog-supplemental, what if the laser is instead at my
>> destination planet?
>
> Ignoring relativity, you'd see a new  pulses originating from earth at
> the rate of one per second once every 7.5 seconds (Doppler effect) .
> Because of time dilation you'd think that there only 3.73 second between
> each new pulse.
>
> You'd run into 7.5 pulses a second for a 1Hz pulse originating on your
> destination planet, and time dilation would made you think that it was
> pulsing at 14.9Hz.
>
> So the Doppler effect is more dramatic than time dialation.

Thanks Bill, very helpful. That's what I'd worked out, but for some
reason I'd convinced myself I was wrong.

> But why bother to ask the question? Darius the Dumb thinks that it is an
> off-topic question - why he should feel the need to point this out when
> you have marked it as off-topic in your subject line is a question worth
> asking, but Darius the Dumb is remarkably dumb.
>
> Asking an off-topic question isn't actually proof positive that you are
> a troll. Most people who post here post off-topic question that they
> correctly imagine will still be of interest to the people who habitually
> post here, but that's a more complicated idea than Darius can get his
> head around.

I don't see his posts. I very rarely killfile, but life's too short to
bother with sad attention-seekers.

--
Cheers
Clive

On 26/02/2024 10:59 pm, Clive Arthur wrote:
> On 26/02/2024 04:10, Bill Sloman wrote:
>> On 26/02/2024 10:30 am, Clive Arthur wrote:

<snip>

> Thanks Bill, very helpful.  That's what I'd worked out, but for some
> reason I'd convinced myself I was wrong.

I'm no authority. There are a couple of people who post here who might
give you a more reliable opinion.

<snipped a reference to Darius the Dumb>

> I don't see his posts.  I very rarely killfile, but life's too short to
> bother with sad attention-seekers.

He does waste a lot of bandwidth. He's even more inane than Skybuck
Flying, and he posts a whole lot more.

--
Bill Sloman, Sydney