[[Mod. note -- I think "floating-point rounding errors" is the phrase
you're looking for. If v/c is very close to 1, then the formula for
gamma tends to be very sensitive to rounding errors, causing the sorts
of anomolous behavior you noticed.
20-JUN-2021
hello -
I find a maximum value for the Lorentz gamma factor,
gamma = ((1-((v)^2/c^2))^(1/2))^-1 = 54794158.005943767726,
for a relative velocity v = 299792457.99999997 m/s.
For an electron with mass m_e = 510998.91 ev/c^2 and momentum p_e=m_ev
the max velocity is v_e = p_e/m_e = 299792457.9999999404 m/s.
Plugging v_e into the gamma equation yields the same gamma max.
Computing a higher velocity past the eighth decimal place does
not change the gamma value either; until it blows up as gamma = inf.
Is there a good turn of phrase to explain this limit?
Cheers,
mj horn
[[Mod. note -- I think "floating-point rounding errors" is the phrase
you're looking for. If v/c is very close to 1, then the formula for
gamma tends to be very sensitive to rounding errors, causing the sorts
of anomolous behavior you noticed.
The computation can be reorganized to be less sensitive to rounding
errors, but the easy solution is to just use brute force, i.e., use
higher precision in the computation. For example, software systems
such as Sage, Maple, and Mathematica can all easily do computations
in higher precision than standard C "double" (which typically gives
about 16-digit accuracy). For example, in Sage:
sage: gamma(v_over_c) = 1/sqrt(1 - v_over_c^2)
sage: gamma(1 - 1/(10**20)) 100000000000000000000/199999999999999999999*sqrt(199999999999999999999)
sage: n(gamma(1 - 1/(10**20)), digits=50) 7.0710678118654752440261212905781540809584467771981e9
sage:
As to what relevance this has for *physics*: the current record for the highest-energy cosmic ray has a gamma factor of over 10**20, corresponding
to v/c of over 1 - 10**-40.
-- jt]]
mark horn <toadast...@gmail.com> schrieb:
[[Mod. note -- I think "floating-point rounding errors" is the phrase you're looking for. If v/c is very close to 1, then the formula forI would suggest that everybody who does floating-point calculations
gamma tends to be very sensitive to rounding errors, causing the sorts
of anomolous behavior you noticed.
should read the famous "Goldberg paper", "What Every Computer
Scientist Should Know About Floating-Point Arithmetic" available
from https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html
(it is not absolutely necessary to follow the proofs as a user).
[...]
Decimal('1.414213562373095048801688724')from decimal import *
two = Decimal(2)
Decimal.sqrt(two)
getcontext().prec = 50
Decimal.sqrt(two) Decimal('1.4142135623730950488016887242096980785696718753769')
On 6/21/21 3:01 AM, mark horn wrote:
[...]
For extended-precision arithmetic, rather than "Sage, Maple, and Mathematica", people may find it easier to use Python and its decimal
module.
Tom Roberts <tjroberts137@sbcglobal.net> schrieb:
On 6/21/21 3:01 AM, mark horn wrote:
[...]
For extended-precision arithmetic, rather than "Sage, Maple, and
Mathematica", people may find it easier to use Python and its decimal
module.
Alternatively, you can use the oldest scientific programming
language, Fortran.
Fortran lets you declare a real variable with at least
n valid decimal digits, and common compilers (ifort and
gfortran, among others) allow up to IEEE 128 bit numbers,
with 33 valid digits. This is described in Michael
Metcalf's Wikiedia article on Fortran 95 features under https://en.wikipedia.org/wiki/Fortran_95_language_features#REAL .
20-JUN-2021
I find a maximum value for the Lorentz gamma factor,
gamma = ((1-((v)^2/c^2))^(1/2))^-1 = 54794158.005943767726,
for a relative velocity v = 299792457.99999997 m/s.
For an electron with mass m_e = 510998.91 ev/c^2 and momentum p_e=m_ev
the max velocity is v_e = p_e/m_e = 299792457.9999999404 m/s.
As to what relevance this has for *physics*: the current record for the highest-energy cosmic ray has a gamma factor of over 10**20, corresponding
to v/c of over 1 - 10**-40.
-- jt]]
Ofcourse you could claim that the speed of light is constant.
Maybe this document gives a glue, because it mentions Supernova 1987A https://www.abc.net.au/science/articles/2011/11/25/3376138.htm
Neutrino's win but Einstein has not lost yet
"but the supernova observations showed that neutrinos and photons
generated at the same time by the supernova, 160 thousand light years
away, arrived here at the same time as well."
But if they arrive simultaneously here how do you know that
they were emitted simultaneously overthere?
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Ofcourse you could claim that the speed of light is constant.
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
[...] with those declared variables you can actually compute things
in a fast way, since Fortran is a compiled language while Python is interpreted.
On 6/26/21 5:36 AM, Jos Bergervoet wrote:
[...] with those declared variables you can actually compute things
in a fast way, since Fortran is a compiled language while Python is
interpreted.
While true,
this is a red herring, except for very-old-school
programmers who don't understand how to use Python [#]. Modern
software development involves using libraries rather than coding stuff yourself.
Python libraries numpy and scipy are every bit as fast as
FORTRAN when using arrays for large computations.
...
The real win for Python, however, is in improved programmer productivity
The real win for Python, however, is in improved programmer productivity compared to Fortran.
Nicolaas Vroom <nicolaa...@pandora.be> schrieb:
Ofcourse you could claim that the speed of light is constant.The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
In article <sb7pcc$243$3@newsreader4.netcologne.de>, Thomas Koenig <tkoenig@netcologne.de> writes:
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Ofcourse you could claim that the speed of light is constant.
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
The speed of light is now a defined quantity, that is true. However,
that is merely a practical matter. If the speed of light really were variable, that could still be detected just as easily as before the redefinition.
Suppose that the speed of light did drop by a measurable
amount. People would not immediately redefine the length of everything because of that.
Many other SI units were recently redefined, so they are "exact" in that sense. The same caveats apply.
In article <4062c54b-7af6-4f3a...@googlegroups.com>,
Nicolaas Vroom <nicolaa...@pandora.be> writes:
As to what relevance this has for *physics*: the current record for the
highest-energy cosmic ray has a gamma factor of over 10**20, corresponding >>> to v/c of over 1 - 10**-40.
The physics behind this question is very important, because how do you know that the speed cosmic ray is almost the same as the speed of light
but smaller and not larger?
What I want to know is how are both speeds established? (*1)
Maybe this document gives a glue, because it mentions Supernova 1987A https://www.abc.net.au/science/articles/2011/11/25/3376138.htm
Neutrino's win but Einstein has not lost yet
The fact that they arrived at roughly the same time sets strong upper
limits on the mass of the neutrinos involved. For some types of
neutrinos, those were (maybe still are) the best limits. A supernova is
not an instantaneous event, and for various reasons the light and
neutrinos are not produced at exactly the same time or, more
importantly, cannot freely travel from the same time, so some
discrepancy is expected.
It's not clean enough to use for the type of
test which you have in mind.
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>
wrote:
In article <sb7pcc$243$3@newsreader4.netcologne.de>, Thomas Koenig
<tkoenig@netcologne.de> writes:
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Of course you could claim that the speed of light is constant.
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
The speed of light is now a defined quantity, that is true. However,
that is merely a practical matter. If the speed of light really were
variable, that could still be detected just as easily as before the
redefinition.
It is incredible how much misunderstanding there is
on such a simple subject.
To clear things up:
The speed of light cannot 'really' be variable.
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>...
In article <sb7pcc$243$3@newsreader4.netcologne.de>, Thomas Koenig
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Ofcourse you could claim that the speed of light is constant.
It is incredible how much misunderstanding there is
on such a simple subject.
The speed of light cannot 'really' be variable.
Why?
In order for the speed of light to be measurable at all
we need to define both a length and a time unit.
...
Indeed we can agree that basically this is determined by the metric
of space. Any massless field will have a propagation speed defined
by the metric, but any measurement of speed also has to use that
metric. So the result is fixed.
This should make clear that a change cannot be observed using the
local metric, but not everyone will agree that this means it cannot
'really' change.
We know that seen from another point in space, the speed of light
can be different if space-time is curved (as it usually is..) You
may then claim that it is only an 'apparent' slowing down if e.g.
light falls into a black hole, but then we only change the discussion
to the meaning of 'apparent' and 'really'. We can't maintain that it
is unobservable, in that case.
On 6/29/21 1:49 PM, Jos Bergervoet wrote:
Indeed we can agree that basically this is determined by the metricYou mean the metric of spacetime (not space). And this applies only to a massless field -- there is no fundamental reason for the photon to be massless, it's just that its mass is observed to be consistent with zero
of space. Any massless field will have a propagation speed defined
by the metric, but any measurement of speed also has to use that
metric. So the result is fixed.
and an extremely tiny upper limit (< 10^-18 eV).
This should make clear that a change cannot be observed using theThe constancy of the vacuum speed of light applies only locally, so
local metric, but not everyone will agree that this means it cannot 'really' change.
everyone who understands the issues will agree for a massless field.
But of course that's the rub -- we don't really know whether the photon
field is truly massless.
We know that seen from another point in space, the speed of lightThat's just an argument over the meanings of words. Moreover it's an
can be different if space-time is curved (as it usually is..) You
may then claim that it is only an 'apparent' slowing down if e.g.
light falls into a black hole, but then we only change the discussion
to the meaning of 'apparent' and 'really'. We can't maintain that it
is unobservable, in that case.
argument that never comes up because the constancy of the vacuum speed
of light applies only locally.
I have to measure the speed of light and I have to measure the speed
of an electron v_e or of a cosmic ray.[...]
A more advanced question is what to do in case the speed of cosmic
ray is higher than the speed of light i.e 299792458,0995 m/sec versus 299792458 m/sec in vacuum.
[...] The speed of light cannot 'really' be variable. [...]
In article <1pbi21w.1t8rob81hgck0aN%nospam@de-ster.demon.nl>, nospam@de-ster.demon.nl (J. J. Lodder) writes:=20,
=20
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>
wrote:
In article <sb7pcc$243$3@newsreader4.netcologne.de>, Thomas Koenig
<tkoenig@netcologne.de> writes:=20
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Of course you could claim that the speed of light is constant.
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
The speed of light is now a defined quantity, that is true. However=
ethat is merely a practical matter. If the speed of light really wer=
variable, that could still be detected just as easily as before the
redefinition.
It is incredible how much misunderstanding there is=20
on such a simple subject.
I agree. :-)
=20
To clear things up:=20
The speed of light cannot 'really' be variable.
It is true that the metre is now defined as the distance travelled by
light in a certain time and thus by definition the speed of light is=20 constant. However, the metre (and the second) used to be defined=20 differently than they are now.
Back then, it was certainly possible, in=20
principle, to detect a change in the speed of light.=20
One could perform the same experiment today.
Nature doesn't know what the current SI definitions are.
The metre is now defined as it is a) as a practical matter and b)=20
because we assume that the speed of light is constant.
If the speed of light did change, i.e. of one does observations like th=ose
of R=C3=B8mer, Fizeau, etc., using, say, pendulum clocks as a reference=, and
notice that it changes, then one has measured the change. The conseque=nce
would not be to point to the SI definition and say that it cannot=20e
change, therefore we must modify other definitions (perhaps even=20 periodically if the speed of light depends on time), but rather would b=
to realize that our assumptions in the current definition of the metre
are wrong and must be changed.
Think about the definition of the metre and kilogram. Why are the=20 original definitions not used? One reason is because on noticed that
the mass of the reference kilogram has actually changed with time.
By your logic, that should not have been possible, since, by definitio=n,
the reference kilogram has a mass of exactly one kilogram.
Nevertheless, the change was detected.
Op zondag 27 juni 2021 om 00:57:19 UTC+2 schreef Phillip Helbig:That is correct
Nicolaas Vroom.
[[Mod. note -- This topic is a bit tricky, because to measure a speed
in meters/second, we need to know what a meter is, and what a second is.
"The metre is the length of the path travelled by light in vacuumThat is the measurement by person "A" in vacuum.
during a time interval of 1/299 792 458 of a second."
So with this definition, the speed of light is necessarily exactly
299 792 458 meters/second. Experiments to "measure the speed of light"
(e.g., by timing a light pulse over a measured distance) are actually measuring a *length* (in meters). E.g., if your measurement shows
that it takes light 100 nanoseconds to travel a certain distance,
then what you've really done is measure that distance to be
(as 100e-9 seconds * 299 792 458 meters/second) = 29.9792458 meters.
On 21/06/29 8:41 AM, J. J. Lodder wrote:
It is incredible how much misunderstanding there isI think there are several reasons for it. See below..
on such a simple subject.
The speed of light cannot 'really' be variable.
Why?
In order for the speed of light to be measurable at all
we need to define both a length and a time unit.
...
Indeed we can agree that basically this is determined by the
metric of space.
Any massless field will have a propagation
speed defined by the metric, but any measurement of speed also
has to use that metric. So the result is fixed.
This should make clear that a change cannot be observed using
the local metric, but not everyone will agree that this means
it cannot 'really' change.
We know that seen from another point in space, the speed of
light can be different if space-time is curved (as it usually
is..)
All these things explain the ongoing discussion, I think..
Op woensdag 30 juni 2021 om 21:37:22 UTC+2 schreef Tom Roberts:
On 6/29/21 1:49 PM, Jos Bergervoet wrote:
Indeed we can agree that basically this is determined by the metricYou mean the metric of spacetime (not space). And this applies only to a massless field -- there is no fundamental reason for the photon to be massless, it's just that its mass is observed to be consistent with zero and an extremely tiny upper limit (< 10^-18 eV).
of space. Any massless field will have a propagation speed defined
by the metric, but any measurement of speed also has to use that
metric. So the result is fixed.
How is this mass observed? Or should I write upper limit?
Is it not true,
that when it is possible to measure the energy of a light pulse,
that then individual photons also have energy,
and as a consequence individual photons also have a mass?
This implies when a star emits light it also emits mass.
We know that seen from another point in space, the speed of lightThat's just an argument over the meanings of words. Moreover it's an argument that never comes up because the constancy of the vacuum speed
can be different if space-time is curved (as it usually is..) You
may then claim that it is only an 'apparent' slowing down if e.g.
light falls into a black hole, but then we only change the discussion
to the meaning of 'apparent' and 'really'. We can't maintain that it
is unobservable, in that case.
of light applies only locally.
Does that implies that globally, considering a light pulse (explosion) emitted over a long distance, that its speed is not constant?
Is this text from Wikipedia true?:
"Photons are massless,[a] so they always move at the speed of light in vacuum,
299792458 m/s (or about 186,282 mi/s).
[a] The photon's invariant mass (also called "rest mass" for massive particles)
is believed to be exactly zero. This is the notion of particle mass generally used by modern physicists. The photon does have a nonzero relativistic mass, depending on its energy, but this varies according to the frame of reference."
Op dinsdag 29 juni 2021 om 08:51:03 UTC+2 schreef Nicolaas Vroom:
Op zondag 27 juni 2021 om 00:57:19 UTC+2 schreef Phillip Helbig:
Nicolaas Vroom.
[[Mod. note -- This topic is a bit tricky, because to measure a speedThat is correct
in meters/second, we need to know what a meter is, and what a second is.
"The metre is the length of the path travelled by light in vacuum
during a time interval of 1/299 792 458 of a second."
So with this definition, the speed of light is necessarily exactlyThat is the measurement by person "A" in vacuum.
299 792 458 meters/second. Experiments to "measure the speed of light" (e.g., by timing a light pulse over a measured distance) are actually measuring a *length* (in meters). E.g., if your measurement shows
that it takes light 100 nanoseconds to travel a certain distance,
then what you've really done is measure that distance to be
(as 100e-9 seconds * 299 792 458 meters/second) = 29.9792458 meters.
What that means that "A" first places two markers a certain distance away
and then sends a light signal between those two markers.
What "A" measures is that it takes 100 nanoseconds to travel that distance. His conclusion is that the distance is 29.9792458 meters.
Suppose "B" does 'exactly' the same, but "B" measures that it takes less than 100 nanosecs and his is conclusion is that the distance is 29.9792458 meters.
Is that physical possible?
In order for "B" to perform the experiment he has to rely on a very detailed description (supplied by "A" or ?), on how to perform this experiment.
For example it should tell you how to measure the time (everywhere in the universe) and give a clear definition exactly what a vacuum is.
This type of information is of critical importance to calculate the distance travelled by a light pulse and secondly to establish if that distance is everywhere the same.
Implying that the speed of light is a physical constant and also everywhere the same. (Personally I doubt that)
The same type of description is also required if you want to measure
the speed of an electron or a cosmic ray.
In that case you first have to measure the 'fixed' distance using a light pulse, secondly you have to measure the time t2 it takes for the cosmic ray to travel that same 'fixed' distance.
Dividing the 'fixed' distance by t2 gives you the speed of the cosmic ray.
Nicolaas Vroom
On 6/29/21 1:41 AM, J. J. Lodder wrote:
[...] The speed of light cannot 'really' be variable. [...]
You make far too many assumptions to be reasonable.
Certainly the (vacuum) speed of light COULD vary, it's just that in the
world we inhabit, with current technology, it is observed to not vary significantly (when measured using standard clocks and rulers at rest in
some locally inertial frame).
But it certainly is possible that in the
future we will develop technology with greatly improved resolution and discover that it actually does vary in the world we inhabit.
It is also possible we will never find it varies -- science is a JOURNEY,
not a destination.
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Ofcourse you could claim that the speed of light is constant.
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
On 6/29/21 1:49 PM, Jos Bergervoet wrote:
Indeed we can agree that basically this is determined by the metric
of space. Any massless field will have a propagation speed defined
by the metric, but any measurement of speed also has to use that
metric. So the result is fixed.
You mean the metric of spacetime (not space). And this applies only to a massless field -- there is no fundamental reason for the photon to be massless, it's just that its mass is observed to be consistent with zero
and an extremely tiny upper limit (< 10^-18 eV).
This should make clear that a change cannot be observed using the
local metric, but not everyone will agree that this means it cannot 'really' change.
The constancy of the vacuum speed of light applies only locally, so
everyone who understands the issues will agree for a massless field.
But of course that's the rub -- we don't really know whether the photon
field is truly massless.
We know that seen from another point in space, the speed of light
can be different if space-time is curved (as it usually is..) You
may then claim that it is only an 'apparent' slowing down if e.g.
light falls into a black hole, but then we only change the discussion
to the meaning of 'apparent' and 'really'. We can't maintain that it
is unobservable, in that case.
That's just an argument over the meanings of words. Moreover it's an
argument that never comes up because the constancy of the vacuum speed
of light applies only locally.
All this only applies to massless fields, and we don't really know
whether the photon field is truly massless. Of course we never will....
Tom Roberts <tjroberts137@sbcglobal.net> wrote:he
=20
On 6/29/21 1:41 AM, J. J. Lodder wrote:
[...] The speed of light cannot 'really' be variable. [...]=20
You make far too many assumptions to be reasonable.
=20
Certainly the (vacuum) speed of light COULD vary, it's just that in t=
inworld we inhabit, with current technology, it is observed to not vary significantly (when measured using standard clocks and rulers at rest=
dsome locally inertial frame).=20
That's where you are mistaken.
There is no such thing as a god-given 'standard clock'
or 'standard ruler'.
=20
But it certainly is possible that in the=20
future we will develop technology with greatly improved resolution an=
NEY,discover that it actually does vary in the world we inhabit.=20
A meaningless statement.
If variation is found we will have to discover (or decide!)
what it is that varies.
(speed?, rulers?, clocks?, all three?, some 'fundamental' 'constant'?)
=20
It is also possible we will never find it varies -- science is a JOUR=
not a destination.=20
Empty ideology.=20
This is not a matter that can be settled
by means of naive empiricism,
by just 'measuring' the 'speed of light',
Op woensdag 30 juni 2021 om 21:37:22 UTC+2 schreef Tom Roberts:
[...] there is no fundamental reason for the photon to be
massless, it's just that its mass is observed to be consistent with
zero and an extremely tiny upper limit (< 10^-18 eV).
How is this mass observed? Or should I write upper limit?
Is it possible to measure this photon field?
Is it not true, that when it is possible to measure the energy of a
light pulse, that then individual photons also have energy,
and as a consequence individual photons also have a mass?
This implies when a star emits light it also emits mass.
the constancy of the vacuum speed of light applies only locally.
Does that implies that globally, considering a light pulse
(explosion) emitted over a long distance, that its speed is not
constant?
Is this text from Wikipedia true?: "Photons are massless,[a] so they
always move at the speed of light in vacuum, 299792458 m/s (or about
186,282 mi/s).
[a] The photon's invariant mass (also called "rest mass" for massive particles) is believed to be exactly zero. This is the notion of
particle mass generally used by modern physicists. The photon does
have a nonzero relativistic mass, depending on its energy, but this
varies according to the frame of reference."
Why can the speed of light not be different in different places in
the universe
The time unit is the most tricky if the method to measure time
involves light signals and when you want to use time to measure the
speed of light. This looks like circular reasoning.
Exactly what is determined by the metric of space?
This raises also the question how is this metric measured.
In article <a5210321-687e-464d...@googlegroups.com>,
Nicolaas Vroom <nicolaa...@pandora.be> writes:
Op woensdag 30 juni 2021 om 21:37:22 UTC+2 schreef Tom Roberts:
That's just an argument over the meanings of words. Moreover it's an argument that never comes up because the constancy of the vacuum speed
of light applies only locally.
Does that implies that globally, considering a light pulse (explosion) emitted over a long distance, that its speed is not constant?Look up "Shapiro delay".
In article <1pbx48j.1xmowyy1fnnxmfN%nos...@de-ster.demon.nl>, nos...@de-ster.demon.nl (J. J. Lodder) writes:Etc,
Thomas Koenig <tko...@netcologne.de> wrote:
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than
299792458 m/s, recalibrate your measurement devices.
Nonsense.Because, for practical reasons, the metre is now defined as the
In the SI as it stands it is impossible in principle
to measure the speed of light,
distance light travels in a certain time. That is our definition,
In article <a5210321-687e-464d-b84c-9733631c2ee0n@googlegroups.com>,
Nicolaas Vroom <nicolaas.vroom@pandora.be> writes:
tindividual photons also have energy, and as a consequence
individual photons also have a mass?
E = mc^2 so in that sense photons have mass...
This implies when a star emits light it also emits mass.
...and as a result the mass of a star decreases when it emits light.
The question is whether the rest mass is zero.
Tom Roberts <tjroberts137@sbcglobal.net> wrote:
Certainly the (vacuum) speed of light COULD vary, it's just that in the
world we inhabit, with current technology, it is observed to not vary
significantly (when measured using standard clocks and rulers at rest in
some locally inertial frame).
There is no such thing as a god-given 'standard clock'
or 'standard ruler'.
But it certainly is possible that in the
future we will develop technology with greatly improved resolution and
discover that it actually does vary in the world we inhabit.
A meaningless statement.
If variation is found we will have to discover (or decide!)
what it is that varies.
(speed?, rulers?, clocks?, all three?, some 'fundamental' 'constant'?)
This is not a matter that can be settled
by means of naive empiricism,
by just 'measuring' the 'speed of light',
Op woensdag 7 juli 2021 om 19:20:49 UTC+2 schreef Phillip Helbig:
Because, for practical reasons, the metre is now defined as the
distance light travels in a certain time. That is our definition,
[[Mod. note -- An old nautical saying is "never go to sea with two chronometers; always take one or three".
In this context, that means
that people doing precision timing & clock development often use an
an ensemble of co-located clocks (typically 5-10 are used), all of similar construction and method-of-operation, so that they can inter-compare the clocks. Since all the clocks in the ensemble are co-located, they should
all record the same elapsed-time readings; more accurately, any differences in their elapsed-time readings can be ascribed to clock drifts (errors). Inter-comparing the clocks can thus give a statistical estimate of the clocks' accuracy If any clock is an outlier
in the ensemble, it's flagged as not-working-properly (a.k.a "broken").
Your premise is wrong -- we do not use the speed of light to determine
the unit of time. The second is defined as the duration of 9,192,631,770 cycles of the hyperfine ground-state transition of Cs-133.
In article <1pbx3kk.jywko91341fc9N%nospam@de-ster.demon.nl>, nospam@de-ster.demon.nl (J. J. Lodder) writes:
Tom Roberts <tjroberts137@sbcglobal.net> wrote:
On 6/29/21 1:41 AM, J. J. Lodder wrote:
[...] The speed of light cannot 'really' be variable. [...]
You make far too many assumptions to be reasonable.
heCertainly the (vacuum) speed of light COULD vary, it's just that in t=
inworld we inhabit, with current technology, it is observed to not vary significantly (when measured using standard clocks and rulers at rest=
some locally inertial frame).
That's where you are mistaken.
There is no such thing as a god-given 'standard clock'
or 'standard ruler'.
dBut it certainly is possible that in the
future we will develop technology with greatly improved resolution an=
discover that it actually does vary in the world we inhabit.
A meaningless statement.
If variation is found we will have to discover (or decide!)
what it is that varies.
(speed?, rulers?, clocks?, all three?, some 'fundamental' 'constant'?)
NEY,It is also possible we will never find it varies -- science is a JOUR=
not a destination.
Empty ideology.
This is not a matter that can be settled
by means of naive empiricism,
by just 'measuring' the 'speed of light',
One could measure the speed of light via several different types of
rulers and clocks, or by measuring wavelength and frequency, or
whatever, in the lab. It is theoretically possible that the speed of
light could change with time and that we could measure it.
The fact that the speed of light is now a defined quantity does not
somehow magically make it impossible to make a measurement which was
possible with the original SI definitions.
Obviously, if such a change were detected, then it would be a good idea
to change the definition of the metre.
In article <1pbx48j.1xmowyy1fnnxmfN%nospam@de-ster.demon.nl>, nospam@de-ster.demon.nl (J. J. Lodder) writes:
Thomas Koenig <tkoenig@netcologne.de> wrote:
Nicolaas Vroom <nicolaas.vroom@pandora.be> schrieb:
Ofcourse you could claim that the speed of light is constant.
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than 299792458 m/s, recalibrate your measurement devices.
Nonsense.
In the SI as it stands it is impossible in principle
to measure the speed of light,
Because, for practical reasons, the metre is now defined as the
distance light travels in a certain time. That is our definition, which Nature doesn't know about. We cannot magically influence Nature by
changing definitions.
With time, more and more SI units have been defined via fiat values of constants of Nature. This is a purely practical matter, because we
ASSUME that they do not change with time. The definitions are also
coupled with experiments which are relatively easy to reproduce.
Back when the metre was defined as 1/10,000,000 of the distance from
north pole to equator along the meridian through Paris, that did not
somehow make it impossible to measure the change in the size of the
Earth with time.
Op woensdag 7 juli 2021 om 19:20:49 UTC+2 schreef Phillip Helbig:
In article <1pbx48j.1xmowyy1fnnxmfN%nos...@de-ster.demon.nl>, nos...@de-ster.demon.nl (J. J. Lodder) writes:
Thomas Koenig <tko...@netcologne.de> wrote:
The way that the SI units are defined now, the speed of light
in vacuum is indeed constant. If you measure anything else than 299792458 m/s, recalibrate your measurement devices.
Etc,Nonsense.Because, for practical reasons, the metre is now defined as the
In the SI as it stands it is impossible in principle
to measure the speed of light,
distance light travels in a certain time. That is our definition,
You can do that, but now you create a new issue:
How is this CERTAIN TIME defined and more important measured in
detail in practice.
That is a very important issue because we can all measure the same
time, but when we compare all the distances measured,
(which should be identical) they are not.
That means at the most 1 person measures the distance of 299792458
meters correct assuming we all measure 1 second.
It is the same as the above ambiguous advice:
"YOU should recalibrate your measurement device."
But if my measurement also is different from all of the others
how much should I adapt my time measurement device?
The above raised issue about CERTAIN TIME becomes even more important
if you want to measure the speed of a cosmic ray (etc).
[[Mod. note -- An old nautical saying is "never go to sea with two chronometers; always take one or three".
On 7/7/21 3:31 AM, J. J. Lodder wrote:
Tom Roberts <tjroberts137@sbcglobal.net> wrote:
Certainly the (vacuum) speed of light COULD vary, it's just that in the
world we inhabit, with current technology, it is observed to not vary
significantly (when measured using standard clocks and rulers at rest in >> some locally inertial frame).
There is no such thing as a god-given 'standard clock'
or 'standard ruler'.
Of course. Such standards are determined by humans. Organizations such
as ISO have been created to agree upon such standards and publish them.
It OUGHT to be obvious that a standard clock measures its elapsed proper
time using standard seconds, and a standard ruler measures distance
using some standard of length, such as meters.
But it certainly is possible that in the
future we will develop technology with greatly improved resolution and
discover that it actually does vary in the world we inhabit.
A meaningless statement.
If variation is found we will have to discover (or decide!)
what it is that varies.
(speed?, rulers?, clocks?, all three?, some 'fundamental' 'constant'?)
My statement is not meaningless: if the speed of light is measured to
vary, then it is certainly varying -- DUH!
Whether something else is also varying is a different question; to date
no significant variation has been found in any of the things you
mention. Such measurements have excellent accuracy, 9 or more
significant digits.
This is not a matter that can be settled
by means of naive empiricism,
by just 'measuring' the 'speed of light',
How else could one detect a variation in the speed of light????
Naval chronometers could have sudden jumps
in their going rates for various reasons.
Take one, and you hope for the best.
Take two and you go crazy when they start to diverge.
(which one is right, so where am I ????)
Take three and you can hope to identify the faulty one.
J. J. Lodder <nospam@de-ster.demon.nl> schrieb:
Naval chronometers could have sudden jumps
in their going rates for various reasons.
Take one, and you hope for the best.
Take two and you go crazy when they start to diverge.
(which one is right, so where am I ????)
Take three and you can hope to identify the faulty one.
"To measure once leads to knowledge. To measure twice leads to
doubt. To measure three times leads to statistics."
(An unattributed Internet quote which seems to be a generalization
of what you wrote).
Tom Roberts <tjroberts137@sbcglobal.net> wrote:...
On 7/7/21 3:31 AM, J. J. Lodder wrote:
This is not a matter that can be settled
by means of naive empiricism,
by just 'measuring' the 'speed of light',
How else could one detect a variation in the speed of light????
The answer is that one can't.
To be able to measure anything at all
you need to know what the space-time is.
The speed of light can only become variable
if you begin by inventing a new space-time
in which the relativity postulate does not hold.
(so you have to build a new foundation for all of phyics)
Next you have to work out how measurements are to be done
in this new space-time,
and how that relates to real physical measurements.
The problem is to give a meaningful interpretation,
based on deeper lying concepts of space and time,
of what you think that you are doing.
On 7/7/21 12:20 PM, J. J. Lodder wrote:
Tom Roberts <tjroberts137@sbcglobal.net> wrote:
The constancy of the vacuum speed of light applies only locally, so
everyone who understands the issues will agree for a massless field.
But of course that's the rub -- we don't really know whether the photon
field is truly massless.
Why this insistence on photons being or not being 'truly' massless?
Because it is an important aspect of whether the (vacuum) speed of light varies. In our current best models of the world, nonzero photon mass and
a varying vacuum speed of light are equivalent (either both are valid or
both are invalid).
All troubles that might arise from a non-zero photon mass
are easily killed in advance
by adding 'in the limit of infinite frequence'
to the definition of the speed of light.
But a) we don't do that, and b) that would make it impossible to
actually measure the speed of light, one could only measure it
approximately -- hopeless for such a fundamental aspect of the world we inhabit, and one used in so much technology.
The correct way to deal with a nonzero photon mass is to distinguish
between the two quite different meanings of c:
1) the vacuum speed of light
2) the symmetry speed of Lorentzian manifolds
If (1) is found to vary, no fundamental revolution in physics is
involved, we just start using a nonzero photon mass [#]. If (2) is found
to vary, it would refute every theory of physics we have today.
[Historically, in 1905 this distinction was not known
and Einstein intermixed them inappropriately. His
second postulate is actually about (2), not (1).
Today we consider SR to be a theory of geometry,
not electrodynamics (the subject of his 1905 paper).]
[#] See Proca theory.
Since the photon mass cannot be measured,
even the longest radio waves that we can make
still have an 'infinite' frequency.
This is just plain wrong:
a) The photon mass has been measured many times; at present the
best measurements are consistent with zero and an upper limit
of 10^-18 eV.
b) EM waves with frequencies from kilohertz to terahertz have
been measured -- NONE are "infinite".
A photon mass corresponding to a wavelength of the size of the universe cannot be measured in principle.
Hmmmm. Mass does not "correspond" to wavelength in any way.
On 7/7/21 12:20 PM, J. J. Lodder wrote:
Tom Roberts <tjrobe...@sbcglobal.net> wrote:
Because it is an important aspect of whether the (vacuum) speed of light varies. In our current best models of the world, nonzero photon mass and
a varying vacuum speed of light are equivalent (either both are valid or both are invalid).
But a) we don't do that, and b) that would make it impossible to
actually measure the speed of light, one could only measure it approximately -- hopeless for such a fundamental aspect of the world we inhabit, and one used in so much technology.
Your points are completely beside the subject of this sub-thread,
which was the question if the speed of light can vary with -time-,
and if we could in principle measure this.
The correct way to deal with a nonzero photon mass is to distinguish between the two quite different meanings of c:
[[Mod. note -- I apologise to all for the delay in processing this
article, which arrived in the moderation system on 22-Aug-2021. -- jt]]
...... the subject of this sub-thread,
which was the question if the speed of light can vary with -time-,
and if we could in principle measure this.
First you must measure the speed of light. Or better, you have to describe
a general accepted way, how the speed of light is measured.
If you have such a recipe, you can measure and decide if the speed of light is everywhere the same and if this speeed is the same in -time- at a
specific location
The same type of problems exists between: what is mass and how is this directictly measured or calculated based on different measurements.
cribeFirst you must measure the speed of light. Or better, you have to des=
lighta general accepted way, how the speed of light is measured.=20
But we have that! Observing the propagation of light using length and
time units based on the propagation of light. The outcome is fixed.
=20
If you have such a recipe, you can measure and decide if the speed of=
is everywhere the same and if this speeed is the same in -time- at a specific location=20
The only way to change it is to abandon the accepted definition (which
always keeps the speed fixed). And perhaps this could happen, if
for instance all speeds in physics suddenly became 10% higher, except
light. Then most physicists would be open to the idea that actually we
should change this definition.
sThe same type of problems exists between: what is mass and how is thi=
directictly measured or calculated based on different measurements.
In article <e86f7a04-599d-1839-6b5e-1562e76896cd@xs4all.nl>, Jos
Bergervoet <jos.bergervoet@xs4all.nl> writes:=20
cribeFirst you must measure the speed of light. Or better, you have to des=
lighta general accepted way, how the speed of light is measured.=20
But we have that! Observing the propagation of light using length and
time units based on the propagation of light. The outcome is fixed.
=20
If you have such a recipe, you can measure and decide if the speed of=
is everywhere the same and if this speeed is the same in -time- at a specific location=20
The only way to change it is to abandon the accepted definition (which always keeps the speed fixed). And perhaps this could happen, if
for instance all speeds in physics suddenly became 10% higher, except light. Then most physicists would be open to the idea that actually we should change this definition.
Indeed. It could happen. It has happened. The metre used to be=20
defined as 1/10,000,000 of the quadrant of the meridian through Paris. =20 The second used to be defined as a certain fraction of a year. These=20 definitions were changed. Why? In part because other definitions can=20
be reproduced with greater accuracy, but also because they can change. =20
Of course, it would have been silly to say that since the definition of=20 the metre is fixed, the size of the Earth, or the length of the year,=20 could not change, even in principle.
sThe same type of problems exists between: what is mass and how is thi=
directictly measured or calculated based on different measurements.
Like some other units, the kilogramme has recently been redefined. Why?=20 In part because the standard kilogramme in Paris was losing mass. So,=20
it was possible to detect it, even though it was the standard.
The Universe does not care how we define our units. Certain quantities=20 either vary or they don't. We cannot prevent them from varying by using=20 them to define a unit. When we are REASONABLY SURE that something does=20 not vary (at least not detectably over interesting timescales), as a=20 purely PRACTICAL matter one can define units in terms of constants of=20 nature such as the speed of light.
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>
wrote:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
As far as we know now there is no such a thing.
We can formulate all known laws of nature in such a way
that the speed of light doesn't occur in any of them.
What remains is that the 'speed of light' is an artefact
caused by maladroit choices in the definition of our unit systems.
It has no more physical reality than Boltzmann's constant,
or the impedance of the vacuum.
If you want to have a 'speed of light' as a constant of nature
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
In article <1peqwo1.1pvreq6wg61gwN%nospam@de-ster.demon.nl>, nospam@de-ster.demon.nl (J. J. Lodder) writes:
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>
wrote:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
As far as we know now there is no such a thing.
We can formulate all known laws of nature in such a way
that the speed of light doesn't occur in any of them.
What remains is that the 'speed of light' is an artefact
caused by maladroit choices in the definition of our unit systems.
It has no more physical reality than Boltzmann's constant,
or the impedance of the vacuum.
If you want to have a 'speed of light' as a constant of nature
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
Please explain. There are various sources of light. We can measure a distance. We can measure a time. Thus, we can measure a speed. We
find that the speed of light is always the same. Similar results for
the speed of sound at a given temperature and pressure.
Your position seems to be that the speed of light is merely a conversion factor, and might as well be set to 1 (not uncommon in some fields of physics). However, that is possible only if the speed of light is a
constant of nature. Thus, it seems to me that you are the one making
the assumption that the speed of light is some fundamental physical
quantity.
Yes, it is possible to have units where the speed of light is just a conversion factor, or is 1, or whatever, but that is possible only if it
IS a constant of nature.
In article <1peqwo1.1pvreq6wg61gwN%nospam@de-ster.demon.nl>, nospam@de-ster.demon.nl (J. J. Lodder) writes:
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>
wrote:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
As far as we know now there is no such a thing.
We can formulate all known laws of nature in such a way
that the speed of light doesn't occur in any of them.
What remains is that the 'speed of light' is an artefact
caused by maladroit choices in the definition of our unit systems.
It has no more physical reality than Boltzmann's constant,
or the impedance of the vacuum.
If you want to have a 'speed of light' as a constant of nature
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
Please explain. There are various sources of light. We can measure a distance. We can measure a time. Thus, we can measure a speed. We
find that the speed of light is always the same. Similar results for
the speed of sound at a given temperature and pressure.
Your position seems to be that the speed of light is merely a conversion factor, and might as well be set to 1 (not uncommon in some fields of physics).
However, that is possible only if the speed of light is a
constant of nature. Thus, it seems to me that you are the one making
the assumption that the speed of light is some fundamental physical
quantity.
Yes, it is possible to have units where the speed of light is just a conversion factor, or is 1, or whatever, but that is possible only if it
IS a constant of nature.
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>...
Your position seems to be that the speed of light is merely a conversion
factor, and might as well be set to 1 (not uncommon in some fields of
physics).
Not just my position, the position of the people who decide
about those kind of things, at places like NIST and BIPM and ...,
with the approval of the CGPM.
So the position of every competent physicist.
And not just some fields of physics, all of physics.
(at least in principle, and as far as we know now)
This position will not need to be changed
until new laws of nature are discovered.
On 21/08/31 2:01 PM, J. J. Lodder wrote:
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>...
...
Your position seems to be that the speed of light is merely a conversion >> factor, and might as well be set to 1 (not uncommon in some fields of
physics).
Not just my position, the position of the people who decide
about those kind of things, at places like NIST and BIPM and ...,
with the approval of the CGPM.
But are decisions from places like NIST and BIPM (even those with
approval of the CGPM) of eternal validity? (In general, I mean..
have all such decisions as of yet been found to remain unchallenged
and unchanged indefinitely?)
norance-of-experts-richard-p-feynman-35-30-25.jpg>So the position of every competent physicist.
Really? <https://www.azquotes.com/picture-quotes/quote-science-is-the-belief-in-the-ig
And not just some fields of physics, all of physics.
(at least in principle, and as far as we know now)
This position will not need to be changed
until new laws of nature are discovered.
But that usually happens every few decades, or even within a few years..
so doesn't that lead to the opposite conclusion than the one you want?
Jos Bergervoet <jos.bergervoet@xs4all.nl> wrote:
On 21/08/31 2:01 PM, J. J. Lodder wrote:
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>...
...
Your position seems to be that the speed of light is merely a conversion >>>> factor, and might as well be set to 1 (not uncommon in some fields of
physics).
Not just my position, the position of the people who decide
about those kind of things, at places like NIST and BIPM and ...,
with the approval of the CGPM.
But are decisions from places like NIST and BIPM (even those with
approval of the CGPM) of eternal validity? (In general, I mean..
have all such decisions as of yet been found to remain unchallenged
and unchanged indefinitely?)
Of course not, subject to possible change by next CGPM.
However, looking back, it is remarkable how cumulative it all has been,
with ever increasing reproducibility of units
and precision of measurement.
norance-of-experts-richard-p-feynman-35-30-25.jpg>So the position of every competent physicist.
Really?
<https://www.azquotes.com/picture-quotes/quote-science-is-the-belief-in-the-ig
What did -he- measure?
Anyway, it is a highly misleading statement, when taken out of context.
You may want to read the original, in context of 'education science', at <http://www.feynman.com/science/what-is-science/>
And just my opinion: I doubt very much whether Feynman
would say the same thing again, were he alive today.
Anyway, if you want to actually measure things in physics
you don't have a choice.
A measurement is a measurement only if it can be traced back,
at least in principle, to a standard kept by NIST, BIPM, etc.
And not just some fields of physics, all of physics.
(at least in principle, and as far as we know now)
This position will not need to be changed
until new laws of nature are discovered.
But that usually happens every few decades, or even within a few years..
so doesn't that lead to the opposite conclusion than the one you want?
Really? For the behaviour of 'ordinary' matter,
under 'ordinary' circumstances, (which is what the CGPM deals with)
nothing fundamentally new has happened since the Dirac equation
and the Pauli Principle were established.
...
What you are really debating is the process of science. In order to do anything we must have some framework of ideas we are working with, that
we assume are valid. In order for alarge group of scientist to work together they must all talk the same "language", i.e. have the same
framework of assumed theory. Otherwise we are debating kinetic energy
vs phlogiston. That just doesn't work.
Just because we have an assumed theoretical framework doesn't mean that
it is correct and cast in concrete, only that it is the current best understanding. As more experiments are performed and more physical
facts revealed we may come to realize that some aspect of our assumed theoretical framework is not quite correct. Or someone may come up with
a different framework that is compelling enough that everyone adopts it.
Examples include quantum mechanics, relativity, Maxwell's Equations, Newton's Laws, etc.
It is the task of the revolutionary to convince the mainstream consensus
that the new idea is better. That is usually a bit hard because so many people become emotionally invested in the consensus framework, but if
the new idea truely has value and is supported by experimental evidence
than it will eventually prevail.
In article <1peqwo1.1pvreq6wg61gwN%nos...@de-ster.demon.nl>, nos...@de-ster.demon.nl (J. J. Lodder) writes:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
If you want to have a 'speed of light' as a constant of naturePlease explain. There are various sources of light. We can measure (1) a distance. We can measure (2) a time. Thus, we can measure a (3) speed.
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
Op dinsdag 31 augustus 2021 om 07:26:18 UTC+2 schreef Phillip Helbig:
In article <1peqwo1.1pvreq6wg61gwN%nos...@de-ster.demon.nl>, nos...@de-ster.demon.nl (J. J. Lodder) writes:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
If you want to have a 'speed of light' as a constant of naturePlease explain. There are various sources of light. We can measure (1) a distance. We can measure (2) a time. Thus, we can measure a (3) speed.
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
Yes we can measure a speed and a distance and using both calculate a speed. This is rather straight forward when you want to calculate the speed of a car , but very difficult when you want to calculate the speed of light or the speed of a neutrino. The main problem is the reference frame.
The first step is to describe exactly how (1) and (2) are measured such that we all can perform the same experiment and compare the results (at different locations or circumstances).
To measure the time you can use two atomic clocks, but than you have to
agree how to synchronise these clocks.
To measure the distance you could start with two points A,B, a fixed distance apart. Next you can define a point C halfway between these two points AB
and issue a synchronisation flash from C towards A and B.
But the question is do both pulses arive simultaneous?
Next you can issue a pulse from point A and measure the arriving time t1 of that pulse at point B.
You can do the reverse from B to A and measure t2.
The question is: Are these two arriving times t1 and t2 (durations) the same? If they are you can calculate c. If they are not you have a problem.
Next you want to do with the same with a neutrino? Is that physical possible. IMO the best strategy is first to calculate the speed of light at that location and use the points (A,B) to measure the speed of a neutrino.
This strategy is not simple.
Op dinsdag 31 augustus 2021 om 07:26:18 UTC+2 schreef Phillip Helbig:
In article <1peqwo1.1pvreq6wg61gwN%nos...@de-ster.demon.nl>, nos...@de-ster.demon.nl (J. J. Lodder) writes:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
If you want to have a 'speed of light' as a constant of naturePlease explain. There are various sources of light. We can measure (1) a distance. We can measure (2) a time. Thus, we can measure a (3) speed.
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
Yes we can measure a speed and a distance and using both calculate a speed. This is rather straight forward when you want to calculate the speed of a car , but very difficult when you want to calculate the speed of light or the speed of a neutrino. The main problem is the reference frame.
The first step is to describe exactly how (1) and (2) are measured such that we all can perform the same experiment and compare the results (at different locations or circumstances).
To measure the time you can use two atomic clocks, but than you have to
agree how to synchronise these clocks.
To measure the distance you could start with two points A,B, a fixed distance apart. Next you can define a point C halfway between these two points AB
and issue a synchronisation flash from C towards A and B.
But the question is do both pulses arive simultaneous?
Next you can issue a pulse from point A and measure the arriving time t1 of that pulse at point B.
You can do the reverse from B to A and measure t2.
The question is: Are these two arriving times t1 and t2 (durations) the same? If they are you can calculate c. If they are not you have a problem.
In article <384f2c33-591d-4297-934f-1013e9db0068n@googlegroups.com>,
Nicolaas Vroom <nicolaas.vroom@pandora.be> writes:
Op dinsdag 31 augustus 2021 om 07:26:18 UTC+2 schreef Phillip Helbig:
In article <1peqwo1.1pvreq6wg61gwN%nos...@de-ster.demon.nl>, nos...@de-ster.demon.nl (J. J. Lodder) writes:
The problem with your position is that you postulate
that what has to be shown,
namely that there is such a thing as the speed of light,
and that it is a constant of nature.
If you want to have a 'speed of light' as a constant of naturePlease explain. There are various sources of light. We can measure (1) a distance. We can measure (2) a time. Thus, we can measure a (3) speed.
you must invent new, and fundamentally different laws of physics
in which there is such a thing,
Yes we can measure a speed and a distance and using both calculate a
speed. This is rather straight forward when you want to calculate the
speed of a car , but very difficult when you want to calculate the speed
of light or the speed of a neutrino. The main problem is the reference frame.
The first step is to describe exactly how (1) and (2) are measured such that we all can perform the same experiment and compare the results (at different locations or circumstances). To measure the time you can use
two atomic clocks, but than you have to agree how to synchronise these clocks.
To measure the distance you could start with two points A,B, a fixed distance apart. Next you can define a point C halfway between these two points AB and issue a synchronisation flash from C towards A and B. But
the question is do both pulses arive simultaneous? Next you can issue a pulse from point A and measure the arriving time t1 of that pulse at
point B. You can do the reverse from B to A and measure t2. The question is: Are these two arriving times t1 and t2 (durations) the same? If they are you can calculate c. If they are not you have a problem.
Leaving aside the issues that the speed of light is now constant by definition (a few decades now) and that we believe that it is constant
in all frames (more than a century now), as far as normal measurements
go, there is really no problem. Ole R=F8mer measured the speed of light
via timing the eclipses of Jupiter's satellites. Fizea measured it with
two gears.
Bringing back the other issues, we could of course still measure the
speed of light with those old methods, and if it did actually change
with time, we would notice it, despite the facts that it is now defined
to be constant and that we believe that it is constant. I don't think
that likely; my point is merely that we cannot prevent the speed of
light from changing simply by defining it to be constant. Rather, it is defined to be constant as a practical matter because we have evidence
that it is.
c to be constant and that we believe that it is constant. I don't thinkIf that is true you should explain us what that evidence is.
that likely; my point is merely that we cannot prevent the speed of
light from changing simply by defining it to be constant. Rather, it is defined to be constant as a practical matter because we have evidence
that it is.
What Ole Romer really measured was the astronomical unit,
aka the AU, in terms of light seconds.
(that means 'really' in an operational sense)
You may have noted that he can no longer do that.
The AU too has been given a defined value of 149597870700 m (2012)
What a new Romer will be measuring nowadays
is where the Earth really is, with respect to Jupiter.
Bringing back the other issues, we could of course still measure the
speed of light with those old methods, and if it did actually change
with time, we would notice it, despite the facts that it is now defined
to be constant and that we believe that it is constant. I don't think
that likely; my point is merely that we cannot prevent the speed of
light from changing simply by defining it to be constant. Rather, it is defined to be constant as a practical matter because we have evidence
that it is.
There you go again.
You start with the (Platonic) assumption that there 'really'
is some 'speed of light' in some absolute sense,
independently of measurements of it.
(and that we can then 'measure' it in some unproblematic way)
And yes, of course we can define the speed of light to be constant,
and then it really is constant because we defined it to be.
That merely implies that we absorb the changes,
if any, somewhere else.
(so somewhere in our choices about units)
Let me take a conceptually simpler example to make the point clear.
The last CGPM defined Boltzmann's constant, k, to have a fixed value.
That means that the triple point of water now needs to be measured.
There is no point in saying that we cannot define the value of k,
because it 'really' might be changing.
Conversely there is no point in saying that the triple point of water 'really' is at 273.16 kelvin in some absolute unchangeable sense.
There is no physical reality involved,
it is just definitions versus definitions.
Digressing into philosophy: your 'real speed of light'
is an example of a Kantian 'Ding an sich' ,
and as such unknowable.
It has no place in physics.
[[Mod. note -- Please limit your text to fit within 80 columns,
preferably around 70, so that readers don't have to scroll horizontally
to read each line. I have rewrapped the lines in this article. -- jt]]
Op vrijdag 17 september 2021 om 06:16:55 UTC+2 schreef Phillip Helbig (undress to reply):
c to be constant and that we believe that it is constant. I don't think that likely; my point is merely that we cannot prevent the speed ofIf that is true you should explain us what that evidence is.
light from changing simply by defining it to be constant. Rather, it is defined to be constant as a practical matter because we have evidence
that it is.
I prefer to write: because we have no evidence that it is not.
My point is mainly that if 'you' define c to be physical constant,
that means that photons always have the same speed, 'you' should
also define how this speed is calculated.
What Rømer did (very cleverly) was to calculate the average
speed of light between Jupiter and the Earth. This calculation is
a mathematical description of the mechanical Sun, Earth, Jupiter
system.
Is his result valid for the whole of the universe?
We use the speed of light to measure hugh distances.
Is it realy correct to claim that this speed, along that whole path, is everywhere the same?
What about the influence if 'space' is a vacuum?
What about the influence of matter?
On 21/09/02 9:34 AM, J. J. Lodder wrote:
Jos Bergervoet <jos.bergervoet@xs4all.nl> wrote:
On 21/08/31 2:01 PM, J. J. Lodder wrote:
Phillip Helbig (undress to reply) <helbig@asclothestro.multivax.de>...
...
Your position seems to be that the speed of light is merely a conversion >>>> factor, and might as well be set to 1 (not uncommon in some fields of >>>> physics).
Not just my position, the position of the people who decide
about those kind of things, at places like NIST and BIPM and ...,
with the approval of the CGPM.
But are decisions from places like NIST and BIPM (even those with
approval of the CGPM) of eternal validity? (In general, I mean..
have all such decisions as of yet been found to remain unchallenged
and unchanged indefinitely?)
Of course not, subject to possible change by next CGPM.
So you agree that speed of light being "merely a conversion factor"
could possibly be revoked by a next CGPM. So no proof here..
However, looking back, it is remarkable how cumulative it all has been, with ever increasing reproducibility of units
and precision of measurement.
If you find that remarkable, you probably agree that it doesn't
*have to* be like that.
And indeed, when Michelson and Morley did
*not* find variation in the speed of light that was quite disruptive.
So no proof here..
-igSo the position of every competent physicist.
Really?
<https://www.azquotes.com/picture-quotes/quote-science-is-the-belief-in-the
norance-of-experts-richard-p-feynman-35-30-25.jpg>
What did -he- measure?
Who cares? No-one says the speed of light has recently changed or is
about to change anytime soon. Nothing has to be measured, our only
question is whether it can change in principle. we just have to
look at what flexibility there is in our definitions and theories.
Anyway, if you want to actually measure things in physics
you don't have a choice.
A measurement is a measurement only if it can be traced back,
at least in principle, to a standard kept by NIST, BIPM, etc.
I think a measurement is a measurement only if (enough) decoherence
occurs between a set of different sub-spaces of the total Hilbert
space, while at the same time relatively phase-constant saddle-points
remain around some pointer states. (But I'm quite willing to consider
any other non-anthropocentric definition, regardless of what NIST,
BIPM, etc. have written down!)
And not just some fields of physics, all of physics.
(at least in principle, and as far as we know now)
This position will not need to be changed
until new laws of nature are discovered.
But that usually happens every few decades, or even within a few years.. >> so doesn't that lead to the opposite conclusion than the one you want?
Really? For the behaviour of 'ordinary' matter,
under 'ordinary' circumstances, (which is what the CGPM deals with)
But *you* brought in the CGPM! This thread was only dealing with
the question whether the speed of light can change in principle,
so certainly not constrained to "ordinary circumstances."
Jos Bergervoet <jos.bergervoet@xs4all.nl> wrote:
On 21/09/02 9:34 AM, J. J. Lodder wrote:
Jos Bergervoet <jos.bergervoet@xs4all.nl> wrote:
On 21/08/31 2:01 PM, J. J. Lodder wrote:...
However, looking back, it is remarkable how cumulative it all has been,
with ever increasing reproducibility of units
and precision of measurement.
If you find that remarkable, you probably agree that it doesn't
*have to* be like that.
As Einstein is supposed to have said:
'The greatest mystery about the universe
is that it is understandable at all'.
And indeed, when Michelson and Morley did
*not* find variation in the speed of light that was quite disruptive.
So no proof here..
Finding that some of the 'constants' of nature'
(the real ones like alpha, not a pseudo like c)
are indeed variable will be far more disruptive than Michelson ever was.
So the position of every competent physicist.
Really?
<https://www.azquotes.com/picture-quotes/quote-science-is-the-belief-in-the-ignorance-of-experts-richard-p-feynman-35-30-25.jpg>
What did -he- measure?
Who cares? No-one says the speed of light has recently changed or is
about to change anytime soon. Nothing has to be measured, our only
question is whether it can change in principle. we just have to
look at what flexibility there is in our definitions and theories.
That's easy. Our present theories say that we live in a spacetime
in which space and time are the same thing. [1]
(apart from historical accidents with our choices of units)
This is the most rigid aspect of all our theories, IMHO.
We'll break everything else first,
if it would be necessary to break things.
[snip more of the 'Feynman on science education' sideline]
Start another thread about it if you wish to continue.
Anyway, if you want to actually measure things in physics
you don't have a choice.
A measurement is a measurement only if it can be traced back,
at least in principle, to a standard kept by NIST, BIPM, etc.
I think a measurement is a measurement only if (enough) decoherence
occurs between a set of different sub-spaces of the total Hilbert
space, while at the same time relatively phase-constant saddle-points
remain around some pointer states. (But I'm quite willing to consider
any other non-anthropocentric definition, regardless of what NIST,
BIPM, etc. have written down!)
Another red herring. Quantum measurement has nothing to do with it.
On 21/09/25 9:12 AM, J. J. Lodder wrote:
Jos Bergervoet <jos.bergervoet@xs4all.nl> wrote:
On 21/09/02 9:34 AM, J. J. Lodder wrote:
Jos Bergervoet <jos.bergervoet@xs4all.nl> wrote:
On 21/08/31 2:01 PM, J. J. Lodder wrote:...
However, looking back, it is remarkable how cumulative it all has been, >>> with ever increasing reproducibility of units
and precision of measurement.
If you find that remarkable, you probably agree that it doesn't
*have to* be like that.
As Einstein is supposed to have said:
'The greatest mystery about the universe
is that it is understandable at all'.
He didn't imply that everything therefore has to be as we
currently think it is. So what would it prove here?
he-ignorance-of-experts-richard-p-feynman-35-30-25.jpg>And indeed, when Michelson and Morley did
*not* find variation in the speed of light that was quite disruptive.
So no proof here..
Finding that some of the 'constants' of nature'
(the real ones like alpha, not a pseudo like c)
are indeed variable will be far more disruptive than Michelson ever was.
If you agree that what happened was disruptive then why couldn't
disruptive things happen again? Perhaps even more disruptive..
..
So the position of every competent physicist.
Really?
<https://www.azquotes.com/picture-quotes/quote-science-is-the-belief-in-t
What did -he- measure?
Who cares? No-one says the speed of light has recently changed or is
about to change anytime soon. Nothing has to be measured, our only
question is whether it can change in principle. we just have to
look at what flexibility there is in our definitions and theories.
That's easy. Our present theories say that we live in a spacetime
in which space and time are the same thing. [1]
(apart from historical accidents with our choices of units)
This is the most rigid aspect of all our theories, IMHO.
I tend to agree, but still it's a theory..
Jos Bergervoet <jos.ber...@xs4all.nl> wrote:Unimportant.
He didn't imply that everything therefore has to be as weThere is nothing to prove, or that can be proven.
currently think it is. So what would it prove here?
We seem to live in a universe that is ruled by law,We are part of world that is constantly changing.
rather than a lawless or a chaotic one.
Why this is the case is, as Einstein said, a complete mystery.Why this is the case nobody knows. It is a 'fact' based on observations.
That's easy. Our present theories say that we live in a spacetime
in which space and time are the same thing. [1]
[1] This follows from the fact that we can eliminate c completely
from all laws of physics by means of a suitable choice of units.
Op woensdag 27 oktober 2021 om 09:16:18 UTC+2 schreef J. J. Lodder:
Jos Bergervoet <jos.ber...@xs4all.nl> wrote:
Unimportant.He didn't imply that everything therefore has to be as weThere is nothing to prove, or that can be proven.
currently think it is. So what would it prove here?
We seem to live in a universe that is ruled by law,We are part of world that is constantly changing.
rather than a lawless or a chaotic one.
This world can be divided into parts that behave more or less identical.
For example 'everywhere' in the univerese are galaxies. [2]
Why this is the case is, as Einstein said, a complete mystery.Why this is the case nobody knows. It is a 'fact' based on observations.
That's easy. Our present theories say that we live in a spacetime
in which space and time are the same thing. [1]
[1] This follows from the fact that we can eliminate c completely
from all laws of physics by means of a suitable choice of units.
[1] is in conflict with [2] above.
The problem with spacetime is that we define a line segment which
connects two events t1 and t2.
This line segment defines the start and end point of a light signal.
The problem is that line segment c*dt does not physical exist.
The main problem is an unambigous definition of the points x1,y1,z1,t1
and x2,y2,z2,t2 of the two events in space.
That means an event in our Galaxy and in Andromeda Galaxy.
That means you need a clear definition how these events are measured.
This problem would be simpler if one reference frame and one clock is used.
Spacetime is in fact a mathematical approach. To set c to 1 (and to make
it a physical constant) does not 'solve' the issues involved.
Nicolaas Vroom <nicolaa...@pandora.be> wrote:
Op woensdag 27 oktober 2021 om 09:16:18 UTC+2 schreefPerhaps, but physics deals with the things that are not changing,
J. J. Lodder:
We seem to live in a universe that is ruled by law,We are part of world that is constantly changing. This world
rather than a lawless or a chaotic one.
can be divided into parts that behave more or less identical.
For example 'everywhere' in the universe are galaxies.
better known as 'the laws of physics'.
(and their consequences) Astronomers, looking back in time,
can see that the laws of physics have not changed.
(in any way that they can detect)
The first step is to make observations. This includes instruments toThis problem would be simpler if one reference frame and
one clock is used.
Spacetime is in fact a mathematical approach. To set c to 1Precisely the point of Kant. There is nothing to 'solve'.
(and to make it a physical constant) does not 'solve'
the issues involved.
Mathematics must come before all physical theory.
Without a well-understood mathematical framework, you can't evenWe should start with a good physical framework, before
begin measuring things.
(beyond the most naive level) This still holds, despite theWE must understand what that physical means.
mathematical framework having been changed from
Euclidean geometry to relativistic Riemannian geometry,
| Sysop: | Keyop |
|---|---|
| Location: | Huddersfield, West Yorkshire, UK |
| Users: | 342 |
| Nodes: | 16 (2 / 14) |
| Uptime: | 27:40:48 |
| Calls: | 7,512 |
| Calls today: | 9 |
| Files: | 12,713 |
| Messages: | 5,641,820 |
| Posted today: | 2 |
