3 Layman Questions
Lawrence A. Roux
Subject: 3 Layman questions
Organization: Syracuse University, Syracuse, NY
Cc:
Bcc:
Hi, as a layman, please ignore the ignorance of the following
three questions:
1) According to Relativity, the speed of light is constant for
all observers, I take this to mean that the speed of light
in each of their respective systems is constant? For instance,
Would a spaceship accellerating towards a star see the light
from that star shift towards the blueend of the spectrum due
to the ship passing through the photon wave faster, and thus
effectively increasing its frequency? Also, if the ship got
to the speed of light, would the photons that it intercepts
then appear as point-like particles?
2) Does anyone know if the galaxy maps produced by those such as
Tully and Fischer show the PRESENT position of the galaxies,
or where the galaxies were when they emitted their light?
From all of the reports I have read it seems that they show
the galaxies atthe positions where the light was emitted,
thus the maps would be skewed because a galaxy that emitted
its light, oh say 2000 years ago, and moving at some speed
would be nowhere near that spot nowdays. (Of course I mean
"moved" in an expansionist way, though its own movement would
also need to be worked in.
3) If gravity is a byproduct of the warp of space/time, then What
the heck is the reason for the Graviton. The only explanation
I have ever received is that "Well, the other forces all have
their own "-on" such as the photon and the Higgs field Boson,
so why shouldn't Gravity have one?" It seems to me that the
Graviton isin direct conflict with Relativity.
-Larry Roux
lar...@rodan.syr.acs.edu
Mike Gross
>To: sci.physics
>Subject: 3 Layman questions
>Organization: Syracuse University, Syracuse, NY
>Cc:
>Bcc:
> Hi, as a layman, please ignore the ignorance of the following
>three questions:
I don't find these questions to be ignorant at all. In fact, your intuition
looks quite good.
> 1) According to Relativity, the speed of light is constant for
> all observers, I take this to mean that the speed of light
> in each of their respective systems is constant? For instance,
> Would a spaceship accellerating towards a star see the light
> from that star shift towards the blueend of the spectrum due
> to the ship passing through the photon wave faster, and thus
> effectively increasing its frequency? Also, if the ship got
> to the speed of light, would the photons that it intercepts
> then appear as point-like particles?
Special Relativity assumes that the speed of light *always* has the same value,
regardless of which frame you use, when you look, or what "system" you
consider. So, the light from a star that you are accelerating toward would
indeed be blueshifted more and more, until you pass it. This is the reverse of
a standard undergraduate level problem (see e.g. Taylor and Wheeler, _Spacetime
Physics_).
Keep in mind, though, that a continuously accelerating body can never reach
the speed of light, though speed will always be increasing (standard example:
a spaceship undergoing uniform acceleration in its own frame will have a
speed v = c tanh(a \tau / c), where a is the acceleration and \tau is the
proper time). So you can't actually reach the limit you asked about in your
question. But, you can consider the limit in which speed gets arbitrarily
close to c. In this limit, electromagnetic waves appear to be infinitely
blueshifted. The Heisenberg Principle (which is really a property of any wave,
not just quantum mechanical ones) says that any finite-length wavepacket will
pass in infinitesimal time, in this limit.
Notice that I'm avoiding any reference to photons. There does exist a special
relativistic theory of quantized radiation (called QED), but it isn't necessary
to make these points, and it adds a *lot* of complexity.
> 2) Does anyone know if the galaxy maps produced by those such as
> Tully and Fischer show the PRESENT position of the galaxies,
> or where the galaxies were when they emitted their light?
> From all of the reports I have read it seems that they show
> the galaxies atthe positions where the light was emitted,
> thus the maps would be skewed because a galaxy that emitted
> its light, oh say 2000 years ago, and moving at some speed
> would be nowhere near that spot nowdays. (Of course I mean
> "moved" in an expansionist way, though its own movement would
> also need to be worked in.
The spiral galaxy maps of Tully and Fisher, as well as the elliptical galaxy
maps of the Seven Samurai (Dressler, Faber, Lynden-Bell, etc.) and the
Center for Astrophysics redshift survey all report the *apparent* positions
of galaxies. This is not a significant mistake, though, since the maps do
not go very deep compared to the size of the observable universe (i.e. their
apparent recessional velocity is very small compared to the speed of light--
15000 km/sec is the limit for the first CfA survey (c = 300,000 km/s).
You are right that there will be *some* motion of galaxies between emission
and detection, but typical galaxy random velocities are several hundred km/s
with respect to the microwave background. When you multiply that by a few
million years light travel time, you get about 10^13 km motion. My back-of-the-
envelope calculation says that that's about 10 kpc. For comparison, 15000 km/s
corresponds to a distance of at least 15 Mpc for a Hubble constant of
100 km/s/Mpc (or 30 Mpc for H=50). That's a factor of 1000, so the correction
is small.
Also, only one component of velocity is known with any accuracy--the projection
along the line-of-sight. Several authors are currently working on methods to
estimate the other two components (particularly Avishai Dekel), but
uncertainties are very large.
> 3) If gravity is a byproduct of the warp of space/time, then What
> the heck is the reason for the Graviton. The only explanation
> I have ever received is that "Well, the other forces all have
> their own "-on" such as the photon and the Higgs field Boson,
> so why shouldn't Gravity have one?" It seems to me that the
> Graviton isin direct conflict with Relativity.
If gravity is a byproduct (and source!) of a warp of spacetime, then there is
no need for a graviton. The reason people think of it is that there is an
energy regime in which both quantum mechanics and general relativity should
have important influences. That is, quantum uncertainty will make the warping
of spacetime as a description of gravity invalid. This happens when quantum
smearing of a gravitationally significant mass becomes comparable to the
scale of curvature of spacetime. A good example is an "evaporating" black hole,
which sheds mass by creating quantum pairs (electrons and positrons, or perhaps
neutrinos if they have mass) which then have one member tunnel through the
event horizon. This will cause the black hole to shrink, but standard quantum
mechanics will not allow it to go away completely due to the correlation
between the pairs. The end state of such a black hole is an open question, and
its resolution may require a reformulation of quantum mechanics or (more
likely, I think) a quantum theory of gravity that has a "ground state" for the
black hole.
I hope this helps.
>
> -Larry Roux
> lar...@rodan.syr.acs.edu
Mike Gross
Physics Board and Lick Observatory
Univ of California GO SLUGS!!!!
Santa Cruz, CA 95064
gr...@lick.ucsc.edu
Jonathan Thornburg
(Lawrence A. Roux) writes:
| Hi, as a layman, please ignore the ignorance of the following
|three questions:
a question is easily phrased without a lot of mathematics, doesn't mean
it can't be a good question.
A suggestion: in the future, you might try crossposting to
sci.physics.research . The moderators there are quite tolerant,
they're just trying to raise the S/N ratio from sci.physics .
|1) According to Relativity, the speed of light is constant for
| all observers, I take this to mean that the speed of light
| in each of their respective systems is constant?
Yes.
| For instance,
| Would a spaceship accellerating towards a star see the light
| from that star shift towards the blueend of the spectrum due
| to the ship passing through the photon wave faster, and thus
| effectively increasing its frequency?
Yes, but be careful. *Any* observer, regardless of her motion,
measures the speed of light to have the "universal" value. Because
of this, have to be careful in trying to derive the doppler shift
this way. But if you are careful, this all works. In particular,
you can look at "how many cycles of light go by in a second"
or its equivalent.
| Also, if the ship got
| to the speed of light, would the photons that it intercepts
| then appear as point-like particles?
It was contemplation of this very paradox, in ~1903, that lead
Einstein to the special theory of relativity in 1905. (He thought
of it in terms of following a light *wave* at the speed of light,
would the wave then appear to be standing still?) The resolution
is that the observer *can't* get to the speed of light.
Generic comment re question 1. I *highly* recommend the book
Edwin F. Taylor and John Archibald Wheeler
"Spacetime Physics", 2nd edition,
W. H. Freeman, 1992?
(1st edition is 1966)
The first edition was already the best introduction to special
relativity around, and emminently accessable with only high school
algebra. I have not seen the 2nd edition yet, but reviews say it's
even better.
| 2) Does anyone know if the galaxy maps produced by those such as
| Tully and Fischer show the PRESENT position of the galaxies,
| or where the galaxies were when they emitted their light?
They show the positions where they were when they emitted the light.
More accurately, they show the position on the sky we see the light
coming from, with radial velocity as a surrogate for radial distance.
| From all of the reports I have read it seems that they show
| the galaxies atthe positions where the light was emitted,
| thus the maps would be skewed because a galaxy that emitted
| its light, oh say 2000 years ago, and moving at some speed
| would be nowhere near that spot nowdays. (Of course I mean
| "moved" in an expansionist way, though its own movement would
| also need to be worked in.
Yes, this is a source of error. However, it's negligable
relative to the other sources of error involved. Basically, this
is because the objects are far enough away, and the peculiar
velocities (their velocities relative to the smooth Hubble flow
at their locations) are small enough, that the distances moved
during the lookback time are a small fraction of the overall
scale of the largescale structure.
Example:
20,000 km/sec radial velocity @ H=75 km/sec/megaparsec
==> distance = ~250 megaparsecs = ~900 million light years
==> lookback time ~ 900 million years
1000 km/sec peculiar velocity = 200 parsecs/million years
==> travels 200,000 parsecs = 0.2 Mpc = ~1/4 of the way from
here to M31 (Andromeda galaxy) in 900 million years
==> this is ~1/1300 of the radial distance
==> if peculiar velocity is radial, it takes distance from 250
megaparsecs to 250.2 megaparsecs (a negligable change, given
the error levels of extragalactic astronomy), or
==> if peculiar velocity is angular, it would give an angular
displacement on the sky of 1/1300 of a radian = 2.5 arcminutes,
again quite negligable.
| 3) If gravity is a byproduct of the warp of space/time, then What
| the heck is the reason for the Graviton.
Quantum mechanics. We believe that general relativity is the
correct theory of gravity, *if* quantum effects can be ignored.
But under some conditions, gravitation has to be treated quantum
mechanically. Noone knows how to do that properly. If you treat
the gravitational field as weak enough for spacetime to be locally
Minkowskian, and do the usual field theory stuff for gravitation,
you find that the gravitational field is carried by spin 2 particles,
which we call "gravitons".
| The only explanation
| I have ever received is that "Well, the other forces all have
| their own "-on" such as the photon and the Higgs field Boson,
| so why shouldn't Gravity have one?" It seems to me that the
| Graviton isin direct conflict with Relativity.
It is in conflict with the *classical* Einstein equations.
But the classical Einstein equations clearly aren't a complete
description of gravitation, since they're clearly wrong for systems
where quantum effects are important. (Examples: (a) spacetime $10^{-25}$
seconds after the Big Bang, (b) spacetime within $10^{-35}$ m of a
spacetime singularity, eg near the center of a black hole,
(c) a small black hole evaporating due to Hawking radiation,
(d) one could imagine building a gravimeter (something which measures
little $g$, the local gravitational acceleration) sensitive enough
to measure the change in the gravitational field of quantum system
as some quantum transition redistributes the system's mass-energy.
This gives a gravitational version of the famous "Schrodinger's Cat"
(explanations of that will be left for QM experts, i.e. *not* me!)
We conclude that the "true" theory of gravitation must somehow
marry general relativity and quantum mechanics, or some other theories
which reduce to them in the appropriate conditions. A great many
very talented theoretical physicists have worked very hard on quantum
gravity ever since the 1930s, and especially since the development of
modern quantum field theory since WW2. Thus far noone has come up
with a theory which works. Whoever does will undoubtably win a Nobel
prize in physics, and receive many other honors.
- Jonathan Thornburg
<jona...@hermes.chpc.utexas.edu> or <jona...@einstein.ph.utexas.edu>
[until 31/Aug/93] U of Texas at Austin / Physics Dept / Center for Relativity
and [until ~Apr/93] U of British Columbia / {Astronomy,Physics}
john baez
>We conclude that the "true" theory of gravitation must somehow
>marry general relativity and quantum mechanics, or some other theories
>which reduce to them in the appropriate conditions. A great many
>very talented theoretical physicists have worked very hard on quantum
>gravity ever since the 1930s, and especially since the development of
>modern quantum field theory since WW2. Thus far noone has come up
>with a theory which works. Whoever does will undoubtably win a Nobel
>prize in physics, and receive many other honors.
Actually, probably whoever does will NOT win a Nobel prize, because it
is quite likely that whatever predictions they make will not be
sufficiently tested in their lifetime(s) for the Nobel committee to risk
granting an award. After all, Einstein never got the prize for general
relativity.
But hey, I can live without a Nobel prize... (He says, racking up crackpot
points...)
Mark
In article <1oijc7...@darkstar.UCSC.EDU> gr...@maxwell.ucsc.edu (Mike Gross) writes:
>Also, only one component of velocity is known with any accuracy--the projection
>along the line-of-sight. Several authors are currently working on methods to
>estimate the other two components (particularly Avishai Dekel), but
Ah, a perfect lead-in.
Remember the kiddie question: why doesn't the sun fall? Remember the pat
answer always given in a science class? Well, guess what? The question's
back!
All these deliberations are Newtonian, and these are things that Newton could
have and should have deduced, if only he were to have analysed the issue
fully. Ironically, a proper analysis reveals 4 Ultimate answers, only 3
of which are currently known.
Why doesn't the sun fall? The answer: the Earth moves around the sun too fast
for them to fall into each other. The child says, okay, then why doesn't the
solar system and the other stars fall into each other? The answer is that
they too must be in a similar orbital motion around the center of the galaxy.
The child says again, okay. Then why don't the galaxies all fall into each
other? Bingo.
One possible answer could have been that there's an infinity of stars and
galaxies all throughout and all pulling us evenly in every direction and the
forces all cancel out everywhere. But Newton himself recognized that this
cannot possibly be true. An infinity of stars would make the nighttime sky
shine as bright as the day. Outer space would be white, not black.
So something screwey must be going on. Either the law of gravity is valid
everywhere or not. If not, then there's another force at work at large
distances (that's effectively what Einstein postulated when he fudged in
the Cosmological Constant).
If the law of gravity IS valid, then either our premise is right or not. In
other words, maybe everything IS falling into each other. Effectively, the
Big Crunch. The implications are profound: namely when you extrapolate back
in time, you ultimately come to a point where the objects must have been in
temporary stasis as they were making the transition from falling away to
falling back together. Extrapolating further back in time, you invariably
come onto a beginning. Perhaps, even Creation itself.
If our premise is correct, then it is possible that everything is flying apart
at such a high speed that either it hasn't or never will fall back. That's
the third possible answer. And its implications are profound. Again, you
invariably come onto the Beginning.
If neither this beginning or this end is true, then that leaves us with one
last possibility: there is a net Universal Angular Momentum, and everything is
in a Universal Orbit. This can be true, also, in combination with the other
two possiblities of things falling away or falling together.
So, in answering the original question you invariably come to one of four
possibilities: Universal Pressure (Cosmological constant), Universal
Contraction and a Beginning & End, Universal Expansion and a Beginning, or
Universal Rotation.
These are golden opportinities that Newton missed that he could have
concluded with the information he had available at hand. He could have gone
down in history as the Man Who Predicted the Big Bang and/or Universal Rotation.
Now the latter possibility is interesting in its own right (and it proves that
hindsight is NOT always 20/20). And I pose it as both a question and
prediction. What if, instead of the Big Bang model, what we actually have is
a Universe with a net angular momentum which periodically expands and contracts,
reentering a new period of expansion just when it's contracted far enough
that the angular momentum counteracts gravity.
The solutions that were first posed to the Einstein Equations for the Big
Bang model correspond (very closely) to the cases of radial motion under
the influence of gravity for velocities less than/equal to/greater than the
escape velocity. What was never considered, on the other hand, were the
cases that correspond to elliptical/parabolic/hyperbolic orbits -- where there
is also a net angular momentum.
Perhaps someone has constructed such a solution to the Einstein Equations
extending the Big Bang model in this manner?
Prediction: I predict that if and when observations enable astronomers to
measure the tangential component of galactic motions when WILL detect a
very slight net angular momentum, proving that the universe does indeed
rotate.
Jarle Brinchmann
In article <27...@galaxy.ucr.edu>, ba...@ucrmath.ucr.edu (john baez) writes:
|>From: ba...@ucrmath.ucr.edu (john baez)
|>Newsgroups: sci.physics
|>Subject: Re: 3 Layman Questions
And the first person to experimentally verify Quantum Gravity will get a
Nobel Prize within few years. I've never understood why I never wanted to
be an experimentalist, (Well, maybe I do :-)
Jarle.
----------------------------------------------------
I believe in this and it has been proven by research:
He who fucks nuns will later join the church.
The Clash
Jarle Brinchmann
I'm just going to comment on the rotation of the universe, according to the
COBE gang, the COBE results indicate that the universe is rotating less than
3 \times 10^(-24) rad/s or less than 1/10000 times during the last 10 billion
years.
If you want to see why this is so, take a look at Collins & Hawking in
MNRAS 1973,162,302, I've cast a few glances at it and it wasn't too enlightning,
but then the results tell us almost nothing, except that the universe is very
slowly rotating. As far as I remember the paper deals with homogeneous models
and I'm not aware of any studies of rotating anisotrope models.
SCOTT I CHASE
>Actually, probably whoever does will NOT win a Nobel prize, because it
>is quite likely that whatever predictions they make will not be
>sufficiently tested in their lifetime(s) for the Nobel committee to risk
>granting an award. After all, Einstein never got the prize for general
>relativity.
>
>But hey, I can live without a Nobel prize... (He says, racking up crackpot
>points...)
The Nobel prize is not the purpose of life. I am sometimes a little insulted
by people who assume that as a group scientists are motivated primarily by
lowly a motivation as the urge to win prizes. Though it would be fun
to win a Nobel prize, or any other prize for that matter, I really couldn't
care very much less than I already do. I get plenty of acknowledgement from
my colleagues when I do good work. What everybody else thinks is more or
less irrelevant.
Besides, every working scientist knows a dozen tremendously gifted and
productive scientists who are qualified to win for every one who actually
does. Some Nobel prize winners *are* truly exceptional - Feynman, Fermi,
Bethe, Alvarez, and Chandrsekhar pop into my mind immediately. But there
are plenty of simply good physicists. Which of you doesn't know someone
as talented or skillful as Penzias or Wilson (Robert), but not quite as
lucky?
-Scott
--------------------
Scott I. Chase "It is not a simple life to be a single cell,
SIC...@CSA2.LBL.GOV although I have no right to say so, having
been a single cell so long ago myself that I
have no memory at all of that stage of my
life." - Lewis Thomas
SCOTT I CHASE
>
>Prediction: I predict that if and when observations enable astronomers to
>measure the tangential component of galactic motions when WILL detect a
>very slight net angular momentum, proving that the universe does indeed
>rotate.
How slight? You prediction may already be too late. If I remember correctly,
one of the side-effects of the COBE measurements was an upper limit to the
rotation rate of the Universe. I don't remember the limit (sorry!) but it
was small in the sense that the Universe certainly has only made a small
fraction of a single rotation, at best, since the Big Bang. That fraction
*may* have been 10^-5.
Mark
>The Nobel prize is not the purpose of life. I am sometimes a little insulted
>by people who assume that as a group scientists are motivated primarily by
You have no need or reason to take offense at this. I think it's a very
nobel goal to hold.