Transcript here - Starship Asterisk

SPEED OF GRAVITY
by
Robert Nemiroff
Michigan Tech
Physics X: About This Course
• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
o Light on math, heavy on concepts
o Anyone anywhere is welcome
• No textbook required
o Wikipedia, web links, and lectures only
o Find all the lectures with Google at:
 "Starship Asterisk" then "Physics X"
o
http://bb.nightskylive.net/asterisk/viewforum.php?f=39
SPEED OF GRAVITY
Does gravity propagate at the speed of light?
It is clear that gravitational radiation propagates at c, although
this has never been directly experimentally confirmed, since
gravitational radiation has not yet been directly detected.
But what about gravity itself?
SPEED OF GRAVITY: HISTORY
•
Newton (1680s) assumes that the speed of gravity is infinite.
o
•
Laplace (1805) tries to fit Newton's theory with a wave mechanism
where the speed of gravity was equal to the speed of light
o
•
This fits all contemporary observations.
Failed.
Laplace then estimated that gravity moves ~106 times faster than
light.
SPEED OF GRAVITY: HISTORY
•
Lorentz (1904) creates ether theory where gravity propagates at
light speed.
o
o
Laplace problem fixed.
Precession of Mercury too small.
•
Many more people propose many gravitational theories.
•
Einstein proposes General Relativity (1915)
o
In GR gravity propagates a light speed.
ABERRATION OF LIGHT
You see two stars before you in the distance. You start moving
rapidly toward them. What do you see?
1.
2.
3.
4.
The stars appear to move apart.
The stars appear to move together.
The stars appear the same.
The stars get out of the way just to be safe.
ABERRATION OF LIGHT
2. The stars appear to move together.
This is a known effect from special relativity. This is caused by
the finite speed of light. This is in addition to the Doppler color effect where the stars in front would appear more blue,
while the stars behind would appear more red.
ABERRATION OF LIGHT
You are placed in a circular orbit around the Sun. Because of
aberration, does the Sun appear precisely 90 degrees from
your orbital motion?
1. Yes, that is necessary for a circular orbit.
2. No, aberration makes the Sun appear slightly ahead of you.
3. No, aberration 'leaves the Sun behind' and makes the Sun
appear slightly behind you.
ABERRATION OF LIGHT
2. No, aberration makes the Sun appear slightly ahead of you.
The faster you orbit, the more the Sun will appear ahead of
you.
ABERRATION OF LIGHT
OK, the Sun appears slightly ahead of you. Does sunlight
push you back, creating a "drag force" as you orbit the Sun.
1. Yes, that sounds reasonable.
2. No, that would cause the Earth to fall into the Sun.
3. No -- the Sun, being over there, cannot create a force over
here.
ABERRATION OF LIGHT
1. Yes, that sounds reasonable.
This is called the Poynting-Robertson effect and is a primary
reason (for example), why dust particles fall into the Sun. The
effect on the Earth, although real, is very small.
ABERRATION OF GRAVITY
OK, the Sun appears slightly ahead of you. Does the gravity of
the Sun also appear slightly ahead
of you?
1. Yes, since sunlight and gravity move at the same speed.
2. No, gravity is immune to this effect.
3. Does this mean the Solar System is unstable?
ABERRATION OF GRAVITY
2. No, gravity is immune to this effect.
Although few direct experiments have been done, the stability
of the Earth's orbit puts limit on aberrational effects. As
detailed in Carlip (2000), in Einstein's general relativity, there
are velocity dependent terms that cancel the aberration
effect. The calculation is complex but demanded by
conservation of angular momentum, although the emission of
gravitational radiation will make the cancellation inexact.
ABERRATION OF GRAVITY
It is therefore possible to see someone in one direction, and
feel the force of its pull from a different direction!
Strange!
ABERRATION OF AN ELECTRIC FIELD
Does a charged object orbiting an oppositely charged object
see an aberrated electric field?
1. Yes, all electromagnetic effects will feel aberration.
2. No, electric fields are (also) immune to this effect.
ABERRATION OF AN ELECTRIC FIELD
2. No, electric fields are immune to this effect.
There is no aberration of an electric field. This is an observed
fact. Given Noether's theorem, any field that is invariant over in time
will conserve energy (locally) and hence will not show
aberration. When worked out in detail, velocity dependent terms come
in to cancel the effect of the finite speed of propagation of a changing
electric field.