Transcript Newton and Einstein (PowerPoint)

Unit 4 – Newton and Einstein
ASTR 101
Prof. Dave Hanes
Newton Succeeds!
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The orbit of (newly-discovered) Uranus
Halley’s comet (and others, later)
The prediction of Neptune
The prediction of Pluto
Letting Newton drive
I. Uranus:
Found by chance, by William Herschel in 1781
He proposed naming it after King George III.
Newton: Yes or No?
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Uranus’s orbit seemed to be just as Newton would suggest.
His laws seemed still to apply, even far beyond Saturn.
Later, bad news! It became apparent that the orbit was not
exactly what we expect. So what do we do?
Option 1: Conclude that Newton’s laws are not correct that far
out, and we need a new theory (or some modification of his).
Option 2: Assume that there is yet another object, even farther
out, that is ‘tugging’ on Uranus and affecting its orbit. Newton
could still be correct!
II. The Discovery of Neptune
predicted by Adams & Leverrier
How (Undiscovered) Neptune
Affects Uranus
Uranus is the planet
closer to the Sun, and
orbits faster.
As Uranus approaches
Neptune, it feels its
gravity (arrow “a”), and
speeds up a bit.
After it passes Neptune,
it is slowed down a bit,
thanks to tug shown by
arrow “b”.
Success! – 1846
Right Where Predicted
…with a subsequent
dispute over whether
the British or the
French deserve the
glory!
III. Halley’s Comet
Halley studied historical
records, noted the periodic
return of a bright comet every
76 years.
He predicted it would return
in 1758 (but did not live to see
it: he would have been 102.)
Not Like the Planetary Orbits
Success!
This showed that Newton’s gravity also explained the behaviour
of objects in long elliptical orbits, as he had suggested.
IV. The Prediction of Pluto
With the passage of more time, Uranus and
Neptune seemed not to be orbiting quite as they
should. Why not?
One obvious answer: perhaps there was yet
another planet, farther out.
Figure out where it must be, search for it!
Percival Lowell
- ‘canals on Mars’
Clyde Tombaugh
- discoverer of Pluto
The ‘Discovery Image’
- as you can see, it’s hard to deduce its properties!
Not Like the Other Planets!
- orbit much more eccentric, and ‘tipped’
Helpful New Information
Pluto has a moon! Found in 1978; named Charon.
The Moon’s Orbit Gives Us
Pluto’s Mass
Discovery: Pluto is not just small, but much too low-mass to
affect the orbits of Uranus and Neptune.
In fact, careful re-examination of the planetary orbits shows no
significant problems at all.
More Recent Discoveries
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There are many, many objects like Pluto in the
outer Solar System
Pluto’s discovery was
pure serendipity. It was
just in the right place at
the right time.
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‘New Horizons’ on Pluto
Pluto Itself
So Pluto Was Demoted!
V. Letting Newton Drive
- we rely on Newton’s gravity to guide space probes
through the solar system
Gravitational ‘Slingshots’
We ‘steal’ a tiny bit of the planet’s orbital energy, slowing
it down a bit! (The effect on the planet is microscopic.)
The Ultimate Failure
of Newtonian Gravity
What Newton Predicts
If we had only the sun and the Earth, our orbit
would repeat, over and over, forever
But We Are Not Isolated
The gravity of the moon and the other planets (mainly
Jupiter) causes the Earth’s orbit to precess (change in
orientation) very slowly. Newton explains this perfectly!
Mercury is a Problem
Mercury precesses “more than it should,” even considering
the effects of the other planets
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Merc-Prec.mp4
Why?
Three possibilities:
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Is there perhaps another planet closer to the Sun?
(Remember how the misbehaviour of Uranus led us to
Neptune!)
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Do we need to slightly modify Newton’s laws?
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Do we need a fresh way of thinking about gravity?
An Inner Planet
Would be hard to find! We could hope to:
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See it as a dot of light very near the sun (just after
sunset, or just before sunrise)
Look for a transit (a ‘black spot’ moving across the face
of the sun) from time to time
Look for an unexpected dot of light (in addition to the
known background stars) during a total solar eclipse
This Proposed Planet Was
Even Given a Name
(in anticipation of its expected discovery)
The name was
based in mythology
The planet (if real)
would be very hot
So they chose ‘Vulcan’
But it was never found!
A New Way of Thinking About Gravity
Matter Distorts Space Itself
(and also the rate at which time flows!)
Objects simply move freely through the
distorted space, along the curves and
contours
No need to think about gravity as a force
An Analogy
Bigger Lumps make Bigger Distortions
[but the effect is universal!]
Newton’s “law of gravity” is a perfectly useful way of
calculating forces, motions, and so on -- unless you are in a
region where space is quite strongly distorted, such as:
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close to a very large mass,
when matter is densely compressed: neutron stars, black
holes,
or when considering the Universe as a whole!
Take ASTR 102 to learn more!
Einstein’s Important Successes
1. The precession of Mercury was
completely explained – no need for a
‘Vulcan’
2. Einstein was able to predict the effect
that gravity would have on light, a
prediction that was subsequently tested
Why Should Gravity Affect Light?
Matter distorts space,
and light moves
through space along
the distorted contours.
Because light is moving so fast, it undergoes only
tiny changes in direction. (Compare lobbing a
beanbag across the room and firing a bullet!)
With and Without the Sun
Look at a field of distant stars
no sun
sun in between
With the Sun in between, the pattern of stars should
appear to ‘spread out’ a bit (much exaggerated here)
Here is Why
The stars near the Sun’s edge shift outward
(again, very much exaggerated here)
Two Problems
1. The shift is absolutely tiny, since even the
gravity of the sun is pretty feeble – but it
should be (just) measureable to
astronomers.
2. More practical: we can’t see the stars
when the sun is in the way!
Solution: Wait for an Eclipse!
Compare pictures taken with no sun in the way,
and later when the sun is there – but eclipsed!
The Eclipse
Expedition (1919)
Proven Right!
And later confirmed many times
Further Evidence!
Time runs at different rates depending where you
are in a ‘gravitational field’ – not just clocks,
but time itself!!
In day-to-day life, we have to compensate for that
effect when using GPS satellites (since they are
far overhead, where Earth’s gravity is weaker)
The effect is small but not negligible!