Unit 4 – Newton and Einstein

ASTR 101Prof. Dave Hanes

Newton Succeeds!

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? 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! – 1846Right 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 seeit: 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 Clyde Tombaugh- ‘canals on Mars’ - 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 InformationPluto 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

There are many, many objects like Pluto in the outer Solar System

Pluto’s discovery waspure serendipity. It was just in the right place at the right time.

‘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 ProblemMercury 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:

Is there perhaps another planet closer to the Sun? (Remember how the misbehaviour of Uranus led us to Neptune!)

Do we need to slightly modify Newton’s laws?

Do we need a fresh way of thinking about gravity?

An Inner Planet

Would be hard to find! We could hope to:

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:

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 betweenWith 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 EclipseExpedition (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!