The Uncertain Nature of Gravity

An Illustration of Science as a Process of Investigating Nature

The Nature of ScienceThe Nature of Science

• Ideas are supported by testable and verifiable data or observation

• Science is a process or way to investigate nature.

• Scientific knowledge doesn’t mean possessing the truth.

• Science seeks consistency between observations and models

Once Upon a TimeOnce Upon a Time

We were certain that:We were certain that:a)a) The Earth was the Center of the The Earth was the Center of the

UniverseUniverseb)b) The Earth was motionlessThe Earth was motionlessc)c) All things fell toward the Earth All things fell toward the Earth

unless there were in the perfect unless there were in the perfect celestial realmcelestial realm

Aristotle Puts Us in the Center of the Universe

Arrogance and the AnomalyArrogance and the Anomaly

Three Choices:

1. Ignore it 2. Fit it to your Bias 3. Throw out your Bias

Discovery in Science:Discovery in Science:How are understanding of gravity How are understanding of gravity

evolvedevolved Usually occur accidentallyUsually occur accidentally Often is a first response to new instrumentation Often is a first response to new instrumentation

or new forms of thinking or new kinds of cultural or new forms of thinking or new kinds of cultural interchangeinterchange

May often be ignored depending on cultural biasMay often be ignored depending on cultural bias

Newton’s Apple and LawsNewton’s Apple and Laws

Apple always falls down?

Gravity is an attractive force only? (so no anti-gravity!)

All matter attracts each other?

Why hasn’t the Universe collapsed?

Two Kinds of MassTwo Kinds of Mass

Equivalence PrincipleEquivalence Principle

Information and Information and CommunicationCommunication

Physics works by information Physics works by information exchange via particlesexchange via particles

Particles can be screened because Particles can be screened because particles interact with matterparticles interact with matter

But There is no such thing as a But There is no such thing as a GRAVITY SCREEN!GRAVITY SCREEN!

Physics to EngineeringPhysics to Engineering

For all practical aspects we are done For all practical aspects we are done now: Newton’s laws provide the now: Newton’s laws provide the scientific foundation to engineer the scientific foundation to engineer the planet.planet.

But, remember the AnomalyBut, remember the Anomaly 1859 precision observations of the 1859 precision observations of the

orbit of Mercury DISAGREE with orbit of Mercury DISAGREE with Newtonian Mechanics!Newtonian Mechanics!

Now we need a new Now we need a new TheoryTheory

Einstein: Gravity = Curvature of Einstein: Gravity = Curvature of SpacetimeSpacetime

Ouch, Curved Spacetime?Ouch, Curved Spacetime?

More mass = greater curvatureMore mass = greater curvature Affects timeAffects time E= mcE= mc22 means that photons have means that photons have

energy related mass and therefore energy related mass and therefore follow the curvature:follow the curvature:

The Need for Dark The Need for Dark MatterMatter

1970’s: Start to measure rotational 1970’s: Start to measure rotational properties of other galaxies:properties of other galaxies:

Invariably observe “flat” rotation Invariably observe “flat” rotation curves:curves:

Resolution:Resolution:

Galaxies must be surrounded by Galaxies must be surrounded by “Halos” of dark material than “Halos” of dark material than contain 90% of the total mass?contain 90% of the total mass?

But what is this dark matter? But what is this dark matter? We We still don’t know after 30 years of still don’t know after 30 years of detailed investigation and theorydetailed investigation and theory

Is there another explanation?Is there another explanation?

Maybe we still don’t Maybe we still don’t understand Gravity?understand Gravity?

Maybe gravity works differently on Maybe gravity works differently on different acceleration scales?different acceleration scales?

This means the effective acceleration on This means the effective acceleration on an object changes so the force law an object changes so the force law changes changes no need for dark matter no need for dark matter

Modified Newtonian Dynamics (MOND): Modified Newtonian Dynamics (MOND): empirically fits the data as well as the empirically fits the data as well as the Dark Matter TheoryDark Matter Theory

But MOND violates the Equivalence But MOND violates the Equivalence Principle Principle therefore if its correct, therefore if its correct, Einstein’s theory has a major flawEinstein’s theory has a major flaw

Reconciliation: An Extra Reconciliation: An Extra DimensionDimension

Gravitons Gravitons the exchange particles the exchange particles in the gravitational force are in the gravitational force are different; they leak to another different; they leak to another dimensiondimension

Explains why the Gravitational force Explains why the Gravitational force is so weakis so weak

The Mysterious Dark EnergyThe Mysterious Dark Energy

ACTS LIKE ANTI GRAVITY!

SummarySummary

We have a good understanding of how We have a good understanding of how gravity works on the Earthgravity works on the EarthWhat is the nature of the Dark Matter?What is the nature of the Dark Matter?If MOND is correct alternative, what is the If MOND is correct alternative, what is the nature of SpaceTime now that the nature of SpaceTime now that the equivalence principle is violatedequivalence principle is violatedDoes the extra dimension explain how Does the extra dimension explain how gravity works throughout the Universegravity works throughout the UniverseWhat is the nature of the Dark Energy?What is the nature of the Dark Energy?

Bottom LineBottom Line

Apparently Life is not so Simple anymore.

There is much still to learn!

Our Current ResponseOur Current Response

Will we Acquire Global Wisdom in Time?

The Coming Resource The Coming Resource ScarcityScarcity

The Decline of Oil:The Decline of Oil:

How the Universe HelpsHow the Universe Helps

It’s a big place, with much to discoverIt’s a big place, with much to discover People think about it in scientific, People think about it in scientific,

philosophical and artistic termsphilosophical and artistic terms It is your ultimate environment – life is It is your ultimate environment – life is

made from star dustmade from star dust It reminds us of the great unknown and It reminds us of the great unknown and

may help to humble us during our more may help to humble us during our more arrogant moments arrogant moments

The value of UniversitiesThe value of Universities

Provides a safe haven for discovery (both Provides a safe haven for discovery (both personal and intellectual)personal and intellectual)Instills critical thinking in terms of conflicts Instills critical thinking in terms of conflicts between discovery, comfort zones, and between discovery, comfort zones, and potential paradigm shiftspotential paradigm shiftsHelps students appreciate the intrinsic Helps students appreciate the intrinsic value of research and asking questionsvalue of research and asking questionsAstronomy is a good venue to teach these Astronomy is a good venue to teach these attributes of science and discoveryattributes of science and discovery

Discovery is EverywhereDiscovery is Everywhere

New Kinds of Solar SystemsNew Kinds of Solar Systems

A Tenth Planet?A Tenth Planet?

Origin of Life?Origin of Life?

STAR DUSTSTAR DUST

PerspectivesPerspectives

Science is DiscoveryScience is Discovery Most is UnknownMost is Unknown Certainty is unwarrantedCertainty is unwarranted Knowledge is contextualKnowledge is contextual Cultural Bias Drives our Cultural Bias Drives our

DecisionsDecisions Finite Resources require Finite Resources require

wisdom to Managewisdom to Manage Can we re-acquire this Can we re-acquire this

Wisdom?Wisdom?

The End