Name: einstein’s gravity and mercury’s orbit
Uploaded: 2023-04-30T11:07:53.000Z
Duration: 1 min 17 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Earth and Space Science

Space Systems

3.3.9-12.B

space-time-curvature-and-the-general-theory-of-relativity

Space-Time Curvature and the General Theory of Relativity

W teorii grawitacji Newtona siła grawitacji działa natychmiast w pustej przestrzeni między dowolnymi dwiema znacznie oddalonymi od siebie masami. Jednak prędkość światła jest górną granicą dla każdej fizycznej interakcji.
Newton nie potrafił wyjaśnić, dlaczego masa użyta w prawie ciążenia jest taka sama jak masa bezwładności użyta w jego drugiej zasadzie dynamiki.
Einstein stwierdził, że skoro grawitacja i przyspieszenie są równoważne, to te dwie masy są takie same. Odniósł się do sprzeczności dotyczącej prędkości światła, wyobrażając sobie przestrzeń jako niewidzialną tkaninę zwaną czasoprzestrzenią, mającą trzy wymiary przestrzenne i jeden wymiar czasowy.
Zasugerował, że w obecności dużej masy tkanina ta zakrzywia się i owija wokół niej. W związku z tym każda mniejsza masa podąża najkrótszą ścieżką wokół większej masy na zakrzywionej tkaninie czasoprzestrzennej.
Podczas całkowitego zaćmienia Słońca w 1919 roku, gwiazda znajdująca się za Słońcem była widoczna nieco dalej od Słońca, zgodnie z przewidywaniami ogólnej teorii względności. Obserwacja ta dowiodła, że światło od gwiazdy podąża najkrótszą ścieżką na zakrzywionej czasoprzestrzeni wokół Słońca.

krzywizna czasoprzestrzeni i ogólna teoria względności

W 1905 roku Albert Einstein opublikował swoją szczególną teorię względności. Zgodnie z tą teorią żadna materia we wszechświecie nie może osiągnąć prędkości większej niż prędkość światła w próżni, co stanowi w ten sposób granicę prędkości wszechświata.

Potwierdzono to w wielu eksperymentach. Jednak przestrzeń i czas nie są już absolutne. Dwóch obserwatorów poruszających się względem siebie nie jest zgodnych co do długości obiektów i upływu czasu. Mechanika obiektów oparta na prawach ruchu Newtona, choć niezwykle dokładna nawet przy prędkościach wielu tysięcy mil na sekundę, zaczyna zawodzić, gdy względny ruch pomiędzy obiektami zbliża się do prędkości światła w próżni. Co więcej, szczególna teoria względności ujawnia fundamentalne ograniczenia praw Newtona.

Zgodnie z prawami ruchu Newtona i prawem grawitacji Newtona wszystkie działania zachodzą natychmiast. Ponieważ szczególna teoria względności Einsteina stwierdza, że ​​we wszechświecie istnieje ograniczenie prędkości, takie natychmiastowe działanie na skończonej odległości nie jest w zasadzie możliwe.

W 1915 roku Einstein zaproponował rozwiązanie tego problemu w ogólnej teorii względności, w której sformalizował zasadę równoważności w kategoriach matematycznych. Zgodnie z teorią grawitacja nie jest siłą działającą pomiędzy dwoma obiektami; zamiast tego jest to wpływ dwóch obiektów na otaczającą je czasoprzestrzeń, co z kolei determinuje ich dynamikę.

W szczególnej i ogólnej teorii względności przestrzeń i czas są traktowane na równi. Zakrzywienie nie dotyczy samej przestrzeni, ale połączonej całości – czasoprzestrzeni.

W przypadku słabych pól grawitacyjnych wyniki ogólnej teorii względności nie różnią się znacząco od prawa ciążenia Newtona. Jednak w przypadku intensywnych pól grawitacyjnych wyniki są rozbieżne i wykazano, że ogólna teoria względności pozwala przewidzieć prawidłowe wyniki. Efekty te zaobserwowano w stosunkowo słabym polu grawitacyjnym naszego Słońca, w odległości orbity Merkurego. Od połowy XIX wieku dokładnie mierzono eliptyczną orbitę Merkurego. Jednakże, chociaż jest eliptyczna, jej ruch komplikuje fakt, że położenie peryhelium elipsy powoli się przesuwa. Większość postępu wynika z przyciągania grawitacyjnego innych planet, ale niewielkiej części tego postępu nie można wytłumaczyć prawami Newtona. Szukano nawet „towarzysza” planety, która wyjaśniałaby rozbieżność w jednym czasie. Jednak ogólna teoria względności poprawnie przewiduje pomiary.

Ten tekst jest adaptacją <a href="https://openstax.org/books/university-physics-volume-1/pages/13-7-einsteins-theory-of- grawitacja">Openstax, University Physics, tom 1, sekcja 13.7: Teoria grawitacji Einsteina.</a>

Krzywizna czasoprzestrzeni i ogólna teoria względności

W 1905 roku Albert Einstein opublikował swoją szczególną teorię względności. Zgodnie z tą teorią, żadna materia we wszechświecie nie może osiągnąć prędkości większej niż ...

Education

JoVE Core

Physics

Fizyka

Unit 1

Jednostka 1

Gravitation

Grawitacja

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

einstein’s gravity and mercury’s orbit

Einstein’s theory of gravity explains how space-time affects motion. In 1905, Einstein published his special theory of relativity. It says that no object can travel faster than the speed of light in a vacuum, which is the speed limit of the universe.

Special relativity also changed the way scientists think about space and time. Two observers moving relative to each other do not agree on object length or on how time passes. Newton’s laws still work well for many cases, even for objects moving at many thousands of miles per second. But they begin to fail when motion gets close to the speed of light.

Special relativity also showed a limit in Newton’s laws of motion and gravitation. In Newton’s model, actions happen instantly. But if the universe has a speed limit, instantaneous action across a distance cannot be fundamental. In 1915, Einstein solved this problem with the general theory of relativity and the principle of equivalence.

General relativity says gravity is not a force pulling two objects together. Instead, mass and energy affect the space-time around them, and that curved space-time guides motion. Space and time are treated together, and the curvature is in space-time, not in space alone. For weak gravitational fields, the theory gives results very close to Newton’s law of gravitation.

The differences appear more clearly in intense gravitational fields. The theory has been tested in the Sun’s weaker gravitational field at Mercury’s orbit. Since the mid-1800s, scientists have measured Mercury’s elliptical path. Its perihelion, or closest point to the Sun, slowly advances, and most of that shift comes from other planets.

A small part of Mercury’s perihelion advance could not be explained by Newton’s laws. Scientists even looked for a companion planet to account for it. General relativity predicts the missing part correctly. This match with Mercury’s orbit helped confirm Einstein’s theory of gravity.

Einstein’s Gravity and Mercury’s Orbit

gravitation

Newton’s Law Behind Planet Motion

Newton’s law of gravitation links falling objects, the Moon’s orbit, and planetary motion. Before Newton, many people thought the sky and Earth followed different laws. Kepler later described the three laws of planetary motion, but their connection to a deeper force was still unclear. Isaac Newton made that connection in 1665 to 1666. He showed that the same phenomenon explains why objects fall to Earth and why the Moon moves around Earth. He also found that the Moon’s centripetal acceleration...

/files/media/science-education/science-education-thumbs/14716.jpg

newton's law of gravitation

Newton's Gravity and the Motion of Bodies

Newton's gravity links the fall of objects on Earth with the motion of the Moon and planets. People once thought the downward pull seen near Earth was unique to our planet. Later, Nicolaus Copernicus proposed the heliocentric theory, which placed Earth and the other planets around the Sun, with the Moon orbiting Earth. Isaac Newton connected these motions in the 17th century. He reasoned that the same attraction that keeps the Moon on its path around Earth also makes an apple fall from a tree.

gravitation between spherically symmetric masses

gravitation-between-spherically-symmetric-masses

Gravitational Potential Inside a Shell

Gravitational potential energy for spherically symmetric bodies depends on their masses and the distance between them. The idea is to treat each body as if its mass were concentrated at its center of mass. This makes the calculation easier for high school physics. A spherically symmetric mass can be thought of as many concentric spherical shells. Now place a point mass at a distance r from the center of one shell. Every particle on a given spherical ring is the same distance from that point...

/files/media/science-education/science-education-thumbs/14717.jpg

gravity between spherical bodies

Point-Mass Approximation in Spherical Gravity

Newton’s law of gravitation gives the force between two point masses. For large spherical objects, such as the Earth, the Moon, and other planets, the same law can be used by treating each body as if its mass were concentrated at its center. This idea can be proven by comparing gravitational potential energy. The transcript shows that a hollow sphere of mass M and a point mass m give the same energy result as two extended solid spherical objects. To set up the proof, a tiny ring on the shell...

reduced mass coordinates: isolated two-body problem

reduced-mass-coordinates-isolated-two-body-problem

Center of Mass in the Two-Body Problem

Reduced mass coordinates help simplify the classical two-body problem. In classical mechanics, this problem describes the motion of two interacting bodies under gravity or another central force. The key idea is that the system can be treated like a single-body problem when reduced mass is used. For this description, one body is imagined to move with the reduced mass while the other body is fixed in place with infinite mass. The reduced mass is a property of the two-body system and is defined...

/files/media/science-education/science-education-thumbs/14740.jpg

acceleration due to gravity on earth

Gravity on Earth: Mass, Direction, and Value

Gravity near Earth gives objects a downward acceleration called acceleration due to gravity. It comes from Earth’s gravitational pull and points toward the center of the Earth. Newton’s law of gravitation shows that the gravitational force depends on mass, while Newton’s second law shows that the acceleration produced by a force depends on how that force acts on the object. Because of this relationship, the acceleration due to gravity is independent of an object’s mass. An object and the Earth...

acceleration due to gravity on other planets

acceleration-due-to-gravity-on-other-planets

Measuring Gravity on Distant Planets

Gravity near Earth’s surface is measured as acceleration due to gravity. On Earth, students can test it with simple experiments. On other planets, that kind of direct experiment is not possible. Instead, astronomers use observations to find a planet’s gravity. They study how the planet affects nearby objects, especially its satellite. The key idea is that the acceleration depends on the planet’s gravity, not on the object’s mass. To make the calculation, scientists track a satellite’s path...

apparent weight and the earth's rotation

Earth's Spin and Apparent Weight

Earth’s spin changes how weight is measured on the surface. Because the Earth turns once every 24 hours, objects on the surface move in a circle. That motion requires a net centripetal force, which points toward the center of the circle. The north and south poles are the only exception. At the equator, the force that provides this inward pull comes from Earth’s gravity. It is the difference between an object’s true weight and the normal force from the ground. The normal force is the support...

variation in acceleration due to gravity near the earth's surface

variation-in-acceleration-due-to-gravity-near-the-earth-s-surface

How Earth’s Shape Affects Apparent Weight

Apparent weight changes slightly on Earth because of rotation and the planet’s shape. Apparent weight is the weight measured by a spring balance at a location. It is different from true weight, which is the force Earth pulls on an object with. Earth’s rotation reduces apparent weight by adding a centripetal force. Centripetal force is the inward force that keeps an object moving in a circle. The apparent weight equals the true weight minus this force. The difference depends on the square of...

potential energy due to gravitation

Gravitational Binding and Potential Energy

Gravitational potential energy describes how gravity stores energy between two masses. Because gravity is a conservative force, the work needed to move an object between two points depends on the starting and ending positions, not on the path taken. That is why a gravitational potential energy function can be written in terms of spatial coordinates. For a mass bound by gravity to another object, the potential energy is negative near the attracting body and rises toward zero as the distance...

the principle of superposition and the gravitational field

the-principle-of-superposition-and-the-gravitational-field

Adding Gravitational Forces with Superposition

Gravitational forces can be added using the principle of superposition. For objects that are sufficiently far apart, the net gravitational force on a point object is the vector sum of the forces from each mass. This means each force is found first, and then the forces are added as vectors. Superposition is useful because gravity from more than one object can combine in ways that are not always easy to see. One force may be increased or reduced by another force, or the final effect may be a...

/files/media/science-education/science-education-thumbs/14754.jpg

escape velocity

Launching a Satellite Beyond Gravity

Launching a satellite beyond gravity requires enough escape velocity to leave a planet or other body and never return. Escape velocity is the minimum initial speed needed to break free from the gravitational pull of an object that is holding the satellite in orbit or near its surface. For Earth, the idea is often shown with a satellite launched from the ground. The calculation assumes that no energy is lost to friction. In real life, Earth’s atmosphere also slows the satellite down, so the...

circular orbits and critical velocity for satellites

circular-orbits-and-critical-velocity-for-satellites

Orbital Speed and Period in Circular Paths

Orbital speed and period in circular paths explain how the Moon, planets, and satellites stay in motion around larger bodies. The Moon orbits Earth. Earth and the other planets orbit the Sun. The space above our atmosphere also contains many artificial satellites. A circular orbit is the simplest orbit to study. It helps show how an object’s speed and its period, or the time for one full orbit, depend on where it is and what it circles. Nicolaus Copernicus first suggested that Earth and the...

energy of a satellite in a circular orbit

energy-of-a-satellite-in-a-circular-orbit

Satellite Orbits and Energy in Space

Satellite orbits and energy in space depend on altitude, orbit shape, and gravity. Thousands of artificial satellites circle Earth every day at many different distances. Satellites below 1,600 km are in low-Earth orbit, or LEO. Research satellites and Earth observation satellites are usually placed in LEO. They mostly move in elliptical orbits, which are oval-shaped paths. Navigation satellites are placed in medium-Earth orbit, or MEO, which ranges from 2,000 km to 36,000 km above Earth’s...

kepler's first law of planetary motion

Planet Orbits as Ellipses

Planet orbits are not perfect circles. Johannes Kepler used planet data from Tycho Brahe to show that each planet follows an ellipse, with the Sun at one focus. This idea came from studying Mars, which did not fit the circular path expected in the Copernican heliocentric model. Copernicus had proposed that the solar system is heliocentric, meaning the Sun is at the center and the planets, including Earth, move around it. His model treated those paths as circles. Brahe later recorded the...

kepler's second law of planetary motion

Kepler's Equal-Area Planet Motion

Kepler's second law explains how a planet moves faster and slower in its orbit around the Sun. Johannes Kepler, a German astronomer and mathematician in the early 17th century, developed three laws for planetary motion. His first law says that planets travel in elliptical orbits, with the Sun at one focus of the ellipse. Because the planet-Sun distance changes during the orbit, the planet does not move at the same speed all the time. In an elliptical orbit, the total energy of the planet is...

kepler's third law of planetary motion

Kepler’s Planet Distance Law

Kepler’s third law links a planet’s average distance from the Sun to the time it takes to orbit the Sun. This rule gives a clear mathematical relationship between orbital distance and orbital period, which is the time for one full revolution. Johannes Kepler developed this law in the early 17th century after building on the observations of Nicolaus Copernicus and Tycho Brahe. Kepler first formed his first two laws from those earlier observations. He later published the third law of planetary...

tidal forces

How the Moon Shapes Earth’s Tides

Earth’s ocean tides are driven by gravitational tidal forces. Scientists have studied the origin of tides for more than 2000 years, and Newton’s work marked the start of a proper understanding. These same tidal forces also act on other astronomical bodies. They can produce internal heat that leads to volcanic activity on Io, one of Jupiter’s moons, and they can tear apart stars that pass too close to black holes. Newton’s law of gravitation says that gravitational force is inversely...

schwarzschild radius and event horizon

Escape Velocity Limits and Black Holes

Escape velocity helps explain how black holes and event horizons form. It is the minimum speed a projectile needs to leave the surface of an object and eventually escape its gravitational field. In this context, escape velocity does not depend on the mass of the object being launched. The key idea is that no object with finite mass can move faster than the speed of light in a vacuum. When escape velocity is compared with that speed limit, the Schwarzschild radius becomes important. If an...

detection of black holes

How Astronomers Find Black Holes

Black holes are found through indirect astronomical evidence. They were first proposed in the 1920s, but they stayed outside observational astronomy until the 1970s. Their closest known cousins are neutron stars, which are made mostly of packed neutrons and are extremely dense. A neutron star can have the mass of the Sun while its diameter is only a few kilometers. Because of that, the escape velocity from its surface is close to the speed of light. Interest in black holes rose again in the...

principle of equivalence

Weightlessness and Einstein’s Equivalence

Weightlessness and Einstein’s equivalence principle are closely connected in gravity. A person in free fall under gravity, such as someone diving toward Earth from an airplane, would feel completely weightless. A person descending in a lift may feel partially weightless. The principle of equivalence says that an object in a uniform gravitational field and an object accelerating at a constant rate with no gravity are under the same experimental conditions. In other words, no experiment can tell...

space-time curvature and the general theory of relativity

Einstein’s theory of gravity explains how space-time affects motion. In 1905, Einstein published his special theory of relativity. It says that no object can travel faster than the speed of light in a vacuum, which is the speed limit of the universe. Special relativity also changed the way scientists think about space and time. Two observers moving relative to each other do not agree on object length or on how time passes. Newton’s laws still work well for many cases, even for objects moving...

key terms and concepts

Explore how Einstein’s general relativity links gravity to curved space-time and explains Mercury’s orbit, including its shifting perihelion.

Einstein’s Gravity and Mercury’s Orbit | JoVE K12

physics

JoVE Core Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

3.3.9-12.D

Louisiana Student Standards for Science

High School

Earth Science

Earth’s Place In The Universe

HS-ESS1-1

HS-ESS1-2

Virginia  Standards of Learning

HIgh School

ES.2

ES.2.a

PH.9

PH.9.c

Alabama Course of Study Science

Earth's Place in the Universe

9-12.ESS.1.3

9-12.ESS.1.3.b

Next Generation Science Standards

Earth’s Place in the Universe

HS-ESS1-4

Florida Department of Education

SC.912.E.5

SC.912.E.5.10

SC.912.E.5.11

Physical Science

SC.912.P.12

SC.912.P.12.8

Georgia Standards of Excellence

Astronomy

SAST4

SAST4.1

The Principle of Superposition and the Gravitational Field

The principle of superposition applies to gravitational forces of objects that are sufficiently far apart. It states that the net gravitational force on a ...

Escape Velocity

The escape velocity of an object is defined as the minimum initial velocity that it requires to escape the surface of another object to which it is ...

Circular Orbits and Critical Velocity for Satellites

The Moon orbits around the Earth. In turn, the Earth (and other planets) orbit the Sun. The space directly above our atmosphere is filled with artificial ...

Energy of a Satellite in a Circular Orbit

Thousands of artificial satellites orbit the Earth every day at various distances from the Earth. Satellites that orbit the Earth below an altitude of ...

Kepler's First Law of Planetary Motion

In the early 17th century, German astronomer and mathematician Johannes Kepler postulated three laws for the motion of planets in the solar system. He ...

Kepler's Second Law of Planetary Motion

In the early 17th century, German astronomer and mathematician Johannes Kepler postulated three laws for the motion of planets in the solar system. His ...

Kepler's Third Law of Planetary Motion

In the early 17th century, German astronomer and mathematician Johannes Kepler postulated three laws for the motion of planets in the solar system. In ...

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

Atomic Structure

All matter is composed of atoms, the smallest individual units of elements. Each atom is made up of three subatomic particles: protons, neutrons, and ...

Radioactivity and Nuclear Equations

Nuclear chemistry is the study of reactions that involve changes in nuclear structure. The nucleus of an atom is composed of protons and, except for ...

Nuclear Stability

Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10&#8722;15 meters, a nucleus is ...

Students who demonstrate understanding can develop a model based on evidence to illustrate the life span of the sun and the role of nuclear fusion in the sun's core to release energy in the form of radiation.  Clarification Statement: Emphasis is on the energy transfer mechanisms that allow energy from nuclear fusion in the sun's core to reach Earth.  Examples of evidence for the model include observations of the masses and lifetimes of other stars, as well as the ways that the sun's radiation varies due to sudden solar flares ("space weather"), the 11-year sunspot cycle, and non-cyclic variations over centuries.  Assessment Boundary: Assessment does not include details of the atomic and sub-atomic processes involved with the sun's nuclear fusion.

3.3.9-12.A

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

atomic-spectroscopy-absorption-emission-and-fluorescence

Atomic Spectroscopy: Absorption, Emission, and Fluorescence

Atomic spectroscopy is a vital tool in elemental analysis, both qualitatively and quantitatively. It can be broadly divided into optical spectroscopy, ...

Detection of Black Holes

Although black holes were theoretically postulated in the 1920s, they remained outside the domain of observational astronomy until the 1970s.
Their ...

In 1905, Albert Einstein published his special theory of relativity. According to this theory, no matter in the universe can attain a speed greater than ...

Schwarzschild Radius and Event Horizon

No object with a finite mass can travel faster than the speed of light in a vacuum. This fact has an interesting consequence in the domain of extremely ...

The Wave Nature of Light

The nature of light has been a subject of inquiry since antiquity. In the seventeenth century, Isaac Newton performed experiments with lenses and prisms ...

Nuclear Binding Energy

The difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium-4, the mass defect ...

Metal Flame Emission

Metal Flame Emission
Flame testing is an analytical technique where a sample is placed in a flame, and the characteristic flame color is used to identify ...

molecular-spectroscopy-absorption-and-emission

Molecular Spectroscopy: Absorption and Emission

Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational ...

atomic-emission-spectroscopy-instrumentation

Atomic Emission Spectroscopy: Instrumentation

The instrumentation of atomic emission spectrometry (AES) involves various components, including atomization devices that convert samples into gas-phase ...

atomic-absorption-spectroscopy-radiation-and-light-sources

Atomic Absorption Spectroscopy: Radiation and Light Sources

Atomic absorption spectroscopy (AAS) relies on the Beer-Lambert law, which requires that the radiation source emits a narrow range of wavelengths to match ...

Students who demonstrate understanding can construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.  Clarification Statement: Emphasis is on the astronomical evidence of the red shift of light from galaxies as an indication that the universe is currently expanding, the cosmic microwave background as the remnant radiation from the Big Bang, and the observed composition of ordinary matter of the universe, primarily found in stars and interstellar gases (from the spectra of electromagnetic radiation from stars), which matches that predicted by the Big Bang theory (3/4 hydrogen and 1/4 helium).  Assessment Boundary:

Spectrophotometry: Introduction

Spectrophotometry is the quantitative measurement of the absorption, reflection, diffraction, or transmission of electromagnetic radiation through a ...

Atomic Emission Spectroscopy: Overview

Atomic emission spectroscopy (AES) is an analytical technique used to determine the elemental composition of a sample by analyzing the light emitted from ...

ultraviolet-and-visible-uvvis-spectroscopy-overview

Ultraviolet and Visible (UV&ndash;Vis) Spectroscopy: Overview

Ultraviolet&#8211;visible (UV&#8211;visible or UV&#8211;Vis) spectroscopy is an analytical technique that investigates the interaction between matter and ...

Students who demonstrate understanding can communicate scientific ideas about the way stars, over their life cycle, produce elements.  Clarification Statement: Emphasis is on the way nucleosynthesis, and therefore the different elements created, varies as a function of the mass of a star and the stage of its lifetime.  Assessment Boundary: Details of the many different nucleosynthesis pathways for stars of differing masses are not assessed.

3.3.9-12.C

Einstein’s Gravity and Mercury’s Orbit

Description

Transcript

Tags

More videos by Standards