Reading Unit 26, 27, 28, 29, 30

The Michelson-Morley Experiment Two scientists devised an experiment to detect the motion of the Earth through the “aether” –Light should move slower in the direction of the Earth’s motion through space –Detected no difference in speed! –No aether, and the speed of light seemed to be a constant!

Einstein’s Insights Albert Einstein started from the assumption that the speed of light was a constant, and worked out the consequences –Length does indeed contract in the direction of motion, by a fraction equal to the Lorentz factor –Time stretches as well, also by the Lorentz factor Moving clocks run slow Moving objects reduce their length in the direction of motion

Special Relativity Time dilation and length contraction depend on the observer! –To an observer on Earth, the spacecraft’s clock appears to run slow, and the ship looks shorter –To an observer on the ship, the Earth appears to be moving in slow-motion, and its shape is distorted. The passage of time and space are relative!

Possibilities for Space Travel Example: A spacecraft leaves Earth, heading for a star 70 light- years away, traveling at.99c –To an observer on Earth, it takes the spacecraft 140 years to get to the star, and back again –To passengers on the ship, it only takes 20 years for the round-trip! This means that high speed travel to the stars is possible, but comes at the cost of friends and family…

General Relativity: Mass Warps Space Mass warps space in its vicinity The larger the mass, the bigger “dent” it makes in space Objects gravitationally attracted to these objects can be seen as rolling “downhill” towards them If the mass is large enough, space can be so warped that objects entering it can never leave – a black hole is formed.

Another underappreciated female in science? Mileva Maric Einstein Nobel Prize was for photoeffect, which was the theme of diploma work by Mileva. To what extend she inspired Einstein or collaborated with him is a subject of debates.

Telescopes Telescopes have been used for hundreds of years to collect light from the sky and focus it into an eyepiece. An astronomer would then look through this eyepiece at planets, nebulae, etc. The human eye is not very sensitive to dim light, and was replaced in astronomy by the film camera. Film is sensitive to only around 10% of the impinging light, and is usually replaced by a…

The Charge-Coupled Device (CCD) The CCD, similar to those found in commercial digital cameras and phones, utilizes the photoelectric effect to collect around 75% of the visible light that is focused on it! It has revolutionized astronomy – images can be recorded and downloaded to a computer anywhere in the world for analysis The science of developing new methods for sensing, focusing and imaging light in astronomy is called instrumentation

Outside the visible spectrum Many objects of astronomical interest are visible only in wavelengths other than the visible! Much can be learned from studying a star, planet or nebula in multiple wavelengths. Radio telescopes can be used from the ground to image pulsars and other bodies Observations in other wavelengths require instrumentation to be lifted above the Earth ’ s atmosphere. X-ray, Gamma ray and infrared wavelength telescopes are currently in orbit!

Modern Telescopes Modern telescopes are designed to collect as much light as possible, and must be built to exacting standards. Collected light is of nanometer wavelength, so the telescopes must be extremely precise to keep the waves coherent for maximum efficiency

Radio Telescopes Radio telescopes, like the one in Arecibo, Puerto Rico, collect radio waves from astronomical objects and events

Radio Telescopes Radio telescope arrays to achieve large collecting areas National Radio Astronomy Observatory (U.S.A.)

Size Matters! Aperture size is very important when collecting light! A large collecting area allows astronomers to image dim and distant objects. For a telescope with an aperture a distance D in diameter,

Refracting Telescopes Telescopes that use lenses to focus light are called refracting telescopes, or refractors. Large refractors are difficult to build! –Glass is heavy, and glass lenses must be supported only by their rims, a difficult engineering problem –Glass sags under its own weight, defocusing the light! –Refractors suffer from chromatic aberration, a blurring effect due to changes in the focal plane of the lens for different wavelengths of light

Reflecting Telescopes Reflecting telescopes, or reflectors, use a curved mirror to focus light Mirrors can be supported from behind, and so can be much larger than refractors Larger sizes mean that more light can be collected and focused, allowing astronomers to image dimmer or more distant objects Most modern telescopes are reflectors.

Different styles of reflectors

X-Ray reflectors X-rays only reflect at glancing angles, otherwise they are absorbed or pass through the mirror! X-Ray mirrors are designed to gently reflect the incoming photons, focusing them at the end of a long tube-shaped array of mirrors

Chandra

Very Large Mirrors Reflectors can be made very large if multiple mirrors are used as the primary mirror. The Keck Telescope uses 36 large mirrors to create a single huge primary. The positions of the mirrors are precisely measured by lasers, and can be individually adjusted to keep them perfectly aligned.

Diffraction and Resolution Some stars that appear to be single bodies to the unaided eye are, when viewed through a telescope, found to be two separate stars. The telescope is able to separate the two stars, while the human eye is not. The telescope, then, has better resolution than the human eye. The telescope’s resolution is better because it has a larger aperture, and light is diffracted less as it passes through it. Diffraction is a rippling effect due to the finite size of an aperture. Light waves approach the aperture as flat plane waves, similar to the straight water waves seen above. As the waves pass through the aperture, the waves become curved.

Diffraction Effects Diffracted light waves can interfere with, or cancel, each other. This results in a diffraction pattern, a blurring of the image as it passes through the telescope. Larger apertures have less diffraction, and therefore higher resolution than smaller apertures. For observing light of wavelength nm, the smallest separation angle  arcsec a telescope can resolve is related to the telescope aperture D cm by:

Interferometers To counter diffraction effects (and build telescopes with higher resolution), astronomers use interferometers. Signals from these arrays of widely-separated telescopes are added together to create images with very high resolution. In fact, the resolution is equivalent to that of a single telescope with an aperture as large as the separation in the array!

Before and After Before: –What looks like a single star… After –…is actually two stars!

Atmospheric Absorption The Earth’s atmosphere absorbs most of the radiation incident on it from space This is a good thing for life – high energy photons would sterilize the planet! This is not a good thing for astronomy, however! Visible, radio and some infrared wavelengths are not absorbed readily by the atmosphere –Optical and radio telescopes work well from the ground Gamma Rays, X-rays, and UV photons are absorbed –Observatories for these wavelengths must be kept above the Earth’s atmosphere!

Ground- and Space-based Observatories

Light Pollution Ambient light from cities are a real problem for optical astronomy. This light pollution washes out images in telescopes. Research telescopes are built far from cities to reduce the effects of light pollution It is getting harder to find good locations for telescopes!

Atmospheric Effects Air refracts light just like glass or water, but to a lesser degree. –Cool air refracts light more than warm air –Pockets of cool air in the atmosphere create moving lenses in the sky, shifting the light rays randomly –This causes a twinkling effect, called scintillation. A stable atmosphere causes less scintillation –We say the seeing is good.

Adaptive Optics Some observatories measure the amount of atmospheric turbulence with lasers, and then adjust the mirrors in their telescopes with tiny motors to eliminate the effect This technique is called adaptive optics

Spitzer Space Telescope James Web Space telescope

Hubble Telescope Edwin Hubble

Observatories in Space