Q&A With Dr. Harry Ferguson
Astronomer, Space Telescope Science Institute

Steven Skaggs
Louisville, Kentucky

My question deals with a famous example explaining the relativity of time. Two twins say goodbye to each other before a rocket. One gets in, blasts off at close the speed of light and when the onboard clock shows she has been gone one year she returns to Earth to find that her twin has aged 50 years. My question is this: can't you imagine that from the standpoint of the spaceship, it was Earth that zoomed backward at near the speed of light and therefore the Earthbound twin that should have experienced a 'time warp'?

In this classic paradox, the roles of rocketeer and stay-at-home are not interchangeable. The reason is that the twin who flies off to another star at nearly the speed of light must accelerate as she leaves the Earth, then eventually decelerate to turn around, accelerate again to come back home, and decelerate again to stop when arriving at Earth. When accelerating and decelerating, the space-traveler is not at rest with respect to the "inertial frame" of the universe, and indeed must expend considerable energy (rocket fuel or whatever) to accomplish this. While the twin on the rocket ship might look backward and imagine that it is the Earth (and her brother there) that is accelerating away from her, this is not actually the case -- the lazy brother on Earth just sits there at rest with regard to the inertial frame of the universe and lets his sister do all the work. The "proof" of this is that the sister will feel "G-forces" as her rocket accelerates, while her brother won't feel any such thing, demonstrating that their situations are not strictly reversible.
M. D.

Panagiotis Panos
Northport, N.Y

My question pertains to the blackhole in the universe. How many are there? How are they formed? And it is my understanding that the magnetic field prohibits light from passing thru thus enabling us from seeing thru? Is this correct?

We don't know how many black holes there are in the universe -- while the concept of the black hole has been around for quite some time, it is only very recently that astronomers have found solid (if still somewhat indirect) evidence that they truly exist. It is believed that black holes form when a very massive star (much more massive than our sun) exhausts its nuclear fuel and collapses under the force of its own gravity. During this collapse, the core of the star is crushed, achieving such high density that its gravitational field prevents all light and matter from escaping it. It is gravity, not magnetism, that prevents us from seeing a black hole directly. However, this gravity has a powerful effect on matter nearby the black hole, and observational evidence for their existence comes from measurements of material (gas and/or starlight) moving at tremendously high velocities in the vicinity.

Ultra-massive black holes apparently also exist in the cores of some, perhaps many, galaxies -- black holes with masses ranging from a million to a billion times that of our sun. Exactly how these formed, we do not know: it may taken place during the original gravitational collapse of material during the formation of the galaxy itself, or perhaps such later when gas and other matter gets funneled to the center of the galaxy due to interactions with other neighboring galaxies. Recent Hubble Space Telescope observations, as well as other data from ground-based telescopes, have shown that their presence in some nearby galaxies is almost a certainty.
M. D.

George Williams
Sterling, VA

The way I understand it Einstein says that nothing can exceed the speed of light. If a Black hole sucks everything into it including light, would not the things being sucked in be going faster then 186,000 per second, owing to the fact of something like the venturi effect?

No, as the t-shirts say, "186,000 miles per second -- it's not just a good idea, it's the law." Even falling into a black hole, the speed of light represents a maximum speed limit in the universe. Objects may accelerate toward the speed of light as they fall into a black hole, but they cannot exceed it.
M. D.

Name Withheld
Mill Bay. British Columbia

It is known that the universe is expanding from the implications of the red shifts of observable light. What could be seen or found out if one was to observe the shift of light further up the spectrum. For example, is there such a thing as an infrared shift. (either in reality or hypothetically.)What would the implications of this type of observation be? Also is there any evidence for anything beyond the obsevable universe and if so how can it be studied if nothing can go faster then the speed of light.

Light is just part of the spectrum of electromagnetic radiation. Electromagnetic waves with wavelengths shorter than light are called ultraviolet radiation, and shorter than this are the x-rays and gamma rays. At wavelengths longer than that of visible light we find infrared radiation, and then microwaves and radio waves.

The Doppler shift, or "red-shift," produced when an object is moving away from us, affects all radiation -- not just visible light, but also radio waves, microwaves, infrared and ultraviolet radiation, x-rays and gamma rays. All are shifted "redward," that is to say that their wavelengths are "stretched" longer. Blue light from a moderately distant galaxy is shifted to become red light, while red light is shifted into the infrared. The infrared light from a galaxy is shifted too. For the most distant galaxies we know today, their visible light is shifted all the way into the infrared, requiring special instruments to observe them at those wavelengths, and their infrared light gets shifted into radio wavelengths known as the "sub-millimeter" regime.
M. D.

Jacob
Wisconsin

I have a couple of questions. First if the universe is expanding were is it expanding to? Next were is the Milky Way in the universe and what is it's closest galaxy? How far away is it?

One can imagine two answers to your first question. First, if the universe is infinite, then it doesn't really "need" anything to expand into. After all, two times infinity is still infinity! An infinite universe can keep expanding indefinitely and remain infinite all the time. Note that the universe was infinite beginning immediately from the time of the Big Bang -- it has *always* been infinite, even as it expands!

The second answer requires one to imagine another dimension beyond the four (three of space and one of time) which we inhabit. Not an easy task, imagining a dimension which we cannot perceive! Let's think of it by analogy. Imagine, for a moment, that the universe is merely two-dimensional, and is confined to the surface of a sphere. (So think of the universe as the surface of a balloon -- a universe in which we can only move right, left, forward and backward along that surface -- up and down don't exist in this imaginary world -- but "space" in this universe is curved in another dimension which its inhabitants can't really perceive.) If the balloon inflates, then this model "universe" expands -- the surface of the balloon grows with time, and any two points on the balloon move apart, just as galaxies in our universe expand away from one another. The inhabitants of this imaginary universe cannot perceive this extra dimension into which the universe is expanding -- in fact, for them, it simply does not exist, except perhaps as a mathematical construct. But they might find this extra dimension a useful idea when trying to explain how their universe is expanding. For us, it is the same way -- we may imagine a higher dimension "into" which our universe is expanding.

Concerning your second question: according to the Big Bang theory, our galaxy, the Milky Way does not occupy any particularly special place in the universe -- there is no center or edge of the universe to define any such special place. The Milky Way is a galaxy like billions of others. We do know quite a bit about its immediate neighborhood, however. Galaxies tend to gather together in groups, and occasionally in rich clusters of thousands of galaxies. The Milky Way is situated in a rather modest collection of a few dozen galaxies which we call (somewhat unimaginatively) "The Local Group." There are two fairly big galaxies in the Local Group -- the Milky Way, and its somewhat larger neighbor, the Andromeda Galaxy (also called M31). There are quite a few smaller galaxies in the Local Group, mostly quite small dwarf systems. The closest galaxies to the Milky Way are two "big dwarfs" which orbit it as satellites, bound by its gravity. These are called the Large and Small Magellanic Clouds: they cannot be easily seen in the sky from the continental United States but are visible from the southern hemisphere. (They are named after Ferdinand Magellan, the first European explorer to see them on his travels to the southern hemisphere).

The Large Magellanic cloud is about 180,000 light years away, and the Small Magellanic Cloud is about 220,000 light years away. The Andromeda Galaxy, M31, is about 2,300,000 light years distant.
M. D.

Tony Immarco

I am aware of the experimental evidence supporting Einstein's theory explaining the bending of starlight as it traverses the sun's gravitational field. But, conceptually I have great difficulty accepting the idea of the theory, e.g, the warping of space surrounding the sun due to its intense gravitional field. But, my instinctive response always is how can you warp what isn't there? What is being warped?

Was the possibilty of weak interactions between gravitational and photon fields ever considered? Would this alternative approach necessarily violate the relativistic requirement that only particles of zero mass move at "c" velocities.? And. therefore, forbids the existence of such interactions.

Can you shed any light on my dilemma?

Curved space is quite difficult to visualize, it's true! It is perhaps easiest to do it by analogy. Imagine a two-dimensional universe, where beings live in a "Flatland" (there is a classic novel with this title which imagines just such a thing) where there is only left and right, forward and backward, but no up and down -- a universe like a sheet of paper. If that paper is "flat," then for those beings that live there geometry would work in the classical fashion imagined by the Greek philosopher Euclid -- the "Euclidean" geometry we learn in school. But you could also imagine that this "Flatland" was curved -- for example, like the surface of a sphere. In this case, geometry behaves differently, and objects following "straight line" trajectories on this surface will move differently than they would if the surface was flat. The curved space envisioned by Einstein's theory of general relativity is like this -- space itself can be curved, warped and distorted by the presence of large masses, and light, which travels in "straight lines" through space, can actually be bent as it passes by such masses because space itself is curved in the vicinity.

It is one of the great hopes of physics that a "unified theory" can be formulated which joins together the four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. Such a theory would offer a complete explanation for the interactions between gravity and (for example) photons of light. Despite much effort throughout this century, however, we have yet to formulate such a complete theory.
M. D.