Questions and Answers 5

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You can scroll through the questions yourself or click below:
  1. Is there a equation for time? How come time does not exist if you go the speed of light and in the center of a black hole?
  2. How does gravitational redshift work?
  3. Is it bright inside a black hole?
  4. If pulsars pass within this solar system,the Milky Way will the two radioactive jets out of the sides burn the world as we know it?
Q Eternity writes, "I was wondering how time works? Is there a equation for time? How come time does not exist if you go the speed of light and in the center of a black hole?"

A You could derive an equation for time from lots of different equations, but all you would have at best is something that describes the relationship of time to other things. For example, you could take v=d/t (velocity equals distance over time) and figure out that t=d/v. This equation tells you how long it takes you to go a certain distance at a certain speed, but it does not define time.

So what is time? My science dictionary defines time as "the dimension of the physical universe which, at a given place, orders the sequence of events." (source: Parker, Sybil P., ed. McGraw-Hill Concise Encyclopedia of Science & Technology. New York: McGraw-Hill, 1994.) That's pretty good, but that definition seems either to leave human beings out or assume that the universe revolves around us. I prefer a definition more like: our perceived order to the sequence of events.

How come time does not exist at the speed of light? As far as I know, time does exist in all space-time (which we think of as just regular space), even at the speed of light. But once you start comparing how time passes for observers at different speeds, time doesn't remain as constant as we think it is. If you suddenly accelerated away from a clock at the speed of light, the light forming the image of the clock in your eyes would be traveling with you. The clock would appear to be frozen in time. But your watch would run normally. Time is not the same for things moving at different speeds.

Why doesn't time exist at the center of a black hole? A black hole singularity is where matter is compressed into zero volume. Matter without volume suggests infinite density, but Nature abhors infinites. At the singularity, something called quantum gravity takes over, rips space and time apart, and leaves only a timeless remnant of space called quantum foam. (For more detailed information, see Thorne, Kip. Black Holes & Time Warps: Einstein's Outrageous Legacy. New York: Norton, 1994.)

If I was a light ray, why would space seem flat? Light rays appear to bend in the warped space surrounding massive objects. That's how a gravity lens works. But light is supposed to take the shortest distance and travel in a straight line. If you were a light ray, however, you would be going in a straight line because in curved space-time, a straight line is curved. So to you, you are going straight, but to a distant observer, your path appears to bend.

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Q Jessica Solomon writes, "Could you please explain how gravitational redshift works?"

A Gravitational redshift happens when light goes from a strong gravitational field to a weaker one. An ordinary object would slow down because it looses energy to gravity. But light cannot slow down--it must travel at light speed. Light's energy loss shows up as a decrease in its frequency, which increases its wavelength. An increase in wavelength means that the light gets shifted toward the red end of the spectrum.

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Q Jeff writes, "If you were to go into a black hole and survive, would it be very bright because all the light would be trapped in?"

A At a certain distance from the event horizon, light rays can go into a circular orbit. This is called the black hole's photosphere. The photosphere is not very stable, and any variation in the gravitational field will cause the orbiting light to either enter the black hole or set it free. Any light that enters the black hole doesn't hang around. Its energy contributes to the black hole singularity's mass.

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Q Ryan Stensrud writes, "If pulsars pass within this solar system,the Milky Way will the two radioactive jets out of the sides burn the world as we know it?"

A That same scenario was on a recent episode of *Sliders.* A pulsar is a neutron star, which has more mass than the sun. If a pulsar came our way, it would probably form a binary star system with the sun way before it got anywhere near the earth. The sun and the pulsar would orbit each other; the Earth would continue to orbit the sun, probably totally unaffected by the pulsar.

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