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Space Physics:
Gravity in Space

Gravity in Space

Is there gravity in space?

There is gravity in space... only it's very, very small. It's called "microgravity" because it is so small. Gravity affects everything, everywhere, although in space it is usually such a small effect that it can be ignored.

Dr. Louis Barbier
(November 2002)


The Fabric of Space

Assume that the presence of matter curves space, and that space has some sort of a structure to it. If the Earth, for example, did not experience a push from the fabric of space, which would create a tension in the fabric of space, would it fall in space? This would be applicable to all objects in the Universe. If there was a push or tension in the fabric of space, would this mean that the Universe is bounded, and there is some sort of force that is keeping space in tension?

Although it is true that matter does curve spacetime, that is really only an analogy that should not be taken too literally. While imagining the Earth as a bowling ball on a rubber sheet does provide a good visual representation of how objects and light move with respect to a more massive body, it is important to realize that this is a two-dimensional view, and that the "tension" on that sheet is not pertinent to the Universe.

The question of whether or not the Universe is bounded is somewhat different. It refers to the relationship between the amount of matter in the Universe and the gravitational pull that all objects exert on one another as they move away from one another.

Lauren Scott
(March 2003)


Gravitons and Quantum Gravity

Suppose there were only one particle in space. Would this particle still emit or release gravitons, or is the presence of a second particle essential for this?

I should preface my answer by saying that we have no quantum theory of gravity yet, so any comments about gravitons should be taken with some caution. And by quantum gravity I mean a theory that incorporates both the large-scale description of the universe (including the force of gravity) and elementary particles. (i.e. unifies general relativity and quantum theory)

The graviton - if it exists (and there is no direct evidence to date for it) - represents the quantization of the gravitational field. This is the particle that carries the Newtonian gravitational force.

Every form of matter (and energy too!) emits gravitons. So the answer to your question is a simple: Yes, a lone particle would emit gravitons.

Dr. Louis Barbier
(June 2003)


Is Light Affected By Gravity?

Is light affected by gravity? If so, how can the speed of light be constant? Wouldn't the light coming off of the Sun be slower than the light we make here? If not, why doesn't light escape a black hole?

Yes, light is affected by gravity, but not in its speed. General Relativity (our best guess as to how the Universe works) gives two effects of gravity on light. It can bend light (which includes effects such as gravitational lensing), and it can change the energy of light. But it changes the energy by shifting the frequency of the light (gravitational redshift) not by changing light speed. Gravity bends light by warping space so that what the light beam sees as "straight" is not straight to an outside observer. The speed of light is still constant.

Dr. Eric Christian

Gravity and the Speed of Light

If the Sun were to suddenly lose half of its mass, wouldn't the Earth instantly feel a change in gravity? Doesn't that mean that gravitons move faster than the speed of light?

Sorry, this doesn't work. Gravitational potential and the gravitons that carry it travel at the speed of light. So, even if you could instananeously change the mass of the Sun, there would be an 8 minute delay before it would be noticed at the Earth. It's all in the theory of General Relativity, which is our best understanding of the way gravity works.

Dr. Eric Christian
(April 2002)


Moving Around in Space

If you are free-floating in the middle of space with no gravitational forces nearby, is it possible for you to turn around by moving any body parts (like hitting yourself in the shoulder or swinging your arms)?

Remember that in order to move your body, you must be acted on by an external force (a body at rest remains at rest, according to Newton's Law). If you are stationary in space with nothing around you to push off on, you cannot move yourself by pushing on yourself. But you can twist your body around its center of gravity - you just can't move your center of gravity. On Earth, we can walk forward because we push on the Earth and it pushes back on us.

So the answer to your question is yes, you can turn yourself around, but you can't move from where you are.

Dr. Louis Barbier
(February 2003)


Detecting Motion in Space

Can motion be detected in space? If you're in a box in space and it's spinning, would you be able to tell that it's moving? What if the box moved in a straight line?

This question is at the heart of Einstein's Theory of Relativity. Einstein must have asked himself similar questions.

First, it is impossible for an observer in a closed box (nothing coming in from outside) to determine the motion of the box if it is moving at constant speed in a straight line. The laws of physics are the same in all such boxes moving at any speed. This constitutes the foundation of Einstein's Special Theory of Relativity. Consequences of this include the constancy of the speed of light.

A spinning box is more difficult. The speed is not constant and there are consequences of acceleration.

If the closed box is not spinning but is simply accelerating uniformly (the speed is changing at a constant rate in a straight line), an observer in the box would be unable to determine whether the box is accelerating or whether it is subject to a uniform gravitation (such as on the surface of the Earth, neglecting the rotation of the Earth). This is the basis of Einstein's General Theory of Relativity.

A spinning box is more complicated. It is accelerating, but the acceleration is different at different places in the box. The result is not simple and I am unable to answer it here. I believe that the spinning motion could be detected by experiments inside the box.

You might want to read the PBS Nova web page.

Dr. Randy Jokipii
(February 2003)