See Physicist & SF Author Lesley L. Smith's website

 NASA's Astronomy picture of the day

Every day a fun new picture!

Featured Topics

Special Relativity:

The special theory of relativity, often just called special relativity, is the branch of physics that applies to very fast things. It is one of Albert Einstein's main claims to fame. Special relativity relates space and time and is based on two primary postulates: the laws of physics are the same in all reference frames without acceleration, and the speed of light in the vacuum of space is the same for everyone. This speed, c, is specifically 186,000 miles per second.

The latter postulate also referred to as the principle of invariant light speed, means the speed of light is the greatest possible speed. This is not a technological limit; it's a physics limit. In other words, spaceships or any other technology cannot exceed the speed of light. Thus, science fictional concepts such as warp drive, hyperdrive, jump drive and others violate this rule of physics. Thus, c is a universal speed limit. Invariant light speed preserves causality; this means causes must precede effects. If faster-than-light motion was allowed, we could see effects preceding causes--which would be truly bizarre. It's worth noting that invariant light speed is a bedrock of modern astronomy; astronomers rely on it to measure the immense distances across our universe.

Possibly Einstein's most famous theory, the equivalence of mass and energy, expressed as E=mc² is a consequence of special relativity. Nuclear fusion, such as that that powers our sun, depends on mass-energy equivalence.

The postulates of special relativity also have some more unusual consequences. Time dilation: When two people are in reference frames with relative speeds, they can observe different time lapses between the same two events. Sometimes this is referred to as moving clocks run slowly. In other words, speed affects time. This effect is only seen, however, at very high speeds approaching the speed of light. Note this does mean the rate at which time passes for you depends on or is relative to, your frame of reference. Sometimes this is referred to as the relativity of simultaneity--depending on your frame of reference, you might not see two events occur simultaneously. You might think this effect is irrelevant to your everyday life, but you would be wrong. GPS satellites in orbit must take special relativity into account to keep the correct time with respect to Earth.

Length contraction: When two people are in reference frames with relative speeds, they can observe different lengths for the same object. This can be referred to as moving objects contract or shrink. In other words, speed affects length. Again, this effect is seen at very high speeds approaching the speed of light. Again, note this does mean the lengths you observe depend on or are relative to, your frame of reference.

Spacetime: The three dimensions of space and the one dimension of time are actually parts of one four-dimensional spacetime continuum. In other words, space and time are not distinct quantities but elements of the same thing.

Relativistic mass: An object in a reference frame with a relative speed can have a mass greater than its mass at rest. This can be referred to as moving masses increase at high speeds. Thus, speed also affects mass. Mass depends on, or is relative to, your frame of reference. This effect is only seen at very high speeds.

Sometimes special relativity is described as 'everything is relative.' This is a little misleading because it's vague. Relative to what? As stated above, the values of time intervals, lengths and masses are relative to your frame of reference. Thus a more accurate description would be that there are no absolute positions in time and space.

Special relativity only applies to reference frames without acceleration. When we add acceleration, we need to add General Relativity, but that's a discussion for another day...