The speed of light in vacuum sets the local limit for signals in relativity. This limit concerns spacetime and cause and effect, rather than only the properties of visible light.
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What the number means
The exact value is 299,792,458 metres per second. The Short rounds this to about 300,000 kilometres per second. Physicists use the symbol c for the value.
Vacuum means empty space without a material such as water or glass. Light travels more slowly through those materials. The universal local limit concerns c, rather than every material-dependent light speed.
Calling it the speed of cause and effect describes the fastest local propagation of a controllable influence. A signal carries information between events. This meaning differs from a pattern that merely appears to move across a surface.
Why light reaches that speed
Light consists of electromagnetic radiation. Its photons have no rest mass and propagate at c in vacuum. Gravitational waves also propagate at c in general relativity.
The agreement does not make gravitational waves a kind of light. It shows that different physical phenomena share the spacetime limit.
Why more thrust does not reach c
A spacecraft has nonzero rest mass. Its total energy increases according to a factor called gamma as its speed approaches c. Gamma equals one divided by the square root of one minus the squared speed fraction.
The speed fraction is the spacecraft's speed divided by c. Near one, the denominator becomes very small. Total energy then increases rapidly for a very small additional speed.
| Speed | Total energy relative to rest energy |
|---|---|
| 99 percent of c | About 7.09 times |
| 99.99 percent of c | About 70.71 times |
The Short rounds those factors to seven and seventy-one. They describe total energy, which includes rest energy. Kinetic energy is the amount above that rest value.
The calculation does not say additional thrust has no effect. Energy and momentum still increase. It says no finite energy increase takes a massive spacecraft all the way to c.
Why faster messaging threatens causal order
Relativity does not give all distant events one observer-independent time order. Observers moving relative to one another can order spacelike-separated events differently. A light signal cannot connect those events within their time separation.
Ordinary causal processes remain consistent because signals stay within the permitted limit. A cause and its directly connected effect retain their causal order.
If observers could exchange controllable faster-than-light signals, some frames would put arrival before transmission. Suitable return signalling could then place a reply before the original message. That is the issue behind the Short's answer-before-question example.
The argument concerns controllable causal signals under relativity's usual assumptions. It does not imply that every rapid-looking motion sends information into the past.
What this means
Light makes the local signal limit visible, but does not create it by itself. The speed limit, energy relationship, and causal structure belong to the same relativistic account.
FAQ
Does light always travel at c?
It travels at c in vacuum. Its propagation through a material can be slower.
Does infinite energy mean a very large practical fuel supply?
No. It means the mathematical requirement has no finite value at c for nonzero rest mass.
Is every apparent faster motion a faster signal?
No. Moving patterns and controllable signals require separate analysis.
Sources
- NIST: definition and history of the metre
- Caltech: special relativity
- Michigan State University: muons and relativity
- LIGO and partners: gravitational waves and gamma rays
- New York University: special relativity
- UCLA: cosmology questions
- NIST: quantum communication and its limits
- CERN: OPERA timing correction
- IceCube: Cherenkov detection
- CERN: LHC numerical examples
- NASA: Voyager distances and light travel time