Time dilation Guide, Meaning , Facts, Information and Description
Time dilation, according to Albert Einstein's special theory of relativity, is the slowing-down of the passage of time as experienced by people or objects moving in relation to an observer. Gravitational time dilation is the slowing down of the passage of time anywhere in the gravitational field.
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2 Gravitational time dilation 3 Time dilation and space flight 4 See also |
When one accelerates towards the speed of light, time slows down with respect to the rest of the Universe. That is, a stationary observer would see the traveling objects slow down their activity. For them, time passes slower. The effect is of course symmetrical: an observer fixed on the "moving" object sees the "stationary observer" slowing down. See twin paradox.
It is important to note that this effect is extremely small at ordinary speeds, and can be safely ignored for all ordinary situations. It is only when an object approaches speeds on the order of 30,000 km/s (still 1/10 of the speed of light), that it becomes important.
The formula for determining time dilation involves the Lorentz factor and is:
Velocity time dilation
Where T0 is the passage of time measured by a stationary observer and T1 is this passage of time measured by an observer travelling at velocity v.
| v (%c) | length due to length contraction | relativistic Mass | time due to time dilation |
|---|---|---|---|
| 0 | 1.000 | 1.000 | 1.000 |
| 10 | 0.995 | 1.005 | 0.995 |
| 50 | 0.867 | 1.155 | 0.867 |
| 90 | 0.436 | 2.294 | 0.436 |
| 99 | 0.141 | 7.089 | 0.141 |
| 99.9 | 0.045 | 22.366 | 0.045 |
| 99.999 | 0.00448 | 224.658 | 0.00448 |
Notice how dramatically the time dilation effect increases as v approaches c. Taken to the extreme, an observer travelling at the speed of light (which, according to special relativity, is impossible for any object with a non-zero rest mass) would be frozen with respect to the outside world. Massless particles (which are forced by relativity to travel at the speed of light) include photons and gluons. Recently it was determined that neutrinos have a mass, unlike previously thought.
Gravitational time dilation is a verified effect of general relativity,
and has been experimentally measured using atomic clocks on airplanes.
The clocks that traveled aboard the airplanes were slightly fast with respect
to clocks on the ground.
The effect is significant enough that the Global Positioning System
needs to correct for its effect on clocks aboard
artificial satellites, providing a further experimental
confirmation of the effect.
An extreme example of gravitational time dilation occurs near a black hole. A clock falling towards the event horizon would appear (to observers far away) to slow down to a halt as it approached the horizon. A small and sturdy enough clock could conceivably cross the horizon without suffering adverse effects at the horizon, but to far away observers it would "freeze" and be flattened out on the horizon.
Time dilation around a black hole may be described using the following equation:
The following chart details the effects of time dilation caused by a black hole (with a circumference of its event horizon of 10,000 km) for an entity orbiting that black hole, relative to an outside observer. For each day that passes for the stalwart black hole orbiters, we can determine the amount of time that would pass for an outside observer.
Gravitational time dilation
Where is time for the object undergoing dilation, is time for an observer outside the system, is the circumference of the event horizon, and is the circumference of the object's orbit about the black hole.
| Circumference of orbit | Time experienced by outside observer per orbiter day |
|---|---|
| 20,000 km | 1.41 days |
| 15,000 km | 1.73 days |
| 12,000 km | 2.44 days |
| 11,000 km | 3.32 days |
| 10,500 km | 4.50 days |
| 10,250 km | 6.40 days |
| 10,050 km | 14.18 days |
| 10,025 km | 20.02 days |
| 10,005 km | 44.73 days |
| 10,000.75 km | 115.47 days |
| 10,000.50 km | 141.42 days |
| 10,000.25 km | 200.00 days |
| 10,000.125 km | 282.84 days |
| 10,000.050 km | 447.21 days |
| 10,000.001 km | 3162.28 days |
Time dilation could make it possible to travel "into the future": if we could accelerate a starship enough, one year aboard the ship might correspond to ten years outside. Indeed, a constant 1g acceleration would permit humans to circumnavigate the known Universe (with a radius of some 15 billion light years) in under a subjective lifetime. A more likely use of this effect would be to enable humans to travel to nearby stars without spending their entire lives aboard the ship. However, any such use of this effect would require an entirely new method of propulsion. A relativistically accelerated object also gains mass, so further acceleration would require increased amounts of fuel. A further problem with relativistic travel is that the interstellar medium would turn into a stream of cosmic rays that would destroy the ship unless stark radiation protection measures were taken.
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