Time dilation and interstellar travel?

Question

I'm having trouble understanding time dilation.

Suppose that a space ship leaves earth and travels at (nearly) the speed of light to a star 10 light years away. An observed on the earth would see the space ship complete the trip in ten years and observe that the ship's clock was running slow - when the ship reached the star it's clock would thus show less than ten years and the ship would appear to have travelled faster than the speed of light - surely this is not correct.

Also should not the pilot see clocks on the earth as running slow?

Answer 1

Let me actually answer the questions.



question 1: would the voyagers think they traveled faster than the speed of light.



answer 1: well yes, sort of, but only from a naive point of view. When they boosted up to speed, the distance to the star contracted, so the trip took much less time (without traveling at superluminal speed). When they boost back down, they might say, "Wow, we went 10 light years (in regular space) in less than 10 years ship time. We went faster than the speed of light." The fallacy, though, is that you are dividing regular space by ship time. You have to pick a reference frame and stick with it. There is no reference frame in which the ship went faster than c.



Question 2: Should not the pilot see earth clocks as slow?



Answer 2: Yes, he will! And earth sees his clocks as slow. This "paradox" occurs because the ship and the earth don't agree on simultanaity, so they can't agree on what a fixed interval of time is. But once the ship gets to its destination and goes back into the earth's reference frame, then the ship and earth can compare notes again. At this point, they will both agree that the ship clock ran slower than the earth. This point is sometimes illustrated by having the ship send home a letter every day on the way to earth and on the way back. The frequency of the letters gets shifted when the ship accelerates or decelerates. They get sent once per week, but they get delivered to earth (because they travel at only light speed) much less frequently (on the way out) or more frequently (on the way back). You can use the letters to get straight in your head whose clocks are doing what and reconcile them in the end. Th

Answer 2

The pilot on the space ship should also see the clocks on Earth running slow. But only the clocks on Earth are correct. The key lies in how clock checking is done - the concept of synchronization. True synchronization can ba made only when the two bodies do not move relative to each other, which is not the case when the ship is moving. Special relativity (which tells you about the slowing of the clocks) is only valid when the relative speed is constant (the systems are called inertial). But it tells nothing about what happens when the ship is accelerating to close to the speed of light, or slowing down at destination. During those times, the ship is not an "inertial system", and nothing can be said about the ship's clocks (within the special relativity, that is). The Earth clock, however, is always in an inertial system. So when the ship arrives at destination, it will see that its clocks are late.



Rigorously speaking, the Earth is not an inertial system, being accelerated towards the Sun, and the Sun is accelerated to the center of the Galaxy. But these accelerations are not large enough to affect the previous reasoning. To find out what happens in an accelerated (non-inertial) system, you need to apply the general relativity theory



There are many paradoxes generated by the special relativity theory, but they can all be solved by synchronization or non-inertial systems

Answer 3

The ship, from the point of view of those onboard, would seem to have reached its destination, 10 light years away, in much less than 10 years, so they would consider themselves to have covered the distance in less subjective time than the proper time that light takes to cover the same distance. But when they disembarked they would discover that more than 10 years of time had passed on the destination planet, on the earth and throughout their observable universe. Yes, the pilot would see clocks running slow on earth, based on observations of light coming from the earth- but remember that the rate at which the light reaches you from earth will be affected by your velocity as well- the delay between the photons leaving earth and reaching you will increase as your distance increases.

By utilising time dilation, you can cover distances at unlimited APPARENT velocity, from your own point of view, but relative to the passage of time in the wider universe, you cannot exceed c. On a related subject, I suggest you read the wikipedia article 'Twin Paradox'

Answer 4

The clock on the spaceship would appear to observers on earth to be running slow, you are correct, however, to anyone on the ship the clock would appear to be running perfectly normal. The ship travelling at near light speed would arrive at the star in slightly over 10 years of earth time (it is travelling slower than the speed of light) but the ship's clock would show significantly less than 10 years (at light speed it would show 0 time elapsed). To an observer on the ship the clocks on earth would appear to be running fast. (remember the twin paradox? the twin on earth would have aged much more than the space twin).

Answer 5

Ah, Point of View!

No, the pilot of your near-lightspeed ship would see the clocks on Earth running very fast as he nears C (the speed of light in a vacuum).

So while the Earth would age ten years while the pilot was zooming off, he would age less (how much less would depend on how fast he was going).

Answer 6

The flow of time in curved space (gravitational fields) is diffirent then the flow of time in open space.

In Albert Einstein's theories of relativity time dilation is manifested in two circumstances:



In special relativity, clocks that are moving with respect to an inertial system of observation (the putatively stationary observer) are found to be running slower. This effect is described precisely by the Lorentz transformations.

In general relativity, clocks at lower potentials in a gravitational field — such as in close proximity to a planet — are found to be running slower.Gravitational time dilation

Answer 7

Time dilation is the phenomenon whereby an observer finds that another's clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. This is often taken to mean that time has "slowed down" for the other clock, but that is only true in the context of the observer's frame of reference. Locally, time is always passing at the same rate. The time dilation phenomenon applies to any process that manifests change over time.



In special relativity, the time dilation effect is reciprocal: as observed from the point of view of any two clocks which are in motion with respect to each other, it will be the other party's clocks that is time dilated. (This presumes that the relative motion of both parties is uniform; that is, they do not accelerate with respect to one another during the course of the observations.)



In contrast, gravitational time dilation (as treated in General Relativity) is not reciprocal: an observer at the top of a tower will observe that clocks at ground level tick slower, and observers on the ground will agree. Thus gravitational time dilation is agreed upon by all stationary observers, independent of their altitude.



Time dilation has been tested a number of times. The routine work carried on in particle accelerators since the 1950s, such as those at CERN, is a continuously running test of the time dilation of special relativity.



Time dilation would make it possible for passengers in a fast moving vehicle to travel further into the future while aging very little, in that their great speed retards the rate of passage of onboard time. That is, the ship's clock (and according to relativity, any human travelling with it) shows less elapsed time than stationary clocks. For sufficiently high speeds the effect is dramatic. For example, one year of travel might correspond to ten years at home. Indeed, a constant 1 g acceleration would permit humans to circumnavigate the known universe (with a radius of some 13.7 billion light years) in one human lifetime. The space-travellers could return to earth billions of years in the future (provided the Universe hadn't collapsed and our solar system was still around, of course). A scenario based on this idea was presented in the novel Planet of the Apes by Pierre Boulle.



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 application of time dilation would require the use of some new, advanced method of propulsion. A further problem with relativistic travel is that at such velocities dispersed particles in the rarefied interstellar medium would turn into a stream of high-energy cosmic rays that would destroy the ship unless extraordinary radiation protection measures were taken. Strong electromagnetic fields that could ionize and deflect any interstellar matter has been suggested as one way to avoid these potentially disastrous consequences.



Current space flight technology has fundamental theoretical limits based on the practical problem that an increasing amount of energy is required for propulsion as a craft approaches the speed of light. The likelihood of collision with small space debris and other particulate material is another practical limitation. At the velocities presently attained, however, time dilation is not a factor in space travel. Travel to regions of spacetime where gravitational time dilation is taking place, such as within the gravitational field of a black hole but outside the event horizon (perhaps on a hyperbolic trajectory exiting the field), could also yield results consistent with present theory. This enables 'time travel' to the future, but not backwards in time as in some pop culture science fiction.



Dr. H

Answer 8

Pilot sees clocks on earth running fast. The ships clock shows ships time, relative to itself. If it took one day, it would show one day.