The Andromeda galaxy is so far away that it takes the light from it, as measured with our Earth-bound clocks, over two million years to reach us.
More accurately, they have no "rest mass".
Why does time dilation occur
Now of course an electron cannot go much faster than this, but it can have a lot more energy. The satellites' handlers managed to raise and circularize the pair's orbits over time. So, when we move, at whatever speed, time slows down relative to a stationary observer. But in many modern devices, electrons are accelerated to higher speeds than this. Gravitational fields slow the passage of time; the closer a clock is to a massive object, the more slowly its hands will move, as seen by an outside observer. This steady flow is so reliable that we fit our lives around it. Pictures taken with powerful telescopes show it to be composed of millions and millions of stars, many of which are just like our own Sun. These annoyances apart, it seems that time flows along smoothly; never bending, never changing its rate: always "on time". When the plane returned, the clock that took the plane ride was slower by exactly the amount Einstein's equations predicted. For these scientists relativity is a routine part of their job, and indeed would be difficult if not impossible without it. The two research teams measured how fast the clocks on Galileo 5 and 6 were ticking at various points in their elliptical orbits — at both the near-Earth and more-distant positions — over the span of about three years.
But note that, for example, the occupants of a rocket travelling at very high speeds would still experience time passing normally. Take a look at Fig 4 and Fig 5 below.
Under all possible proper times between two events, the proper time of the unaccelerated clock is maximal, which is the solution to the twin paradox.
It is only when the clock is in motion with respect to the other clock that the time dilation occurs. See the original paper by J.
Gravitational time dilation
Keep in mind that when the clock returned, it had recorded less time than the ground clock. It is only when the clock is in motion with respect to the other clock that the time dilation occurs. See the original paper by J. For the sake of completeness I have included the relevant equation below but you can skip over it and move on to the graph below it if you prefer. If you are confused, look at Fig 4 and you'll see that both the sent beam and received beam occur under the sun, thus the clock is not moving. Not only that but all the experiments have indicated that light is indeed mass-less. Even if we go at high enough speeds to bring about a large slowing down of local time we wouldn't notice because our own body clocks would also be running just as slowly. So the question is how can it do this, and why doesn't it need infinite energy to go that fast? For example, it still takes light around 1.
Time dilation isn't just theoretical; global-positioning satellites have to take it into effect to provide accurate readings to users here on Earth.
Because space and time are part of the same entity it's impossible to move in space without moving in time.
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