How does a clock go slower if it is travelling at high speed?
Dec 24
MajorSparky asked:
Einstein says that a clock travelling at high speed (near the speed of light) will go slower than a `stationary’ clock. It has been demonstrated that a clock returning to earth from an orbiting satellite shows less time than a stationary, earth bound, clock. How come? Velocity is relative. I.e. the earth bound clock is moving at high speed w.r.t. the satellite bound clock, such that the latter could be considered stationary, and the earth bound clock to be moving at high speed. So why doesn’t the earth bound clock go slower?
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Einstein says that a clock travelling at high speed (near the speed of light) will go slower than a `stationary’ clock. It has been demonstrated that a clock returning to earth from an orbiting satellite shows less time than a stationary, earth bound, clock. How come? Velocity is relative. I.e. the earth bound clock is moving at high speed w.r.t. the satellite bound clock, such that the latter could be considered stationary, and the earth bound clock to be moving at high speed. So why doesn’t the earth bound clock go slower?
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Have a look for ‘time dilation’ on wikipedia.
I think that law has to do with time bending in on itself. The clock isn’t really going slower, it appears to go slower, but it is actually going faster (at the speed of light too). After the speed of light stops it seems as though the clock folds in on itself, and catches up to itself.
But that actually has to do with your perception, not really anything to do with speed. The speed of light just distorts things so it appears that it is moving that way to us. If our eyes (and our physical body) could move at the speed of light the clock would actually look normal.
Clocks only go slower in certain reference frames. If a person is moving at high speed relative to you then it appears to you as though the clock of the person is running slow. However, to the person it looks like the clock is running normally, and that your clock is running slow. This is a direct consequence of the fact that the speed of light is the same in all inertial reference frames.
Comparing clocks on a satelite to see if they are slow does not actually work. Think about it this way. If you left on a rocket from Earth traveling very fast everyone on Earth would observe your clocks going very slowly. However, when you turned around your clocks would be going much faster, and by the time you reached Earth again you would have aged the same as everyone on Earth. You only appear to go slower if you are traveling at a constant velocity (that means same speed AND same direction). If you accelerate, by turning around, inertial reference frames don’t apply.
Think about it another way. If you were flying away from a clock traveling at the speed of light, the photons from the clock would never reach your eyes, so it would appear as though time stopped. But to a person on the clock looking at you the photons from your body never reach their eyes, so it appears to them as though you have stopped. However, when you both look at yourselves time appears to be passing normally.
The theory of relativity only works under two assumptions:
The speed of light is the same in all inertial reference frames, and it is constant.
The laws of physics are of the same form in all inertial reference frames.
One other consequence of relativity is that lengths also appear shorter. That is, the closer you get to the speed of light the more compressed everything is in the axis of motion.
If you want more information try looking up “time dilation” and “length contraction” with special relativity.
You are correct as far as what special relativity says. If both clocks are moving with uniform motion (i.e. no accelerations) there is no actual difference in time. That’s not to say a “stationary” observe looking at the “moving” clock does not see it running slow – he does. But if one could bring both clokcs back together, one would find they are reading teh same time.
The operative word in the last sentence is “if”. Both observers have to be moving with constant velocity. Velocity is a vector – it has magnitude and direction. So both the magnitude (speed) and direction must remain unchanged for a body to be in uniform motion. If that is so, you can’t bring the two clocks we just talked about together more than once. They either start from the same place and separate due to relative motion, or they pass each other at some point in time. If you want to bring them together again, one of the clocks has to accelerate (change direction and speed). Once a clock is accelerated, then there is an external measure that allows an observer to decide if he is at rest relative to that clock (or that the observer with the clock is in motion with respect to another observer). In that case, the stationary observe does not experience time dilation, only the moving observer does. So when the clocks are brought together again, teh moving clock will have “lost time” due to running slower than the stationary clock.
Now clocks orbiting the earth are in constant acceleration. Theyare changing direction at every instant along the orbit – the linear velocity is tangential to the orbit so as the clock moves, the linear velocity vector points in a different direction. The acceleration is v^2/r where v is teh linear velocity. Since the clock is accelerating, it will experience time dilation relative to the clock on earth.
Your thinking of the twin paradox in special relativity. The problem with the twin paradox, is that it doesn’t work with special relativity alone, you need general relativity to make it work properly.
In the twin paradox, one twin stays on earth and the other flys to a distant star at a high speed, turns around, and comes back to earth. The twin on earth sees the twin traveling at a high speed as having a slower clock, so when that twin comes back to earth, he’s younger than the twin on earth. Special relativity ignores the perspective of the twin traveling in the rocket ship because that twin is not in an inertial frame. When he turns around, there is tremendous acceleration, from close to the speed of light in one direction to close to the speed of light in the other. Special relativity can’t deal with acceleration.
In general relativity, you can look at the perspective of the twin in the rocket ship and find that when he turns around he sees the twin’s clock on earth as moving very very fast, so when he gets back to earth, they end up agreeing that the twin on the ship is younger.
It’s similar for satellites. They accelerate around the earth and experience less gravity, this affects their perspective differently and it cannot be properly accounted for with special relativity. You need general relativity. In general relativity, different reference frames aren’t equivalent because some are more or less inertial. Frames that have the same inertia are equivalent. Others are not.
Relativity can be difficult to grasp.
The most important thing you need to remember is the REFERENCE FRAME. It will affect all your measurements.
For example, in the case you specify, the clock will run slowly according to someone on earth looking through the satellite’s window (I know, just ignore the logistics of this!). Also, according to a person on the satellite, a clock on earth will go slower than his. Of course, anyone looking at a clock that is stationary with respect to themselves, it will go at exactly the correct speed.
Think of this: suppose someone on earth and someone moving at 0.99c (99% the speed of light) timed some experiment that should take exactly the same amount of time, regardless of where it is performed. For example, some person takes exactly 30 seconds to tie his shoes. If he times himself on earth and on the satellite, he will get EXACTLY the same result.
The rate at which time progresses never literally changes. It will appear to change if an EVENT is moving relative to it’s OBSERVER.
Time dilation is a result of Einstein’s concepts of relativity. Although twins may age differently because one has approached the speed of light, neither is aware of the difference in the passage of time (until they compare their identical ‘atomic’ clocks).
Velocity is relative but acceleration is not. For the clocks to return to the same place to be compared, one must accelerate. The change of reference frames caused by the acceleration is what causes one clock to be behind the other when they come back together.
The clock is not physically going slower. It’s just the notion of time is slower.
Don’t know how it happens. But it does happen.
There are experiments with high speed planes and synchronised cesium clocks.
The differences are measurable but tiny at plane speeds.
The fastest thing when Einstein worked it out was a train!!!
the simplist way i can put it is this
when an object moves, it moves through space and time. if it is moving at 1 metre per second then it is moving through time at an equivilant of the speed of light minus 1 metre per second. you are converting your time movement into spatial movement. so, if the object moves at the speed of light through spatial dimensions, then it’s time movement is the equivalant of the speed of light minus the speed of light which is zero, so therefore the objects obsevered time has stopped. from the objects point of view it has crossed the entire universe instantaniously but the universe has aged billions of years instantly as well.