# Time slows down when you approach the speed of light?

Doc Al
Mentor
See, now that is why I can't understand the general scientific acceptance of this area.

If the destination is seven light years away from Earth, i.e. it takes light severn years to reach us from there, the speed of light is constant etc. then we can calculate the physical distance between the Earth and this destination.

If a spaceship is travelling to this destination it still has to travel that same physical distance irrespective of its speed. How can it do anything other? If the distance is the same and the speed of light is constant, then how can the time taken be anything other than seven years...?
You miss the point that the measured time and distance for the voyage of the spaceship depends on what frame is doing the measuring. In the Earth frame, of course the distance is 7 light years and the travel time is a bit greater than 7 years. But in the spaceship frame, the time and the distance are quite different.

pervect
Staff Emeritus
See, now that is why I can't understand the general scientific acceptance of this area.

If the destination is seven light years away from Earth, i.e. it takes light severn years to reach us from there, the speed of light is constant etc. then we can calculate the physical distance between the Earth and this destination.

If a spaceship is travelling to this destination it still has to travel that same physical distance irrespective of its speed. How can it do anything other? If the distance is the same and the speed of light is constant, then how can the time taken be anything other than seven years...?
The problem you have is that you are incorrectly assuming that distance is "physical" and therfore the same for all observers.

The point of relativity is that distance depends on the observer. There is a quantity which involves both space and time defined between two events which is independent of the observer doing the measuring, but this quantity is not distance, but rather the Lorentz interval.

Philosophically, this means that distance is not very fundamental (because it depends on who measures it). The Loretnz interval is a more fundamental and physically significant quantity, just because it is independent of the observer.

Of course distance is physical, the distance between the two points is a physical constant irrespective of who observes the event or what the observers measure the distance to be.

The spaceship has to travel that same distance between the two points that light travels. If it it travelling at light speed or just below then it can only take seven years or a little more as observed from the Earth. What the guy in the spaceship observers may be different but it is still seven years irrespective of what he interprets the time to be - don't go with this time slows as you approach light speed theory I'm afraid. It's just a speed, why should biological processes slow just because you increase the speed at which the body is travelling let along time itself - what is observed is just that, an observation.

Of course distance is physical, the distance between the two points is a physical constant irrespective of who observes the event or what the observers measure the distance to be.
"Observes" in this case means "measures". Distance is not invariant, unless with "distance" you mean "distance in the ref. frame in which the two points are stationary".

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Distance in the ref. frame - yes, the actual, real distance, which doesn't change.

Forget all these different frames of reference since they produce incorrect 'observed' measurements.

Doc Al
Mentor
Distance in the ref. frame - yes, the actual, real distance, which doesn't change.

Forget all these different frames of reference since they produce incorrect 'observed' measurements.
Enough already. You're behind the times by over a century.