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# Time dilation

We'll start thinking about time dilation in special relativity and how a Loedel diagram can help up appreciate symmetry between inertial frames.

## Want to join the conversation?

• I keep seeing two different formulas for time dilation, one puts the initial time OVER the Lorentz factor, and the other MULTIPLIES it. So, which one is right? Or are they both right?
• WAIT! So, if someone is moving at 0.9 or 0.99 percent the speed of light, their time relative to a stationary observer will slow down, but this says that the stationary observer will also slow down from the point of view of the person travelling at almost the speed of light. This makes sense because it agrees with the fact that everything is relative and someone moving can be treated as a stationary person with the universe moving around them. BUT, when the person travelling at almost the speed of light stops, then they must arrive further in the future as they have experienced time dilation, if throughout the journey they have seen the stationary persons clock slow down, how can they then arrive further in the future?
• You are describing what it usually called "The Twin Paradox". This apparent discrepancy between the two observers is because you are only using Special Relativity that doesn't deal with the effects of acceleration.

The observer on the earth is not experiencing acceleration where as the person in a rocket travels away and then had to accelerate to return to earth. This acceleration breaks the equivalences between the two observers frames of reference.
• In the end Sal says that the blue event happens BEFORE the yellow event, in B frame of reference, and that the blue event happens AFTER the yellow event, in A frame of reference. This reasoning looks right, but does it take in account the time it takes for a light ray to be emitted and to come back, in order for the event to be actually seen (by observer in B, in the first case, or by observer in A, in the second case)? Or maybe is this all actually due to this very "waste of time" between emitting a light ray and receiving it back?
• The order of the events will not change because the time it takes for the light to travel back and forth between the events. When the light from the event gets to the observer they can calculate when it was at the event and even with this correction there will still be a discrepancy in the order of events between the two reference frames.
• but when you travel at high speed time flow slower for the traveler but here in A's perspective delta t prime is larger than delta t.
• When someone travels at significant fractions of the speed of light, relative to themselves, time passes normally. Likewise, an observer would also say that time is passing normally. However, the observer would say that relative to himself, the traveler is experiencing time slower.
• if a person is moving in a space time at 20 m/s and another person is moving in same space time at 30 m/s then will the second person will move forward in time ?
does theory of relativity does not act here
• as far as i know, for both of them, time will continue to 'move forward' within their own frames of reference at any speed.
But if they are moving relative to one another, then the time for one person will be moving more slowly when seen from the other persons point of view.

However, this difference will be VERY small, since they have such small relative velocities compared with maximum velocity. (c)
• Why special theory of relativity is only taken in inertial frame not a non inertial frame?
• I am pretty sure that this is why its called special:

the general theory involves accelerating frames whereas the special (special case of...) refers only to non-accelerating frames

ok?
• Correct me if i am wrong.
The faster we move and approach the speed of light the slower the time gets for us viewed from a stationary frame of reference and events occur later for us.

But from our frame of reference the stationary frame is approaching the speed of light so time goes slower for it. What is going on here? I am so confused
(1 vote)
• There's no such thing as "a stationary frame of reference". You can be stationary in a particular frame of reference, or moving in that frame of reference.
If you are zooming along in your spaceship, you are stationary in the ship's frame of reference, but the universe around you is moving. Inside the spaceship, everything is normal for you. Time passes just as you expect. Outside the spaceship, maybe you see your friend zooming past you. If you could watch her clock you would see it tick slowly. Your friend is stationary in her reference frame, and her clock ticks normally for her, but if she could see YOUR clock while you are zooming past her, she would say that your clock is slow. Both of you think the other one's clock is slow. How can it be? Because time is not absolute, it's relative.
• This is more challenging then I thought. Can anyone explain this in a more understandable way?
(1 vote)
• I'll give a quick and dirty explanation first and see if you're interested in a longer one...
Inside the frame of reference of the spaceship (or whatever moving frame), time flows normally for you. You notice nothing different. But for an observer watching you from a stationary frame of reference relative to yours, they see time for you travelling more slowly.
• There's one thing I don't understand: the phenomenon described here is called 'time dilation', but why 'dilation' and not 'speeding up' or 'change'? In the examples given, some event that is judged to happen at a certain time in frame A, happens at a later time in frame B, so doesn't this imply that time is passing faster in B than in A, and not slower? I suppose it all depends on the perspective from which you're looking at it, but that is why I don't understand why we always talk about time dilation, as if the only thing that ever happens is that time slows down, whereas it seems like in some comparison we may say that time actually goes faster in one frame than in another. Or have I misunderstood something?
• The term dilation is used because it is a mathematical term for a transform/function that scales a set of coordinates. Since in special relativity there is a change in distances in space and time this is a mathematical scaling of coordinates.