# Special Relativity and observation

Ok. So I know the following scenario is impossible in practice however I am trying to get my head around the concept of temporal shift.

I am in a spaceship travelling at close to the speed of light. Time within my spaceship (my time) moves slower than that of another person on earth who is walking in the park, yet my temporal dimension moves normally relative to me (I do not perceive myself to moving more slowly within my ship)

What happens when we observe each other? If my ship is made of glass will my observer observe me moving in slow motion? and will I observe him to be walking faster. IE do our temporal dimensions remain distinct?

I am guessing that our temporal dimension at the point observation becomes 'hitched' and in this sense we will observe each other to be doing things within a 'normal' but new temporal dimension that has been established at the point of observation?

(please apply some restraint if my question appears stupid or has an obvious answer, I am not a Physicist)

• Well, you can imagine The Flash(you) looking at a normal guy. You would see him in slow motion. The faster you move, the slower time is. – QuIcKmAtHs Jan 4 '18 at 12:08
• Xcoderx. Is there evidence for the fact that I might see him to be behaving in an impossible manner? If I might observe him to be moving slower, why might my consciousness not equally observe the clock on my ship to be moving slower, given that there is no evidence to apply a temporality to my consciousness? IE how do we know that my perception or my consciousness (observation itself) remains independent of time. Is it not equally true to assume that I might observe him to be walking at normal speed but that my observation is confined to his Past? – Marcus de Brun Jan 4 '18 at 12:23
• See for example my answer at physics.stackexchange.com/q/307573 – WillO Jan 4 '18 at 12:34

First of all, the phrase „I am in a spaceship travelling at close to the speed of light“ strictly speaking does not make sense. Why not? Because you cannot simply travel at close to the speed of light. You can only travel at close to the speed of light relative to something. In your case, the spaceship travels at close to the speed of light relative to the earth. On the other hand, the earth “travels” at close to the speed of light relative to your spaceship. The situation is symmetrical. This is crucial for understanding relativity.

Secondly, the phrase “time within my spaceship moves slower than that of another person on earth” does not make sense either. Why not? Because you are comparing apples and oranges. In order to compare the time passing on your watch with the time a person on earth sees on his watch passing you need to define simultaneous events you both agree upon that they happened at the same time (you need to agree upon the point in time you both need to take a look at your watches in order to compare your readings). This however, is not possible: According to relativity, you will not find agreement: Events which the person on earth sees as happening at the same time you will not consider as happening at the same time (provided that the events occur at different locations).

What you probably mean instead is the following: The time in your spaceship moves faster than the time on a clock located on earth as you observe it. Please note the difference: In the first case the person on earth checks the time on the clock on earth, in the second case you are watching the clock on earth from your spaceship. This makes a crucial difference.

Of course, when you observe the clock from your spaceship, you will receive the time from this clock only at a delay since moving at close to the speed of light your distance to the clock will increase (or decrease) rapidly and since nothing can travel faster than light the info about the time on this clock will not arrive at your spaceship faster than that. After you correct the incoming signals for this delay you will find that the the clock on earth moves slower than your clock.

On the other hand – the situation is symmetrical – the person on the earth will find out the same thing about your clock in the spaceship as he observes it: Also, this clock is moving slower relative to his clock.

How does this fit together? Isn’t this a contradiction? In one case, your clock is going slower, in the other the clock on earth? Again, you are comparing apples and oranges: Since you and the person on earth will never agree on the simultaneity of events when they happen at a distance, you cannot compare the readings of you and the person on earth directly.

But of course, you can return your spaceship, go back to earth and compare the clocks directly: Guess you started from earth, made sure that your clock and the clock of the person on earth were synchronized. After that you travelled to a distant start and now you return, always travelling at close to the speed of light relative to the person on earth. After your return you will find out that on the clock on earth much more time has passed than on your clock. For the person on earth this sounds reasonable: He saw your clock running slower all the time during the trip. But you also saw his clock running slower all the time during the trip. Where did the extra time go that passed on the clock on earth? Where did you miss something?

As I said in order to read and compare the time you need to have a concept of simultaneity. You can define a consistent concept of simultaneity within your spaceship frame while you are travelling at constant speed: So you can compare your local time with the time you see on the clocks on earth. Likewise the person on earth can define for himself in his frame which is at rest relative to the earth a consistent concept of simultaneity. This concept, however, will not fit to your concept. The more far away from each other events occur the more you and the person on earth will disagree about the time difference between these events.

Now, at some point on your journey you need to return: you need to decelerate your spaceship and accelerate it in the opposite direction in order to be able to return to earth. During this period of deceleration and acceleration your concept of simultaneity “changes”. This involves events which happen far away from you (e.g. on earth). That means - during your deceleration period - an event on earth you regarded as simultaneous to your here and now you will not anymore regard as simultaneous a second later but as happening some time, maybe hours, days or even years ago depending on the distance and the strength of your deceleration. That, essentially, is were the “lost” time is going. And this is why the twin paradox at the end can be resolved.

• plumbipes: That is a fantastic answer!! I am going to diegest the content and get back to you!! – Marcus de Brun Jan 4 '18 at 14:11
• Why does everyone always consider this asymmetrical case where 'actual' time dilation can apparently be attributed to the acceleration of one party? Isn't the more relevant case to consider where both parties undergo acceleration in opposite directions, and therefore both must apparently reason in relation to each other that the other has 'actually' slowed, and yet that leads to a contradiction when they return and find that, since both have experienced the acceleration, they must still be in sync with each other? That can't be the proper interpretation of SR. – Steve Jan 7 '18 at 0:10
• In the symmetrical case the same analysis applies: The shift of the simultaneity concept during acceleration will now apply to both of them. At the end, both clocks show the same time after return in this case – plumbipes Jan 7 '18 at 11:55

You are using some terminology with which I am unfamiliar. But the situation as I understand it is the following:

You observe the earth-based person moving slower through time. Likewise, they also observe you moving slower through time. The real brain bender is what happens if you "instantaneously" turn around and travel back to earth and compare clocks?

• You cannot say that. Light from him travels from him to you, which is the same speed which you travel. However, a clock to you moves slower than a clock to him. – QuIcKmAtHs Jan 4 '18 at 12:27
• Please read up on this: en.wikipedia.org/wiki/Time_dilation – QuIcKmAtHs Jan 4 '18 at 12:30
• Thanks xcoder I have gone through the time dilation math (much easier than anything QM). I suppose I am left with a more philosophical question. How does human consciousness react to time dilation? If we assert that my observer will be observed to walk faster then we assert than my consciousness is time dependent that it is aware when time speeds up or slows down. Therefore my consciousness should be aware that time has slowed down on my ship? – Marcus de Brun Jan 4 '18 at 13:09
• @MarcusdeBrun, how would your consciousness be aware that time had slowed down, if indeed it had? To it, everything would continue to happen at a normal pace, because all the biological processes which support consciousness would also have slowed. – Steve Jan 7 '18 at 0:06
• Steve: you are applying the assumption that consciousness is entirely dependent upon biological processes, yet there is no evidence (that I am aware of) to support this assumption Various medical conditions that affect biological process have not been shown to affect one's experience of time. A drop in body temperature slows metabolism and I know of no research that sugessts a resultant effect upon ones experience of time. The mind/body question has not been settelled. I am not aware of a proven link between metabolism and temporal experience. – Marcus de Brun Jan 7 '18 at 0:18

@plumbipes: Your comment makes a lot of sense. However your analysis is focused upon the practicalities or more correctly the impracticalities of the thought experiment itself.

The fundamental issue at hand is the effective relationship between the act of observation and the function consciousness. If it were possible for two observers to observe each other within the context of the temporal dilation that is mandated by SR at a relative velocity; What would either observer actually observe? Of course the experiment is impossible in principle however I suspect the answers offered by SR and QM are equally implausible.

I don't believe that one observer would actually observe another observer 'move' faster or slower as a consequence of time dilation. I suspect that in reality consciousness exherts control over ones experience of time and consciousness controls the temporal experience of a time dilation. This is very important if we realty the the thought experiment to the uncertaintity principle, or the 'observer effect' or the double slit with detection.

If whilst on board my ship, if I could continually observer my observer I might conintually observe him within the temporal dimesion that exists between observer and observed. I would see him walk at a 'normal' pace for as long as I am detecting him or am conscious of him. When I am on board my ship my consciousness maintains a fixed temporal relationship between me and the time within my ship ie whilst on my ship time moves normally as percieved by my consciousness. My point being that temporality and or time dilation is possibly dependent upon consciousness or detection.

If consciouness and or detection between two objects fixes the temporal dimension at a constant, this might suggest that the Universe is in reality a-temporal and consequently superdetermined in that the Universe exists as a complete form (devoid of time) consciousness or detection might represent the point application of temporality between two objects. This would have implications for Bells inequality, QM and the Double slit?

Is it not possible that a piece of the puzzle that is not being considered thouroughly is the relationship between thought or detection and the imposition of temporal dimension.