If we are looking out "into the past" when we view planets/stars away from the solar system, does that mean they are looking at our past when/if viewing us? In which case there is no present? Does that mean we would have to communicate at the speed of light, to communicate in "real time?"
Actually if we travel the speed of light it would only be instantaneous for the person/message traveling...right?
Well ...If an observer anywhere looks at you, they see you at the time when light signals from you reach them, so they see you in the past with respect to your current present - irrespective of whether they are in a distant galaxy or, say, right there talking with you.
There is a bit of an exception to this though, under certain circumstances - because of entanglement, or 'instantaneous' perception (for lack of a better word) as per Bell's inequality or Bell's theorem. Something A can be instantly affected by something else B, anywhere in the Universe.
It's treated in my forthcoming book, due out soon.
Technically, our present only belongs to us in our own frame of reference. The further we look away, the earlier we go back in time (for that entity). We look at the sun what it was ~8 minutes back. We look at the galaxies what these galaxies were billion years ago. The reverse also holds. For example, in its own frame of reference, the sun looks at us what we were ~8 minutes back. The technical details of the following argument require an understanding of special theory of relativity (STR), the concept of light cone, the world point and the world line.
According to the special theory of relativity, the space and time can be defined in terms of a 4-D Minkowski space-time continuum. All the physical acts that are related by the cause and effect principle can be represented within a 'light cone' in the space-time graphical presentation. The boundary of the light cone defines the speed of light that is a fundamental constant and the maximum speed that can be acquired by any physical influence. Please note that only a particle with a zero rest mass can acquire this velocity. The non-zero rest mass particles can never acquire this velocity. The 'world point' of any entity defines it present position in space and time. The world point evolves within the light cone along a 'world line'. Different entities define distinct world lines. Only the intersection of the distinct world lines at any instant defines identical space and time perceptions for the various entities defining these world lines.
Agreed S. Sahijpal,
with the proviso nevertheless that Bell's theorem, along with Alain Aspect's and Nicolas Gisin's and others' experimental confirmation of it, has made the notion of light cones imposing stark insuperable boundaries obsolete - or at the very least not the whole tale.
As a teacher, I feel that it would be better to introduce the concepts of STR in the initial stages. STR (along with GTR; General theory of relativity) continues to be the most profound physical theories of the universe in terms of their validity in literally almost every physical act of universe. The concepts of quantum entanglement are far to complex, and their practical validity or applicability to the physical universe at a grand scale continues to be questionable. Under these circumstances it would not be advisable to introduce these concepts right in the beginning. The inquisitive mind will eventually hit at it but before that let it venture through the elegance of STR (and may be GTR ?). The journey is more important than destination............
The original questions were:
1."If we are looking out "into the past" when we view planets/stars away from the solar system..does that mean they are looking at our past when/if viewing us?"
Yes.
2. In which case...there is no present?!
No. That is not what that means. It means that they cannot see our present and we can't see theirs. However, the present exists everywhere. That "past" that we're viewing is they light that traveled here over time.
3. Does that mean we would have to communicate at the speed of light, to communicate in "real time?"
Depending on the distance, it would require faster than the speed of light velocities to communicate in real time.
"My head hurts."
Aspirin...
Re.: "there is no present"...
Look at your open hand. Clench your fist. When you can see (and feel) that your hand is closed, open it. Repeat - faster.
This illustrates an effectively real-time sensory-control mechanism...
While there is propagation delay in the transmission of nerve signals and light, it's negligible in comparison to the time it takes to contract muscles...
The speed of light is also a finite quantity, so if the source and the observer are 6 lac kms apart, there would be a time lag of 2 secs for communication. I think there can never be absolute 'real time' communication, only the lag of time can be vanishingly small.
Also if we talk about the arrow of time, then as far as I know only the thermodynamic concept of the increase of entropy can explain the passing of time or it is the only thing that can distinguish between 'past and 'present'.
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