"Now Besso has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion"
Since there exist in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.
Why is motion in spacetime impossible? It has to do with the definitions of space and time and the equation of velocity v = dx/dt. What the equation is saying is that, if an object moves over any distance d x, there is an elapsed time d t. Since time is defined in physics as a parameter for denoting change (evolution), the equation for velocity along the time axis must be given as v = dt/dt which is self-referential. The self-reference comes from having to divide dt by itself. dt/dt always equals 1 because the units cancel out. This is of course meaningless as far as velocity is concerned.
To emphasize, it is logically impossible for the t coordinate of an object to change because such a change is self-referential. Et voilà! It is that simple. No time travel, no motion in spacetime, no spacetime and no time dimension. They are all abstract mathematical constructs without any counterpart in nature.
Efforts to understand time below the Planck scale have led to an exceedingly strange juncture in physics. The problem, in brief, is that time may not exist at the most fundamental level of physical reality. If so, then what is time? And why is it so obviously and tyrannically omnipresent in our own experience? “The meaning of time has become terribly problematic in contemporary physics,” says Simon Saunders, a philosopher of physics at the University of Oxford. “The situation is so uncomfortable that by far the best thing to do is declare oneself an agnostic.”
The trouble with time started a century ago, when Einstein’s special and general theories of relativity demolished the idea of time as a universal constant. One consequence is that the past, present, and future are not absolutes. Einstein’s theories also opened a rift in physics because the rules of general relativity (which describe gravity and the large-scale structure of the cosmos) seem incompatible with those of quantum physics (which govern the realm of the tiny). Some four decades ago, the renowned physicist John Wheeler, then at Princeton, and the late Bryce DeWitt, then at the University of North Carolina, developed an extraordinary equation that provides a possible framework for unifying relativity and quantum mechanics. But the Wheeler-DeWitt equation has always been controversial, in part because it adds yet another, even more baffling twist to our understanding of time.
“One finds that time just disappears from the Wheeler-DeWitt equation,” says Carlo Rovelli, a physicist at the University of the Mediterranean in Marseille, France. “It is an issue that many theorists have puzzled about. It may be that the best way to think about quantum reality is to give up the notion of time—that the fundamental description of the universe must be timeless.”
Yes the effect of Earth's gravity on clocks on the Earth's surface is known to be about 45ms/day slower than GPS satellites due to gravity and about 7 ms/day faster due to the satellites' velocity, so the net effect is the difference of 38ms/day.
originally posted by: bigfatfurrytexan
a reply to: Arbitrageur
question:
I understand how velocity affects time
What about mass/gravity? Has there been calculations/considerations regarding the effect of the gravity of local bodies and their relation to "CMB time'?
I like you coining the CMB time concept, as it is far more eloquent than my own musings of "Real Universe Time".
The equivalence principle was properly introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g (g = 9.81 m/s2 being a standard reference of gravitational acceleration at the Earth's surface) is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1g. Einstein stated it thus:
we [...] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.
—Einstein, 1907
If the light actually reached us, it was probably traveling through a vacuum so the speed never deviates from the speed of light regardless of how much time slows down. The light can get distorted in gravitational lensing however, as in this simulation of an Einstein ring forming around a black hole, but it has little effect on arrival time to Earth.
Example: the light from a supermassive galaxy reaches us. We are told it took millions of years to reach us due to distance. However, what about the variance in relative time due to travelling through various fields of gravitational effect (wells)?
I haven't tried to calculate it, but since 45ms is the difference between time on Earth's surface and the GPS satellite due to gravity alone, and the satellite is still in the Earth's gravitational field, I'd guess something more than 45ms, but still maybe less than 1 second per day, like off the top of my head and this might be wrong but half a second a day perhaps difference between Earth's surface and a low gravity location. I searched a little bit for a figure and didn't find one and don't have time to make calculations right now, but I think the half second per day may be a decent guess for how much time would speed up completely away from Earth's gravity.
If you find a place where there is the least amount of matter in the universe, what is the difference in the traversing of time there vs here? Not considering velocity, just gravity. What does this mean for the concept of intergalactic travel?