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  • Writer's pictureKirk

Space Travel - Gravity and Time

Last week we looked at how gravity affects space:

But, what about time? What does gravity do to time? Well, it at least gives the appearance of slowing it. The greater the mass of an object, the more it appears to slow time. In fact, if you measured time with a precise instrument on Jupiter, a planet with 318 times the mass of earth, and compared that time to earth, you would find the timepiece on Jupiter runs about 20 nanoseconds slower for each second on earth. Not much, but still enough to measure and to noticeably warp time. But, it takes a very, very big chunk of mass to appreciably slow time.

So how does it work? Why does gravity slow time? If you were to travel and live on Jupiter you could live a little longer, right? Not only would a clock's ticking hand move slower but your entire metabolism would slow down. The flow of entropy from low to high state also slows. It's what is known as time dilation.

Any attempts I have had in figuring out time dilation breaks down. It is not an intuitive thing. I understand why time dilation is necessary since the speed of light is constant in all reference frames, but this is a tough one to get my head around. Whenever I read where someone thinks they understand it, I will simply shake my head. They might believe they understand on their terms, but their understanding is probably wrong.

Ok, so I'll take a shot at my understanding of this. I'm a simplistic thinker so this will be as simple as simple can get on such a complex idea as time dilation.

Einstein proved that light speed is universal. It travels at the same speed no matter what the reference frame (this is rather a lengthy topic and well documented so I won't get into details here). But to illustrate, let's go back to the bowling ball on the trampoline example. Let's say the trampoline was wide enough that it took one second for light to travel across (ok that's a huge trampoline as it would extend nearly to the moon). Now if you stretch the trampoline by placing the bowling ball (or earth is a better example for a trampoline this size), the light beam would follow the contour of the extended "warped" fabric, but still arrive at the other end at the same time. It doesn't matter to light if you warp space and it has to take a longer route or not, it's going to make its appointment at the other end exactly the same "time" (quotations because time seems meaningless when compared this way, to me anyway).

Well, how can that be? If light travels the same speed regardless, and if it has to take a longer route, how can it arrive at the same time as the unbended light beam? That's where time dilation comes in. The only way to make this possible is if one second lasts longer in the warped space. Time must be slowed (or distance shortened) for this to eliminate the paradox.

Another way to look at it that might be useful is thinking of the trampoline fabric as a section of space-time. It is a fixed amount of space-time. If you stretch it with the bowling ball it does not change the amount of space time in the fabric. So, if you warp it, or stretch it, to make the distance something must cross the warped surface a longer distance, then time has to shrink to make up for it. That way the space-time remains constant regardless how much you stretch the fabric. The more you stretch it (by gravity), the more time must slow to compensate.

But, for me to go farther than this and explain time dilation would further increase the entropy in my brain, as Nick and I jested in a comment section of a past post. This is just not intuitive stuff. It's beyond the scope of any of our experiences. If you read this and don't understand it, well you're like almost everyone else. On the other hand, if you think you do understand time dilation, you're one of the arrogant fools out there who has yet to be sufficiently humbled by the vast complexities of physics.

Gravity is a cool thing. It is the least understood of the forces known to man. It is the only one of the four known forces of the universe for which a quantum theory does not exist. The mystery of gravity will likely one day be revealed by another bright Newton or Einstein, but we've had a number of these bright guys (Dirac, Feynman, and even Sheldon Cooper), who could never crack the code. Not even Einstein himself could do it.

Maybe a reader here can? I'm sure my posts are attracting the brightest minds of our time.

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