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Does gravity actually slow down time?
Time does not slow due to gravity. Instead, your time is dilated relative to matter in a lower gravitational potential. What is the distinction? We’ll take a look at this.
Synchronized GPS Time
Imagine, for a moment, that you’re an astronaut on the International Space Station.
You brought with you a big telescope and your GPS time synced phone. You wanted to a have a little test of Relativity. So you decided to point your lens to none other than Big Ben in London.
You watch as the historic clock ticks and notice it’s 1/100th of a second behind yours. But as you recall in Relativity, that is to be expected. The question is, though, did Big Ben actually tick only when you observed it or did it tick at precisely the same time as your phone’s clock? The obvious answer is the latter. The difference between the time was mostly due to the time it takes light to travel – which is slightly due to differences in gravitational potential and to differences in velocity.
Through a day, countless people ask others, “What time is it?” “Do you have the time?” You see, with General Relativity, we can have universal time where all of our clocks tick simultaneously despite differences in distance, gravitational potential and velocity (inertial frames). This happens every day. Just look at the top of your phone to see it in action. We know that when we observe an event is not precisely when the event occurred and, with the right knowledge, we can determine exactly when the event took place.
NASA uses this insight of Relativity to provide communication to distant spacecraft precisely when and where it needs it.
Time Dilation Due to Gravitational Potential
Suppose we took a clock and placed it on Mercury (the closest planet to the Sun). Assuming the clock was not GPS synced or designed to adjust for relativistic effects, the clock would tick slower. Let’s be very clear, though: aside from the extra Vitamin D, everything would proceed exactly as though you were on Earth – i.e. you wouldn’t see anything going slower(locally, that is). Because of this, we don’t call it time slowing – we call it time dilation. This means a stretching or widening of time. This also implies it is only occurring locally and is not a global effect.
Now if you were unaware of relativity, you might believe that this reflected true time and come to expect the clocks everywhere else to read the same. But when you return to Earth, you would find our clocks were further ahead. Time did not change or slow whatsoever, only the wave speed propagation and hence time evolution of matter on Mercury slowed.
To really hit this point home, consider an hour glass.
Suppose we timed this hour glass at 200 degrees Celsius and again at -100 Celsius. Would both have taken an identical amount of time to empty? Clearly the colder temperature hour glass would empty slower. Did time slow at -100 Celsius? Just because reactions occurred slower does not equate to a slowing of time – only a time dilation.
Why go through all the effort to make this distinction? This now relates to your first question.
Time Stops for No One
Some have suggested that time is fluid and bidirectional. I’ve even seen people say that if you could move faster than the speed of light, you would violate causality. One example I’ve seen given is a woman who shoots a man with a bullet that can travel faster than light. Would she be guilty of the crime since she pulled the trigger after he died? But what this story doesn’t tell is that the time violating order of events was only the observation for “some” observers but not all. Just like we can sync our clocks with knowledge of relativity, a court could easily hold her liable for the gun shot by applying Einstein’s equations to the circumstances. The story truly breaks down when you ask how fast the bullet should travel so it would have killed him 5 seconds prior to pulling the trigger. This calculation is impossible, given the distance you could shoot someone with a gun from. The notion that time is somehow disrupted by faster than light travel is absurd.
Observation is Not Reality
Consider the light from the Sun that reaches our eyes every day. It began it’s journey approximately 8 minutes ago. If you could somehow teleport to the Sun to see the light leaving right now, would that mean you had gone 8 minutes into the future to do so? That assumption would only hold true if observation equates to reality. But when you observe something is not necessarily when it took place, just like when you first noticed as a child that thunder trailed lightning. Thus we know at an early age that observation does not equal reality.
An Invariant Coordinate Frame
Because of this, we can know that time proceeds forward in a Universal frame, absent any gravitational potential, and we can use relativity to calculate this frame, which is equal for all inertial frames. I’m referring to the spacetime interval(ds2). We calculate that frame in most cases with an equation similar to Pythagora’s theorem:
We can use this to calculate a frame which is invariant to all observers and would be calculated the same by all of them. Having a clearer understanding of relativity allows us to sync all of our clocks and dispel any absurd notions of a loss of causality(i.e. time travel). Now, don’t get me wrong – I’m not implying matter could ever proceed at or greater than light speed. This fact is simply due to the connection matter has with light.
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