# why time moves differently while closer to a massive body?

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why time moves differently while closer to a massive body?

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Mass warps spacetime: the usual analogy is of a bowling ball sitting on a trampoline. The fabric close to the bowling has been stretched, so the distance increases. Since a light beam cannot speed up to compensate, time must expand with the distance.

You have a rubber sheet stretched out flat. You also have a magical marble that rolls across the sheet at a steady rate. You mark two dots on the sheet, and time how long it takes the marble to get from one to the other.

Now put a bowling ball between the two dots. The sheet stretches downwards from the massive body warping it, leaving it down in a bit of a well.

Now let the magical marble roll from one dot to the other again. It will take it much longer to make the trip, because the massive body is pulling on the sheet around it.

to paraphrase neil degrasse tyson: “when you ask someone to meet at your house, they’ll ask ‘what time’. If you ask someone to meet you at 5pm, they’ll ask ‘where'”

since space and time are linked, this “spacetime” can be thought of as one “fabric”. If you warp the fabric via gravitational effects then you are warping space *and* time

This requires a couple of a leaps, but it is the most intuitive explanation.

Space and time are intrinsically connected. Just like up and down are in the same coordinate system as left and right and forward and back, time is part and parcel of that same coordinate system. Here is where the leap comes in: everything is moving through space time at the speed of light. If you are stationary, you are hurling through the time direction at the speed of light. If you are moving through space at the speed of light, you are not experiencing time. The two are intrinsically linked, and that’s what E = mc^2 tells us. There is kinetic energy (mass * acceleration), potential energy, and this intrinsic energy (mc^2) due to this phenomenon. (Side note: E = mc^2 is the special case when kinetic and potential energy are equal to zero.)

If you do some advanced math, you can see that it would take infinite energy to accelerate anything with mass all the way up to speed of light. That is, to exchange all of the mc^2 energy from the time direction into a spatial direction requires more and more energy the closer you get to the speed of light.

The exact mechanism that causes mass is mostly understood, but very complex. A good way to visualize gravity due to mass is that mass creates a type of drag pressure on things that are nearby in space time. Anything that gets close is “attracted” due to this curvature in 4 dimensions.

We still don’t have a full bridge between the quantum world and relativity, so this is where the second leap comes in. The universe of the small behaves probabilistically, whereas the large scale is more classical due to the law of averages. The interactions between particles (gluons, specifically) are actually responsible for the overwhelming majority of mass (>98%) in the universe. It is clear then that particle interactions are somehow causing this warping of space time, but I do not think this is fully understood how this directly interplays with space time.

Many particle physicists want there to be a new force mediating particle dubbed the “graviton”, but that seems to be more fantasy than science. Einstein was hoping that there was some fundamental curvature in space time which would further support relativity, but the universe appears to be frustratingly “flat”. There is still a lot to discover about our universe.

A few people have referenced the rubber sheet. I prefer to think of space-time as a sponge. Picture a decent-sized sponge. Like the size of a coffee table. This is a representation of space-time. Cut a hole and put a grain of sand in there. Doesn’t do much. Now squeeze a bowling ball into the middle of it. All the sponge surrounding it gets squished. This is space-time getting compressed – it’s displaced by the mass of the bowling ball (or, in real size, space-time is displaced by the mass of the earth). That’s how I visualize space-time getting squished in 3D.

Now, picture a big 3D grid in the sponge. The distance from one space on the grid to the next is a unit – let’s call that one light-year. Based on Einstein’s theories, *the speed of light* is the constant. No matter how squished those boxes of the grid are, nor how stretched they might get – regardless of how far apart they might seem, it *ALWAYS* takes a beam of light one year to cross a unit. That’s the key to why time slows down. But let’s get one more example to visualize this.

Let’s just say that one unit is the width of the entire United States. Light going across that one unit moves pretty fast to get across it. And remember time changes, so we can’t really use units of real time, and we can call it whatever we want, but for now let’s call that distance across our reference year. Now, let’s squish that one unit that crosses the entire United States down so that it’s the distance to your neighbor’s house. Because it’s still one unit, just squished, it’s STILL one light-year across – *it will take light the same amount of time to cross the original US unit as it does to cross to the next-door unit*. That’s one unit of space-time, just squished – even though it’s just to your neighbor’s house, *it’s still one year*. *Light still travels at the same speed in both*. *So light still travels the same distance in both*. Light needs to cross both of those distances *in the same amount of time, because they’re both one unit across*. How can light possibly go “one unit” which are both different “sizes” (just one is squished) in the same amount of time? One way would be for light to speed up – but that’s not possible – *the speed of light is a constant*. The other way, as impossible as it may seem, is that light slows down. It’s harder for light (really, time) to travel, because the “sponge” here is so dense, it’s been compressed. So the time moves more slowly in this compressed area of space-time – the “next door year” has to move slower than our “reference year” to end up in the same place at the same “time”.

Hope this helps!