# Time dilation question from a layperson

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1. Nov 8, 2014

### Graham2

I have what might be a silly question about time dilation caused by gravity. My understanding (very basic) of it thus far is as follows:

1. A clock (time) will tick slower when close to a gravitational source that a clock that's positioned farther away.

2. This affect has been measured in the real world using atomic clocks that are driven by atomic vibrations, which are constant.

3. My understanding (could be wrong) is that higher gravity is physically slowing down the atomic vibration inside the clock, therefore causing it to tick slower.

My question is this:

- Has this been measured using any other method than with atomic clocks which function on a physical movement (however small the scale) of atomic particles that can be physically slowed down by gravity?

- How do we know that "time itself" is being affected and not simply the instrument that's being used to measure it - in this case, an atomic clock? After all, our conscienceness and perception of time is not affected by the atomic vibrations that affect atomic clocks.

So, for example: in a scenario where two synchronized atomic clocks are placed in two different gravitational environments; and let's say for simplicity sake that clock #1 ticked 10x faster that clock #2 (let's say this is a MASSIVE difference in gravity). Would an observer next to clock #2 be able to see that the seconds seem to be ticking by much slower than what he/she is accostomed to? Moreover. If two observers wanted to conduct an experiment in this scenario - let's say both people could, say, type at a consistent and reliable 50 wpm (as measured on Earth's surface). Observer 1 sits next to clock 1 in the low gravity environment. The other is with clock 2 in the high gravity environment. They both sit and transcribe the same 3000 word article and agree to meet at some spot equidistant from eachother when they are finished. Would they show up at the meeting place simultaneously - only that stopwatch 2 reads 6:00 and stopwatch 1 reads 60:00?

Or, put another way...If typist 1 and stopwatch 1 were on a spacestation in geosynchronous orbit over a SUPER HIGH GRAVITY planet (10:1 time dialation for the purposes of this question), and had a telescope powerful enough to look down and seen the fingers of typist 2. Would typist 1 see them as moving in super slow motion, or at normal speed?

Everything I've read thus far seems to indicate that the answer would be that typist 1 would see typist 2 moving in super slow motion - but how do we actually KNOW? Again - I think my big hangup is that the only instrument that seems to have been used to measure this has physical inner workings that suseptible to changes in gravitational conditions. How do we know that human consciousness is subject to it in the same way??

PLEASE use very simple terms! I'm definitely not a physicist or a scientist, just a curious enthusiast - and a layperson.

Thanks!

2. Nov 8, 2014

### DaveC426913

That is a lot of questions. Some of the subsequent ones will probably go away as you firwst one are answered, so my advice is to ask a couple of succinct questions, and see how that goes.

I'm going to try to answer your question about what it means to perceive time dilation and whether it is dependent on our observational vantage point.

One very nearby way we see time dilation is in the shower of particles from space. Cosmic rays from space collide with particles in the upper atmosphere and produce muons, which streak Eaerthward. These muons have a half life of known duration. They should not reach the ground before decaying, yet they do. The reason is because they are time-dilated, their half life is stretched out long enough that they reach the ground before decaying.

This does not really address all your questions directly, but I hope you can see that time dilation is not merely an "observational" phenomenon. The muons must cross a known distance and their time to do so is well-known.

3. Nov 9, 2014

### Staff: Mentor

No.

How do you know? It's true that the clocks aren't coupled directly to your brain, but your brain is a physical system just like the clocks are.

We don't know directly because we can't (yet) run experiments on humans involving time dilation factors large enough to be consciously perceptible. But, as I said above, human bodies and brains are physical systems just like atomic clocks, so our physical theories predict that human bodies and brains will be affected by time dilation just like all other physical systems are (and every physical system we've tested so far is affected).

It's worth thinking about why our physical theories--specifically General Relativity--predict this. GR predicts it because time dilation is built into spacetime itself: different paths through spacetime can have different lengths, and the length of a path (more precisely, of a timelike path) is simply the time perceived by an observer following that path. It's just geometry. Any physical object (including a human) that moves along a given path will record the same elapsed time, so if a human and an atomic clock that are moving together, the human will perceive the same elapsed time that the atomic clock records. Anything else would contradict the way spacetime works in GR.

4. Nov 9, 2014

### A.T.

The Shapiro-delay implies that a light clock would also run slower near a mass.
http://en.wikipedia.org/wiki/Shapiro_delay

If all types of clock agree, then we simply define "time" to be what all clocks measure. "Time itself" is irrelevant if "time itself" cannot be measured.

5. Nov 9, 2014

### DaveC426913

But to be clear, time dilation is not some far-flung future spaceship phenomenon.

We experience it right here on Earth all the time. The GPS in your smart phone is programmed to compensate for time dilation - it would not work if we were not in a gravity well.

But this - this is gold:

This site http://leapsecond.com/great2005/ documents a man's trip with his family in a camper van to the top of Mt. Ranier - equipped with 3 cesium clocks. He beautifully and incontrovertibly demonstrated a change in the time passing for him and his kids during their stay on the mountain. It was a glorious moment for home-brewed science.

Last edited: Nov 9, 2014