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This starts from my basic understanding that time slows down as gravity increases.  Gravity is "caused by?" mass.

This must mean that time is faster where there is less gravity which leads me to think that somewhere way out there in deep space there is a point at which all gravitational forces are equal and it makes me wonder what happens to time in the absence of gravity?

Does time exist in the absence of gravity?

and my second question relates to statements like "the universe is 13 billion years old".  But a year is a quantity of time and if time is much faster in one place and much slower in another and possibly non existent in yet another, how can we say how 'old' something is?

Please concentrate on helping me with the first question though.

|   Dave   |    I've been around for most of my life.

One day I was feeling suicidal and so I got up on the roof and ran and jumped off the edge but I accidentally tripped, did a triple somersault and bounced off the awning below.  No one saw it except these two little kittens and one said to the other "See? That's how you do that." -- Steven Wright

22 minutes ago, sightseer said:

This starts from my basic understanding that time slows down as gravity increases.  Gravity is "caused by?" mass.

This must mean that time is faster where there is less gravity which leads me to think that somewhere way out there in deep space there is a point at which all gravitational forces are equal and it makes me wonder what happens to time in the absence of gravity?

Does time exist in the absence of gravity?

and my second question relates to statements like "the universe is 13 billion years old".  But a year is a quantity of time and if time is much faster in one place and much slower in another and possibly non existent in yet another, how can we say how 'old' something is?

Please concentrate on helping me with the first question though.

I remember seeing it on stack exchange: general relativity - Does time exist without Gravity? - Physics Stack Exchange 🙂

• 1

Victor Roos

Ok. thanks.

Edited by sightseer

|   Dave   |    I've been around for most of my life.

One day I was feeling suicidal and so I got up on the roof and ran and jumped off the edge but I accidentally tripped, did a triple somersault and bounced off the awning below.  No one saw it except these two little kittens and one said to the other "See? That's how you do that." -- Steven Wright

Time has no relevance as the physical universe knows no time. Time is a creation of our being, not that of the physical cosmos. That being the case, "time" in what we call the universe has no direction. Hence, describing something as "X" years old is only meaningful in our frame of reference. Time, as we define it, therefore exists regardless of the presence, or absence, of gravity.

A point to ponder though is this: If time knows no direction, why can we remember the past but not the future?

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46 minutes ago, W2DR said:

Time is a creation of our being, not that of the physical cosmos.

So you're saying time is man-made?  That's what I used to say but people kept telling me I was wrong.  It seems to me that see variance in the half life of a Cesium atom and say 'time is slowing down'.  I can't argue with GPS or other satellite functionality though.

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why can we remember the past but not the future?

I think some people can.  Maybe everyone can but some are more sensitive.  Its like empathy.  Some are more empathic than others. (empathetic?)  I currently think that our brains are quantum devices and quantum entanglement may just mean that some are aware of things that 'may be yet to happen'.  Maybe everything that ever will happen has happened all at the same time but the existence of mass affects the rate at which we become aware.

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|   Dave   |    I've been around for most of my life.

One day I was feeling suicidal and so I got up on the roof and ran and jumped off the edge but I accidentally tripped, did a triple somersault and bounced off the awning below.  No one saw it except these two little kittens and one said to the other "See? That's how you do that." -- Steven Wright

12 hours ago, W2DR said:

A point to ponder though is this: If time knows no direction, why can we remember the past but not the future?

Actually, I suspect some people can occasionally 'remember' the future; we call it a premonition. Of course I know some people think there is no such thing, but having actually had what I strongly suspect was one of these once, I am fairly sure they really can occur.

In my case, it was a very specific 'preview' of an unusual incident; an aviation accident where someone was killed by being chopped up by a helicopter main rotor blade when they parachuted down onto it whilst it was on the ground at an airfield doing an engine run up. As you can imagine, this was a pretty horrific accident, but the relevance here is that it is an unusual-enough occurrence for me to consider having 'seen it' some hours before, as a premonition of a specific incident, rather than something run-of-the-mill merely occurring to me as so many mundane thoughts do with everyone throughout the day. Beyond this, I have absolutely no explanation for why it occurred, but it was very bizarre and quite disturbing too, obviously.

Naturally enough this is kind of hard to believe, so you'd just have to take my word for it that this really did occur. I am aware that, unusual and coincidental though it would be, there is the slim possibility that it could just have been a pure coincidence that I envisioned such a very bizarre and unusual accident a few hours before one really occurred, but I'm inclined to think not based on details I 'saw' in my mind and what I subsequently learned about the accident. Bit off topic i know, but it did, and indeed still does occasionally make me wonder how many things there are about our capabilities which we might not be aware of, and trust me, I'm a real skeptic where that sort of stuff is concerned, but as I say, having personally had that happen, I do know it is possible.

Edited by Chock
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17 hours ago, sightseer said:

This starts from my basic understanding that time slows down as gravity increases.  Gravity is "caused by?" mass.

According to Einstein, yes, gravity is a result of the warped geometry of spacetime. Matter tells space how to curve and space tells matter how to move. But there's something else to consider. Gravity can also be considered in terms of time dilation.

17 hours ago, sightseer said:

This must mean that time is faster where there is less gravity which leads me to think that somewhere way out there in deep space there is a point at which all gravitational forces are equal and it makes me wonder what happens to time in the absence of gravity?

Does time exist in the absence of gravity?

I would say "an absence of gravity" is impossible. Gravity fills the universe. Would it exist in a gravity free realm? I say yes. Special Relativity includes time but no gravity.

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and my second question relates to statements like "the universe is 13 billion years old".  But a year is a quantity of time and if time is much faster in one place and much slower in another and possibly non existent in yet another, how can we say how 'old' something is?

Time is never non-existent according to special relativity. The age of the universe is determined with a frame of reference. Cosmic time, relating to the Hubble flow.

In the theory of Relativity, "proper" time is a precisely defined quantity. Your own proper time is the time that a clock would show that you take along on your journey. It depends on your velocity (present and past) and the trajectory that you took on your journey, which includes the effects of gravity. For yourself, your proper time just clicks away as it does every day, even if you move very fast or would fall into a black hole (a large black hole; for small black holes, shear forces would rip you apart if you get close).

The tricky part is that everything has its own proper time. Time progresses at different rates for someone who stays at rest far away from a black hole or a planet, and for someone who moves fast (relative to the first person) or who falls towards it. This has been observed very often (e.g., in the experiments by Pound and Rebka, or by Hafele and Keating). It needs to be taken into account in the design of experiments and in technology. For instance, fast moving muons (a kind of electron, but heavier) are generated by cosmic rays in the atmosphere (10-15 km high). They decay within 0.1 microseconds and thus should only be able to travel less than 100 meters, even at the speed of light. However, since they are fast, their clocks move slower, so that they can reach the surface of the Earth.

A particularly impressive demonstration of these effects is the GPS system. It basically works by measuring the time it takes a signal from a GPS satellite to reach your cell phone. Now, that satellite is higher up (less gravity) and moves at several km/s, which means its proper time is different from the one shown in your cell phone. If Relativity is not taken into account, these times are not synchronized anymore. They deviate by several microseconds every day, which translates into a distance measurement error of 100m daily. Engineers actually learned that the hard way: the very first GPS satellites didn't correct for relativistic effects. As a consequence, their measurements were woefully inadequate.

It gets more extreme near black holes: as I wrote above, your time clicks away even if you fall into a black hole. If you could somehow survive that trip, you would pass through the horizon and reach the center of the black hole (the singularity) in a finite amount of your own proper time. However, an observer resting far away from the black hole would conclude that, in his own proper time, it would take you an infinite amount of time to reach the horizon. The reason is that all signals that you send him would get more and more delayed as you approach the horizon, but for you locally, that delay doesn't matter.

With this in mind, let me turn to the questions of the OP:

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Does time exist in the absence of gravity?

Yes. Your proper time would still depend on your velocity, but any gravitational effects would be absent.

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"the universe is 13 billion years old".  But a year is a quantity of time and if time is much faster in one place and much slower in another and possibly non existent in yet another, how can we say how 'old' something is?

That is a very good question, and the full answer is extremely complex. The short answer is: the oldest light of the visible universe travelled 13.6 billion years to us, in our own proper time, but taking into account the expansion of the universe.

The more complex answer depends on how we measure distances. For something on Earth, we can use a ruler. What about the Moon and other planets? We can send light back and forward to determine how far away they are. But things get more tricky when we want to measure the distance of different stars or galaxies. Astronomers use "standard candles" for that. From the spectrum of light that a star emits, they can determine how large it is and what nuclear processes keep it alive. For a certain class of stars, the intensity of light they emit is very well defined, so that you can use their brightness as a measure of how far they are. This even works with nearby galaxies. If we go even farther away, we enter the realm of cosmology. Technically, that's not science anymore but natural philosophy, simply because we cannot verify hypotheses anymore by repeating a cosmological experiment. However, physical cosmology is extremely successful in using well-established physical laws (gravity, thermodynamics, ..) to make predictions about galaxies. The standard model of cosmology is therefore accepted by most (but not all) experts as a good explanation of how the universe works. Within this model, you can calculate the proper time (as measure on Earth) that light needs to travel from any point of the universe to us.

Peter

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20 hours ago, sightseer said:

So you're saying time is man-made?

The measurement of time on earth is man-made.  That is not to say time does not exist.  It does.  Would entropy exist in the absence of time?

Noel

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