How would you know you're not in time dilation?

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and debonair
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Reading the recent discussion about the redshift evidence that has created the need for dark matter to explain the observations, I thought of another possible way that one might observe the universe seeming to be older and slowed down than what it appears: Time dilation.

When I first saw Interstellar I wasn't aware of the time dilation of planets orbiting black holes. It had simply never come up in my reading, so I presumed they had gotten that part wrong because they traveled in and out of a dilated reference frame with very little fuel consumption. I had assumed that all dilated frames required an equal commitment to acceleration as a spaceship in straight line flight - or what would be required to come to a hover over a black hole. But I was forgetting that a reasonably vector would put you in line to be accelerated by the black hole's gravity into an orbit that is as dilated as coming to a dead stop above the black hole, but the acceleration was "free" because gravity provided it. (Just as regular earth orbits are rather spectacularly high energy, being a balance between gravity and a Mach 32 escape velocity.) Which is why the Interstellar crew didn't realize how long they would be gone - it was just as easy to travel to a dilated planet as a regular one.

But I digress.

What if the black hole(s) at the center of our galaxy were much more massive than we thought? The obvious effect would be that the galaxy itself would have to revolve more quickly than we currently measure it to maintain a stable orbit with a much higher escape velocity. That velocity would in turn cause the objects orbiting in the galaxy to be in an accelerated, and therefore dilated, reference frame. Which would make the rest of the universe look weird because we'd be receiving light within our galaxy that comes from similarly dilated sources, but light from outside the galaxy would seem different because we are slowed.

I can think of any of number of problems with this happening - and I very much doubt I have a Nobel Prize in the offing. But has anyone read anything about how we measure our own orbital dilation? Is there a good methodology for observing one's dilation compared to some theoretical place with effectively zero dilation? It's fun to think about.
 
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I do not believe that acceleration has anything to do with time dilation. Time dilates due to velocity and due to gravity. As gravity falls with the square of the distance, the effects drop quickly. Given the distances involved in the universe, I don't expect that the gravitation pull of the center would have noticeable affects on the majority of the universe.

There are multiple factors that affect time dilation at any point in space. I don't feel gravity from the center of the universe would be noticeable.
 
I do not believe that acceleration has anything to do with time dilation. Time dilates due to velocity and due to gravity. As gravity falls with the square of the distance, the effects drop quickly. Given the distances involved in the universe, I don't expect that the gravitation pull of the center would have noticeable affects on the majority of the universe.

There are multiple factors that affect time dilation at any point in space. I don't feel gravity from the center of the universe would be noticeable.
Velocity is relative, acceleration is not. If two objects pass each other at high velocity, the one that accelerated to that velocity will be the one dilated, not both. That's why the twin 'paradox' exists - the traveling twins relative velocity to the earth changed due to acceleration.

Gravity is acceleration.
 
Gravity is acceleration.
The apple on the table in front of me is not changing velocity at 9.8m/s/s even though gravity is working on it in exactly as the same way as the apple that fell off the tree onto Newton's head.
Acceleration is not the same thing as gravity.
 
The apple on the table in front of me is not changing velocity at 9.8m/s/s even though gravity is working on it in exactly as the same way as the apple that fell off the tree onto Newton's head.
Acceleration is not the same thing as gravity.
It isn't the same thing in the way you're using it, but in discussing the level of time dilation yielded by being in a gravity well, gravity is identical to the acceleration and velocity change necessary to leave the planet.
 
Velocity is relative, acceleration is not. If two objects pass each other at high velocity, the one that accelerated to that velocity will be the one dilated, not both. That's why the twin 'paradox' exists - the traveling twins relative velocity to the earth changed due to acceleration.

Gravity is acceleration.
These are some interesting assertions. Let me provide an alternate perspective.

  • Acceleration is relative. It is defined as a change of velocity over some change in time. It is a comparison of before and after velocities; acceleration is the change in velocity relative to the initial velocity.
  • Physics does not have memory. Time dilation is not affected by when or how long it took an object required to achieve its velocity.
  • Saying things are relative does not mean that the same laws of physics apply in both frames of reference. As a general practice, one chooses a frame of reference that allows the simplest model to describe it is the one chosen. It is equally valid to say the Earth is fixed and the Sun and planets move about it as it is to say the Sun is fixed and the Earth and planets move about it. Models of planetary movement were created using Earth as the frame of reference. They were accurate, but quite complex. Describing planetary motion using the Sun as the frame of reference simplifies things greatly.
  • Gravity is one of the defined forces. It does not require a change in position over time. Acceleration is not a force, it is a reaction to applied force. Acceleration is a description of the rate of change in position.
 
These are some interesting assertions. Let me provide an alternate perspective.

  • Acceleration is relative. It is defined as a change of velocity over some change in time. It is a comparison of before and after velocities; acceleration is the change in velocity relative to the initial velocity.
  • Physics does not have memory. Time dilation is not affected by when or how long it took an object required to achieve its velocity.
  • Saying things are relative does not mean that the same laws of physics apply in both frames of reference. As a general practice, one chooses a frame of reference that allows the simplest model to describe it is the one chosen. It is equally valid to say the Earth is fixed and the Sun and planets move about it as it is to say the Sun is fixed and the Earth and planets move about it. Models of planetary movement were created using Earth as the frame of reference. They were accurate, but quite complex. Describing planetary motion using the Sun as the frame of reference simplifies things greatly.
  • Gravity is one of the defined forces. It does not require a change in position over time. Acceleration is not a force, it is a reaction to applied force. Acceleration is a description of the rate of change in position.
I'm not interested in the semantics as much as the facts - time dilation is not relative. It happens for objects whose velocity is changing relative to their previous velocity, not from an observer. This happens due to force being applied to that object - either propulsion or gravity. And this is a plus or minus situation (as if physics has a memory) with dilation reducing in proportion to the accelerating force running opposite to the previous velocity vector.

As velocity or gravity increases, time dilation eventually produces effects that the crew of the ship can observe in the appearance of the outside universe. So two objects passing each other at large percentages of C may be experiencing time in different ways, and can tell if they are the one dilated from the degree of those effects.

Gravity is a defined force. It is also a wave. And it is also an effect - one that is experienced in a way that is identical to acceleration. An atom cannot discern a difference between gravity acceleration effects and propulsion acceleration effects at any given moment.


So we can argue about definitions and points of view, but it is a fact that time dilation is not a relative effect based on velocity differences between two objects. And that's what this thread is about.
 
"Acceleration is not the same thing as gravity".

What did Einstein say about gravity and acceleration in 1905?
 
Just plug Jim C's question into search and Voila!


What he means is the effect is the same. The cause is different - one is by speeding up in space and the other is by experiencing gravity on a massive object.
 
I'm not going to pretend I know anything about it. So just point and laugh.
Whenever these things are discussed there is the question of who does what relative to whom. The twin that stays at home can be considered to accelerate away from the other twin, as far as he is concerned, or possibly not. If acceleration is just the rate of change of velocity, velocity is measured in terms of different frames of reference then why not. and the earthbound twin can be thought to be travelling away from the "stationary" spacebound twin with a constantly changing velocity.
Hence relativity.
So much for the little I think I understand.

So:
If there are real time dilations going on, then one frame of reference has to be more important than another.
In which case the only absolute frame of reference has to be the "site" of the big bang. (Which I've also heard astronomers on television say we can define as here for all practical purposes, at least calculations of red shift etc)

But if we're all moving away from the site of the big bang, (against a gravity which may be enough to create the big crunch, or not) and we each experience time dilation relative to it, can time itself simply be defined as a sort of uber-dilation relative to it?

And black is white and I have terrible trouble with zebra crossings.
 
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I'm not going to pretend I know anything about it. So just point and laugh.
Whenever these things are discussed there is the question of who does what relative to whom. The twin that stays at home can be considered to accelerate away from the other twin, as far as he is concerned, or possibly not. If acceleration is just the rate of change of velocity, velocity is measured in terms of different frames of reference then why not. and the earthbound twin can be thought to be travelling away from the "stationary" spacebound twin with a constantly changing velocity.
Hence relativity.
So much for the little I think I understand.

So:
If there are real time dilations going on, then one frame of reference has to be more important than another.
In which case the only absolute frame of reference has to be the "site" of the big bang. (Which I've also heard astronomers on television say we can define as here for all practical purposes, at least calculations of red shift etc)

But if we're all moving away from the site of the big bang, (against a gravity which may be enough to create the big crunch, or not) and we each experience time dilation relative to it, can time itself simply be defined as a sort of uber-dilation relative to it?

And black is white and I have terrible trouble with zebra crossings.
This seems to cause many people problems, but it works like I've stated - the object that is changing velocity via a force that causes acceleration is the one which experiences the change in dilation. Not the other one that no new force acts upon.
 
The relativity notion can be difficult to picture. The title says 'how do you know you're not in time dilation' (right now). For that matter, how do we know the whole solar system is not traveling at the speed of light? Or the Galaxy? How do you measure the relative velocity of two galaxies and then what are the two galaxies' velocities relative to Earth? Who is really going faster?
 
In which case the only absolute frame of reference has to be the "site" of the big bang.

This is a quite interesting statement. I wonder how Einstein would answer this. I see the logic being used. Every time I have asked the question: Where is the center of the universe? Wiki tells me there is no such thing. "The center of the Universe is a concept that lacks a coherent definition in modern astronomy; according to standard cosmological theories on the shape of the universe, it has no center." History of the center of the Universe - Wikipedia
 
In a sense, the answer to the original question is that everything is in time dilation relative to every other thing that is in a different frame of reference. The amount of dilation can be calculated by comparing both thing's vectors (velocity plus direction) and their gravitational differences.

There reason the idea of a centre of the universe is meaningless in modern cosmology is that there are no privileged frames of reference. This is one of the tenets of relativity.
Every centre is arbitrary.
 
There reason the idea of a centre of the universe is meaningless in modern cosmology is that there are no privileged frames of reference. This is one of the tenets of relativity.
Every centre is arbitrary.

Yes, I've heard this. For some reason my brain makes this connection: Every centre is arbitrary = Infinite universe. (Not observed, Not proven)
What can you do when you can't see the edge?

The other question my brain asks (just blame it on my brain) Expanding universe. Expanding from where? This goes back to what @farntfar was asking "moving away from the site of the big bang". The answer is - everywhere is expanding all the time, so the 'site' is where we are now (We are still in the center. It has changed quite a lot since the BB). If there was nothing else around when the big bang went off, a 'center' would not have had any meaning then either. Technically, we were there at the big bang, but that energy transformed a zillion times over to get where we are today. We can't even put a stick of dynamite back together. That's Entropy for you.
 
In a sense, the answer to the original question is that everything is in time dilation relative to every other thing that is in a different frame of reference. The amount of dilation can be calculated by comparing both thing's vectors (velocity plus direction) and their gravitational differences.

There reason the idea of a centre of the universe is meaningless in modern cosmology is that there are no privileged frames of reference. This is one of the tenets of relativity.
Every centre is arbitrary.
Well, we have done that and concluded that the space is expanding, causing distant galaxies to appear to be moving more quickly away from us than we previously thought.

I was just wondering if we missed that we are living in (say) 20% time dilation, which would also explain why the outside universe is moving faster than we thought it should.
 

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