# Inertia and resting time rate

questionator89
Hi PF,

I am trying to get a good explanation regarding time dilation and how it works.
I want to understand why we can't slow an objects inertia down enough to see it age faster than we age at our velocity.

I will try to use a reference which is understandable to everyone.
So, an easy reference would be "earth time" which is 1 second per earth second in my question.
But if we were to be going 65% c (is this how we represent the speed of light?) we would be experiencing time as; for every 5 earth seconds we feel 1 second. ( i dont know the math)

It seems that as you reach close to the speed of light ( this is my interpretation) that the discrepancy between what we perceive as time traveling fast, and what our original resting frame of perceived time, is a big difference.

But it doesn't work like this in the opposite direction? I intuitively feel this doesn't follow a normal pattern of physics (but i know nothing)

It would seem that, if we were to place a clock somewhere in space, which to us is seemingly not moving at all
that the difference would be something like 1.0000000123 seconds per earth second get recorded.
This seems like a very small discrepancy.
It should be that we can decrease our velocity and inversely experience 5 seconds our time for every 1 earth second, and see everything slowly meander about.

we could be perceived from another object to be experiencing extreme time dilation. Its all about perception and reference points.

But obviously if we cant make something "rest" enough where it ages a considerable amount,while we experience 1 second per second does this imply a resting velocity? or inertia maybe is the right word?
This dimension has inertia built into it for a resting object to experience such a small discrepancy between units of time at a resting speed and a fast orbiting speed.

In many scenarios it is suggested that you could rip about in a spaceship for 5 years at an incredibly fast rate and return to earth to see that they have aged 20 years. Is this wrong? not the math I have said because I honestly have no idea, but the concept in general?

It almost sounds like it is easier to slow time down by traveling incredibly fast, than to speed time up by traveling incredibly slow.

I don't know how much science fiction I read has any credibility to it at all, but could high density electromagnetism shield an object from inertia and cause it to age to infinity?
maybe forget this last question....

Staff Emeritus
Hi PF,

I am trying to get a good explanation regarding time dilation and how it works.
I want to understand why we can't slow an objects inertia down enough to see it age faster than we age at our velocity.

We always have zero velocity in our own inertial frame. Something moving, in any direction, would have a positive velocity and would be time dilated. The least momentum an object can possibly have is zero, in which case it will be perfectly still with respect to our frame of reference.

I will try to use a reference which is understandable to everyone.
So, an easy reference would be "earth time" which is 1 second per earth second in my question.
But if we were to be going 65% c (is this how we represent the speed of light?) we would be experiencing time as; for every 5 earth seconds we feel 1 second. ( i dont know the math)

From your frame time on Earth would be passing at about 3/4's what it does for you. IE your clock ticks 4 seconds for every 3 seconds on earth. From the Earth's frame, YOU are ticking at 3 seconds for every 4 seconds of their time.

It seems that as you reach close to the speed of light ( this is my interpretation) that the discrepancy between what we perceive as time traveling fast, and what our original resting frame of perceived time, is a big difference.

Yes, time dilation becomes extreme as you approach c.

It would seem that, if we were to place a clock somewhere in space, which to us is seemingly not moving at all
that the difference would be something like 1.0000000123 seconds per earth second get recorded.
This seems like a very small discrepancy.
It should be that we can decrease our velocity and inversely experience 5 seconds our time for every 1 earth second, and see everything slowly meander about.

Ignoring GR effects, the time dilation according to SR would be zero if the clock is not moving with respect to your frame. Now, you ask what happens if we decrease our velocity. But... decrease in respect to what? The clock is already stationary to us and we to it. We can't decrease our velocity any further! If we were to accelerate in any direction we would be moving with respect to the clock and time dilation would occur.

This dimension has inertia built into it for a resting object to experience such a small discrepancy between units of time at a resting speed and a fast orbiting speed.

Ignore inertia. It is irrelevant. Momentum is what you get when an object is moving.

In many scenarios it is suggested that you could rip about in a spaceship for 5 years at an incredibly fast rate and return to earth to see that they have aged 20 years. Is this wrong? not the math I have said because I honestly have no idea, but the concept in general?

No, this is absolutely true.

It almost sounds like it is easier to slow time down by traveling incredibly fast, than to speed time up by traveling incredibly slow.

It is. It is impossible to "speed up time". The fastest time will pass for two observers in different frames is when they are both stationary with each other. If they are moving at all then they will be experiencing time dilation with respect to each other.

I don't know how much science fiction I read has any credibility to it at all, but could high density electromagnetism shield an object from inertia and cause it to age to infinity?
maybe forget this last question....

Yeah, I'm going to ignore this question, as nothing about it makes sense. :tongue:

1 person
questionator89
We always have zero velocity in our own inertial frame. Something moving, in any direction, would have a positive velocity and would be time dilated. The least momentum an object can possibly have is zero, in which case it will be perfectly still with respect to our frame of reference.

Yeah, I understand this. But what I am saying is, isn't it strange that we cant speed up time but we can slow it down?
I get how ridiculous this must sound too.
but to a guy like me it seems like god had a big tennis ball, and then he gave it a fast throw, then within this tennis ball our universe was created.
even at rest we experience the passage of time. Why doesn't this translate to we as a universe have a built in momentum that allows this base rate of time passage.

I wonder does momentum have to be the whole unit traveling in one direction? could the expansion of the universe (multiverse? i dont know whats proper anymore) count as momentum?

From your frame time on Earth would be passing at about 3/4's what it does for you. IE your clock ticks 4 seconds for every 3 seconds on earth. From the Earth's frame, YOU are ticking at 3 seconds for every 4 seconds of their time.

With what you described the total difference in time between the two objects is that there is no difference. I dont feel like this fits into what I am saying.
Is this because light takes time to reach us and the doppler effect or something?

In a time dilation scenario, doesn't the speedy astronaut come back to earth a few seconds younger then the people at rest on earth?

It is. It is impossible to "speed up time". The fastest time will pass for two observers in different frames is when they are both stationary with each other. If they are moving at all then they will be experiencing time dilation with respect to each other.

Staff Emeritus
but to a guy like me it seems like god had a big tennis ball, and then he gave it a fast throw, then within this tennis ball our universe was created.
even at rest we experience the passage of time. Why doesn't this translate to we as a universe have a built in momentum that allows this base rate of time passage.

Because the universe simply doesn't work that way.

I wonder does momentum have to be the whole unit traveling in one direction? could the expansion of the universe (multiverse? i dont know whats proper anymore) count as momentum?

Hmm. I'm not sure.

With what you described the total difference in time between the two objects is that there is no difference. I dont feel like this fits into what I am saying.
Is this because light takes time to reach us and the doppler effect or something?

I think this is getting above my knowledge level for SR, sorry.

In a time dilation scenario, doesn't the speedy astronaut come back to earth a few seconds younger then the people at rest on earth?

Just looking at SR, yes. But they key here is that he left our frame and then came back. Let's say you and your buddy are out in space in your own spaceships. He accelerates and travels for a little while and then comes back. His clock will read as having had less time pass than yours. BUT, if he takes off, then stops, and then YOU take off and go to where he is and stop, both clocks will read as having had the same amount of time pass. Make sense?

questionator89
Yeah that last paragraph does make sense. The astronaut has to come back, because if we accelerate to catch up with him then we dilate our own time frame to do so.

What is the opposite of dilate? contract? lets use contract.

How can we create an experiment where time is contracted beyond the normal resting point?

I suppose we cant really.

If we were to just drop a clock off in orbit, have it seemingly not move, pick it upon completing a full orbit, we would see very little time difference.

My brains like, "dude, its not moving you cant slow it down no more"

but then my hearts all like " dude, how come we can speed up so much, which slows time down, but we cant slow down enough to speed time up"?

cognitive dissonance i tell ya

Last edited:
Staff Emeritus
Yeah that last paragraph does make sense. The astronaut has to come back, because if we accelerate to catch up with him then we dilate our own time frame to do so.

What is the opposite of dilate? contract? lets use contract.

How can we create an experiment where time is contracted beyond the normal resting point?

I suppose we cant really.

You are correct. We cannot.
Your best chance of understanding this is to pick up a book on the subject. Hell, I'd even recommend looking at a translation of Einsteins original papers on SR. The way he describes it is crazy simple for such a high impact theory.

questionator89
alright I will google that. I doubt I can interpret it but I am sure there is a translation somewhere.
Do you have any idea what it is I am misunderstanding to think such a thing should be possible?

Last edited:
Staff Emeritus
alright I will google that. I doubt I can interpret it but I am sure there is a translation somewhere.
Do you have any idea what it is I am misunderstanding to think such a thing should be possible?

That something exists outside the universe?

1 person
questionator89
thank you Drakkith.
BTW I dont think something exists outside of the universe because when we use the word universe the definition should just encompass all I would think.
I am just trying to understand the connections between space and time and momentum and mass and light and how they are apparently all tied up in some equilibrium that only shows a big change when we speed up.

If we were to assume we were in the middle of some spectrum where at one end time stops because your going so fast, and on the other end everything ages at an incredibly fast rate because you are moving so slow, this would be more comprehensive to me.

Another thing I have realized.
Einsteins theory of SR says that the closer you get to the speed of light the more mass you will have and if you were to reach the speed of light the amount of energy it would take to move a mass this large would be infinite.

And yet, when something is seemingly standing still it still has mass?
its all convoluted. We gain mass when we speed up but we don't lose mass by slowing down would mean that we would just continue to gain mass.
We reach an infinite amount of mass at the speed of light, but at rest we don't lose our base mass.
I don't really understand how someones perspective could translate to reality in these scenarios anymore.
Why would the astronaut actually not age.
If the speed of light is constant why the hell would our mass be INFINITE. Lets be realistic here.
So we start out with 5lbs and accelerate it to 90% the speed of light.
then we take 10 lbs and accelerate it to 90% of the speed of light.
They both do not have the same mass at this point I would assume.
but go 10% more and they both have an INFINITE mass? that takes an INFINITE amount of energy to move?

It seems like all of this is based on one fact that the speed of light is constant, so if you are moving fast the light is actually going slower than when you were standing still.

But where does light and time connect?
I see how light and PERCEPTION connect but what we perceive isnt always the reality.
When you see someone from a distance kick a soccer ball and it takes the sound a bit of time to reach you we dont think, oh wow I have changed reality forever! I made the soccer ball make sound after it left his foot because that is what I perceived to happen!
Why wouldnt the astronaut speed off at 60% the speed of light for what seems to him to be 5 mins, turn around and come back to earth at the same speed in what seems to him to be 5 mins, and find when he gets back it went from 12:00pm to 12:10 pm? on his clock and on earths

Staff Emeritus
If we were to assume we were in the middle of some spectrum where at one end time stops because your going so fast, and on the other end everything ages at an incredibly fast rate because you are moving so slow, this would be more comprehensive to me.

I don't know what to tell you other than it doesn't work that way.

Another thing I have realized.
Einsteins theory of SR says that the closer you get to the speed of light the more mass you will have and if you were to reach the speed of light the amount of energy it would take to move a mass this large would be infinite.

Not quite. The term mass is used to refer to "invariant mass", which is the mass an object has in its own frame of reference. This never changes no matter how fast it goes.

If the speed of light is constant why the hell would our mass be INFINITE. Lets be realistic here.
So we start out with 5lbs and accelerate it to 90% the speed of light.
then we take 10 lbs and accelerate it to 90% of the speed of light.
They both do not have the same mass at this point I would assume.
but go 10% more and they both have an INFINITE mass? that takes an INFINITE amount of energy to move?

No, if you try to use the speed of light in the equation it simply doesn't work. That's why you get an infinity. The reality is that we cannot reach c. Whether an infinite amount of energy would get us there or not is irrelevant. We can't do it. Period. So it doesn't matter that the math says infinity as an answer.

It seems like all of this is based on one fact that the speed of light is constant, so if you are moving fast the light is actually going slower than when you were standing still.

Any observer in an inertial frame of reference will always measure the speed of light to be c.

But where does light and time connect?

I don't even know what you're asking here.

Why wouldnt the astronaut speed off at 60% the speed of light for what seems to him to be 5 mins, turn around and come back to earth at the same speed in what seems to him to be 5 mins, and find when he gets back it went from 12:00pm to 12:10 pm? on his clock and on earths

Because it doesn't work that way.

1 person
questionator89
I don't know what to tell you other than it doesn't work that way.
Yah, no worries. I am very aware that it doesn't work this way.... but it is a bit peculiar.

Not quite. The term mass is used to refer to "invariant mass", which is the mass an object has in its own frame of reference. This never changes no matter how fast it goes.

I understand I think. I didn't quite get the gain in mass. I thought maybe it had something to do with thermodynamics where somehow all the energy that it took to move the object which is moving is still inside the object, which can translate into mass somehow... where a 25 lb object moving at x speed makes an impact of 60lbs.. i dunno.
what you said makes sense here

No, if you try to use the speed of light in the equation it simply doesn't work. That's why you get an infinity. The reality is that we cannot reach c. Whether an infinite amount of energy would get us there or not is irrelevant. We can't do it. Period. So it doesn't matter that the math says infinity as an answer.
Okay. I can understand that.
I am just wondering to myself how infinity shows up in an equation? Like, I know what the symbol looks like∞ but at what point during a mathematical equation do we just go aahhh i dunno INFINITY.
Is there some sort of equation which we do that sums to 0? that we then translate the meaning to be infinity?

Any observer in an inertial frame of reference will always measure the speed of light to be c.
I don't understand this. The speed of light is constant though.
So if i am standing still and turn on a flashlight the light leaves me at 299 792 458 m / s
but when I am traveling at 90m/h and turn on a flashlight wouldn't the light leave me at 299 792 458 m / s MINUS 90m/h?
meaning that light is traveling slower to me at a faster momentum?
I should say I perceive the light to be traveling slower because I also have a rate of speed which has to leave the total speed because light cannot exceed 299 792 458 m / s

I don't even know what you're asking here.
DUDE i dont know what I am asking here either.
Or in any of these questions thats the whole point.
but it seems like when we see adams clock moving slower because hes moving super fast that this is simply due to an optical illusion because the speed of light is constant.
I dont understand where in these perspectives he comes back and has actually experienced a different rate of time.
it seems like as he moved back toward us time would just speed back up and return to normal perspective.

Because it doesn't work that way.
i knoww.. it should tho. maybe

Last edited:
questionator89
I dont want that last post to sound disrespectful in any way to scientists or Einstein or whatever. Well it does sound disrespectful but I do not mean it that way. I just don't understand because I haven't been educated

I am trying to get a good explanation regarding time dilation and how it works.
I want to understand why we can't slow an objects inertia down enough to see it age faster than we age at our velocity.
You can, in a sense.

For example, you could launch your grandfather clock into space at a speed of 100km/s, wait for a little bit, and then travel yourself in the same direction at 1000km/s. When you pass your grandfather clock, you will find it ticking faster than your wrist watch which you happen to be wearing.

Of course, in SR, this requires you to first obtain a speed from your Earth rest frame to make this happen, so it is a slightly contrived example.

If you are willing to look at GR, the example is more realistic. Being on Earth's surface, you are actually already traveling at some speed in a way (as being in a gravitational potential is akin to having a velocity). You could now put your grandfather clock on a GPS satellites circling the Earth, and sit back on Earth surface. You would find that the clock on the GPS is actually ticking faster than your wrist watch because it is at a lower gravitational potential (which means traveling slower in a sense).

(Not to complicate things, but there is another part of the story. The real orbital velocity of the GPS satellite (much higher than your velocity from Earth's rotation) actually reduces some of this clock rate difference, but the gravitational potential difference is the dominant factor in this case, and you will see the clock on the GPS being faster on the whole.)

Staff Emeritus
Okay. I can understand that.
I am just wondering to myself how infinity shows up in an equation? Like, I know what the symbol looks like∞ but at what point during a mathematical equation do we just go aahhh i dunno INFINITY.
Is there some sort of equation which we do that sums to 0? that we then translate the meaning to be infinity?

I don't mean a single equation, I mean the math as a whole. If you wanted to calculate the amount of energy you needed to accelerate an object to c, you would end up calculating more and more energy needed to accelerate up to a velocity closer and closer to c. And it never ends. You just keep throwing more energy into your equation and you just get closer to c, but never reach it. So we say that it takes an infinite amount of energy to get to light speed.

I don't understand this. The speed of light is constant though.
So if i am standing still and turn on a flashlight the light leaves me at 299 792 458 m / s
but when I am traveling at 90m/h and turn on a flashlight wouldn't the light leave me at 299 792 458 m / s MINUS 90m/h?
meaning that light is traveling slower to me at a faster momentum?
I should say I perceive the light to be traveling slower because I also have a rate of speed which has to leave the total speed because light cannot exceed 299 792 458 m / s

No, this is incorrect. You will ALWAYS measure the speed of light as c, no matter how fast you are traveling. Not only that, but an observer moving relative to you will ALSO measure the speed of the light you emitted as being c.

but it seems like when we see adams clock moving slower because hes moving super fast that this is simply due to an optical illusion because the speed of light is constant.
I dont understand where in these perspectives he comes back and has actually experienced a different rate of time.
it seems like as he moved back toward us time would just speed back up and return to normal perspective.

It's no illusion, it's a very real effect. Like I said, you really really need to dig into the basics of SR to understand it. There are plenty of articles online. Heck, I just linked you the actual papers by Einstein, so you can start there if you'd like.

Staff Emeritus
For example, you could launch your grandfather clock into space at a speed of 100km/s, wait for a little bit, and then travel yourself in the same direction at 1000km/s. When you pass your grandfather clock, you will find it ticking faster than your wrist watch which you happen to be wearing.

How so?

If you are willing to look at GR, the example is more realistic.

Bringing GR up will not help him right now.

How so?
Since you are traveling faster your time is slower. This is the 'invariant' differential aging, not including Doppler effects.

Staff Emeritus
Since you are traveling faster your time is slower. This is the 'invariant' differential aging, not including Doppler effects.

Are you talking about the clock ticking faster as you pass it, or are you saying that more time will have elapsed as per the clock when you pass it?

Are you talking about the clock ticking faster as you pass it, or are you saying that more time will have elapsed as per the clock when you pass it?
I am talking about the grandfather clock ticking faster (than your wrist watch) as you pass it, when you are co-located, since it is traveling slower than you.

That isn't true from your frame of reference. You will see the clock ticking slower than your own.
http://en.wikipedia.org/wiki/Time_d...nce_of_time_dilation_due_to_relative_velocity
That's not true when the two clocks are co-located and Doppler effect doesn't play any part in the observations. There is no requirement for the clocks to be at 'relative rest' when co-located (even briefly) for 'local' observations.

Once you eliminate the Doppler effect, you are left with the 'invariant' differential aging, in which case the grandfather clock is ticking faster than your wrist watch, because you are traveling faster.

Staff Emeritus
That's not true when the two clocks are co-located and Doppler effect doesn't play any part in the observations. There is no requirement for the clocks to be at 'relative rest' when co-located (even briefly) for 'local' observations.

Once you eliminate the Doppler effect, you are left with the 'invariant' differential aging, in which case the grandfather clock is ticking faster than your wrist watch, because you are traveling faster.

You are not traveling faster than the clock. The clock is the one doing the traveling to you.

You are not traveling faster than the clock. The clock is the one doing the traveling to you.
Which clock?

'... you could launch your grandfather clock into space at a speed of 100km/s, wait for a little bit, and then travel yourself in the same direction at 1000km/s. When you pass your grandfather clock, you will find it ticking faster than your wrist watch which you happen to be wearing.'

Remember that at the time of the observation the two clocks are co-located, so there are no Doppler effects to consider.

Staff Emeritus