# Time Dilation. I don't get it!

#### MeJennifer

I don't know! Isn't it rather true, that at v=c the length of the whole universe appears contracted to 0. Locks like a singularity to me! So either you allow v=c which means that you accept, that the ship is everywhere at once (earth and alpha centauri) or you declare that v=c is not permissible.
Ii is not about accepting something.
Light always escapes at c regardless of the relative speed of the traveler. One way to look at it is that the traveler always travels at 0c relative to light.

#### JesseM

One way to look at it is that the traveler always travels at 0c relative to light.
Probably better not to get into another discussion of this, but for eiapeteides' sake I want to note that this way of looking at it seems to be unique to MeJennifer--no physicist talks this way, and personally I don't see any way of making sense of this claim (normally in physics when you say something like 'A travels at velocity v relative to B', you mean that in B's rest frame A is moving at velocity v, but light doesn't have a rest frame of its own, and even if you try to construct a pseudo-frame for it the speed of sublight objects would not be 0).

#### MeJennifer

Probably better not to get into another discussion of this, but for eiapeteides' sake I want to note that this way of looking at it seems to be unique to MeJennifer--no physicist talks this way, and personally I don't see any way of making sense of this claim (normally in physics when you say something like 'A travels at velocity v relative to B', you mean that in B's rest frame A is moving at velocity v, but light doesn't have a rest frame of its own, and even if you try to construct a pseudo-frame for it the speed of sublight objects would not be 0).
Remember that a frame is just a mathematical construct, not something that exists.

All movement between objects of mass is relative, including acceleration and all movement between objects of mass and light is absolute, including acceleration.
Feel free to show a case where this is not true.

#### eiapeteides

I think it would be better we leave the v=c case alone. That doesn't lead anywhere. Just introduces divisions by zero and thous singularities in the transformations.

#### JesseM

Remember that a frame is just a mathematical construct, not something that exists.
An inertial coordinate system can be defined in physical terms, in terms of measurements made on a system of rulers and clocks. In any case, I don't think you have any well-defined mathematical construct in mind that provides a general definition of what you mean by "A travels at velocity v relative to B", such that if you plug in A=the traveler and B=a light beam, you conclude that v=0.
MeJennifer said:
All movement between objects of mass is relative, including acceleration and all movement between objects of mass and light is absolute, including acceleration.
That's not exactly true, since in an accelerated coordinate system the speed of light need not be constant. But even if we ignore acceleration, it's true in some sense that "all movement between objects of mass and light is absolute", but that's because in any sublight object's rest frame the speed of light is c, there's no way to make sense of your statement that "the traveler always travels at 0c relative to light" in terms of any existing concept or definition in physics.

But like I said, it's probably better not to get into another discussion of this--unless you can provide some rigorous definition of what you mean, any further verbal explanations would be pointless and "not even wrong".

#### JesseM

I think it would be better we leave the v=c case alone. That doesn't lead anywhere. Just introduces divisions by zero and thous singularities in the transformations.
I agree, let's stick with v=0.999c as in your example. Any further thoughts on my last post to you (post #20)?

#### MeJennifer

That's not exactly true, since in an accelerated coordinate system the speed of light need not be constant.
Coordinate systems are not physical either. Saying that something is not constant in some coordinate system does not by itself mean anything.

During acceleration, e.g. a != 0 the speed of light is not constant! Hence in conformance with what I stated. Light speed changes during the acceleration of a mass object.

#### JesseM

Coordinate systems are not physical either.
You didn't address my point that they can be defined in terms of physical measurements on rulers and clocks. Anyway, even if you don't call them physical, the one-way "speed of light" cannot be defined outside of coordinate systems. Did you mean "all movement between objects of mass and light is absolute, including acceleration" to be a physical statement? If so, what physical measurements were you basing this on?
MeJennifer said:
Light speed changes during the acceleration of a mass object.
Is that supposed to be a physical statement? Again, based on what physical measurements? Certainly if one object is accelerating and is being observed by a second inertial observer, the acceleration of the first doesn't cause any measured change in the speed of light.

#### Darryl

here is my NON technical take on this.

http://www.fourmilab.ch/documents/ohmygodpart.html

this is an article on the "oh my god particle" that was detected. it was travelling at

v = 0.9999999999999999999999951 c

Distance[3] Perceived
Object (light years) Travel Time
=============== ================== ===========
Alpha Centauri 4.36 0.43 milliseconds
Galactic nucleus 32,000 3.2 seconds
Andromeda galaxy 2,180,000 3.5 minutes
Virgo cluster 42,000,000 1.15 hours
Quasar 3C273 2,500,000,000 3 days
Edge of universe 17,000,000,000 19 days

#### RandallB

RB: " From the traveler view the Planet and Earth cannot be "on the same time"! They cannot "be simultaneous".

And yes you can use v=c if your careful, but if you must use 0.9999999
"
eiapeteides said:
I don't know! Isn't it rather true, that at v=c the length of the whole universe appears contracted to 0. Locks like a singularity to me! So either you allow v=c which means that you accept, that the ship is everywhere at once (earth and alpha centauri) or you declare that v=c is not permissible.

In the first case the earth and alpha centauri are stationary at the ships position and so is the rest of the whole universe. Nothing moves at v=c!
I didn’t say anything did have a v = c; I said you can do this by using that.
You should have noted that “instantaneous” speed changes work best when looking the movement of the traveler as accounting for the accelerations is not important to understand the issues involved in SR, at least the part you are looking at. This of course means INFINITE acceleration and that not possible, But I notice you seem to be able to do that, because this is a thought experiment and you can assume the Time Change experienced be traveler for such an acceleration to be “0”.

The biggest mistake people make in working SR is transferring an assumption in one frame and transferring that same assumption to the other frame as you have done here by not being careful as I said you needed to be. You claim the ship “is everywhere at once”, I will accept that as so but ONLY in that one frame and no other. I will even accept that the traveler at that instant can see and read the clocks “everywhere” (everywhere also defined as only the line it travels on no matter how long in other frames). But I suspect you are still assuming that as it simultaneously reads the clocks on both earth and the planet you are still thinking those two reading will be the same just because the ship is everywhere at t = 0; and that I do not accept! Time remains at t = 0 only for the ship and it will see that although Earth and Planet have “synchronized” clocks they cannot read the same time “simultaneously” for the traveler. This is the point not obscure pointless singularity issues that don’t mean anything to this SR problem.

The point of your post is you “don’t understand time dilation” and as JesseM agrees with me: “ …to really "get" all aspects of time dilation you must understand simultaneity, ..” So take the time to work it through CAREFULLY and completely; and I still predict: you will not understand time dilation until you “get” simultaneity. Good Luck, I’ll let you work on it for a few days. If you prefer to use a different approach, that’s OK.

#### Darryl

sorry dont mean to mulit post,

that chart is the perceived travel time of the particle travelling at almost c.

so if you were observing Andromeda galazy 2.18 million light years away.

you then jump in your ALMOST c space ship and start your stop watch, in 3.5 minutes you will arrive at your destination, by YOUR watch. the trip took 3.5 minutes.

but you are now 2 million LY's away from earth, so earth will be 2million years older.

so in the 3.5 minutes you have lived for, the earth you just left will have aged 2 million years. if you shine your headlighs at earth and they see you, it will be 2 million and 3.5 minutes later. than the time frame you left the earth in. (i really hope im about right ).
i would not like to guess what you could see, when you travel at those speeds.

I also think, that if you go faster, and reach c. you cant TRALLEL at all !.
as t=0, (probably too simplistic). but if your time stops when you reach c, then as light does not experience TIME at all, if it does not experience time, and cannot travel and distance in 0 time.

so you cant TRAVEL at the speed of light.

or,
at the speed of light, you cant travel.

so if your at the speed of light, by definition you cannot travel, you would start your trip, and instantly end it. no time would pass, making it impossible to measure your own speed, or location !! ????

if your going ALMOST c, and you travel 2million LY's in 3.5 minutes. your going to have to time your trip VERY accuractly, or you will overshoot or undershoot by ALOT.
i guess infinate at c.

#### eiapeteides

Time remains at t = 0 only for the ship and it will see that although Earth and Planet have “synchronized” clocks they cannot read the same time “simultaneously” for the traveler. This is the point not obscure pointless singularity issues that don’t mean anything to this SR problem.
I rather stick with v=0.999c. Because the whole hubbub with v=c and t=0 does obscure the issue.

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#### eiapeteides

JesseM:
I agree, let's stick with v=0.999c as in your example. Any further thoughts on my last post to you (post #20)?
Well, i did draw the events which occur during that little "Thought experiment" in an x-t diagram.

And your explanation does indeed make sense. The Doppler-shift and the difference between real and observed time and distance is quite evident from those diagrams.

Now what I still don't get is the transformation from the Ships to the terrestrial observers view by means of the Lorentz equations. But I'm working on it!

Thanks!

#### eiapeteides

so in the 3.5 minutes you have lived for, the earth you just left will have aged 2 million years. if you shine your headlighs at earth and they see you, it will be 2 million and 3.5 minutes later. than the time frame you left the earth in. (i really hope im about right ).
Isn't it rather 4 mil years after they've seen you depart?

#### eiapeteides

here is my NON technical take on this.

http://www.fourmilab.ch/documents/ohmygodpart.html

this is an article on the "oh my god particle" that was detected. it was travelling at

v = 0.9999999999999999999999951 c

Distance[3] Perceived
Object (light years) Travel Time
=============== ================== ===========
Alpha Centauri 4.36 0.43 milliseconds
Galactic nucleus 32,000 3.2 seconds
Andromeda galaxy 2,180,000 3.5 minutes
Virgo cluster 42,000,000 1.15 hours
Quasar 3C273 2,500,000,000 3 days
Edge of universe 17,000,000,000 19 days

OK the traveling times make sense mathematically. But here's the catch:
Is the universe really contracted to a diameter of 17 light-days?
Or is the clock of the traveller slowed so that he only measures 19 days?

Obviously if the traveller stays focused on earth while he approaches it from the "edge of the universe" he will still count 17,000,000,000 vernal equinoxes.

In other words: Does the motion of the traveller affect the time-perception of the traveller or does it affect the dynamics of the whole universe?

Are those two statements symmetric? That's what the principle of relative motion states if I understand it correctly.

#### MeJennifer

Did you mean "all movement between objects of mass and light is absolute, including acceleration" to be a physical statement?
In fact it is the core of Einstein's relativity theory!
For an unaccelerated object light aways escapes at a speed of c while during the acceleration of an object this is no longer the case. Again feel free to give a situation that is in contradiction with this.

Certainly if one object is accelerating and is being observed by a second inertial observer, the acceleration of the first doesn't cause any measured change in the speed of light.
Here you are using movement between objects of mass not objects and light. And as I wrote before movements between objects of mass are relative including acceleration.

#### MeJennifer

In other words: Does the motion of the traveller affect the time-perception of the traveller or does it affect the dynamics of the whole universe?
Neither.

Time perception obviously does not change since the laws of physics do not change when something is in relative motion. The dynamics of the universe obviously not change either just because someone is in relative movement. The observed length-contraction is simply an effect of relative motion.

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#### eiapeteides

Time perception obviously does not change since the laws of physics do not change when something is in relative motion and the dynamics of the universe do obviously not change just because someone is in relative movement. The observed length-contraction is simply an effect
of relative motion
Let's look at Length contraction for a moment. A observer moving at 0.999999999... c sees the universe contracted to a radius of 19 light-days.

He sees the radius of the earth-orbit squeezed to nano meters. And yet those changes have no effect on the orbital motion of the earth or the moon or mars? We agreed after all that the traveller observes the same orbital events as the earth bound observer does. How is this possible?

The terrestrial observer does not observe any change in the Doppler-shift of the Microwave background radiation. The traveller on the other hand does, or doesn't he?

Is time dilation the effect of relative motion on space, or is it the effect of absolute motion (against the mw-background) on the visual perception of the traveller?

That's what I can't sort out.

#### MeJennifer

Let's look at Length contraction for a moment. A observer moving at 0.999999999... c sees the universe contracted to a radius of 19 light-days.

He sees the radius of the earth-orbit squeezed to nano meters. And yet those changes have no effect on the orbital motion of the earth or the moon or mars? We agreed after all that the traveller observes the same orbital events as the earth bound observer does. How is this possible?
Because length contraction is an effect of the traveler's relative motion. Just an effect!

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#### eiapeteides

Because length contraction is an effect of the traveler's relative motion. Just an effect!
You said that already. My question is: an effect on what?

a)the visual perception of the traveller

or

b)the geometry of the universe that surrounds him?

#### MeJennifer

You said that already. My question is: an effect on what?
An effect on how he measures distances of objects in relative motion.

#### eiapeteides

An effect on how he measures distances of objects in relative motion.
So it is a sensory effect! It affects the sensors and scales and thereby the process by which the traveler measures lengths and time intervals?

I was, and still am, under the impression that relativity means that motion has an effect on "Space-Time" itself and not just on the way we measure it.

Am I wrong?

#### MeJennifer

So it is a sensory effect! It affects the sensors and scales and thereby the process by which the traveler measures lengths and time intervals?
It does not affect the sensors and the scales, since the sensors and the scales are not in relative motion to the one who measures the results.

It is simply the case that if two objects of mass are in relative motion with each other that from each perspective the other object's time dilates, its mass increases and that the distance between them decreases. Why? Well because of the fact that light always escapes or approaches an object of mass at the same speed regardless or the relative motion between another object of mass.

I was, and still am, under the impression that relativity means that motion has an effect on "Space-Time" itself and not just on the way we measure it.
Motion does not effect space-time, that would be impossible since there is no motion in space-time.

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#### eiapeteides

Motion does not effect space-time, that would be impossible since there is no motion in space-time.
So who should one interpret the content of a previous post:

http://www.fourmilab.ch/documents/ohmygodpart.html

this is an article on the "oh my god particle" that was detected. it was travelling at

v = 0.9999999999999999999999951 c

Distance[3] Perceived
Object (light years) Travel Time
=============== ================== ===========
Alpha Centauri 4.36 0.43 milliseconds
Galactic nucleus 32,000 3.2 seconds
Andromeda galaxy 2,180,000 3.5 minutes
Virgo cluster 42,000,000 1.15 hours
Quasar 3C273 2,500,000,000 3 days
Edge of universe 17,000,000,000 19 days
Does that not mean that the Galaxies and the distances between them appear contracted to an observer co-moving with the particle?

Looks like space-time contraction to me.

#### MeJennifer

Looks like space-time contraction to me.
There is no such thing as space-time contraction.

In relativity we have:
When two objects (of mass) are in relative motion with each other each object will measure the other object's distance to be shorter, clock to run slower and mass to increase compared to if they would not be in relative motion with each other.

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