Time does not run slower or faster

[Tunji]

Maybe I missed something. Can people travel at a speed approaching light without being torn apart?

 

[JesseM]

In the Lorentz transformation a ship, ummm sorry, Bob or Ann approaching light velocity would experience length contraction of their path. They would pass through a shorter path of space-time than the twin or whomever remained on Earth that their age was being judged against. Thus would be younger.

You're confusing different issues here, length contraction is just for the spatial distance between two objects (or the ends of a single object) as seen in a frame where the objects are moving, it doesn't tell you the time measured along a path through spacetime (a 'worldline'). In the Earth's frame, the traveling twin is moving at high velocity during both the trip away from Earth and the trip returning to Earth, so her clock is ticking slower and thus she ages less because of time dilation (not length contraction). You can analyze it from the perspective of a different frame, but it has to be the frame of an inertial observer who doesn't change velocities, so even if the traveling twin is moving slower than the Earth during the trip away from Earth in the frame of this observer (meaning the Earth's clock is ticking slower during this period, from this observer's perspective), she'd then have to be moving faster than the Earth during the trip back to Earth in the frame of the same observer (so her clock would be ticking slower during this period), and the result would be that the average rate her clock was ticking on both parts of the trip would still be slower than the Earth, so this observer would also predict she'd be younger when she returns to Earth.

 

[JesseM]

Maybe I missed something. Can people travel at a speed approaching light without being torn apart?

What would tear them apart? One of the main points of relativity is that there's no absolute notion of speed (the Earth is moving at a 'speed approaching light' right at this moment in some frame, and this frame is just as valid as the frame where the Earth is at rest), and the laws of physics will work exactly the same in the frames of different observers moving at constant velocity (unchanging speed and direction). If you have two observers in sealed rooms which are both moving at different constant velocities in space, they will both feel weightless, and there is no experiment they can perform inside the room that will come out differently depending on their velocity relative to the Earth (or any other landmark), all their experiments will give the same results even if they are moving at 0.99c relative to one another. When you feel G-forces in a moving vehicle, that has nothing to do with your velocity, the G-forces are only produced by acceleration (changing velocity). Even the science-fictional notion of "inertial dampeners" is only meant to prevent starships from getting torn apart by large accelerations, not large velocities (and again, whether a velocity is large or small depends on an arbitrary choice of reference frame, there is no such thing as absolute velocity in physics).

 

[robphy]

Relativistically speaking,
a different way to say this, which corrects an often heard saying "Speed Kills", is
"speed doesn't kill... acceleration does".

 

[pervect]

Relativistically speaking,
a different way to say this, which corrects an often heard saying "Speed Kills", is
"speed doesn't kill... acceleration does".

This reminds me of the saying - it's not the fall that kills you, it's the sudden stop at the end...

 

[Ich]

Hmm... acceleration gradients kill? Killing Tides?

 

[rqr]

Originally Posted by rqr
>>There can be no x, t, and v. No clocks, remember?
>>You cannot use any math, including any that contains
>>the standard variables x, t, and v.

Originally Posted by JesseM:
>This is a truly bizarre argument--

Not for a test of special relativity theory.

Yes, the experiment that I presented was a simple test of SR.
In order to test SR, one must necessarily exclude all that is
SR, including its definition of simultaneity and its slowed
clocks and contracting rulers.

This is why the given experiment contains no clocks and no
rulers. This is why the given experiment contains no definition
of "simultaneity." Indeed, this is why it neither contains nor
needs any coordinate system of any kind, including Einstein's.

Here's an equation to help you see what this means:

SR coordinate system = SR's results guaranteed.

Therefore, it is "illegal" to insert any SR "stuff" into
the given experiment.

This includes SR's relative simultaneity.

Looking back at the given experiment, we see that it contains
only people and their aging.

Originally Posted by JesseM:
>...how can you say that Ann or Bob is "10 years old" if
>you think there is no truth about statements concerning
>time elapsed when there are no clocks to measure it?

Because "10 years old" is a physical state having nothing
whatsoever to do with any coordinate time.

In fact, it doesn't matter if they are 10 or 120; all that
matters is that their ages at their meeting point are the
same.

Originally Posted by JesseM:
>But you must admit that there is absolutely no experimental
>test that could distinguish between [1] and [2], therefore
>it's just a philosiphical view and not one that can be
>justified using physics.

I do not need to "distinguish between [1] and [2]"; all I
need to do is to declare their physical existence. Indeed,
as I said, I included _both_ in the results, so there was
never any need to even consider distinguishing.


Originally Posted by rqr
>>To reiterate, one of these two must occur regardless of any
>>clock synchronization or simultaneity schemes, math, or
>>worldlines. Also to reiterate, this has nothing to do with
>>any whims on anyone's part.

Originally Posted by JesseM:
>No, this is just a philosophical preconception. There is no
>need to believe there is any sort of physical truth about
>which of these is correct, since if there is no absolute
>present then there is no objective truth about how old
>two people are "at the same time", the notion of "same time"
>would be just as coordinate-dependent as "same x-coordinate"
>from the perspective of a four-dimensionalist.

Again, I am not trying to decide which of the two is correct;
both are used in the given results.

Originally Posted by JesseM:
>Would you accept without argument that [[1] and [2]] are
>distinct "physical possibilities", or would you point
>out that "speed" is an intrinsically coordinate-dependent
>notion and that there are no physical reasons to believe
>in absolute velocity? If the latter, why can't you accept
>that the same thing might be true of relative rate of aging?

Speed is not analogous to aging. Aging is a definite physical
process that is absolute. Speed, on the other hand, can vary
depending on the coordinate system used. There is no way that
the use of any coordinate system can control my aging process.

There are no relative speeds in the given experiment because
there are no coordinate systems.

And there was no mention of absolute speeds.

As I said, this is a simple test of special relativity theory;
it is a test of SR's claims that aging (for inertially-moving
people) has something to do with coordinate measurements,
worldlines, odometer analogies, accelerations, frame jumping,
different "space-time paths," or whatever.

The test shows that objective differential aging exists, and
that SR has no physical explanation for it.

Two people who have the same age when they meet in passing
do not have the same age when they meet again.

As I said, this is guaranteed to happen whether we have [1]
or [2]. (Since Copy-Bob is a direct copy of Bob (age-wise),
it is clear that "when they meet again" can apply to either
Bob or to Copy-Bob.)

There is no philosophy here.

rqr

 

[JesseM]

Originally Posted by rqr
>>There can be no x, t, and v. No clocks, remember?
>>You cannot use any math, including any that contains
>>the standard variables x, t, and v.

Originally Posted by JesseM:
>This is a truly bizarre argument--

Not for a test of special relativity theory.

Yes, the experiment that I presented was a simple test of SR.
In order to test SR, one must necessarily exclude all that is
SR, including its definition of simultaneity and its slowed
clocks and contracting rulers.

This is why the given experiment contains no clocks and no
rulers. This is why the given experiment contains no definition
of "simultaneity." Indeed, this is why it neither contains nor
needs any coordinate system of any kind, including Einstein's.

Here's an equation to help you see what this means:

SR coordinate system = SR's results guaranteed.

Therefore, it is "illegal" to insert any SR "stuff" into
the given experiment.

No, you're confused, it would be quite possible to use SR coordinate systems and find that the physical predictions of SR were wrong. For example, if actual physical clocks did not show time dilation, they would not all tick at the same rate as the time coordinate of their rest frame in the SR coordinate systems. More generally, if different observers in relative motion determined what equations would be used to express the laws of physics in their SR rest frame, if the laws of physics were such that the equations they found would differ from one SR frame to another, this would show SR was wrong. If Newtonian physics was correct, for example, the equations to describe it would have to be different in different SR frames.

Originally Posted by JesseM:
>...how can you say that Ann or Bob is "10 years old" if
>you think there is no truth about statements concerning
>time elapsed when there are no clocks to measure it?

Because "10 years old" is a physical state having nothing
whatsoever to do with any coordinate time.

I agree, but you're missing the point--the point is that saying that two people are "10 years old at the same time" is not necessarily a physical state that can be true or not true independent of your coordinate system; there would be an objective truth about whether two events happened at the "same time" if you accept the philosophy of presentism, but if you accept the philosophy of four-dimensionalism, the reality is just a static four-dimensional "block universe" filled with different worldlines similar to a block of ice cube with strings embedded in it, and there is no more of an absolute truth about whether two points in this 4-dimensional structure happen at the "same time" than there is about whether two marks drawn on different strings in the ice cube have the "same height" (which depends on which spatial axis you define as height).

Originally Posted by JesseM:
>But you must admit that there is absolutely no experimental
>test that could distinguish between [1] and [2], therefore
>it's just a philosiphical view and not one that can be
>justified using physics.

I do not need to "distinguish between [1] and [2]"; all I
need to do is to declare their physical existence.

But if there is no physical truth about whether two events happen "at the same time", there is no physical distinction between [1] and [2]. Do you think [1] and [2] would represent distinct physical possibilities if four-dimensionalism were correct and there was no absolute present, just a static four-dimensional spacetime?

Originally Posted by JesseM:
>No, this is just a philosophical preconception. There is no
>need to believe there is any sort of physical truth about
>which of these is correct, since if there is no absolute
>present then there is no objective truth about how old
>two people are "at the same time", the notion of "same time"
>would be just as coordinate-dependent as "same x-coordinate"
>from the perspective of a four-dimensionalist.

Again, I am not trying to decide which of the two is correct;
both are used in the given results.

But you are assuming that one or the other must be correct. Four-dimensionalism would say this is a false assumption--don't you understand that? You're free to disagree with four-dimensionalism, but this is a philosophical belief, and you admit there's no physical way to test between [1] and [2].

Originally Posted by JesseM:
>Would you accept without argument that [[1] and [2]] are
>distinct "physical possibilities", or would you point
>out that "speed" is an intrinsically coordinate-dependent
>notion and that there are no physical reasons to believe
>in absolute velocity? If the latter, why can't you accept
>that the same thing might be true of relative rate of aging?

Speed is not analogous to aging. Aging is a definite physical
process that is absolute.

We're not talking about the aging of one person, we're talking about whether two people at different locations are the same age or different ages. If there is no absolute truth about whether two events happen at the "same time", how can their be an absolute truth about whether two people are the "same age" or not? If the ultimate reality is just a static 4-dimensional spacetime with various worldlines embedded in it like strings in a block of ice, there's certainly a definite truth about how old anyone is at any given point on their worldline, but no truth about whether a point on my worldline and a point on your worldline happen "at the same time" or "different times", so if those two points are me turning 30 and you turning 30, there's no absolute truth about whether we reached these ages "at the same time" or different times".

Speed, on the other hand, can vary
depending on the coordinate system used. There is no way that
the use of any coordinate system can control my aging process.

There are no relative speeds in the given experiment because
there are no coordinate systems.

And there was no mention of absolute speeds.

I wasn't claiming there was, I was using speed as an analogy to point out that it's wrong to simply assume there must be a definite physical truth about whether property X of Ann is different than or equal to the same property X of Bob; you admit that if X=speed there would be no definite physical truth, so I'm trying to show you that the same may be the case if X=age.

As I said, this is a simple test of special relativity theory;
it is a test of SR's claims that aging (for inertially-moving
people) has something to do with coordinate measurements,
worldlines, odometer analogies, accelerations, frame jumping,
different "space-time paths," or whatever.

The test shows that objective differential aging exists, and
that SR has no physical explanation for it.

There is no test of your claim that either [1] or [2] must be physically correct--that is just a philosophical assumption you make. It might be correct if presentism is the right philosophy of time, but if four-dimensionalism is the right philosophy of time it isn't.

There is no philosophy here.

Clearly you don't understand the philosophy then. Can you explain in your own words how even if the four-dimensionalist philosophy of time is true and there is no absolute present, there still must be a physical truth about whether [1] or [2] is correct?

 

[rqr]

2 jessem

Quote:
[JesseM wrote:]
>But you are assuming that one or the other must
>be correct. Four-dimensionalism would say this
>is a false assumption--don't you understand that?
>You're free to disagree with four-dimensionalism,
>but this is a philosophical belief, and you admit
>there's no physical way to test between [1] and [2].

I repeat, there is no need to "test between [1] and [2]."
(More on this below.)

Quote:
[JesseM wrote:]
>We're not talking about the aging of one person,
>we're talking about whether two people at different
>locations are the same age or different ages. If
>there is no absolute truth about whether two events
>happen at the "same time", how can their be an
>absolute truth about whether two people are the
>"same age" or not?

I repeat, there is no need to possesses absolute time
or the "absolute truth" re time as far as the given
experiment is concerned; in fact, that was one of the
prime reasons for the particular design of the given
experiment.

Perhaps a quick example will help:
Suppose there are two inertially-moving rods in
space very far apart in different frames; despite
their physical separation, we know that there are
only three physical possibilities re their intrinsic
lengths, viz., (i) they are equal, (ii) rod A is
longer, or (iii) rod B is longer. Note carefully
that these three physical possibilities have nothing
to do with coordinate measurements; indeed, such
measurements can come up with many different
"lengths" for one and the same rod that is moving
at a constant velocity. Also note the fact that
this has nothing to do with absolute time or with
relative time. And there is no philosophy here.

Similarly, in the given experiment, there are only
two physical possibilities, namely, [1] Ann's and
Bob/Copy-Bob's ages differ when the latter meet in
passing, or [2] they are the same.

But, as I said, I don't care which it is. There is
no need to find out which is the case. There is no
need to test for which is true because I used BOTH
in my solution. (It's not either/or; it's both!)

To reiterate, here is how I used [1]:
If Ann's and Bob/Copy-Bob's ages differ when the latter
meet in passing, then this proves that objective age
differences occur sans acceleration because neither
Ann nor Bob accelerated.

To reiterate, here is how I used [2]:
If Ann's and Bob/Copy-Bob's ages are the same when the
latter meet in passing, then this also proves that objective
age differences occur sans acceleration because neither
Ann nor Copy-Bob accelerated, and yet their ages differed
when they met in passing at the end.

This proves the deficiency of special relativity theory
by showing that it has no physical explanation whatsoever
for this objective differential aging; indeed, if we could
ever get beyond this simple conclusion, we could then see
clearly that the only possible physical explanation must
be that which SR has denied all meaning to.

There are many other ways to show how SR is deficient;
for example, and contrary to all that we have been told,
it cannot correctly measure light's one-way speed. In
order to do this, one must use absolutely synchronous
clocks, but SR has no such clocks.

Quoting Einstein's 1905 paper (from Section 2):
"Observers moving with the moving rod would thus
find that the two clocks were not synchronous,
while observers in the stationary system would
declare the clocks to be synchronous."

This makes it clear that, to Einstein, the phrase
"relative simultaneity" is equivalent to the phrase
"relative synchronization," which cannot be absolute
synchronization. Therefore, SR has no absolutely
synchronous clocks, so it cannot correctly measure
any one-way, two-clock speed, including light's.

rqr

 

[JesseM]

I repeat, there is no need to possesses absolute time
or the "absolute truth" re time as far as the given
experiment is concerned; in fact, that was one of the
prime reasons for the particular design of the given
experiment.

My point is that even if you do not know the "absolute truth" about simultaneity or absolute time, you are still assuming that such an absolute truth exists. From the four-dimensionalist point of view, there simply is no absolute truth about whether two events happen at the "same time" or not, not even an unknowable absolute truth.

Perhaps a quick example will help:
Suppose there are two inertially-moving rods in
space very far apart in different frames; despite
their physical separation, we know that there are
only three physical possibilities re their intrinsic
lengths, viz., (i) they are equal, (ii) rod A is
longer, or (iii) rod B is longer.

No, I don't know that! You keep ignoring the possibility that there simply is no absolute truth about certain questions. Is this idea really so hard to understand? Didn't you already admit that there may be no absolute truth about whose speed is greater?

Note carefully
that these three physical possibilities have nothing
to do with coordinate measurements; indeed, such
measurements can come up with many different
"lengths" for one and the same rod that is moving
at a constant velocity. Also note the fact that
this has nothing to do with absolute time or with
relative time.

Sure there is. The "length" of a moving object is just the distance between the back end and the front end at a single moment. If I say that the event of the back end of the moving rod passing the Earth happened "at the same time" as the event of the front end passing the moon, then I will say the length of the rod is equal to the distance between the Earth and the moon; if you disagree that these events happened simultaneously, and you say instead that the event of the back end passing Earth happened "at the same time" as the event of the front end passing Mars, then you will say the length of the rod is equal to the distance between Earth and Mars. If there is no absolute truth about whether two events happened at the "same time", then there is no absolute truth about an object's "length".

Similarly, in the given experiment, there are only
two physical possibilities, namely, [1] Ann's and
Bob/Copy-Bob's ages differ when the latter meet in
passing, or [2] they are the same.

You keep ignoring the argument that a four-dimensionalist who doesn't believe in absolute time would deny that these are two distinct physical possibilities! Do you not see that by saying there must be an absolute truth about whether [1] or [2] is true, you are saying there must be an absolute truth about whether the event of Ann turning 10 and the event of Bob turning 10 happened "at the same time" or not--in other words, you are assuming there is such a thing as absolute time?

But, as I said, I don't care which it is. There is
no need to find out which is the case.

Do you not understand that the four-dimensionalist would deny that either one is "the case" in any objective sense, just like you would deny that either [1] Bob's speed is greater than or equal to Ann's or [2] Bob's speed is less than Ann's must objectively be true while the other is objectively false, since you don't believe in absolute velocity? Why do you keep assuming that one or the other must objectively be true in your example?

This discussion will never go anywhere if you keep ignoring the basic point of my argument, that your assumption that either [1] or [2] must be true is based on the idea that there's a definite truth about whether two events happen "at the same time", and that since a four-dimensionalist would deny this, he'd also deny that there's any objective truth about whether [1] or [2] is the case. For a four-dimensionalist, time is just another dimension in four-dimensional spacetime, and asking whether two events happen at the "same time" is just as arbitrary as asking whether two dots on a sheet of paper have the "same height along the y-axis"--obviously there is no objective answer to this question, it depends on an arbitrary choice of how you decide to orient your y-axis! If you wish to continue this discussion, you can't just keep on ignoring my argument and asserting that either [1] or [2] must be true without even addressing the reasons a four-dimensionalist would deny that assertion. So if you choose to post again, please give a detailed answer to the final question from my last post, namely: "Can you explain in your own words how even if the four-dimensionalist philosophy of time is true and there is no absolute present, there still must be a physical truth about whether [1] or [2] is correct?"

 

[pervect]

OK, I think it's time to lock this thread. Jesse has done a fine job of presenting the mainstream view, but ultimately, by our formal written guidelines, Physics Forums is a place to explain mainstream science, not to debate it.

I believe that the explanation has been already done by Jesse in his past responses, and that this thread has crossed the line between a discussion and a "debate" - in particular a rather nonproductive debate.