A question about the present!

[mistermack]

The third twin can't do this without accelerating. Once you insist that one thing is ACTUALLY happening you are declaring a preferred frame, and these do not exist in relativity. You can't do an experiment that gives a special result in one frame as compared to another.

No, the way the thought experiment works, is that the third twin is in non-accelerating inertial motion, coming to meet the second twin.

As they pass each other, the clock reading of the second twin is transferred to the third twin, who then continues back to the first twin, where both his clock reading, and that of the second twin, are transferred to the first twin.

So nothing physical is actually accelerated in the whole process.

 

Wikipedia on the subject :

Eventually, Lord Halsbury and others removed any acceleration by introducing the "three-brother" approach. The traveling twin transfers his clock reading to a third one, traveling in the opposite direction. Another way of avoiding acceleration effects is the use of the relativistic Doppler effect (see What it looks like: the relativistic Doppler shift below).

Neither Einstein nor Langevin considered such results to be problematic: Einstein only called it "peculiar" while Langevin presented it as a consequence of absolute acceleration.^{[A 10]} Both men argued that, from the time differential illustrated by the story of the twins, no self-contradiction could be constructed. In other words, neither Einstein nor Langevin saw the story of the twins as constituting a challenge to the self-consistency of relativistic physics.

[swansont]

No, the way the thought experiment works, is that the third twin is in non-accelerating inertial motion, coming to meet the second twin.

As they pass each other, the clock reading of the second twin is transferred to the third twin, who then continues back to the first twin, where both his clock reading, and that of the second twin, are transferred to the first twin.

So nothing physical is actually accelerated in the whole process.

 

Wikipedia on the subject :

Eventually, Lord Halsbury and others removed any acceleration by introducing the "three-brother" approach. The traveling twin transfers his clock reading to a third one, traveling in the opposite direction. Another way of avoiding acceleration effects is the use of the relativistic Doppler effect (see What it looks like: the relativistic Doppler shift below).

Neither Einstein nor Langevin considered such results to be problematic: Einstein only called it "peculiar" while Langevin presented it as a consequence of absolute acceleration.^{[A 10]} Both men argued that, from the time differential illustrated by the story of the twins, no self-contradiction could be constructed. In other words, neither Einstein nor Langevin saw the story of the twins as constituting a challenge to the self-consistency of relativistic physics.

OK. But what is the problem then?

[mistermack]

OK. But what is the problem then?

As I wrote earlier, "both clocks can't run at 0.6 x the rate of the other."

 

The problem is explaining what is special about the stationary clock, compared to the moving clock, when the moving clock can be regarded as stationary, and the stationary clock as moving.

 

I'm not arguing that it's ACTUALLY contradictory. As I said earlier, I can't get my head around the problem. That doesn't mean I'm arguing for errors in SR.

[swansont]

As I wrote earlier, "both clocks can't run at 0.6 x the rate of the other."

 

The problem is explaining what is special about the stationary clock, compared to the moving clock, when the moving clock can be regarded as stationary, and the stationary clock as moving.

 

I'm not arguing that it's ACTUALLY contradictory. As I said earlier, I can't get my head around the problem. That doesn't mean I'm arguing for errors in SR.

 

 

Each clock sees the other as running slow. They do, in fact, each run at 0.6 of the other, according to the local observer. But you can only be one of the observers.

[mistermack]

 

 

Each clock sees the other as running slow. They do, in fact, each run at 0.6 of the other, according to the local observer. But you can only be one of the observers.

But in the case of actual twins, one ages more than the other.

[Strange]

But in the case of actual twins, one ages more than the other.

Only in the special case that one accelerates away and then returns.

[mistermack]

Only in the special case that one accelerates away and then returns.

That's what I would have thought, but apparently not :

Again from wikipedia :

 

"As mentioned above, an "out and back" twin paradox adventure may incorporate the transfer of clock reading from an "outgoing" astronaut to an "incoming" astronaut, thus entirely eliminating the effect of acceleration. Also, according to the so-called "clock postulate", physical acceleration of clocks doesn't contribute to the kinematical effects of special relativity. Rather, the time differential between two reunited clocks is produced purely by uniform inertial motion, as discussed in Einstein's original 1905 relativity paper,^[15] as well as in all subsequent kinematical derivations of the Lorentz transformations. "

 

So they are saying that acceleration is nothing to do with it, it's just the uniform inertial motion that makes one twin younger than the other.

[Strange]

Yes. That is a more accurate statement.

[studiot]

I like Eddington's story of the cigars as a way of avoiding the acceleration in the twin's 'paradox'.

 

 

[swansont]

That's what I would have thought, but apparently not :

Again from wikipedia :

 

"As mentioned above, an "out and back" twin paradox adventure may incorporate the transfer of clock reading from an "outgoing" astronaut to an "incoming" astronaut, thus entirely eliminating the effect of acceleration. Also, according to the so-called "clock postulate", physical acceleration of clocks doesn't contribute to the kinematical effects of special relativity. Rather, the time differential between two reunited clocks is produced purely by uniform inertial motion, as discussed in Einstein's original 1905 relativity paper,^[15] as well as in all subsequent kinematical derivations of the Lorentz transformations. "

 

So they are saying that acceleration is nothing to do with it, it's just the uniform inertial motion that makes one twin younger than the other.

 

 

Acceleration has nothing to do with the dilation (difference in proper time), but it has everything to do with which clock reads slower when compared side-by-side. If you continue reading that section, the two twins each see the other's as slow until one of them undergoes an acceleration.

[mistermack]

 

 

Acceleration has nothing to do with the dilation (difference in proper time), but it has everything to do with which clock reads slower when compared side-by-side. If you continue reading that section, the two twins each see the other's as slow until one of them undergoes an acceleration.

I don't have a problem with the case where acceleration is involved. It's the case where acceleration is removed by the third twin that I struggle with.

And the bit of the wiki page that I quoted in blue, above :

 

"Rather, the time differential between two reunited clocks is produced purely by uniform inertial motion, "

 

Which seems to contradict what you said. Two reunited clocks, and side-by-side clocks, are the same thing.

[michel123456]

Yes, there is nothing in Relativity that forbids Such "apparent" superluminal velocities. Just because you visually see the clock approaching at a greater than c speed, does not mean that you would consider it as moving faster than c.

Do not conflate "what you see" with "what is happening". You can use "what you see" (along with other information) to determine "what is happening", but they are not the same thing.

In this case, you "see" the clock crossing 1 light hr of distance in 15 min, but that is just due to the fact both the light showing the clock 1 light hr away and the clock leave the same point at the same time, and the light beats the clock to you by fifteen minutes. Thus you "see" the trip compressed into 15 min. But since you know that this light took an hour to get to you, you know that both the light and clock left 1 hr before you first see the light from the clock.

If you see the clock leave when your clock reads 1:00, you know that the clock and light both left when your clock read 12:00, and since the clock arrives when your clock reads 1:15, the clock took 1 hr 15 min to reach you, and traveled at 0.8c relative to you, regardless of the fact that you "see" the clock coming at you you much faster

Is there any example of "apparent" superluminal velocity?

[DanMP]

Interesting, so when time slows down for an object moving away from an observer, effectively putting the object behind in time, we also observe this object travelling into the past, being behind in time, by the relayed information delay?

Consider 2 clocks, one stationary and the other moving away. Just before parting, they display the same hour: 00:00. After departure, the clock moving away will show 00:01, then 00:02 and so on, slower than the stationary one, but still going towards future. So it's not "travelling into the past", it's just going towards future at different pace. In fact it's "travelling into the future", for the parting clock/person, because if they get back, they will find people at home passed away or older (twin paradox).

 

Sorry if this was posted before. I didn't read all the posts.

[swansont]

I don't have a problem with the case where acceleration is involved. It's the case where acceleration is removed by the third twin that I struggle with.

And the bit of the wiki page that I quoted in blue, above :

 

"Rather, the time differential between two reunited clocks is produced purely by uniform inertial motion, "

 

Which seems to contradict what you said. Two reunited clocks, and side-by-side clocks, are the same thing.

 

 

There is no way to reunite the clocks without acceleration. What that says is the cause of the time difference is the motion (clock frequency depends on the speed, and the time differential depends on the frequency integrated over the trip duration), not the acceleration. IOW, the details of the acceleration are unimportant, as long as it's an insignificant part of the trip. i.e. it could be essentially instantaneous, or it could be by slingshotting around a star without a change in speed. As long as the time spent at v is the same, the result will be the same.

[mistermack]

 

 

There is no way to reunite the clocks without acceleration. What that says is the cause of the time difference is the motion (clock frequency depends on the speed, and the time differential depends on the frequency integrated over the trip duration), not the acceleration. IOW, the details of the acceleration are unimportant, as long as it's an insignificant part of the trip. i.e. it could be essentially instantaneous, or it could be by slingshotting around a star without a change in speed. As long as the time spent at v is the same, the result will be the same.

Yes, I know that's what it says. But you're not being consistent. Earlier, you said that acceleration has everything to do with which clock reads slower when compared side-by-side. Now you're saying the cause is purely the motion, in line with the article.

 

If you forget clocks, and imagine two new-born babies in relative motion, as in the twin paradox, with the third twin on the return leg.

If the difference in speed is so great, that the time difference is ten to one, on the outward leg, the "static" twin ages ten years, and the "travelling" twin ages one. At that point, the travelling party meets the return ship. They send a photo of the one-year-old to the return ship. At the same instant, a baby is born on the return ship. It makes the return trip at the same speed as the outward one, so the time ratio is again ten to one.

Then when it passes the point of origin, it sends the photo of the original twin, at one year of age, plus the photo of the return twin, again aged one to the static twin. He is now aged twenty, and he can see that in his 20 years, these two babies have only aged one year each.

None of the babies have undergone acceleration, but the ageing for the moving babies was somehow slower than for the "static" one.

 

The question that bothers me is, what decides which is static, and which is moving?

[JohnLesser]

You mostly are telling me there is no time dilation, huh......

A general observation.

Surely we all agree that there is no such thing as time dilation -- it is merely a change or difference in ticking.

 

That is what they are saying.

[iNow]

No, and there's no way you're this unintentionally obtuse. Stop trolling.

[JohnLesser]

No, and there's no way you're this unintentionally obtuse. Stop trolling.

How rude, you are trolling a good thread and trying to ruin it.

 

 

''you'' are insisting both twins always occupy the present, i.e synchronous in time

 

 

Contradictory to a time dilation.

 

I am not sure most of you understand SR.

Edited April 3, 2017 by JohnLesser

[iNow]

''you'' are insisting both twins always occupy the present, i.e synchronous in time

 

Contradictory to a time dilation.

Given that was my first post, I've insisted no such thing.

[JohnLesser]

Given that was my first post, I've insisted no such thing.

Can you read? ''you'' in quotation does not mean you.

[Lord Antares]

How rude, you are trolling a good thread and trying to ruin it.

 

 

''you'' are insisting both twins always occupy the present, i.e synchronous in time

 

 

Contradictory to a time dilation.

 

I am not sure most of you understand SR.

 

No. You don't understand.

Everything is ''occupying'' the present in its state of reference. Nothing is going into the past or the future because time is relative and is ''normal'' for every person's frame of reference.

 

You don't seem to understand that time is relative. You seem to be trying to reconcile time dilation with absolute time.

There is no such thing as absolute time.

[JohnLesser]

 

No. You don't understand.

Everything is ''occupying'' the present in its state of reference. Nothing is going into the past or the future because time is relative and is ''normal'' for every person's frame of reference.

 

You don't seem to understand that time is relative. You seem to be trying to reconcile time dilation with absolute time.

There is no such thing as absolute time.

No, again you are in a way saying there is no time dilation. The two twins can not both occupy the present unless their time lines are equal length.

[Lord Antares]

They would both say that the other twin's time is dilated, but they are both in their present time where time ticks ''normally''.

 

Again, there is no universal time to say that one is in the present and the other is not.

[swansont]

Yes, I know that's what it says. But you're not being consistent. Earlier, you said that acceleration has everything to do with which clock reads slower when compared side-by-side. Now you're saying the cause is purely the motion, in line with the article.

 

 

The acceleration is why one ends up being the slow one. But the different rates are a result of the speed, and they both see the others as slow until there's an acceleration.

''you'' are insisting both twins always occupy the present, i.e synchronous in time

 

 

 

I don't think anyone had advanced this notion.

No, again you are in a way saying there is no time dilation. The two twins can not both occupy the present unless their time lines are equal length.

 

 

There's is only one person who has suggested there is no time dilation, and that is madmac.

 

The problem here is the notion that there is a "present" that has any meaning outside of a single frame of reference. Time is relative. "Now" does not mean someone's watch agrees with someone else's, if they occupy different frames. if my watch reads "1:00" that's the present, for me. If yours reads "11:00", that's the present, for you. We could be standing side-by-side, because you took a fast trip someplace and came back.

[JohnLesser]

 

 

Again, there is no universal time to say that one is in the present and the other is not.

Like I said, the travelling twin drifts into the past. Their relative present being behind in time relative to ground state, then light allows us to see the past.

 

 

The acceleration is why one ends up being the slow one. But the different rates are a result of the speed, and they both see the others as slow until there's an acceleration.

 

 

I don't think anyone had advanced this notion.

 

 

There's is only one person who has suggested there is no time dilation, and that is madmac.

 

The problem here is the notion that there is a "present" that has any meaning outside of a single frame of reference. Time is relative. "Now" does not mean someone's watch agrees with someone else's, if they occupy different frames.

They have.

 

Yes, each present is simultaneity.

Edited April 3, 2017 by JohnLesser