# Ask Me AnythingAMA with the NSF's Dr. Dave Boboltz

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#### David-J-Franks

Hi Dr. David Boboltz,

Thank you for coming to the forum, it's a rare privilege for a member of the public to communicate with a professional scientist.

I learned from reading these forums that light speed can only be measured involving a two-way round trip. The problem is that you cannot guarantee to synchronise two clocks, one for each end of the experiment. If you synchronise two clocks next to each other and then move them apart, the movement and acceleration, according to relativity, will alter the timing. Synchronising the clocks in the middle of the experiment and moving each one an equal distance to each end still does not guarantee the clocks have not been moving in opposite directions through a flowing ether. For similar reasons you can't connect a cable between the clocks and synchronise them either because once again you can't guarantee the electricity will flow at the same speed in opposite directions.

When someone says you can't do something it gets me thinking. I came up with the following thought experiment and I would like your opinion on it, please.

What if you have a mechanical clock with a rotating dial floating in space connected to another dial with a very long shaft, say one kilometre or even 10 kilometres long. Would this mean each end of the shaft is exactly synchronised? If so, all you have to do now is shine a laser beam from one dial to the other and record the time it takes for a one-way trip, job done? You could then rotate the shaft 90 degrees to measure the speed in another direction just to check if there is a flowing ether. Also instead of a dial, you could have slits in a disc as in some of the original experiments used to measure the speed of light.

Also, you could have one end of the shaft nearer a strong gravitational body where time is supposed to slow down so what happens to the other end of the shaft?

Yet again you could also spin one end of the shaft around the other and the different velocities again should produce different times, so what happens to each end of the shaft in that case?

I know next to nothing about relativity, so you may either laugh or be unable to answer it, I'm not sure which.

Thank you

#### DrDaveBoboltz

Verified Expert
That's interesting!

Somewhat similarly, for protostars, seemingly regardless of mass, the bipolar flows are very powerful and very likely attributed to magnetic interactions both within the protostar and with the inner accretion disk. [I'm still reading one of Bo Reipurth's books. ]

1) For red giants, like newborns, is it the magnetic field activity that causes asymmetry, thus aligning with their magnetic poles.?

For evolved stars the mechanism is thought to be a little different than young stars although there are some similarities. For evolved stars, their outer envelope gets so big (think the orbit of Jupiter) that the plasma is only loosely gravitationally bound. The star starts to shed material into its surroundings. This material can sometimes accumulate more toward the equatorial plane of the star than the poles, perhaps cause by a remnant disk of material or planetary system. The subsequent outflowing material is constrained by the denser material along the equator and you can end up with the bipolar shapes that are seen for example in the Hourglass nebula or Eta Carina.

2) Are they even bipolar fields?

The last question presents a puzzle to me. The Sun seems to have two N-S magnetic fields -- one in the northern hemisphere and one is the south. This field flips with the "solar cycle" (~ 11 years), so a full "solar cycle" can be considered to be 22 years, to get the N-S field back to where it was.

Nearly all stars have a dipole field like the Earth or the Sun caused by the dynamo effect. For the Sun the field is generated at the interface between the faster rotating core of the star where most of the nuclear burning takes place and the more slowly rotating outer convective envelope. Because the convective region of the Sun rotates faster at the equator than it does at the poles, the dipole magnetic field gets stretched out into more of a toroidal shape. Material tends to migrate toward the poles in the convective process carrying the magnetic field with it. This causes the field to switch poles as you said about every 22 years. Then the whole process starts over. This is a way over simplification.

3) Do you have some nice paint brushes that can paint what a quad-like mag. field looks like and how it came to be? [Assuming my assertions are even correct.]

Check out this paper which actually measured the quadrupole component of the Sun's magnetic field.

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Helio

#### KC Strom

Hello Dr. Boboltz.

Are radio astronomy observations affected (if that is the correct term) by relativistic, gravitational, or cosmological redshift? If so, in what way.

#### DrDaveBoboltz

Verified Expert
Hi Dr. David Boboltz,

Thank you for coming to the forum, it's a rare privilege for a member of the public to communicate with a professional scientist.

I learned from reading these forums that light speed can only be measured involving a two-way round trip. The problem is that you cannot guarantee to synchronise two clocks, one for each end of the experiment. If you synchronise two clocks next to each other and then move them apart, the movement and acceleration, according to relativity, will alter the timing. Synchronising the clocks in the middle of the experiment and moving each one an equal distance to each end still does not guarantee the clocks have not been moving in opposite directions through a flowing ether. For similar reasons you can't connect a cable between the clocks and synchronise them either because once again you can't guarantee the electricity will flow at the same speed in opposite directions.

When someone says you can't do something it gets me thinking. I came up with the following thought experiment and I would like your opinion on it, please.

Hi @David-J-Franks it is really my pleasure. Everyone here has posted really thought-provoking questions making me think out of my normal box.

You pose some interesting questions. Let's think about them one at a time.

What if you have a mechanical clock with a rotating dial floating in space connected to another dial with a very long shaft, say one kilometre or even 10 kilometres long. Would this mean each end of the shaft is exactly synchronised? If so, all you have to do now is shine a laser beam from one dial to the other and record the time it takes for a one-way trip, job done? You could then rotate the shaft 90 degrees to measure the speed in another direction just to check if there is a flowing ether. Also instead of a dial, you could have slits in a disc as in some of the original experiments used to measure the speed of light.

For the mechanical clock, let's first assume that the separated dials are connected by a massless, infinitely-rigid rod, and there is no delay in transmitting the torque from the clock down the connecting rod to the second dial. I could foresee two issues with this scenario.

First is that the observer would have to view the location of the hands on the distant dial when the laser traverses the distance between the two. Somehow that information would have to travel back to the observer at the near dial so they could measure the offset (maybe they could observe the distant dial with a telescope). The travel time for that information to get back to the observer at the near dial would be limited by the speed of light itself and would interfere with the measurement itself. Basically, the observer can't read the time off of the distant dial instantaneously, so we're kind of back to the round trip measurement.

The second issue is that in addition to time dilation, special relativity results in length contraction. The length of a ruler in a moving frame appears shorter to the observer in the stationary frame. The hand on a clock dial is basically an angular distance measurement. So, I think to the observer at the near dial, the distance between the ticks on the far dial will appear shorter and the total distance that the hand moves shorter due to length contraction.

Now, what if we add mass to the mechanical rod. In this case, it would take a finite amount of time for the mechanical wave that transmits the torque down the rod to move the hands on the distant dial. This mechanical wave cannot travel faster than the speed of light, and would result in an extra delay between the near dial and the far dial impacting the speed of light measurement.

Also, you could have one end of the shaft nearer a strong gravitational body where time is supposed to slow down so what happens to the other end of the shaft?

So in this case, not only would we have to consider the effects of special relativity, but since we are near a gravitational body we would have to consider general relativistic effects, i.e., the bending of space-time near such a body. General relativity considerations would make the measurement even more inaccurate. The Global Positioning System (GPS) satellites, since they are basically atomic clocks orbiting the Earth (a strong gravitational body), must take into account both the effects of special relativity and general relativity.

Yet again you could also spin one end of the shaft around the other and the different velocities again should produce different times, so what happens to each end of the shaft in that case?

I think maybe I covered this one in the length contraction part of question 1. Velocity is change in distance over change in time. I think either length contraction (change in distance) or time dilation (change in time) would get you.

I know next to nothing about relativity, so you may either laugh or be unable to answer it, I'm not sure which.

Thank you

I tried my best. It's been a long while since I had to think about relativity problems.

David-J-Franks

#### David-J-Franks

Hi @David-J-Franks it is really my pleasure. Everyone here has posted really thought-provoking questions making me think out of my normal box.

You pose some interesting questions. Let's think about them one at a time.

For the mechanical clock, let's first assume that the separated dials are connected by a massless, infinitely-rigid rod, and there is no delay in transmitting the torque from the clock down the connecting rod to the second dial. I could foresee two issues with this scenario.

First is that the observer would have to view the location of the hands on the distant dial when the laser traverses the distance between the two. Somehow that information would have to travel back to the observer at the near dial so they could measure the offset (maybe they could observe the distant dial with a telescope). The travel time for that information to get back to the observer at the near dial would be limited by the speed of light itself and would interfere with the measurement itself. Basically, the observer can't read the time off of the distant dial instantaneously, so we're kind of back to the round trip measurement.

The second issue is that in addition to time dilation, special relativity results in length contraction. The length of a ruler in a moving frame appears shorter to the observer in the stationary frame. The hand on a clock dial is basically an angular distance measurement. So, I think to the observer at the near dial, the distance between the ticks on the far dial will appear shorter and the total distance that the hand moves shorter due to length contraction.

Now, what if we add mass to the mechanical rod. In this case, it would take a finite amount of time for the mechanical wave that transmits the torque down the rod to move the hands on the distant dial. This mechanical wave cannot travel faster than the speed of light, and would result in an extra delay between the near dial and the far dial impacting the speed of light measurement.

So in this case, not only would we have to consider the effects of special relativity, but since we are near a gravitational body we would have to consider general relativistic effects, i.e., the bending of space-time near such a body. General relativity considerations would make the measurement even more inaccurate. The Global Positioning System (GPS) satellites, since they are basically atomic clocks orbiting the Earth (a strong gravitational body), must take into account both the effects of special relativity and general relativity.

I think maybe I covered this one in the length contraction part of question 1. Velocity is change in distance over change in time. I think either length contraction (change in distance) or time dilation (change in time) would get you.

I tried my best. It's been a long while since I had to think about relativity problems.
Hi Dr. David Boboltz,

I can see you put a lot of time and thought into your answer, thank you very much for that!

However, I didn't provide enough background details so I don't think you have quite grasped what I'm getting at - my fault.
For the mechanical clock, let's first assume that the separated dials are connected by a massless, infinitely-rigid rod, and there is no delay in transmitting the torque from the clock down the connecting rod to the second dial. I could foresee two issues with this scenario.
The rod is caused to rotate at a constant speed well before the experiment starts. Being in space the rod will continue to rotate indefinitely. With no friction, the rod does not need to be massless nor infinitely rigid, so no issues with torque or mechanical waves.

You raise two issues;

First issue - The purpose of the experiment is to make a one-way measurement for the speed of light. I'm also proposing an observer at the distant disc end, so there is no need for communication between the distant observer and the near end observer. The distant observer only has to note on their dial when the laser pulse was received, I did not say I wanted the answer back from the second distant disc immediately, it does not matter if the distant observer writes it on paper and sends it to the near observer one hour later.

The near observer does not have to view the hands on the distant disc, so no 'speed of communication' problem.

I'm not proposing instant communication by mechanical means. I think what's happening is that because the dials are mechanically linked there is 'instant knowledge of where the dial is on each end of the rod by each observer.

If it is pre-arranged for the near observer to send a laser pulse every time their dial passes the zero mark the distant observer will 'know' this without any communication. So when they measure the time of reception on their disk they can calculate the speed of light without even having to communicate back with the near observer.

The dials of course can be connected to electronic counters.

Second issue - This is also solved because the near observer does not need to observe the distant disc so relativity does not apply here.

Once again the whole point of my thought experiment was because I read you cannot do this measurement by synchronising two clocks and moving them apart because the movement and acceleration cause them to be unsynchronised.

Many thanks once again

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#### DrDaveBoboltz

Verified Expert
Hi Dr. Dave,

Will the magnetic radiation shielding for astronauts will be available on Artemis Moon missions and how feasible it will be?

Thanks

Hi @suneritz, thanks for the question.

I think the magnetic radiation shield discussed in the link you sent is still in the development stage, and wouldn't be likely at least for the early Artemis missions. This article on the NASA web pages talks about how they plan to protect the astronauts against the radiation from both the Sun and from sources outside the solar system that send more energetic cosmic rays our way.

The big problem with any shield is having to lift the necessary mass into orbit and get it to the destination. For the travel to the moon on the Orion spacecraft, the trip is a few days, so the chance of being impacted by a solar storm is decreased (but not zero). By 2025 the Sun will be nearing solar maximum where it is more active and the chance of an eruption is greater. The plan seems to be that the astronauts would hunker down in a temporary shelter made from stuff already on the Orion module. For the orbiting Gateway, I would expect that there would be a more permanent solar storm shelter made from innovative materials for astronaut protection.

For a Mars mission, the exposure time for the trip to and from Mars is on the order of years. In addition, the Sun's magnetic field (which provides some protection from galactic cosmic rays) is weaker the further out you go out, and thus provides less protection during the trip. The NASA page below mentions some of the more active magnetic field shielding mentioned in the link you sent, which might be available for a future Mars mission.

Dave

suneritz

#### DrDaveBoboltz

Verified Expert
Hello Dr. Boboltz.

Are radio astronomy observations affected (if that is the correct term) by relativistic, gravitational, or cosmological redshift? If so, in what way.

Hi @KC Strom, thanks for the question.

Yes, radio observations are affected by both relativity and cosmological redshift. For redshift, one example is the Cosmic Microwave Background (CMB) which is the primordial heat left over from the Big Bang. This approximately 3000K blackbody radiation was originally emitted in the infrared during a period called the Epoch of Recombination about 379,000 years after the Big Bang. In the subsequent 13.3 billion years it took for the light to travel to the observer here on Earth, it was cosmologically redshifted, due to the expansion of the Universe, to the microwave (radio) part of the spectrum with a temperature of about 2.7K. The CMB radiation was first detected in 1964 by Penzias and Wilson using an early radio telescope, for which they won the 1978 Nobel prize.

There are several examples of radio astronomy observations being used to test Einstein's theory of relativity. Several of these have used radio telescopes to measure the precise position shift of the emission from distant quasars caused by the bending of light near a large gravitational bodies in our solar system like the Sun or Jupiter. In addition to bodies in our solar system causing these light bending effects, sometimes when the light from a distant quasar encounters a massive intervening body like a foreground galaxy, a gravitational lens effect can form something called an Einstein ring. The fist image of an Einstein ring was made with the Very Large Array radio telescope in 1986.

Dave

#### Helio

Nearly all stars have a dipole field like the Earth or the Sun caused by the dynamo effect. For the Sun the field is generated at the interface between the faster rotating core of the star where most of the nuclear burning takes place and the more slowly rotating outer convective envelope. Because the convective region of the Sun rotates faster at the equator than it does at the poles, the dipole magnetic field gets stretched out into more of a toroidal shape. Material tends to migrate toward the poles in the convective process carrying the magnetic field with it. This causes the field to switch poles as you said about every 22 years. Then the whole process starts over. This is a way over simplification.

View attachment 27

Check out this paper which actually measured the quadrupole component of the Sun's magnetic field.

Thanks much for the eloquent elaboration! That does indeed help.

#### DrDaveBoboltz

Verified Expert
Hi Dr. David Boboltz,

I can see you put a lot of time and thought into your answer, thank you very much for that!

However, I didn't provide enough background details so I don't think you have quite grasped what I'm getting at - my fault.
The rod is caused to rotate at a constant speed well before the experiment starts. Being in space the rod will continue to rotate indefinitely. With no friction, the rod does not need to be massless nor infinitely rigid, so no issues with torque or mechanical waves.

Thanks @David-J-Franks for the additional info on the thought experiment. To your first point, there is no such thing as a frictionless rod, so you would have to have something driving the hands on the clock in order to maintain the constant speed. That torque would have to be transmitted down the rod to drive the distant hand with some finite time delay. I think that even if it were frictionless, at some point you would have had to have started the clock, and that initial impulse would have to have been transmitted down the rod with some finite delay causing the far hand to be permanently offset from the near hand. That far hand could never catch up because the rod is not infinitely rigid, you 'd always have a bit of twist.

You raise two issues;

First issue - The purpose of the experiment is to make a one-way measurement for the speed of light. I'm also proposing an observer at the distant disc end, so there is no need for communication between the distant observer and the near end observer. The distant observer only has to note on their dial when the laser pulse was received, I did not say I wanted the answer back from the second distant disc immediately, it does not matter if the distant observer writes it on paper and sends it to the near observer one hour later.

The near observer does not have to view the hands on the distant disc, so no 'speed of communication' problem.

I'm not proposing instant communication by mechanical means. I think what's happening is that because the dials are mechanically linked there is 'instant knowledge of where the dial is on each end of the rod by each observer.

If it is pre-arranged for the near observer to send a laser pulse every time their dial passes the zero mark the distant observer will 'know' this without any communication. So when they measure the time of reception on their disk they can calculate the speed of light without even having to communicate back with the near observer.

So, I think that even if there was a second observer, there would be no way for the two observers to agree on when
their dial passes the zero mark. The distant observer has to know when to start counting the ticks.

The dials of course can be connected to electronic counters.

How do you synchronize the counters? An electronic counter is just a clock.

Second issue - This is also solved because the near observer does not need to observe the distant disc so relativity does not apply here.

Once again the whole point of my thought experiment was because I read you cannot do this measurement by synchronising two clocks and moving them apart because the movement and acceleration cause them to be unsynchronised.

I think the second observer would suffer from the same effect as a clock would. The two observers synchronize their watches, then the second observer accelerates in his ship out to the far hand of the mechanical clock, and time is now slightly different for him. He is no longer in sync with the near observer.

Many thanks once again

You are welcome. It is fun to think about these sorts of experiments.

David-J-Franks

#### DrDaveBoboltz

Verified Expert
Hi all,

The Space.com forum moderators were kind enough to leave my AMA open an additional day so I could try to respond to all the remaining questions. Thanks again for all of the really insightful questions and discussion, I really enjoyed myself.
Hopefully they'll let me do this again sometime.

Cheers,
Dave

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