Ask a physicist anything. (3)

[TemperateSeaIsland]

We hear about relativity being taken into account with GPS but does it also need to be taken into account with communication satillites?

 

[Wiccan_Child]

We hear about relativity being taken into account with GPS but does it also need to be taken into account with communication satillites?

The higher up you go, the more noticeable the effects are. It also doesn't help if you're in anything but a geostationary orbit: flying one way round the planet can shift your internal clock in a different way than if you flew the other way, relative to a ground clock.

GPS can have some acceptable delay, but communications satellites have less leeway; both have to make use of relativistic mechanics. Damn you Einstein!

 

[AV1611VET]

What is the average thickness of the dust on the moon in inches?

 

[Wiccan_Child]

What is the average thickness of the dust on the moon in inches?

The lunar regolith, the uppermost geological layer, is 5m thick on the mares, or 10m thick at older regions- so it's about 200 to 400 inches thick.

The actual dusty part of the Moon's regolith is only a few centimetres thick (i.e., about an inch).

 

[tanzanos]

What is the average thickness of the dust on the moon in inches?

INCHES? Who uses imperial measurements? As for your question I seriously suspect that you aim to give us the erroneous answer: "Based on measured rates of planetary dust accumulation, there is too little moon dust for an old moon" or something of that nature.

 

[Wiccan_Child]

INCHES? Who uses imperial measurements? As for your question I seriously suspect that you aim to give us the erroneous answer: "Based on measured rates of planetary dust accumulation, there is too little moon dust for an old moon" or something of that nature.

Now now, he may be legitimately interested

 

[AV1611VET]

INCHES? Who uses imperial measurements? As for your question I seriously suspect that you aim to give us the erroneous answer: "Based on measured rates of planetary dust accumulation, there is too little moon dust for an old moon" or something of that nature.

Not in this thread --

 

[AV1611VET]

The lunar regolith, the uppermost geological layer, is 5m thick on the mares, or 10m thick at older regions- so it's about 200 to 400 inches thick.

The actual dusty part of the Moon's regolith is only a few centimetres thick (i.e., about an inch).

Thank you, sir!

 

[Wiccan_Child]

Not in this thread --

If it's the thread I'm thinking of, I'm keeping my nose well out

Thank you, sir!

My thread's URL is always open

 

[Supreme]

Is it true that standing near large objects, such as the pyramids, slows down time? And could we build a railway across the Earth, and go at nearly light speed, and skip time? (For example, a week would pass at the train travelling near light speed and 100 years would pass outside the train). How does this work? Thanks.

 

[Wiccan_Child]

Is it true that standing near large objects, such as the pyramids, slows down time?

The deeper you are in a gravitational well, the slower time flows for you compared to someone higher up (e.g., someone on Earth has slower clocks than someone on the ISS). Standing next to pyramids would

And could we build a railway across the Earth, and go at nearly light speed, and skip time? (For example, a week would pass at the train travelling near light speed and 100 years would pass outside the train). How does this work? Thanks.

When we a spaceship moving very fast, we see its clocks ticking slower than ours. But this works both ways: those people on the spaceship can look back at Earth and see its clocks tick slower. The trick is acceleration.

If the spaceship (or the train, or whatever) suddenly decelerates, then it sees the world around it suddenly 'go faster through time' - this is because all the light that was emitted by the world at time t=1, t=2, t=3, etc, is suddenly able to catch up with the now-slower train.

So, yes, you could go in a train, accelerate to near lightspeed, then decelerate such that you're not 100 years in the future. 'Course, we've only been able to shave a few nanoseconds off atomic clocks, which isn't quite time travel yet

 

[Supreme]

The deeper you are in a gravitational well, the slower time flows for you compared to someone higher up (e.g., someone on Earth has slower clocks than someone on the ISS). Standing next to pyramids would


When we a spaceship moving very fast, we see its clocks ticking slower than ours. But this works both ways: those people on the spaceship can look back at Earth and see its clocks tick slower. The trick is acceleration.

If the spaceship (or the train, or whatever) suddenly decelerates, then it sees the world around it suddenly 'go faster through time' - this is because all the light that was emitted by the world at time t=1, t=2, t=3, etc, is suddenly able to catch up with the now-slower train.

So, yes, you could go in a train, accelerate to near lightspeed, then decelerate such that you're not 100 years in the future. 'Course, we've only been able to shave a few nanoseconds off atomic clocks, which isn't quite time travel yet

Thanks for the answer (I can't pretend to fully understand, but I'm just not a physics kinda guy).

Stephen Hawkins was right!

(I was watching his fascinating programme on Channel 4 recently)

He also proposed that aliens would obliterate humanity if we came into contact with them, as they could have created ways to slow down ageing and become immortal. He also said that time travel backwards would be impossible, as it could create a paradox. Time travel into the future could be possible by orbiting a supermassive black hole, as due to its size, time slows down near it, although that would be VERY tricky.

 

[Wiccan_Child]

Thanks for the answer (I can't pretend to fully understand, but I'm just not a physics kinda guy).

Stephen Hawkins was right!

(I was watching his fascinating programme on Channel 4 recently)

Channel 4! The closest thing to the BBC since the BBC

He also proposed that aliens would obliterate humanity if we came into contact with them, as they could have created ways to slow down ageing and become immortal.

I'm not convinced that we can make such grand and sweeping statements about alien life. Some people say they must be non-violent (since a violent species would inevitably destroy itself), some say they must be violent (since they would have an exponential need for resources), some say they must be on Earth already (since they're advanced enough to do so), etc.

Short of asking them themselves, we can't really know for sure. Though the recently discovered Earth-like planet a mere 20 lightyears away (!!!) makes me wonder why we haven't been talking to aliens yet. Fermi, you cunning dog...

He also said that time travel backwards would be impossible, as it could create a paradox. Time travel into the future could be possible by orbiting a supermassive black hole, as due to its size, time slows down near it, although that would be VERY tricky.

It isn't any more theoretically problematic than having something in orbit. The trick is getting close enough to time travel practically, without falling in...

 

[tanzanos]

Not in this thread --

I knew you were up to something!

 

[AV1611VET]

I knew you were up to something!

Ain't I always --

 

[AV1611VET]

FYI, WC, you can move up to 'tentmaker' now, eh?

ETA: Oops -- nevermind -- my bad.

 

[tanzanos]

FYI, WC, you can move up to 'tentmaker' now, eh?

ETA: Oops -- nevermind -- my bad.

Bedouin cherish and respect tent makers. After all where would they be without one?

 

[tanzanos]

Time dilation is only observable by an external observer. One cannot see his own time slowing down. His time will always be constant to him but to an external observer it will either look slow or fast depending on the relative speed of either party.

If I could travel at relativistic speeds I would not notice any difference in time. Now if I were to make a round trip a few light years long then on my return I would find that Earth time has overtaken my time and everyone would have aged more than myself. Even though my clock ticked normally according to me I would discover to my surprise that earth clocks and my clock are no longer in sync.

 

[Wiccan_Child]

Time dilation is only observable by an external observer. One cannot see his own time slowing down. His time will always be constant to him but to an external observer it will either look slow or fast depending on the relative speed of either party.

Actually, it will always look slow to an outside observer. Time dilation in special relativity works both ways - both clocks see the other as the one that's moving, and, thus, the one that's slow.

If I could travel at relativistic speeds I would not notice any difference in time.

You'd notice external clocks (by which your speed is measured) to be running slower than your on-board clock. If you decelerated, you'd notice the engine side of your ship tick faster by a tiny fraction.

 

[Chesterton]

Bedouin cherish and respect tent makers.

Except for St. Paul.