Today I Learned

[DrGreg]

No more Whitworth bolts ?

http://britishfasteners.com/threads/bsf.html

That does appear to be one of the exceptions according to Wikipedia: British Standard Whitworth § Current usage

 

[jim hardy]

One gravity is approximately one light year per square year, useful for those relativistic rockets.

I'm still kinda stunned by that thought.
Are photons affected by gravity?
If so, do they arrive here with a wavelength different than they started out with ?

Have a laugh at that question i don't mind
i'm still struggling with modern physics
as Leonard Cohen says, 'waiting for the miracle to come... 'old jim

ps i kept my Whitworth wrenches .

 

[OCR]

That is an amazing story.

Yeah, it is an amazing story....

Here is the video of The Gimli Glider incident ... http://www.dailymotion.com/video/x28kvkj_air-disasters-gimli-glider-season-1-episode-2_tv .

 

[Astronuc]

TIL Russia is doing simulated nuclear attacks on Sweden, and has apparently been violating Swedish airspace. Sweden is a neutral country.
http://nationalinterest.org/blog/th...raft-killer-missile-now-service-its-not-17100

We do know that one test off Scotland sent the missile “well in excess of 100 kilometers,” an MBDA engineer told AINonline.

But Sweden is a major weapons exporter — the 12th largest according to 2014 data from the Stockholm International Peace Research Institute.

Sweden works closely with the NATO alliance during military exercises. And in May, the Swedish parliament ratified an agreement allowing NATO more room to conduct exercises in the country.

 

[Stephanus]

@Stephanus: the 300,000,000 is meter/second. It is m/s^2.

You've slipped up somewhere - I agree that the answer should be c / year, but that should be $\simeq (3\times 10^8 \mathrm {ms^{-1}}) / (\pi \times 10^7 \mathrm s)\simeq 9.5 \mathrm {ms^{-2}}$, about 3% below g=9.81ms^-2.

Yep, you're right. My mistake. I'm off by 000 .

 

[Stephanus]

I just hope that, post-Brexit, we don't abandon the metric system!

Talking about Brexit...
http://www.dailymail.co.uk/news/art...ridlock-Indonesian-junction-named-BREXIT.html
I live in Indonesia, Brebes is some 200 km east of me. Every year in Idul Fitri celebration, millions people from Jakarta are coming to their hometown. And most of them pass the north route. The jam in Brebes Exit Tol is so severe. Cars can get stuck for days in 40 km long traffic jam.

 

[Ibix]

I'm still kinda stunned by that thought.

There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration. However this particular choice of units is very handy for thinking about relativistic rockets for a number of reasons. A light year is an appropriate scale for interstellar distances; a year is an appropriate time scale, and useful for "how old will I be when I get there"; the speed of light is exactly 1 by definition so all the annoying factors of c in relativistic equations drop out; the acceleration due to gravity at Earth's surface is very close to 1, and most people want to accelerate their rockets at 1g so it feels like home inside.

Are photons affected by gravity?

If so, do they arrive here with a wavelength different than they started out with ?

This is unrelated to the above. But yes, light is most certainly affected by gravity. Sometimes astronomers see multiple images of a galaxy because a nearer galaxy pulls the light out of a straight line path. https://en.m.wikipedia.org/wiki/Gravitational_lens. Also, black holes wouldn't be black if their gravity couldn't trap light.

Yes, light can arrive with a different wavelength compared to when it was emitted if the emitter and receiver are at different heights (more formally, different gravitational potentials). Pound and Rebka were the first to verify this experimentally. https://en.m.wikipedia.org/wiki/Pound%25E2%2580%2593Rebka_experiment

I'm drifting pretty far off topic.

 

[Stephanus]

I'm still kinda stunned by that thought.

There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration.

It stuns me, too. Even the units match. Gravity = Distance/time square.
Light year per year square = distance per time square.
[Edit: the unit and the number matches]

 

[jim hardy]

Thanks ibix ! I saw the convenience of the units ,

There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration. However this particular choice of units is very handy for thinking about relativistic rockets for a number of reasons. A light year is an appropriate scale for interstellar distances; a year is an appropriate time scale, and useful for "how old will I be when I get there"; the speed of light is exactly 1 by definition so all the annoying factors of c in relativistic equations drop out; the acceleration due to gravity at Earth's surface is very close to 1, and most people want to accelerate their rockets at 1g so it feels like home inside.

to make things so simple is the mark of genius.

The pound rebka link returns a "bad title " error,
is this it ?
https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

Fascinating, thank you,
my old mentor said that would happen , but we were not aware of the experiment.

I think your post is right on topic for "today i learned"

 

[Stephanus]

...I'm drifting pretty far off topic.

I think your post is right on topic for "today i learned"

Far off?? I'd say deep into.

 

[epenguin]

I'm still kinda stunned by that thought.
Are photons affected by gravity?
.

Yes. That's why black holes are black.

I'm sure you really knew that.

 

[jim hardy]

I'm sure you really knew that.

yes,
you know how something rattles around in the back of your mind and you can't even phrase the question ?
I'm still trying to weigh that with respect to Hubble's Red Shift

plodder that i am a lot of things just never have clicked together.

old jim

 

[Stephanus]

I'm still trying to weigh that with respect to Hubble's Red Shift

Hubble red shift??
Don't tell me.
Today I learned that constant can change??

The universe has expanded and the Hubble Constant has changed its value since then.

 

[jim hardy]

Don't tell me.

Tell ?? I'm only in a position to ask...

 

[fresh_42]

Tell ?? I'm only in a position to ask...

Sokrates?

 

[Stephanus]

No, Jim. It's just that you guys are discussing about Hubble, and just several hours ago I received that reply, that can't help my hand to comment. Frankly, I didn't read all your and Mr. epenguin's comment. It's just that red shift comes up and I recall the answer I had this afternoon (my time).

 

[NascentOxygen]

Yes, light can arrive with a different wavelength compared to when it was emitted if the emitter and receiver are at different heights (more formally, different gravitational potentials). Pound and Rebka were the first to verify this experimentally. https://en.m.wikipedia.org/wiki/Pound%25E2%2580%2593Rebka_experiment

Browsers don't like that URL.

 

[mfb]

Hubble red shift??
Don't tell me.
Today I learned that constant can change??

Hubble red shift probably means "the redshift due to the Hubble constant".

But the Hubble constant is actually changing, it decreases slowly. It is constant in terms of the distance of objects (if they are not too far away), but not constant in time.

 

[Astronuc]

TIL that a third of claims for lightning strikes on boats in Florida!
http://www.boatus.com/seaworthy/swlightning.asp

Thirty-three percent of all lightning claims are from the Sunshine State and the strike rate there is 3.3 boats per thousand. Surprisingly, the second most struck area in the country is the Chesapeake Bay (twenty-nine percent), and those who boat there in the summer can attest to the ferocity of the sudden thunderstorms. Not surprisingly, the majority of strikes are on sailboats (4 per 1000), but power boats get struck also (5 per 10,000); Trawlers have the highest rate for power boats (2 per 1000) and lightning has struck houseboats, bass boats, and even PWCs.

 

[Cdz]

Today I learned that I registered on PF more than one year ago!

 

[Pepper Mint]

Today I experienced or understood not knew or learned how young fresh girls' acts and words can change a straight man's mind completely. It's the power of lust or sexual urge not love. And that does occur in life as always.
So you! all physics ladies should always be aware of how and what your spouses are thinking and want.

 

[Ibix]

The pound rebka link returns a "bad title " error,
is this it ?
https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

Yes.

TIL two things. One, that snipping the URL out of the super-long URL that Google provides doesn't always reduce to the valid URL. Two, I should always check my links...

 

[jim hardy]

It's the power

While raising teenage daughters i realized they are not very aware of it.
It is very potent. Please be careful with it.

 

[ProfuselyQuarky]

Today I learned what Wellie Wanging is--pretty much a sport consisting of throwing boots as far as possible.

 

[fresh_42]

Today I learned what Wellie Wanging is--pretty much a sport consisting of throwing boots as far as possible.

I'll have to scale up my font sizes. I misread this.

 

[OmCheeto]

Today I learned that humans emit black body radiation in the visible wavelengths. Though, it is at a rate too low to see.

 

[zoobyshoe]

Today I learned that humans emit black body radiation in the visible wavelengths. Though, it is at a rate too low to see.

I wonder if those fish that live 5 miles deep in the ocean with huge eyes could see it.

 

[jim hardy]

Today I learned that humans emit black body radiation in the visible wavelengths. Though, it is at a rate too low to see.

Well now -
That explains "Women's Intuition" .

Vive la difference !

 

[1oldman2]

visible wavelengths. Though, it is at a rate too low to see.

Dang, that sounds contradictory

 

[Ibix]

Dang, that sounds contradictory

Not really. Your retina needs (from memory) ~20-30 visible light photons on one cell to trigger a detection event. So to form an image you need 20-30 photons on each light sensitive cell. Anything emitting at a lower rate is not really emitting detectably to an unaided human, although it may be technically emitting.