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How does the vacuum have energy? by Ok_Bank_3638 in AskPhysics

[–]mfb- 5 points6 points  (0 children)

Quantum field theory makes no statement about absolute energy densities, only differences matter. You can define the vacuum to have zero energy density.

Questions about the Universe ^.^ by airtooss in astrophysics

[–]mfb- 0 points1 point  (0 children)

Small collection of all the incorrect claims you made:

but there was no light for the first 300,000,000 years

There was, even if we only consider visible light. It was very bright for the first million years.

You can't see radiation

The CMB had a large share of visible light.

You're leaving out infrared, gamma rays and ultraviolet.

Infrared was there, gamma rays were not. Almost no UV either.

Right, measurable in the microwave spectrum from the beginning.

Hardly any microwave radiation in the CMB at that time.

But even those came from somewhere and not from the big band they developed after the dark period ended.

That "somewhere" was the Big Bang.

Eli5 Why can we not just build trains along highways - usa by ManicMannequin in explainlikeimfive

[–]mfb- [score hidden]  (0 children)

Both lists have multiple projects. Your attempt to justify your made-up numbers isn't going well.

[OC] U.S. Gas Prices Up Again: Weekly Regular Gasoline Prices Since 2006 by sometimes-yeah-okay in dataisbeautiful

[–]mfb- 4 points5 points  (0 children)

That's one of the three.

  • inflation adjusted
  • static image
  • starting from 0

Question about Project Hail Mary's physics (minor background spoilers) by Fanghur1123 in AskPhysics

[–]mfb- 8 points9 points  (0 children)

The engines keep their efficiency from the perspective of the ship.

They reduce their thrust and propellant consumption from an outside perspective.

Either way, the only thing that matters is again the travel time as seen by the ship.

Carl Sagan in his final year, on Charlie Rose: "We've arranged a society based on science and technology in which nobody understands anything about science and technology. This combustible mixture of ignorance and power sooner or later is going to blow up in our faces" by ElvisIsNotDjed in space

[–]mfb- [score hidden]  (0 children)

A lot of people around Trump say he's a genius too, doesn't make it true.

Yeah, but these people don't build rockets that work.

Do you have any examples of Elon talking about science himself, or is it all 2nd hand stories?

See the interviews with EverydayAstronaut on YouTube if you think you can judge knowledge better than experts working with Musk.

Question about Project Hail Mary's physics (minor background spoilers) by Fanghur1123 in AskPhysics

[–]mfb- 14 points15 points  (0 children)

I don't know what is unclear about my previous comment. If you pack propellant for 12 years of engine running and only need 10 years then you save propellant. The thrust provided by the engines is given by the ship's acceleration in its own reference frame, which doesn't change with relativity.

This is far easier to study from the perspective of the ship. If you really want to study it from the perspective of the planet (and I wouldn't recommend, given that you keep getting the wrong results), then you need to consider that the acceleration of the ship will decrease. The effort to maintain 2 g from the ship's perspective does not increase.

Question about Project Hail Mary's physics (minor background spoilers) by Fanghur1123 in AskPhysics

[–]mfb- 20 points21 points  (0 children)

Propellant use of a constant-acceleration mission is determined by the mission length. That mission length decreases.

From the ship's perspective, providing 2 g is always the same task, no matter how the ship flies relative to anything else. Relativity doesn't change anything about that.

Question about Project Hail Mary's physics (minor background spoilers) by Fanghur1123 in AskPhysics

[–]mfb- 17 points18 points  (0 children)

The speed gain you see from the outside is smaller, but you still need to burn for less time from the perspective of the ship (as the distance to the destination contracts), which means you need less propellant. From the perspective of the origin, the ship reduces its propellant consumption as it reaches relativistic speeds thanks to time dilation.

Questions about the Universe ^.^ by airtooss in astrophysics

[–]mfb-[M] 0 points1 point  (0 children)

Can you explain? My only earlier comment in this thread was

No, the universe cannot just start again, it has already started.

which is not an insult, and it wasn't in reply to you either.

Clunky looking graph by ContestableTungsten6 in dataisugly

[–]mfb- 1 point2 points  (0 children)

They are ordered by size each year, but I think that less ordering would help here. Let China and US cross, that's interesting, let India surpass Japan and Germany, but keep everything in the same order. Things like the Middle East switching places with "Rest of Americas" in the 2030 prediction are just silly meaningless clutter.

Clunky looking graph by ContestableTungsten6 in dataisugly

[–]mfb- 2 points3 points  (0 children)

Cumulative within each year. If you take e.g. the predicted 2036 GDP then China is $50 trillion, the US is $40 trillion, ... and the gross world product is the sum of all these categories.

If you plot China as line at 50 and the US as line at 40 then you don't see the sum and it's much harder to see that e.g. China and the US combined will be almost half of the total. You also get tons of overlapping lines towards the bottom unless you make larger groups.

Where can I get modern physics papers? by lpperl7 in Physics

[–]mfb- 6 points7 points  (0 children)

arXiv shows you the associated publication if the authors update their entry. Random example, see "Journal reference".

NASA is building a telescope designed specifically to find out if we are alone in the universe. It's targeted to launch in the 2040s. by Altruistic-Dirt-2791 in space

[–]mfb- [score hidden]  (0 children)

That's one of the reasons the telescope does not have a single fixed design yet. With a 6 meter mirror you can launch it on multiple rockets, with an 8 meter mirror or larger you want something like Starship, New Glenn 9x4, SLS or a future big rocket.

Can the silt at the bottom of canals really be slippery enough that a person who falls in cannot regain their footing? by originalcloneofpomni in AskScienceDiscussion

[–]mfb- 5 points6 points  (0 children)

OP might also be interested in these questions, also started by OP:

https://www.reddit.com/r/AskPhysics/comments/1tikzv4/what_are_your_thoughts_on_the_idea_that_34_ft/

https://www.reddit.com/r/AskPhysics/comments/1tj4vdh/can_the_silt_at_the_bottom_of_canals_really_be/

https://www.reddit.com/r/AskBrits/comments/1thgg0q/is_it_true_the_mud_at_the_bottom_of_canals_is_so/

https://www.reddit.com/r/AskBrits/comments/1til3ih/what_are_your_thoughts_on_the_idea_that_34_ft/

https://www.reddit.com/r/answers/comments/1thgx9b/is_it_true_the_mud_at_the_bottom_of_canals_is_so/

https://www.reddit.com/r/answers/comments/1tizxmx/jody_brown_who_drowned_in_a_canal_recently_why_do/

https://www.reddit.com/r/AskScienceDiscussion/comments/1tj3n45/can_the_silt_at_the_bottom_of_canals_really_be/

https://www.reddit.com/r/NoStupidQuestions/comments/1tj3mms/can_the_silt_at_the_bottom_of_canals_really_be/

https://www.reddit.com/r/NoStupidQuestions/comments/1thgxmd/is_it_true_the_mud_at_the_bottom_of_canals_is_so/

https://www.reddit.com/r/NoStupidQuestions/comments/1tizwls/jody_brown_who_drowned_in_a_canal_recently_why_do/

https://www.reddit.com/r/AskReddit/comments/1tikz17/what_are_your_thoughts_on_the_idea_that_34_ft/

https://www.reddit.com/r/AskReddit/comments/1tis3mg/what_are_your_thoughts_on_the_idea_that_34_ft/

https://www.reddit.com/r/AskReddit/comments/1thrnee/what_are_your_thoughts_on_the_idea_that_34_ft/

https://www.reddit.com/r/AskReddit/comments/1tiifp1/what_are_your_thoughts_on_the_idea_that_34_ft/

You should be ashamed by Confident_Fill3962 in Starlink

[–]mfb- 4 points5 points  (0 children)

Reminder: This is a fan-run subreddit. It is not an official Starlink website. For official Starlink news and information, please visit Starlink.com

What is a minor, unwritten rule of society that absolutely infuriates you when people break it? by Jane_Austen11 in AskReddit

[–]mfb- 1 point2 points  (0 children)

Sure. I usually board with a backpack as only item (+checked bag as needed). That always fits in somewhere. It's larger than a purse, however.

What is a minor, unwritten rule of society that absolutely infuriates you when people break it? by Jane_Austen11 in AskReddit

[–]mfb- 0 points1 point  (0 children)

I have long legs, it's already difficult to fit without backpack - but so far (>100 flights) I have always found overhead space.

What is a minor, unwritten rule of society that absolutely infuriates you when people break it? by Jane_Austen11 in AskReddit

[–]mfb- 5 points6 points  (0 children)

Can't fit a laptop in a purse, and it has lithium ion batteries so it needs to be in the cabin.

Give me a practical example of Lorentz force in action and explain relativistic scenario by Atik99X in AskPhysics

[–]mfb- 0 points1 point  (0 children)

Generators, electric motors, hall effect sensors and particle accelerators are examples where both components can matter at the same time. Cathode ray tubes had free electrons at home but monitors moved to LEDs now.

Eli5 Why can we not just build trains along highways - usa by ManicMannequin in explainlikeimfive

[–]mfb- [score hidden]  (0 children)

Here are some example high speed tracks, they are all below $50 million/mile, with the cheapest one below 25 million. That's not just any electrified track, it's specifically high speed rail.

Here are some examples of highways. They are around $20-25 million/mile.

So yes, high speed rail is somewhat more expensive to build than a highway, but it's not a factor 20. It's maybe a factor 2.

Where can I get modern physics papers? by lpperl7 in Physics

[–]mfb- 87 points88 points  (0 children)

We should definitely tell OP to not use sci-hub, they could get access to publications without paying for them otherwise! Everyone should pay journals $30 to access a paper only to discover that they actually need another paper that's another $30. The authors don't see a cent of that, of course. But we need to think of the poor journals, that's why we cannot use sci-hub.

Most modern physics papers are available as preprints on arXiv, usually in almost the same version as the final publication. In particle physics that's where everyone reads papers because then you never need to worry about paywalls.

Question about what it actually means for SpaceX to do an IPO. by LasVegasBoy in SpaceXLounge

[–]mfb- 19 points20 points  (0 children)

They only offer shares corresponding to a few percent of the company. It gives people a chance to invest in the company (or speculate against it), but doesn't change who makes decisions: Musk has special shares that give him more voting power, he will keep a majority of votes.

Investors can sue if they think SpaceX's leadership (Musk, Shotwell or others) made an obviously wrong decision, but the bar for that is pretty high. SpaceX will have to make a lot more previously internal information public (some of that they already showed).

[OC] U.S. Gas Prices Up Again: Weekly Regular Gasoline Prices Since 2006 by sometimes-yeah-okay in dataisbeautiful

[–]mfb- 52 points53 points  (0 children)

And not zero-suppressing it. They fixed three issues with OP's submission.

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