Gravity

Quantum gravity is a castle in the air

When you spend some time digging into the history of physics, you find yourself uncovering the foundations of physics, and then you come to appreciate a few things. You come to appreciate how gravity works, and why an electron falls down. It isn’t because gravitons are flying back and forth: Graviton image by Julie Peasley, see http://www.particlezoo.net/ You also come to appreciate that light interacts with light to form electrons and positrons in gamma-gamma pair production. You come to appreciate that the electron is not a…

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Dark matter

There’s plenty of evidence for dark matter, ranging from velocity dispersion, flat galactic rotation curves, and gravitational lensing. The evidence for dark matter is so good we can even map it out: Image credit NASA, ESA and R. Massey (California Institute of Technology), see spacetelescope.org There’s also plenty of papers and articles about dark matter candidates. Maxim Khlopov refers to WIMPs, axions, neutrinos, mirror-world particles, extra-dimensional particles, and black holes. Andreas Ringwald refers to neutralinos, gravitinos, sterile neutrinos, and axions. Kim Griest refers to MACHOs, molecular…

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Dark matter candidates

Everybody loves a mystery, and one of the best mysteries in physics is dark matter. As to where it starts, check out Brian Koberlain who said its origins can be traced to the 1600s. Or Alexis Bouvard who in 1821 said anomalies in the orbit of Uranus could be caused by dark matter. Or see the Ars Technica history of dark matter article by Stephanie Bucklin. She said in 1884 Lord Kelvin concluded that “many of our supposed thousand million stars, perhaps a great majority of…

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Firewall!

The black hole firewall is a relatively recent idea. On Wikipedia you can read how it “was proposed in 2012 by Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully as a possible solution to an apparent inconsistency in black hole complementarity”. Their proposal is known as the AMPS firewall, and the title of their paper is black holes: complementarity or firewalls? They cannot all be true They start by saying “we argue that the following three statements cannot all be true: (i) Hawking radiation is…

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Hawking radiation

Everybody knows about Hawking radiation. It’s been on TV. See the YouTube clip from the Channel 4 documentary Stephen Hawking: master of the universe. That’s where Kim Weaver of NASA says black holes offer an ultimate test of the physics of the universe. She says they’re cauldrons of heat and light with jets and particles coming out at half the speed of light, and an enormous amount of stuff going on. She finishes up saying and that’s exciting. I absolutely agree. Because yes, I think black…

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Black holes

We can trace black holes all the way back to John Michell in 1783. He's the man who devised the torsion balance used by Henry Cavendish to determine the mass of the Earth. Michell was something of an expert on gravity. He talked about "dark stars"  which were dark "in consequence of the diminution of the velocity of their light". He also said this: "if there should really exist in nature any bodies, whose density is not less than that of the sun, and whose diameters…

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The principle of equivalence and other myths

Once you know that an optical clock goes slower when it’s lower because light goes slower when it’s lower, you soon understand why light curves. Not because it follows the curvature of spacetime. Because the speed of light is spatially variable, like Einstein said. Then once you know about the wave nature of matter and electron spin, you soon understand why matter falls down, and why the Newtonian deflection of matter is only half the deflection of light. Then once you know how gravity works, you…

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How gravity works

Once you understand that there is no time flowing inside an optical clock, then you understand that an optical clock goes slower when it’s lower because light goes slower when it’s lower. Because the speed of light varies in the room you’re in. After that you understand that light waves curve downwards in a gravitational field for the same reason that sonar waves curve downwards in an ocean. Search the ES310 sonar propagation webpage and you find where the US Navy said it: “Recall how differences…

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The speed of light is not constant

The speed of sound in air is sometimes said to be 343.2 metres per second or 768 miles per hour. But actually, it varies. It usually decreases with altitude up to about 11 kilometres above sea level. That’s about 36,000 feet, which is typical for a passenger jet. At that altitude the speed of sound is circa 295 m/s or 660 mph, which is one reason why passenger jets don’t fly as fast as you might like. Interestingly enough, the speed of sound typically decreases with…

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The nature of time

I think it’s important to understand the nature of time. That’s because I think it leads to other things. I also think you can gain an understanding of the nature of time by being empirical. For example we use phrases like time flows and time passes, but when you look for the empirical evidence of time flowing or passing, you can’t find any. That’s because there isn’t any. I can hold my hands up a foot apart and show you the gap, the space between them.…

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