John Duffield
  • Member for 6 years, 10 months
  • Last seen more than a month ago
Does a gun exert enough gravity on the bullet it fired to stop it?
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102 votes

Does a gun exert enough gravity on the bullet it fired to stop it? No. Would, given a large enough amount of time, the bullet fall back to the gun? No. Or is there a limit to the distance ...

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Do black holes violate the first law of thermodynamics?
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35 votes

Do black holes violate the first law of thermodynamics? No. See Wikipedia re the first law of thermodynamics: "The first law of thermodynamics is a version of the law of conservation of energy, ...

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What is time, does it flow, and if so what defines its direction?
19 votes

What is time? As Einstein said, time is what clocks measure. And if you take a look at what a clock actually does, if you open up a clock and take a cold scientific look at the empirical evidence, ...

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Can we find actual rest mass of things on Earth
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12 votes

Earth moves around the Sun and the Sun moves around the galaxy and the galaxy moves with unknown speed and direction. We have speed so the mass of us all altered. The relativistic mass is altered, ...

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Could the black hole in the center of the galaxy be a white hole?
11 votes

Could the black hole in the center of the galaxy be a white hole? I think not. IMHO there are no white holes. IMHO white holes are a mathematical fantasy. In the center of the galaxy there is a ...

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Is time an illusion?
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10 votes

Is time an illusion? No. I think it's best to think of it as something like heat. You know what heat is, especially if you put your hand on a stove: szzz aaargh! Heat is definitely not some illusion. ...

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How many dimensions does electricity have?
9 votes

How many dimensions does electricity have? Three. The same as everything else. In physics however we often talk of 3+1 dimensions, the +1 being the time dimension. However this is a dimension in the ...

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Work done in assembling a point charge is infinite
8 votes

Griffiths was referring to "the problem of infinities". See the Wikipedia article on the history of quantum field theory which has a section on it here. People often say this issue stemmed ...

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How can a black hole reduce the speed of light?
8 votes

How can a black hole reduce the speed of light? Because a gravitational field is a place where the speed of light varies. See Einstein talking about it in paragraph 2 here: The speed of light varies ...

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Why does red light travel faster than blue light?
8 votes

Jon Custer hinted at something, which I think is best explained via an analogy. Imagine you can walk along a pavement at 4mph. When the pavement is empty, it takes you an hour to travel four miles. ...

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If an electron is an excitation of the electron field, what causes the excitation to be stable?
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7 votes

If an electron is an excitation of the electron field, what causes the excitation to be stable? I think the best way to say it is to take a tip from topological quantum field theory: "Although TQFTs ...

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Do the electric and magnetic components of an electromagnetic wave really generate each other?
7 votes

Do the electric and magnetic components of an electromagnetic wave really generate each other? No they don't. Like Andrea said, they're two "aspects" of the same thing. And like you said, it's an ...

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Why does moving through time not require energy?
7 votes

Moving through the other three dimensions necessitates energy. But why doesn't moving through time necessitate energy? Like OrangeDog and peta said, it doesn't take any energy to move through space. ...

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Electron as a standing wave and its stability
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6 votes

Electron as a standing wave Yes, the electron is a standing wave. See atomic orbitals on Wikipedia: "The electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead ...

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Forces and the light
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6 votes

Do external forces affect light? Yes. See for example Faraday rotation: GNUFDL image by Dr Bob, see Wikipedia Can any external force make the light accelerate? Yes. See for example Compton ...

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Why don't electrons collaspe into black holes?
6 votes

Why don't electrons collapse into black holes? Because the electron isn't a point-particle. Its field is what it is. It isn't some speck that has a field, it is that field. There's energy in that ...

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Do rainbows have ultraviolet bands and infrared bands?
6 votes

Is it possible that rainbows have ultraviolet bands and infra red bands and we are not able to see? Yes, see engineer's answer. As for whether we can see them, take a look at aphakia: "Aphakic people ...

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Does General Relativity imply loops in space?
6 votes

Everyone who has been interested in modern science has heard explanations (certainly simplifications) of general relativity, mostly that space is curved. I'm afraid that those explanations that say ...

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What is time dilation really?
5 votes

What is time dilation really? A reduced rate of local motion. See What is time, does it flow, and if so what defines its direction? As Einstein said, time is what clocks measure. And if you take a ...

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Are there waves of time?
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5 votes

In a way, gravitational waves are "waves of time". Or maybe it would be better to say "waves of time dilation". Take a look at this physicsworld report on today's LIGO announcement: "The LIGO ...

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What happens if a super fast rotating ball accelerates near speed of light?
5 votes

What happens if a super fast rotating ball accelerates near speed of light? The ball breaks up even before you accelerate it linearly. This is a bit of a problem with flywheels. In practice you just ...

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Kinetic energy and Potential of a photon
5 votes

I'm going to have to give an answer that's very different to Jimmy360's. Apologies. How does the potential and kinetic energy of a photon relate? They don't. The photon is all kinetic energy. Do ...

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How do we know that the cesium-beam frequency used in atomic clocks is always the same?
5 votes

But what experiment showed that cesium-beam's period was so terribly consistent? They compared it with other clocks. That frequency is terribly consistent, because it isn't actually constant, because ...

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How do we know that gravity is spacetime and not a field on spacetime?
4 votes

How do we know that gravity is spacetime and not a field on spacetime? We don't because it isn't. I should explain that spacetime is an abstract mathematical space, not real space. The Earth is ...

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Does law of inertia has anything to do with speed of light?
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4 votes

Does law of inertia has anything to do with speed of light? Yes. Inertia is resistance to change in motion. It doesn't take much effort to get a skateboard moving, or to stop it moving. But it takes ...

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How can photons/particles/objects/things be massless?
4 votes

How can photons/particles/objects/things be massless? Photons aren't massless the way people think. A photon has a non-zero "inertial mass" and a non-zero "active gravitational mass". But it doesn't ...

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Photon Emission/Absorbsion from the Photons Perspective
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4 votes

Let's start with your assumptions: 1) Photons travel at the speed of light. Right. No problem with that. 2) From the photon's reference spacetime is contracted to 0 length in the direction of ...

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Is there evidence to support that the Earth has a molten core?
4 votes

Is there evidence to support that the earth has a molten core? Yes. We have these things called volcanoes: ;)

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Gravitational field strength and potential different signs
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4 votes

Because of a convention wherein zero gravitational potential is said to be at infinity. See Wikipedia: $V(x) = \frac{W}{m} = \frac{1}{m} \int\limits_{\infty}^{x} F \ dx = \frac{1}{m} \int\limits_{\...

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Why does the minus sign in the Minkowski metric mean that nothing can move backwards in time?
4 votes

No, because it isn't true. Nothing can move backwards in time because nothing can move forwards in time. Moving in or through time is just a figure of speech. So is “the flow of time”. If you open up ...

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