# Tag Info

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It travels forwards instead of backwards in an accelerating car for the same reason that a helium balloon travels upwards instead of downwards under the influence of gravity. Why is that? In an accelerating car, for all intents and purposes the acceleration can be considered a change in the amount and direction of gravity, from pointing straight down to ...

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When your car accelerates forward, the air inside moves back relative to the car. This creates a slightly high pressure in the rear of the vehicle and a low pressure up front. Since helium is lighter than air, it moves away from the region of high pressure. A similar balloon filled with $CO_2$ would move back, since it is heavier than the surrounding air

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The view of most physicists is that asking "How can it be that the speed of light is constant?" is similar to asking "How can it be that things don't always go in the direction of the force on them?" or "How can it be that quantum-mechanical predictions involve probability?" The usual answer is that these things simply are. There is no deeper, more ...

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Short answer: no. Explanation: Many introductory text books talk about "rest mass" and "relativistic mass" and say that the "rest mass" is the mass measured in the particles rest frame. That's not wrong, you can do physics in that point of view, but that is not how people talk about and define mass anymore. In the modern view each particle has one and ...

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"Relativity" is actually a misleading word that Einstein didn't like. It doesn't mean "every vantage point is equivalent and it's all relative". It really means only inertial, non-accelerating vantage points are equivalent. You could think of it as, prior to relativity, people believed that there was an absolute position/speed to the universe. Special ...

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This is just a footnote to Crazy Buddy's answer (which is correct! :-): Length contraction is a real phenomenon, and indeed the RHIC observes this every day because the nuclei are moving so fast that the collision is between two disks not two spheres. However to see something you need to have light emitted from the object reach your eye, and the light from ...

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Everything moves in geodesics, unless acted on by a non-inertial force. Ok. Everything moves in geodesics This just means it takes a path in spacetime, with this path satisfying the geodesic equation: $$\frac{\mbox d^2x^\rho}{\mbox ... 10 In relativistic mechanics, there is a conserved quantity, relativistic momentum: \vec p = \gamma m \vec v \gamma = \dfrac{1}{\sqrt{1-\frac{v^2}{c^2}}} where m is the invariant mass or less precisely, the rest mass. Now, one interpretation is to identify \gamma m as the relativistic mass, a speed dependent mass. But this is actually unnatural as it ... 10 Your question is a natural one to ask, but it has no answer. It is a bit like asking by what mechanism the angles of a triangle always wind up adding to 180 degrees (in Euclidean geometry). There is no mechanism for that - no one is going around checking all the triangles to make sure their angles add up right. It is just a logical consequence of the theory ... 10 It acts precisely like water in a cup. Or, more specifically, like the air in the cup. Since the helium is a much lower density than the nitrogen and other gasses in your car, it can be visualized like an air bubble in a bottle. The container for the helium(the balloon) has negligible mass. When you accelerate forward, the water in a bottle will move ... 10 Fun question. Here's my "me-too" answer. Suppose the car has just emerged from a river, so there's a lot of water in it, and the balloon is tied to the floor. Then you drive away. The air in the car is just like a bunch of water :) 9 The real force at work is centripetal force, or a force pushing inwards. Imagine you have a bucket on a string, and you swing that around in a circle: As you swing the bucket, it travels in a circle. The red line shows the path the bucket takes. In order to make it swing like this, you have to apply a constant force on the rope -- this is the green arrow ... 9 You have successfully discovered that the kinetic energy depends on the reference frame. That is actually true. What is amazing, however, is that the fact that kinetic energy is conserved is NOT reference frame-dependent. So, when you balance your conservation of energy equation in the two frames, you'll find different numbers for the total energy, but ... 8 Centrifugal force is a particular example of a fictitious force. It is introduced so that Newton's second law holds in a rotating reference frame. Newton's second law says$$F = ma$$This means that whenever we find an object accelerating (speeding up, slowing down, turning, or some combination), we can look around and find a physical reason why this ... 8 John Norton at Pitt relates the story quite nicely. In Einstein's own words: After ten years of reflection such a principle resulted from a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with a velocity c (velocity of light in a vacuum), I should observe such a beam of light as a spatially oscillatory ... 8 Because spacetime includes both multiple points in space and multiple moments in time, you have to think of a particle as a line (or a 1D curve) through spacetime, not a point. The line is called the world line. It's made up of all the (x,t) points at which the particle exists: in other words, if you, as an external observer, measure the particle's ... 8 You can't travel at the speed of light. So it's a meaningless question. The reason some people will say that time freezes at the speed of light is that it's possible to take two points on any path going through spacetime at less than the speed of light and calculate the amount of time that a particle would experience as it travels between those points along ... 8 Both are right. Any moving clock is slower than a clock at rest, from the perspective of the frame at rest. Maybe this simplified freehand graphic (apologies for its lack of precision) helps to see that both A and B feel the same about each other's time dilation: Let's say that the red axis represents A and its proper time measured in minutes (first ... 8 The sphere is contracted in the horizontal axis and perceived as an ellipsoid. This is what we believe about length contraction and this happens only, when we take Einstein's simultaneity into account. But, the stationary observer would see the sphere appearing as the sphere always (i.e) the circular outline would still be there at any velocity relative to ... 7 Intuition and perception (or the lack of there of) can be a big problem when you're trying to comprehend the implications of special/general relativity. You must understand that in everyday life which fuels our intuition is pretty slow. Most people don't move faster than 900 km/h or 250 m/s. And that's a luxury for most, to travel by a fast jet. The ... 7 To elaborate on Mark M's answer: If you consider an accelerating reference frame with respect to Rindler coordinates (where time is measured by idealized point-particle accelerating clocks, and objects at different locations accelerate at different rates in order to preserve proper lengths in the momentarily comoving reference frames), then light may not ... 7 In special relativity there is no way you can see someone elses time going faster. This is because in SR all motion is relative. There is no notion of an absolute state of rest. In dmckee's example of the muon experiment, we see time moving more slowly for the muons. However the muons (if they were sentient) would see time moving more slowly for us. This ... 7 You assert that special relativity is valid only under constant speed while the speeds of electrons in quantum mechanics vary with positions This is a very misleading statement. What is true is that via Poincare transformations, special relativity describes precisely how the observations of different inertial observers are connected. It is also the ... 7 It's one of the postulates of the special theory of relativity that the speed of light is c=299,792,458\,{\rm m/s} in all inertial reference frames, regardless of the speed of the source and the speed of the observer. This would conflict with other principles in Newtonian physics because one may always make light move faster or slower by adding the speed ... 6 Yes, what you are suggesting is exactly what is happening, but that is if you have an expression which transforms like an axial vector which you can identify with the the spin of the photon. The inherent spin property of photons (1\hbar) and electrons (\tfrac12\hbar) is of course reference frame independent. Maybe without realizing you brought forward ... 6 In actual fact, the relative speed rule does not apply, ever. The relativistically correct speed addition rule is the following:$$s=\frac{v+u}{1+\frac{vu}{c^2}} When $\frac{vu}{c^2}$ is close to zero (in other words when the velocities invloved are much less than the speed of light, then the correct formula reduces to the Galilean version $s=u+v$. ...

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There is a substantial literature on this positivist point of view. The key to looking it up is "Mach's principle". Wikipedia has a page on it and the Stanford Encyclopedia of Philosophy discusses the question amongst other issues on a page on "Early Philosophical Interpretations of General Relativity". If you want to go to the opera on the question the SEP ...

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(I will assume in my answer that people have read the discussion on the old question, linked to by the OP.) No, it is not like the aether. It is still true that locally, there is no preferred reference frame. You don't even really need to think about spacetime to see what is going on. Consider a two-dimensional plane, parametrised by $(x,y)$, and roll it ...

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When it comes to light any analogy with water is doomed to be completely wrong. I word this so strongly because it is a common misconception that frustrated all of physics for several decades back around the turn of the 20th century. People kept trying to make analogies with water, and the result was always wrong. What you are describing is what is known ...

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