# Tag Info

## Hot answers tagged reference-frame

50

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 ...

33

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

11

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 ...

11

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 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 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 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 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 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 ... 6 Suppose you pick two people at random. From one, you pluck a single hair from their head. Is it possible to tell who had the hair plucked by weighing the people? Technically, plucking a hair makes a person very slightly lighter, so you get a tiny bit of information about who had the hair plucked by weighing the people. But the information is very slight ... 5 From the comments to user16307's answer I'm guessing you're fairly new to special relativity. Until you get familiar with the subject it's very dangerous to throw around concepts like time dilation and length contraction because you can easily fall into traps like the pole in a barn paradox. The only safe way to work out what happens is to use the Lorentz ... 5 Objects, defined as things with mass, don't move at the speed of light. The time dilation factor is$$\gamma = \frac{1}{\sqrt{1 - v^2/c^2}} and it has no limit - it diverges at $v\to c$. For speeds very close to the speed of light, we could define $\epsilon = \frac{c - v}{c}$, then we'd have $\gamma \sim \frac{1}{\sqrt{2\epsilon}}$ This shows how much ...

5

I put my negative comment in the from of the answer. Here are the very basics of the simultaneity implications (or causes) of relativity: According to the special theory of relativity, it is impossible to say in an absolute sense whether two distinct events occur at the same time if those events are separated in space, such as a car crash in London ...

5

There can be at least two different flavors of paradoxes. In one, a result such as 2+2=5 is proved, and the problem must be either incorrect reasoning or a set of assumptions that was invalid. In the other type, exemplified by the EPR paradox, the correct result of an argument is so surprising that it seems like it must be a mistake. Based on the ...

5

That is exactly right. A fundamental tenet of physics is that all inertial reference frames are equivalent and indistinguishable.1 Furthermore, given one inertial frame (standing at rest2), any other frame moving with respect to it with a constant velocity is also inertial. The frame "moving at terminal velocity" is just as inertial as "sitting still" and so ...

5

Fermat's principle is a bit more complicated than what you state: it says that in travelling from $A$ to $B$, light will go along the paths that will minimize the time taken to get there - and these may or may not be straight lines. (See e.g. Wikipedia.) That said, the gravitational lensing of light does not operate quite like that. Since it is in vacuum, ...

5

User Sahil Chadha has already answered the question, but here's the math and a pretty picture for anyone who is unconvinced that you're right. Since the train is accelerating, from the perspective of an observer on the train, the ball will experience a (fictitious) force in the direction opposite the train's travel having magnitude $ma$ where $m$ is the ...

4

The three-torus geometry or, more precisely, the $R\times T^3$ spacetime geometry, breaks the Lorentz invariance to nothing. The identification $x\sim x+2\pi R_x$ and similarly for $y,z$ needed to define the torus is an identification that doesn't stay the same when $t,x$ are mixed by the Lorentz transformation, so the spacetime is just not Lorentz-symmetric ...

4

The effect you're talking about is called gravitational time dilation. The effect is easily calculated from the metric (typically the Schwarzschild metric) but when you ask "why is this?" I'd guess you're asking if there is a way to understand why this happens without working through all the algebra. The answer is no, not really, but I can attempt to give a ...

4

in relation to anything else that can make such measurements. As the speed of light is universal, nothing can see any other massive field moving at the speed of light (which is reserved for massless fields) your 0.51 number suggests that you expect that naive addition of velocities holds when velocities approach the speed of light. This is wrong. Here is ...

4

Moving away from earth at some speed can mean moving faster in space OR moving slower in space and thus also mean slower or faster time flow than the one on the earth. There is no speed relative to space, there is just relative motion between objects. But since the earth already travels at some always varying speed and thus is not standing ...

4

It is an axiom of general relativity. A basic assumption. A more accurate statement would be that light always travels along a spacetime geodesic. The classical addition of speeds in newtonian meachanics doesn't work inrelativity (light is the most extreme case). That is the source of all weird stuff in relativity

4

This actually is not so much a relativistic effect but only a consequence of light travelling at a finite speed What you see is the image of the object as it was 10 years ago, this doesn't mean the object is actually "in the past", it only means that any information (photons) sent by the object will reach you 10 years after it has been emitted. (and ...

4

General relativity is not going to help you 'live forever'. Sure, you might develop the technology to hover a tiny distance above the horizon of a black hole. Your aging will slow down. People on earth will observe you outlive your parent by many millions of years. By any number of years really. However, this doesn't help you live 'forever'. All physical ...

4

GR doesn't have global frames of reference. No global or local frame of reference is needed in order to define coordinates in GR. Picking a local inertial frame is one possible way of defining coordinates locally, but it isn't the only way, and it doesn't typically suffice as a way of defining coordinates globally. In GR, coordinates are just labels for ...

4

The principle of relativity states that there is no preferred inertial frame. All frames are equivalent and that the motion between two frames is relative. You can choose any of the frames and call it at rest and the other in motion. If you conduct an experiment on a platform of a station and also in a train moving with constant velocity, no experiment will ...

Only top voted, non community-wiki answers of a minimum length are eligible