# Tag Info

1

Well, I'm not going to tell you my opinion, because that would be irrelevant to the actual science. But, what I can do is assess your premises and conclusions. What you should note first, and I'm unsure if you know this or not already, is that the four dimensions of spacetime are the three spatial dimensions and time. We can define a velocity through the ...

2

In "Adventures in Friedmann cosmology: A detailed expansion of the cosmological Friedmann equations" by Robert J. Nemiroff and Bijunath Patla in the American Journal of Physics volume 76, on page 265 (2008); http://dx.doi.org/10.1119/1.2830536 the authors call them "cosmic strings" But this is in the context of cosmology, so its for a universe that on very ...

1

Ether theories did not yield consistent results, and they were not complying with experimental results. In this situation, Einstein presented a theory which allied the relativity principle with the postulate of the constancy of light. The price for this solution (that nobody dared to pay before him) was the abandon of the absoluteness of simultaneity of ...

0

It isn't the aether that Einstein rejected, but the notion of absolute simultaneity; and this rejection was forced by taking the speed of light as constant from all inertial or stationary frames; and this is a deduction from Maxwells EM theory. Lorentz, I take it, introduced length contraction as a way of making classical mechanics and EM consistent - or ...

4

Asked by lucas: I know nothing about relativity but I cannot accept that there is a phenomenon called time dilation. However I have no problem with it because of mathematics behind it. I have no problem if time is dilated, because I don't know what time is. But I wonder when they say a clock will work slowly with respect to the other same clock if its ...

3

Time dilation is not clocks running faster or slower. It occurs when two observers measure a different elapsed time between the same two spacetime points. Time is what clocks measure i.e. a clock is a measuring device that measures elapsed time between two points in the same way that a milometer measures the distance in space travelled between two points. ...

11

Yes, there is a simple explanation! The pair production process $\gamma \to e^+ + e^-$ is forbidden by energy-momentum conservation, so it doesn't happen in either frame. One way to see this is that, as a massless object, the photon has "the most possible momentum for its energy", as it doesn't have the extra rest mass-energy. So if you try to make the ...

8

Pair production cannot just happen. It requires some other particle/object to balance out the momentum. Lets suppose this object is a nucleus. So the interaction looks to the fast moving observer like a blue shifted photon scattering off a stationary nucleus and producing an $e\bar{e}$ pair. To us it looks like a nucleus moving at relativistic speed ...

0

In the book The Elegant Universe, Brian Greene states on page 27, the "Einstein proclaimed that all objects in the universe are always traveling through space-time at one fixed speed, that of light. ........ We are presently talking about an object's combined speed through all four dimensions, three space and one time, and it is the object's speed in this ...

1

In Figure 1 the system $\bbox[#99FFFF,2px]{S'\equiv O'x'y'z'}$ is moving with velocity $\mathbf{b}=(b,0,0)$ relatively to $\bbox[#E0E0E0,2px]{S\equiv Oxyz}$. In Figure 2 we build two systems $\bbox[#E0E0E0,2px]{\Sigma\equiv K\rm{uvw}}$ and $\bbox[#99FFFF,2px]{\Sigma'\equiv K'\rm{u'v'w'}}$ as shown therein. The system $\bbox[#E0E0E0,2px]{\Sigma\equiv K\rm{... 5 It's a good question and it isn't obvious that both observers will agree on the magnitude of their relative velocity. It's fairly straightforward to show this using the Lorentz transformations, but let me attempt a more intuitive approach (if you want to see the maths comment to this answer and I can append it). You have chosen an inertial frame$S_1\$ where ...

2

If in one frame you have equal steady positive charges densities in the two wires and equal steady negative charge densities in the two wires but the negative charges are moving at a steady speed in the same direction in the two wires. Then, in the frame of the negative charges, the two wires are positively charged. So yes, the negative charges feel a force ...

0

I'd just like to emphasize the point - touched on in several of the other answers - that the core of the "paradox" here is our natural, nonrelativistic tendency to implicitly assume a universal time. Such an assumption indeed is contradicted because, if true, the well orderedness of time intervals would be blatantly contradicted by the situation where both ...

1

Suppose George and Gracie are moving toward each other, each claiming to be stationary. Their clocks happen to be set so they'll both chime 10PM at the moment they meet. When Gracie's clock chimes 2PM, she says: "I see that George has his clock set to 4PM. He'll be here in 8 hours, with his clock chiming 10PM. That slow clock of his will chime only 6 ...

0

The situation is symmetrical, so both will see the other one* undergo time dilation. This seems contradicting at first sight, but if any of them turned around for a meetup, they would undergo acceleration and thus change in which inertial frame of reference they are stationary in. This will result in a "speed-up" of time for the accelerating spaceship.

0

Yes and no. Coordinate acceleration doesn't need to be relative, but proper acceleration is always invariant.

-2

Consider a positively charged particle, if it comes in upward motion its velocity increases as a result its mass decreases(according to conservation of momentum i.e. m1v1=m2v2), as mass decreases its number of atoms decrease i.e. equal number of protons and electrons are decreasing which reside in an atom.thus the charge on the particle is unaffected by its ...

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