# Relaitivity Definition and 80 Discussions

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1. ### Special Relativity/Energy/Photon Question

The Process of a single, isolated electron initially at rest emitting a photon and recoiling in the opposite direction clearly violates the conservation of energy. On the other hand, a single, isolated, moving electron has kinetic energy, and can easily emit a photon. Or can it? Explain.
2. ### Relativity - Pion

Homework Statement After being produced in a collision between elementary particles, a positive pion (π+) must travel down a 1.00 km -long tube to reach an experimental area. A π+ particle has an average lifetime (measured in its rest frame) of 2.60×10−8s; the π+ we are considering has this...
3. ### How exactly does Hawking radiation work?

Homework Statement For school I'm doing a project on hawking radiaton but I have very big difficulties trying to understand it. I'm trying to understand the matter about: Unruh effect, particle pair (antimatter - matter) and the theory of relativity regarding vaccuum. Homework Equations none...
4. ### Decay of a particle of mass M into two particles

Homework Statement A particle of mass M and 4-moment P decays into two particles of masses m1 and m2 1) Find the total energy of each particle (lab frame). 2) Show that the kinetic energy T1 of the first particle in the same reference frame is given by T_1= \Delta M (1 - \frac{m_1}{M} -...
5. ### Calculating rest mass and energy (in an inertial frame)

Homework Statement A particle is accelerated so it has a total energy of 10GeV measured in the accelerator’s rest frame. The particle's momentum is 8GeV/c in the same frame. Calculate... a.) Rest mass of the particle b.) Energy in an inertial frame in which its momentum is 6GeV/c c.) The speed...
6. ### Doppler shift derivation from Lorentz Transformations

Homework Statement Show how one can obtain the Doppler transformation for the frequency of a receding source just using the Lorentz transformations for the energy (where E=h). Homework Equations Relativistic transformations for momentum and energy: E = γ(E' + vp'x) pc/E = v/c = β The Attempt...
7. ### B Laser pointer at speed of light

This is just kind of an odd question that has to do more with concept than practice but if you were going 1m/s slower than the speed of light and were holding a laser pointer would the laser only point one meter ahead of you? I know that light doesn't add like normal vectors but this is...
8. ### Two-particle collision

Homework Statement In a two-particle collision the square of the invariant mass is. M is total mass of the system (M2 is the square of the total mass) m is the mass of each particles E is the energy of each particles p is momentum vector of eache particles Homework Equations M2 = (E1+E2)2 -...
9. ### B Dimension of the photograph of a four dimensional world?

If time is a dimension, what would be the dimension of a photograph in such a space?
10. ### Directional Acceleration at Relativistic Speeds

Homework Statement A particle flies along in the positive +x direction. It has a constant force F applied 30º clockwise to the x-axis. It is moving at .6 c. What is the angle of acceleration? Homework Equations a = F/(mγ3) The Attempt at a Solution [/B] I'm pretty sure I know how to do...
11. ### Rest, Mass, and Kinetic Energy

Homework Statement I really don't have a homework question just a thought. Is rest energy "maximum energy" for a particle? As to say a particle at rest has a given energy, so when it is in motion it transfers some mass energy to kinetic energy, where both the mass and kinetic energy together...
12. ### Homework Help -- Relative Velocity

<Moderator note: moved from technical forums, so no template> Problem: Two trains (A and B) are moving along parallel tracks at different speeds. A person sitting on train A looks out the window and sees two things happen: a firecracker explodes right outside his window, and, exactly 1.0...
13. ### Perpendicular relativistic velocities

Homework Statement Imagine two motorcycle gang leaders racing at relativistic speeds along perpendicular paths from the local pool hall, as shown in Figure 1.21. How fast does pack leader Beta recede over Alpha’s right shoulder as seen by Alpha? Solution Figure 1.21 shows the situation as seen...
14. ### Relativistic particle decay

Homework Statement Unstable particles cannot live very long. Their mean life time t is defined by N(t) = N0e−t/τ , i.e., after a time of t, the number of particles left is N0/e. (For muons, τ=2.2µs.) Due to time dilation and length contraction, unstable particles can still travel far if their...
15. ### Taylor expansion of the relativistic Doppler effect?

[Note from mentor: this thread was originally posted in a non-homework forum, therefore it does not use the homework template.] I have been given an equation for the relativistic doppler effect but I'm struggling to see this as a function and then give a first order Taylor expansion. Any help...
16. ### B Quantum communication will allow view the future and past?

If a quantum communication device based in interlakced particles equipped with powerful telescope and radio receiver is placed far away some millions is years light. Then the images and radio signals detected, could belong to the future or past. And this info from the future or past can be...
17. ### Relativity train Problem

1. Two long trains pass each other head on with a relative speed of 0.97c. Bob, the driver of the first train notices that it takes 5.8 secs to pass the entire second train. How long do people on the second train say it takes for Bob to go by their train? 2. t = t0/(1-v2/c2)1/2 3. I...
18. ### A Relativistic conduction current density?

Hello, I am an electrical engineer rather than a physicist, however, I am trying to understand the physics of a twin wire transmission line in terms of the charge and current density. Let's say we have a lossless, infinite length, twin wire transmission line, a step current is induced into the...
19. ### I Einstein's Train Example Seems to Exceed c....?

Youtube vid of an Einstein's Train example: From the perspective of the man on platform, the rear of the train seems to be approaching the front lightning's light at greater than c (train's speed + c). Is this correct? I thought there was a rule against that...
20. ### B How do we know we are not inside a blackhole?

If you fall into a black hole, that what is closer to the singularity, will fall faster that than what is further. However, space before and after you, is falling in with you. So from each General Relativistic perspective of "you", because there would be no inertial frame to orient yourself (you...
21. ### E-field of a moving charge at constant velocity

I came upon this page that provides the answer: http://farside.ph.utexas.edu/teaching/em/lectures/node125.html But I can't figure out what gamma(u) is. Could anyone help?
22. ### Conditons at Big Bang, comparison with Light Speed travel

According to Einsteins Theory of relativity if an object traveled at the speed of light, it's mass would become infinite, time would seem to stop relative to others and it length would become absolute zero. The same conditions were thought of at the Big Bang, when time didn't seem to have...
23. ### What if earth were massive enough to cause gravitationl lensng

If the Earth was really massive, then is it possible that no sunlight will reach Earth due to gravitational lensing?
24. ### Phase transition at high energies?

Hi,I'm new here, and created my account solely for following question: Does the incompatibility between QM and Relativity indicate a phase transition at high energies? (as in: indicate more than a "could be")
25. ### Relativistic decay

Homework Statement Hi- I've been doing some questions and came across the following A high energy particle, traveling towards an observer with velocity v decays to produce a photon with energy E in the rest frame of the particle. Find the frequency at which this photon would be detected by...
26. ### B Speed Of Light

I have seen this before, and just saw a thread about the sped of light. The problem is that they say, "light has no mass" then conclude incorrectly on a number of issues. What may be worse is that no replies seem to address the source of confusion, light does have mass, zero rest mass yes, but...
27. ### Force exerted on the palm of your hand by a beam of light

Compute the force exerted on the palm of you hand by the beam from a 1.0W flashlight. (a) if your hand absorbs the light, and (b) if the light reflects from your hand. What would the mass of the particle that exerts that same force in each case would be if you hold it at Earth's surface? On...
28. ### In a radiation dominated era, how do I show this?

As above. In a radiation dominated how do I show this equation? http://i.imgur.com/nMwxvD5.png (It's easier for me to post the image to imgur than use latex. I know g is the relativistic degrees of freedom. I think that the scale factor a(t) is proportional to 1/t and that a(T) is proportional...
29. ### Does quantum gravity argue that space is not singularly....

Does quantum gravity argue that space is not singularly occupied by rectilinear motion? I am reading a general relativity intro sort of thing and this inquiry popped up in my mind. The text was explicating space being occupied by only rectilinear motion and I wondered if quantum gravity could...
30. ### Klein-Gordon eqn: why dismiss messages at phase velocity

Hi All, I've heard it said that the superluminal phase velocity of the KG eqn is not a problem for relativistic causality because signals travel at the packet/group velocity, which is the inverse of the phase velocity (c being 1). I'm a bit skeptical of this. We can strip away all the quantum...