# Tag Info

110

If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes after the reactions ceased. The Sun however would continue to shine for tens of millions of years at roughly its current luminosity. The power source is not "...

70

Photons can't have a perspective. If we have a particle with mass, we can imagine taking a frame of reference in which that particle is at rest. We can then see things "from the particle's perspective". But there is no frame of reference in which a photon is at rest. Photons always move at the speed of light in every frame of reference. If I try to set ...

32

It doesn't work like that. An observer at the light source (and indeed any observers anywhere else) will always see light travelling (in vacuum) at the speed of light locally. There is also a major problem with your thought experiment. It is not possible for you to have a stationary light source within the event horizon of a black hole. It, and everything ...

10

First, the Sun will not end up as a supernova - only a star $>8$ times the mass of the Sun will end its life in that way. You also have the wrong idea about "trapped light" (photons bouncing around and gradually working their way to the surface). Photons are constantly emitted and absorbed again and don't travel very far (compared to the radius of the ...

9

In principle yes, but in practice no. At the so-called photon sphere, gravity is exactly so strong that a photon on a tangential trajectory would stay in orbit. For a non-rotating black hole of mass $M$, the radius of the photon sphere is $3/2$ times the radius of the event horizon — i.e. the "surface" of the black hole — which itself is given by $r_s \... 9 You cannot exceed the speed of light "locally". But you can -see- imagine* distances increase quicker than the speed of light. If by traveling you mean "moving compared to local space time", then light cannot travel quicker than the speed of light. In your example, the distance increases faster than the speed of light, because spacetime is dragged along ... 8 It is simply not true that gravity can only interact with mass. Rather, any long-range spin-2 force interacts with all energy-momentum equally, and it source is the stress-energy-momentum tensor. That is one way to state the equivalence principle. Note that a massive object in its own rest frame has an associated energy$E = mc^2$, which under ordinary ... 8 So, assuming my assumptions aren't assinine (they very well could be) ... Your words, not mine. Your "very well could be" is the case. Assume the universe is infinite The universe might well be infinitely large, but the observable universe is all that we can possibly see. By all signs, the universe is 13.8 billion years old. We can't see the stuff that is ... 8 For any massive object the gravitational potential energy is given by Newton's law: $$V(r) = -\frac{GMm}{r}$$ The gravitational potential energy is due to the attractive gravitational force, but for an orbiting object there is also a (fictitious) centrifugal force pushing it outwards. If we calculate the potential energy due to the centrifugal force and ... 6 The "common theory" you're reading is not about the processes that produce light in stars, it's just intended as a demonstration of the speed of light through space. When it talks about the Sun "shutting down", it's not talking about the nuclear fusion processes stopping, it means that the Sun as a whole stops shining. I'm not a physicist, but I don't think ... 6 The anisotropies in the CMB are caused by four effects; three at the surface of last scattering (SoLS), and one after: Temperature differences Denser regions will be more compressed and thus hotter, on average. Hence, an overdensity will result in a hotter spot, with a fractional fluctuation$\Delta T/T_0$. Gravitational redshift Photons climbing up (or ... 5 There isn't really a minimum size. All black holes have an event horizon from which nothing, not even light can escape. The (Schwarzschild) radius of this event horizon is 3km for a black hole of a solar mass and scales linearly with mass. There is a region just outside the event horizon called the photon sphere, within which light can briefly orbit the ... 5 There is a quantity in relativity of$s^2$which is defined as$t^2-x^2$, where$t$is the difference in time between two events, and$x$is the difference in position (measured in units such that$c=1$). If$s^2$is positive, then the square root of it is the proper time. If it's negative, then the square root of$-s^2\$ is the proper distance. (BTW, proper ...

4

Time does not exist for photons. Quoting Wikipedia's somewhat unnecessarily technical page: In relativity, proper time along a timelike world line is defined as the time as measured by a clock following that line. The proper time between any two points along any photon's path in empty space is zero. So the photon "experiences" no passage of time. It's ...

4

How LIGO, LISA, etc. Detect Gravitational Waves The point of instruments like LIGO and LISA is to measure time-varying changes in the distance within different arms of the instrument. In the case of an arm oriented in the direction of an incoming gravitational wave (GW), the length of the arm will increase and decrease, while an arm oriented perpendicular ...

4

What we call "gravity" is really just the distortion of spacetime. If spacetime was completely straight, there would be zero gravity. But the existence of a massive body is one of the things that can distort spacetime, so that "straight" lines are not straight anymore. We perceive that distortion as "gravity". It is the distortion that actually generates the ...

4

I think there is a missing piece of information. The BAT is a coded mask telescope. The imaging is done by photons passing through a mask and falling onto an array of 32768 detectors. http://swift.gsfc.nasa.gov/about_swift/bat_desc.html The "mask-weighted" light curve is produced after a complex ray tracing exercise using an estimate of the position of the ...

4

What are the velocity, mass, and charge distribution of the solar wind. Velocity The solar wind speed has a large range of variation, between ~250–820 km/s [e.g., Chen et al., 2014; Gopalswamy, 2006; Jian et al., 2011, 2014; Kasper et al., 2012; Maksimovic et al., 1998; Marsch, 1983; McComas et al., 2013; Schwenn, 1983; Stverak et al., 2008, 2009] near the ...

4

No. Or at least such an effect has never been observed, neither in the locality of the Earth or in light detected from distant sources. If a photon has an interaction with a quantum field (such as an electromagnetic field) this causes a scattering. Scattering would cause a blurring and dimming of distant sources. This is not observed. Such an effect, if ...

4

I believe that the answer is mainly the first (directly from stars), although there will certainly be some of each of the others. The diffuseness comes from the fact that there are many more relatively dim stars than rods in the retina of the human eye. My argument for this is that magnified photographs of most of the Milky Way at least, break down into ...

3

The CMB consists of photons that were emitted very shortly before and during the recombination of electrons with hydrogen and helium nuclei. [Side note - why is it called the epoch of recombination when the atoms were never "combined" to begin with?] Those photons were then able to travel, unabsorbed across the universe until they encountered our microwave ...

3

This is a very broad question and though a comprehensive answer lies outside the scope of this simple Q&A format, I give you a couple of examples where "neutrino telescopes" would revolutionary. There is a predicted cosmic neutrino background, analogous to the cosmic microwave background. Neutrinos decoupled at 1 second after the big bang, filling the ...

3

The (late time) ISW is caused by the evolution of cosmic structures as photons of the cosmic microwave background traverse them on their way to our detectors. It may cause a redshift or blueshift with respect to the redshift predicted for a homogeneous expanding universe. A bit more detail: If a photon "falls" into a potential well, its frequency and energy ...

2

Your statement "most of the universe is composed of mostly empty space" is very vague. In reality, the Universe is embedded in diffuse background photon fields, from low energy (like the so-called cosmic microwave background which is relic from the big bang) to very high energy (from the extreme compact objects, like AGNs). Of course, there is also unknown ...

2

If photon is mass-less and gravity can interact only with matter, then how gravity can alter the trajectory of light? The second part of your "if" clause is incorrect. It's mass-energy, not just mass, that gravitates.

2

Even vacuum is seething with virtual particle pairs, so photons always interact with something. Check Wikipedia on the Scharnhorst effect for a bit of an explanation: Owing to Heisenberg's uncertainty principle, an empty space which appears to be a true vacuum is actually filled with virtual subatomic particles. These are called vacuum fluctuations. As a ...

2

No. Photons can't come from the singularity of a black hole, or from beyond the event horizon. While some very good candidates for black holes exist, none have been certainly observed. We have never observed Hawking radiation: it remains theoretical. We have observed radiation from the accretion disk of likely black holes, however the matter in this disc is ...

2

As stated in my answer to Would we have more than 8 minutes of light, if the sun "went out"? , the Sun would cool at roughly constant luminosity for some tens of millions of years. That means "we" would not notice anything change on human timescales (apart from the lack of neutrinos, which has no measurable effect on anything apart from the odd ...

2

The short answer is, "gravity". The time of "co-moving observers" (observers moving with the expansion of the universe") is expanding, but our "proper time" is not, because we're within a local group of galaxies that are bound together gravitationally. The principles involved are detailed in the Wikipedia article "Co-moving and proper distances", and its ...

2

They are so dark they absorb almost every bit of light from any star and receive no photon velocity momentum or electromagnetic radiation. The momentum that a planet receives or does not receive from photons is negligible for its orbit. These dark masses are extremely high in mass and do not interact with light like other planets and moons. A dark ...

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