Special & General Relativity Questions and Answers
This information was graciously provided by the NASA-sponsored "Ask the Space Scientist" web page and its author, Dr. Sten Odenwald. The following is a reproduction of the "Special and General Relativity" section of his work. Please visit the site directly for more Q & A.
- How do astronomers tell the difference between the different kinds of redshifts?
- Could an astronaut use a wrist watch to measure relativistic time dilation?
- What is it about quantum mechanics that is incompatible with general relativity?
- How did Roemer measure the speed of light back in the 17th century?
- Does the star DI Herculis prove that general relativity is breaking down?
- What property of matter causes geodesics to curve near them?
- Exactly how does light get red shifted in a gravitational field?
- Can space exist by itself without matter or energy around?
- How did Bradley determine the speed of light in 1779 from stellar parallax?
- What objects have we seen traveling near the speed of light either natural or man-made?
- Do gravitational fields produce their own gravity?
- What happens in the space between gravitating bodies?
- Is space-time orientable?
- Is the gravity of distant objects the cause of inertia?
- If space exists, what is it?
- We used to think that the speed of sound was impassable, isn't the speed of light the same thing?
- What is 'sub-space'?
- How can matter just appear out of nowhere?
- What are P-branes?
- Do any theories require more than 1 dimension of time?
- What does light travel time have to do with looking into the past or future?
- Do time and space exist for a photon on which eternity should pass as an instant?
- If space ends, what's on the other side?
- Why does the warping of space near the Earth make its orbit elliptical, but has no effect upon light?
- Are all the postulates in General Relativity accepted or does there exist room for alternatives to General Relativity with different outcomes?
- If astronomers can use the cosmic background radiation as a reference frame doesn't that invalidate special relativity?
- If there is so little friction in space, why can't we reach the speed of light with a big rocket?
- Why do the laws of physics break down in singularities?
- What proofs are there for general relativity?
- What is the relationship between space and time?
- If I could put myself in the same point in spacetime where I was yesterday at noon, would it still be yesterday at noon?
- If you travel fast in space will you REALLY come back younger than the friends you left behind?
- Can gravitational lens events cause gamma ray bursts to reoccur in the same part of the sky, rather than repeated explosions of the same object?
- If gravity travels at the speed of light, why don't the equations used in celestial mechanics include time delays?
- Is space really quantized?
- Does space have more than 3 dimensions?
- When your speed exceeds light, how does your mass become 'imaginary'?
- As you approach the speed of light does the universe seem to disappear into a bright point of light ahead of you?
- Why can't we travel faster than light?
- If the universe is finite, can gravity waves have already traveled completely around the universe and back?
- If space can be bent, can it also be sliced?
- What does the word 'spacetime' mean?
- Does faster than light travel mean time travel is possible?
- If I shined a flashlight into the sky, would someone in another galaxy eventually see it?
- If I passed the Sun at near the speed of light, it's mass would be higher, so why wouldn't the planets fall in?
- Are there any other astronomical systems where general relativity can be tested other than the Hulse-Taylor binary pulsar?
- What is the simplest evidence that there are more than 4 dimensions?
- If Observer A was accelerating relative to Observer B, would they agree that it is Observer A that gained mass?
- What is it like to be in a place with no gravity?
- Why isn't our 4-dimensional space-time curved in the 5th dimension?
- Are there really such things as gravitons?
- How do you explain Einstein's relativity theory to a 12 year old?
- If a marshmallow traveled at 99.99 percent the speed of light hit the Earth, what would happen?
- Why don't gravitational lenses also produce redshifts?
- Can gravity waves be used to carry information?
- What prevents densities higher than the so-called Planck Limit from happening?
- If gravity is a distortion in space-time, why do we call it a force?
- According to relativity, how much younger are astronauts in earth-orbit after 6 months?
- How do you really know that the limits to general relativity for strong fields are set inside black holes?
- Does light travel at the same speed back and forth along a straight line?
- Because of general relativity, do stars age at the same rate inside and at the surface?
- What is meant by the 'Euclidean geometry of space'?
- What would happen if the speed of light were 10 times faster?
- If an object travels near light speed, what does its image look like?
- Is there any common sense way to think about the Twin Paradox?
- Does time stop when you travel at the speed of light?
- Could you tell me the name of the NASA astronomer who discovered two gaps in time?
- Is there a theory that predicts the velocity of light?
- If we traveled 100 light years into space, would we really see the Earth as it was 100 years ago?
- Why is the Doppler shift the only explanation for cosmological redshifts?
- Does gravity produce a Doppler shift?
- Why is the gravitational field of the universe another name for space-time?
- How are scientists trying to detect gravity waves?
- What is curved space-time?
- Don't objects steadily evaporate over time by emitting gravity waves?
- Why do photons produce pressure even though they are massless?
- Has it been experimentally proven that the speed of light is the fastest?
- Why do Singularities have to have zero volume?
- Can loops in time exist in nature?
- Is there a static condition against which the speed of light is measured?
- How do you reconcile the universe not having an expansion center with observations that galaxies are moving away from us?
- Does anyone really understand general relativity well enough to have an intuitive grasp of the universe?
- If nothing can travel faster than light, why does gravity take affect instantly?
- If two ships approach at 2/3 the speed of light, would they see each other?
- In layman's terms, why does your rate of speed affect time?
- What kind of experiment would show that time is quantized?
- Why dose the decay of an orbit mean that gravitational radiation is being produced?
- Will we ever be able to move through time?
- How soon after you start to move do you see the relativistic contraction effect?
- If the speed of light is a constant, why is it slower in glass and water?
- Do Newtonian and Einsteinian gravity yield the same results for the same input parameters?
- What binary star is producing gravity waves?
- Is there any way we can measure our true speed through the universe?
- Can light be altered by a magnetic field?
- Why can't a rigid body exist?
- Why does time have to change at high speeds?
- What does the equation look like that shows how gravitational radiation is lost from the binary pulsar system?
- When an object falls under gravity, is it really feeling a force, or is it just moving along a curved path through space?
- Under what circumstances can a particle exceed the speed of light?
- What would a universe be like with two time dimensions?
- What happens to rotating solid objects in relativity theory?
- How is gravitational lensing used to search for dark matter?
- Why does the 'Twin Paradox' exist as a physical phenomenon?
- Has the constancy of the speed of light been measured?
- Do radio waves get red shifted?
- Exactly why is the speed of light a constant in all reference frames?
- If you could slow everything down to a complete stop, would time pass infinitely fast to you?
- What is the fourth dimension?
- How is it said that relativity is tested billions of times a year, when there are only a few accelerator labs on Earth?
- Why can't time be considered a force?
- How does general relativity incorporate rotation?
- Can you use the geometry of gravitational lenses to determine the distances to the lensed quasar?
- If spacetime is a consequence of the deformation of space by gravity, why is gravity a universal phenomenon rather than a local one?
- What is the momentum of a photon?
- How could you get Doppler shifts with light if the speed of light never changes?
- How do we know gravitational lenses aren't just refraction off of intergalactic clouds?
- In the problem of the marshmallow moving at the speed of light, didn't you forget about its kinetic energy?
- Can you exceed the speed of light by manipulating space-time in some way?
- Do time and space become meaningless at the speed of light?
- Why is time so different from space in the way we experience it?
- How does a body actually bend space-time?
- Does relativity apply to 'sideways' angular motion or just motion along the line of sight?
- What the ding dang are gravitons?
- Why aren't all gravitational lenses perfect rings?
- Is gravity the exchange of gravitons, or the curvature of space-time?
- Do photons carry mass as suggested by the equation m = hf/c2 ?
- When a photon falls in a gravitational well, does its speed exceed 'c'?
- If gravity is a 'wave' of energy, does the concept of frequency have any meaning?
- Would the collision of two photons cause them to slow down as a result?
- Do astronomers take into account gravitational redshifts when they compute the cosmological redshift of a distant galaxy?
- If we see a galaxy moving near the speed of light, as they would also see ours, whose mass is getting bigger?
- Why are multiple gravitational lenses more common than rings?
- If you slowed down light, would it become less of a wave and more of a particle?
- If an object approached the speed of light, would it be transformed into light?
- Does gravity travel faster than the speed of light?
- Does matter really get infinitely compressed in a singularity?
- How does gravity propagate through space?
- Does the explanation for superluminal motion in some quasars work for galaxies moving away from us at high speeds due to the expansion of the universe?
- If there is no such thing as velocity except with respect to a reference object, how can there be speed limits in deep space?
- What is it that physically prevents faster than light speeds?
- Are we gravitationally affected by where an object is now, or where it was when the light we see started its journey to us?
- In Robert Forward's book 'Dragon Egg' the inhabitants of a neutron star see an approaching rocket stand still in space. Does this violate relativity?
- How are electromagnetic fields and space-time related, and can electromagnetic fields exert a force on space- time?
- How do you reconcile classical quantum mechanics with general relativity?
- Why isn't it possible to travel in time?
- Is the constancy of the speed of light dictated by special relativity invalidated by the expansion of the universe?
- Where can I get a copy of Misner, Thorne and Wheeler's 'Gravitation' book published in 1973?
- If a photon travels at the speed of light, why isn't its mass infinite?
- What is the speed of light in megaparsecs per planck time?
- What fraction of the perihelion advance of Mercury is due to special relativistic effects?
- Why can't you choose the reference frame where time moves fastest as the one at absolute rest?
- What is the Einstein Shift?
- What is time?
- What is Dark Matter and does it produce gravitational lensing?
- If two objects approach at 75 percent the speed of light, why are they not traveling 1.5 times the speed of light?
- If light has traveled thousands of years, how could it possibly have aged according to relativity?
- Is it ever possible to determine whether a body is at rest?
- Does time pass for all massive bodies at the same rate?
- If you travel near the speed of light, do you enter another dimension?
- If you traveled at the speed of light, would the universe appear instantaneous?
- Do objects really contract when they travel near the speed of light?
- What happens to the fabric of space-time when an object moves through it near the speed of light?
- Is it true that the speed of light is slowing down?
- What visual distortions occur as you approach the speed of light?
- Is the speed of light a barrier of the same kind as the speed of sound?
- Without a fixed reference frame for judging speed in the universe, how do we know which 'twin' did the moving in the famous Twin Paradox?
- If Einstein says that everything is relative, how can there exist any firm constants upon which to base a Unified Theory of Nature?
- Could the displacement of star images near the Sun be caused by refraction in the atmosphere of the sun, not by general relativity?
- What can you learn from studying gravity waves?
- What studies have been made for gravity waves?
- Why does time change when you travel near the speed of light?
- Could energy ever come to a point and create mass?
- What does Stephen Hawking currently think about time travel?
- Would our system of time change if we went to another solar system?
- Do worm holes allow all times to exist simultaneously?
- Does 'now' have a size instead of being a point in spacetime?
- Do electrons interact with past and future representations of themselves?
- If a blue shifted clock runs fast, why do fast moving particles decay slowly?
- Can gravity affect the speed of light?
- Is there such a thing as hyperspace?
- How do we know that the speed of light is the same in every galaxy?
- Is our perception of distances and sizes in the universe just an illusion?
- What is a space anomaly?
- What is a vortex, and how does it work?
- How do distant objects get magnified by space curvature?
- Why can't matter travel back in time?
- Is time linear, or could there be other dimensions to it as for space?
- Is it possible to exceed the speed of light?
- Can you see Cherenkov radiation in empty space?
- What is a space time continuum?
- Do the different estimates for the age of the universe imply that the speed of light may have changed?
- Is the amount by which light is bent important in cosmology?
- How is it that, according to general relativity, space can be curved?
- Is the cosmological redshift real, or just an optical illusion caused by dust?
- Is it difficult to know what is happening now if it takes light time to get here?
- Why does a particles mass increase as it moves?
- How do you actually use Einstein's famous equation E = mc-squared?
- Could you explain how gravitational lenses work, and what determines the number of images you see?
- Is anti-matter the same as negative mass?
- If we could travel faster than light, could we go back in time?
- Why is the speed of light a constant?
- Is there a limit to how fast you can go using gravitational boosts from the sun and planets?
- Is there a simple way to measure the speed of light?
- Does gravity travel at the speed of light?
- What is light, how does it move, and how do we measure its speed?