160 FAQs about Relativity (The Astronomy Cafe)

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  1. Are there any other astronomical systems where general relativity can be tested other than the Hulse-Taylor binary pulsar?
  2. What is the simplest evidence that there are more than 4 dimensions?
  3. If Observer A was accelerating relative to Observer B, would they agree that it is Observer A that gained mass?
  4. What is it like to be in a place with no gravity?
  5. Why isn't our 4-dimensional space-time curved in the 5th dimension?
  6. Are there really such things as gravitons?
  7. How do you explain Einstein's relativity theory to a 12 year old?
  8. If a marshmallow traveled at 99.99 percent the speed of light hit the Earth, what would happen?
  9. Why don't gravitational lenses also produce redshifts?
  10. Can gravity waves be used to carry information?
  11. What prevents densities higher than the so-called Planck Limit from happening?
  12. If gravity is a distortion in space-time, why do we call it a force?
  13. According to relativity, how much younger are astronauts in earth-orbit after 6 months?
  14. How do you really know that the limits to general relativity for strong fields are set inside black holes?
  15. Does light travel at the same speed back and forth along a straight line?
  16. Because of general relativity, do stars age at the same rate inside and at the surface?
  17. What is meant by the 'Euclidean geometry of space'?
  18. What would happen if the speed of light were 10 times faster?
  19. If an object travels near light speed, what does its image look like?
  20. Can information between pairs of particles travel faster than light?
  21. Is there any common sense way to think about the Twin Paradox?
  22. Does time stop when you travel at the speed of light?
  23. Could you tell me the name of the NASA astronomer who discovered two gaps in time?
  24. Is there a theory that predicts the velocity of light?
  25. If we traveled 100 light years into space, would we really see the Earth as it was 100 years ago?
  26. Why is the Doppler shift the only explanation for cosmological redshifts?
  27. Does gravity produce a Doppler shift?
  28. Why is the gravitational field of the universe another name for space-time?
  29. How are scientists trying to detect gravity waves?
  30. What is curved space-time?
  31. Don't objects steadily evaporate over time by emitting gravity waves?
  32. Why do photons produce pressure even though they are massless?
  33. Has it been experimentally proven that the speed of light is the fastest?
  34. Why do Singularities have to have zero volume?
  35. Can loops in time exist in nature?
  36. Is there a static condition against which the speed of light is measured?
  37. How do you reconcile the universe not having an expansion center with observations that galaxies are moving away from us?
  38. Does anyone really understand general relativity well enough to have an intuitive grasp of the universe?
  39. If nothing can travel faster than light, why does gravity take affect instantly?
  40. If two ships approach at 2/3 the speed of light, would they see each other?
  41. In layman's terms, why does your rate of speed affect time?
  42. What kind of experiment would show that time is quantized?
  43. Why dose the decay of an orbit mean that gravitational radiation is being produced?
  44. Will we ever be able to move through time?
  45. How soon after you start to move do you see the relativistic contraction effect?
  46. If the speed of light is a constant, why is it slower in glass and water?
  47. Do Newtonian and Einsteinian gravity yield the same results for the same input parameters?
  48. What binary star is producing gravity waves?
  49. Is there any way we can measure our true speed through the universe?
  50. Can light be altered by a magnetic field?
  51. Why can't a rigid body exist?
  52. Why does time have to change at high speeds?
  53. What does the equation look like that shows how gravitational radiation is lost from the binary pulsar system?
  54. When an object falls under gravity, is it really feeling a force, or is it just moving along a curved path through space?
  55. Under what circumstances can a particle exceed the speed of light?
  56. What would a universe be like with two time dimensions?
  57. What happens to rotating solid objects in relativity theory?
  58. How is gravitational lensing used to search for dark matter?
  59. Why does the 'Twin Paradox' exist as a physical phenomenon?
  60. Has the constancy of the speed of light been measured?
  61. Do radio waves get red shifted?
  62. Exactly why is the speed of light a constant in all reference frames?
  63. If you could slow everything down to a complete stop, would time pass infinitely fast to you?
  64. What is the fourth dimension?
  65. How is it said that relativity is tested billions of times a year, when there are only a few accelerator labs on Earth?
  66. Why can't time be considered a force?
  67. How does general relativity incorporate rotation?
  68. Can you use the geometry of gravitational lenses to determine the distances to the lensed quasar?
  69. If spacetime is a consequence of the deformation of space by gravity, why is gravity a universal phenomenon rather than a local one?
  70. What is the momentum of a photon?
  71. How could you get Doppler shifts with light if the speed of light never changes?
  72. How do we know gravitational lenses aren't just refraction off of intergalactic clouds?
  73. In the problem of the marshmallow moving at the speed of light, didn't you forget about its kinetic energy?
  74. Can you exceed the speed of light by manipulating space-time in some way?
  75. Do time and space become meaningless at the speed of light?
  76. Why is time so different from space in the way we experience it?
  77. How does a body actually bend space-time?
  78. Does relativity apply to 'sideways' angular motion or just motion along the line of sight?
  79. What the ding dang are gravitons?
  80. Why aren't all gravitational lenses perfect rings?
  81. Is gravity the exchange of gravitons, or the curvature of space-time?
  82. Do photons carry mass as suggested by the equation m = hf/c2 ?
  83. When a photon falls in a gravitational well, does its speed exceed 'c'?
  84. If gravity is a 'wave' of energy, does the concept of frequency have any meaning?
  85. Would the collision of two photons cause them to slow down as a result?
  86. Do astronomers take into account gravitational redshifts when they compute the cosmological redshift of a distant galaxy?
  87. If we see a galaxy moving near the speed of light, as they would also see ours, whose mass is getting bigger?
  88. Why are multiple gravitational lenses more common than rings?
  89. If you slowed down light, would it become less of a wave and more of a particle?
  90. If an object approached the speed of light, would it be transformed into light?
  91. Does gravity travel faster than the speed of light?
  92. Does matter really get infinitely compressed in a singularity?
  93. How does gravity propagate through space?
  94. 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?
  95. If there is no such thing as velocity except with respect to a reference object, how can there be speed limits in deep space?
  96. What is it that physically prevents faster than light speeds?
  97. Are we gravitationally affected by where an object is now, or where it was when the light we see started its journey to us?
  98. 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?
  99. How are electromagnetic fields and space-time related, and can electromagnetic fields exert a force on space- time?
  100. How do you reconcile classical quantum mechanics with general relativity?
  101. Why isn't it possible to travel in time?
  102. Is the constancy of the speed of light dictated by special relativity invalidated by the expansion of the universe?
  103. Where can I get a copy of Misner, Thorne and Wheeler's 'Gravitation' book published in 1973?
  104. If a photon travels at the speed of light, why isn't its mass infinite?
  105. What is the speed of light in megaparsecs per planck time?
  106. What fraction of the perihelion advance of Mercury is due to special relativistic effects?
  107. Why can't you choose the reference frame where time moves fastest as the one at absolute rest?
  108. What is the Einstein Shift?
  109. What is time?
  110. What is Dark Matter and does it produce gravitational lensing?
  111. If two objects approach at 75 percent the speed of light, why are they not traveling 1.5 times the speed of light?
  112. If light has traveled thousands of years, how could it possibly have aged according to relativity?
  113. Is it ever possible to determine whether a body is at rest?
  114. Does time pass for all massive bodies at the same rate?
  115. If you travel near the speed of light, do you enter another dimension?
  116. If you traveled at the speed of light, would the universe appear instantaneous?
  117. Do objects really contract when they travel near the speed of light?
  118. What happens to the fabric of space-time when an object moves through it near the speed of light?
  119. Is it true that the speed of light is slowing down?
  120. What visual distortions occur as you approach the speed of light?
  121. Is the speed of light a barrier of the same kind as the speed of sound?
  122. 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?
  123. If Einstein says that everything is relative, how can there exist any firm constants upon which to base a Unified Theory of Nature?
  124. Could the displacement of star images near the Sun be caused by refraction in the atmosphere of the sun, not by general relativity?
  125. What can you learn from studying gravity waves?
  126. What studies have been made for gravity waves?
  127. Why does time change when you travel near the speed of light?
  128. Could energy ever come to a point and create mass?
  129. What does Stephen Hawking currently think about time travel?
  130. Would our system of time change if we went to another solar system?
  131. Do worm holes allow all times to exist simultaneously?
  132. Does 'now' have a size instead of being a point in spacetime?
  133. Do electrons interact with past and future representations of themselves?
  134. If a blue shifted clock runs fast, why do fast moving particles decay slowly?
  135. Can gravity affect the speed of light?
  136. Is there such a thing as hyperspace?
  137. How do we know that the speed of light is the same in every galaxy?
  138. Is our perception of distances and sizes in the universe just an illusion?
  139. What is a space anomaly?
  140. What is a vortex, and how does it work?
  141. How do distant objects get magnified by space curvature?

    142. iWhy can't matter travel back in time?

  142. Is time linear, or could there be other dimensions to it as for space?
  143. Is it possible to exceed the speed of light?
  144. Can you see Cherenkov radiation in empty space?
  145. What is a space time continuum?
  146. Do the different estimates for the age of the universe imply that the speed of light may have changed?
  147. Is the amount by which light is bent important in cosmology?
  148. How is it that, according to general relativity, space can be curved?
  149. Is the cosmological redshift real, or just an optical illusion caused by dust?
  150. Is it difficult to know what is happening now if it takes light time to get here?
  151. Why does a particles mass increase as it moves?
  152. How do you actually use Einstein's famous equation E = mc-squared?
  153. Could you explain how gravitational lenses work, and what determines the number of images you see?
  154. Is anti-matter the same as negative mass?
  155. If we could travel faster than light, could we go back in time?
  156. Why is the speed of light a constant?
  157. Is there a limit to how fast you can go using gravitational boosts from the sun and planets?
  158. Is there a simple way to measure the speed of light?
  159. Does gravity travel at the speed of light?
  160. What is light, how does it move, and how do we measure its speed?

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