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Ask a Physicist

Here you’ll find questions readers have submitted about the science surrounding gravitational waves, along with Physicist Ben Owen’s responses. Although we’re no longer accepting new questions, we encourage you to browse through those listed here and check out the Resources section if you have other questions on gravitational wave science.

Question Topics

LIGO
Sources of Gravitational Waves
General Theory of Relativity
The Speed of Light

Gravitational Waves
Miscellaneous

LIGO

Q: How can one know that the length of one arm is exactly 4km (or so)?
A: Actually, what the interferometer measures is the difference between the lengths of the two arms…

Q: I understand that the gravitational waves scientists believe we can detect are from extremely massive events in space such as binary systems, pulsars, and supernovae. Is Einstein@Home searching for gravitational waves from a particular one of these source types?
A: The LIGO Science Collaboration is implementing searches for all those sources and more, but the one Einstein@Home is running is the "all-sky pulsar search"…

Q: Is the baseline between LIGO and the German detector long enough to be able to pinpoint the source of the waves? The Einstein@home screensaver displays the location on the sky of where it is searching, so how is this directionality achieved?
A: Physically, LIGO is passive. It sits where it is, and that's it. The “pointing” is done in the processing of the data…

Q: Have we had any positive detections of gravitational waves other than the detection of their effects?
A: Not yet. We just passed the thirtieth anniversary of the discovery of the first indirect evidence...

Q: What will physicists do if there is still no sign of gravity waves when advanced LIGO is operational? Will they abandon the search or modify the theory?
A: If advanced LIGO detects nothing after a year or two, that will be even more revolutionary than if it does. But I wouldn't bet on it...

Sources of Gravitational Waves

Q: Would so-called solid quark stars that undergo "glitches" produce gravity waves due to the changing moment of inertia. Could such a detection be used to prove or rule out the existence of strange quark stars?
A: A "glitch" is what radio astronomers call it when they see a pulsar's frequency suddenly jump up a little bit…

Q: In order to detect gravitational waves from a pulsar, wouldn't that pulsar have to precess about its axis since symmetrical configurations would not produce gravity waves?
A: Precessing stars do radiate gravitational waves, but they're not the only ones…

Q: Is there an estimate of the speed of propagation of gravitational waves?
A: Einstein's answer is that gravitational waves should propagate at the same speed as light...


General Theory of Relativity

Q: I had always thought that there is no up or down in the outer spaces of the Universe. This now seems not to be correct if matter causes curvature and other objects fall towards the curves. What am I forgetting here?
A: Matter does indeed cause curvature, and this curvature is present even out in space. But that's not the same as the notion of up or down…

Q: Do white holes exist? If so, how are they formed? Does time exist in white holes, since they are inverse black holes?
A: A white hole is indeed the inverse of a black hole, but we don't expect to find one in real life...

Q: Does the graviton exist? If so, why has it never been detected? How does this fit with Einstein's belief that gravity is actually the curving of space/time?
A: First, what's a graviton? On the microscopic level, the other fundamental forces in the universe work by exchanging "carrier" particles. The photon is the carrier of electromagnetism.

Q: I was always under the impression that gravity was a field. How can something
be both a field and a wave?
A: Waves are just a certain behavior of a field when it changes in a certain
way with respect to location in space and time...

Q: Do gravitational waves distort both space and time? I can understand the
warping of space and trying to measure the difference in stretch, but how
do you know a time distortion would not affect the LIGO split laser beam?
A: You could say that gravitational waves distort both space and time...

The Speed of Light

Q: If you were traveling at the speed of light and turned on the lights what would appear to happen? Also --- Does darkness have a traveling speed?
A: My experiments indicate that darkness leaves the refrigerator at least as fast as I can open the door…

Q: It appears that the speed of light isn't constant. At least it can be demonstrated that light can be manipulated by humans at room temperature (up to c times 1.4).
A: You hear a lot of claims like this in the news, and they often come with a claim of breaking the rule that nothing can move faster than the speed of light, "c"…

Q: Why does combining the principle of relativity with the invariance of the speed of light lead one to conclude that no medium is required for light to propagate?
A: James Clerk Maxwell's equations for the electric and magnetic fields imply special relativity, though it wasn't obvious at the time…

Q: I have heard that there are particles that travel faster than light emitted in nuclear reactors. Is this true? How can things travel faster than the speed of light?
A: Yes, it's true. In fact it's why radioactive stuff glows...

Gravitational Waves

Q: What amplitude, frequency and wavelength gravity waves are we expecting to find?
A: Each detector can find them only in a certain range of frequencies...

Q: If gravitational waves are successfully detected, can we expect that knowledge
to lead to new kinds of technological advancements?
A: Even though we haven't detected gravitational waves yet, there have already
been technical advancements spun off from the search effort...

Miscellaneous

Q: Is there any truth to the idea that dark colors attract heat and light colors do not?
A: You could say something like that, though "attract" isn't really the right word…

Q: Which would be considered more fundamental - the constant "c" (speed of light in vacuum), or the constant "epsilon_0" (permittivity of free space), or the constant "alpha" (fine structure)? Are they equally fundamental?
A: These constants all include the same information - they can be expressed in terms of each other (and some other constants)…

Q: Do you think that Albert Einstein had a big influence on the way people live their lives today? Do you think German people have a different opinion of him than English?
A: I don't know how opinions differ from one country to another, but Einstein has a lot of influence on how we live today, even if you only look at the more practical things...

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