Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Faster than the Speed of Light? How do we know?

Learn more physics!

Most recent answer: 04/09/2012
Q:
Would we be able to detect greater speeds than the speed of light if there were any?
- Szabo Sorin (age 21)
West University, Romania
A:
According to relativity, nothing can go faster than the speed of light in a vacuum (in space).

However, particles can move faster than the speed of light in a material, which is less than the speed of light in a vacuum.

When this happens, we get something called Cerenkov radiation. Cerenkov radiation is similar to a sonic boom. When a plane moves faster than the speed of sound, it creates a big "boom" that is a shockwave of energy. When a particle goes faster than the speed of light through a material, it gives off a shockwave of energy, and that is Cerenkov radiation.

Light can be slowed down in materials because light (photons) interacts with particles of the material. Then it is possible for a particle to move faster than the slowed down light, and Cerenkov radiation occurs.

We can measure this radiation and identify particles that travel faster than the speed of light in a material.

(published on 10/22/2007)

Follow-Up #1: Velocity of neutrinos as a standard?

Q:
In light of the CERN Gran Sasso result, should we not be using the velocity of the neutrino? If the neutrino interacts very weakly with matter, and we know that photons do, could this could make sense of the results? Didn't Willebrord Snell explain refractive index by the absorption and reemission of photons?
- Ernie Bennett (age 72)
Buderim, Queensland, Australia
A:
Hello Mr. Bennet. 
Welcome to our site:  you are our first customer from Buderim, Queensland, Australia.

You have an interesting suggestion but there are several problems in replacing the velocity of light with the velocity of neutrinos as a standard. 
1.  The OPERA experiment may be wrong.   It is the only one in which superluminal neutrinos have been detected.  Other experiments both at CERN and at Fermilab are planned that will verify or refute that result.  
2. The best relevant experiment I know of  is the concurrent observation of both neutrino and light induced flashes from a very distant supernova explosion SN1987A. The neutrinos were detected in  the Japanese Kamiokanda detector and others in the US and Russia.  If the neutrinos had traveled at the same speed as the CERN neutrinos they would have arrived months earlier.   See the nice discussion of this at:
3.  One can argue that the supernova neutrinos were of rather low energy whereas the CERN neutrinos were much higher energy and that makes a difference.  If so then that precludes the speed of neutrinos as being a standard.  Lorentz invariance is violated and speed of  "neutrino light" would depend on which inertial frame you were in.    Not good for a standard.

My personal belief is (and I very much hope so) that the OPERA result is flawed in some way.   The systematic errors in the timing are very subtle and involve corrections due to general relativity effects.  Keep tuned.

LeeH

Feb. 22: By a coincidence, just today the news came out that a loose connection in the experiment caused a 60ns timing error. It could turn out to be that simple. Mike W.

(published on 02/21/2012)

Follow-Up #2: revisiting CERN neutrinos

Q:
Why are you hoping that the CERN experiment is wrong? Is not science about truth and not about personal preference? Is it not more exciting to find out something new? Are we to assume that Einstein will be right forever? He is like all other scientists fallible. I fear his iconic status could even hinder the progress of science. You may be right now but some day another "Einstein" will come along. I would think scientists would look forward to progress.
- Chris (age 57)
Glen cove NY nassau
A:
We aren't committed to Einstein always having been right. He was almost certainly wrong when he decided that the cosmological constant was zero. He was wrong when he asserted that there would be some local hidden variables underneath the apparent randomness of quantum mechanics. His General Relativity will, it's generally assumed, turn out to ultimately be wrong as a universal theory, instead becoming an excellent approximation for events above some size scale, like most of classical physics. So our tastes aren't concerned with persons.

What would be weird would be for an extremely well-established theory of extraordinary logical simplicity and accuracy (Special Relativity) to be wrong on one specific fact right in the middle of a sea of facts on which it is exactly right. I can't think of any other scientific revolution that occurred that way. Even the little problem with the orbit of Mercury (the first symptom of problems with Newtonian gravity) was off at one edge of the phenomena, the largest gravitational potential for which precise data were available.

At any rate, our guess that there was an experimental problem in the neutrino results has been fully confirmed.

As for general conservatism in physics, the current conservative position is that there are really 10 space dimensions, not 3. We have a satellite up trying to explore whether there are traces of events from before the Big Bang, a possible collision in those higher dimensions. Einstein showed successfully that the laws of Euclid's geometry don't apply to our universe. We have in my lifetime concluded that for sure the universe cannot obey any form of Einstein's local realism. It's a recognized open question whether the quantum events that occur all the time have unique outcomes or a huge range of different outcomes. In other words, we don't know whether vastly more yous finished this sentence than started it. You're sure that we're too conservative?

Mike W.

(published on 04/06/2012)

Follow-Up #3: keeping up with new science

Q:
I thank you for that answer. It puts things in better perspective. I especially like the last part of your answer. That totally set me straight about science becoming conservative. It is hard to keep up with the latest information and i find the things you mentioned very exciting. Thanks
- chris (age 57)
Glen cove NY nassau
A:
Thanks for the kind words.

Mike W.

(published on 04/09/2012)

Follow-up on this answer.