# Space is Cold, But If You Move Faster, Does Space Heat Up?

The Unruh effect is basically the prediction that an accelerating observer will observe blackbody radiation, whereas an inertial observer wouldn’t observe any. In other words, it means is that empty space should feel relatively cold to anyone moving along at a constant speed, but one who’s accelerating would find that same space hot. Once thought to be impossible to measure, four theorists have just claimed to have devised an experiment that may well confirm the underlying physics of it.

But, not everyone believes it can be done and one such skeptic is Vladimir Belinski, a theorist at International Network of Centers for Relativistic Astrophysics in Pescara Italy.  He says, “The Unruh effect is nonsense, it’s based on a mathematical mistake.”  As observed and written in Einstein’s theories of special and general relativity, things appear differently to people in motion relative to one another. For example, if you held a meter stick in one hand and a watch in the other and had a friend run past at near-light speed, they would observe that your watch was ticking slower than normal and that the stick appeared much smaller than one meter. Also, if they were to carry the watch and the stick you would see the stick appear smaller and the clock tick slower.

If an observer accelerates things change slightly as the observer will find the vacuum hotter whereas one moving at a constant speed would measure the empty space temperature to be absolute zero. This was the idea behind William Unruh’s theory. He argued that an accelerated observer will detect photons and other particles whereas, with a non-accelerating observer, the vacuum is devoid of particles.  Physicists are unable to measure this effect directly as is too weak. So instead they decided to detect the fog of photons and other particles seen by the observer and the way in which they did this was by studying the light emitted by electrons.

The way that would work is like this:  Imagine you shot a bunch of electrons laterally across a magnetic field. According to basic physics, the electron will begin to turn circles in the field.  If we then applied a vertical electric field to give the electrons a shove upwards, the electrons would both circulate and accelerate upwards simultaneously, defining two frames of reference. Daniel Vanzella is a theorist at the University of Sao Paulo in Sao Carlos, Brazil, and has recently been conducting research in this area. Vanzella and colleagues began their studies in the accelerating frame where they anticipate the circulating electrons will encounter the fog of photons. The fog of photons heated up the electrons in the accelerated frame, making them radiate more in the lab frame.  Therefore, by observing the excess of long wavelength photons in the lab frame, you’ll know that the accelerated frame space is filled with photons.

Some say the experiment won’t work, but no one can agree as to why. Detley Bucholz is a theorist at the University of Gottingen in Germany, and he believes the that the vacuum may appear hot to the accelerated observer, but that’s because of the friction at play during the acceleration. So, the experiment may show the desired effect, but it wouldn’t reveal a fog of photons in the accelerating frame. On the other hand, Robert O’Connell, a theorist at Louisiana State University, says there is a fog of photons here, but it’s not possible to produce any extra radiation in the lab frame. He argues that although the fog of photons may well exist, the experiment won’t produce the signal. So it seems the Unruh debate is far from over…